- All upgrades to an engine will require careful
- Serious power increases will always require
- Engine reliability should always come before
power on a road car.
Upgrading a forced induction engine or converting an
NA engine into a forced induction unit requires careful planning and will
involve some form of engine modification. The higher the boost levels the more
consideration that needs to be given to the engine.
reliability and integrity are a major factor and can’t be underestimated. When
Toyota add a blower to their engines, even with mild boost wheels, they
strengthen their engines in various ways despite the need to build to a
forced induction, small engines are producing higher output of power and are
subject to higher mechanical and thermal stresses. Even when the heat and
pressure in the cylinders isn’t much higher, they stay to the peak for much
longer on the crank stroke. Parts like the cylinder head, valves, head gasket,
pistons, rings and upper cylinder area will be subject to a greater heat load
as there will be less time to shed combustion heat. Con rods are the
exception. They are generally put under more load on naturally aspirated
engines. But on excessive boost, these parts must also be considered.
components will suffer the normal distortions, but the distortion will be held
for longer so they may be inclined to remain distorted. An out of shape piston
creates more friction against the cylinder wall. So along with the additional
heat due to heat soak, it is also generating heat by contacting the cylinder
wall. It may appear to be easy to fix these type of problems by reverting to
stronger, heavier internal engine components but this will introduce its own
set of problems. If we retain the same rev limit, then the heavier components
will place heavier loads on the piston pins, con rods and con rod bolts and to
a lesser extent on the crank.
A lot of
supercharged engines suffer detonation. This is normally blamed on excessive
boost when in fact it is caused by ineffective piston ring seal. The blow-by
caused due to ineffective rings will blow into the crankcase and disturb the
lubricating oil on the cylinder face, spraying a mist of oil into the inlet
track, thus lowering the octane rating of the mixture, leading to detonation.
It also forces the rings away from the cylinder wall causing a further
build-up of heat on the piston, further encouraging detonation.
engine is designed and built properly, with attention to every detail then
reliability won’t be an issue.
Cylinder Block Preparation and Crank Testing.
cylinder block must be addressed first and foremost. Inspect the water
passages for rust and scale. Boil and clean in a chemical bath with all the
Welch and oil gallery plugs removed. Finish the oilways and stud holes with
brushes. Clean and dry with high pressure air. Green blocks (new) should get
the same treatment, ensuring the water passages are free of casting sand.
visually check for cracks and do a pressure crack test with the Welch plugs
attached and the head in place. All water passages should be blocked off.
Gradually build the air pressure to 40psi, watching out for Welch plugs or
other items flying out under pressure. Hold the pressure for 5min. If its
holding firm then go to 50psi. Anything after this can be dangerous, but if
you are using a heavy duty, thick wall race block and you’ve taken the right
precautions (protecting yourself from projectiles) then continue up to 100psi.
Slowly spray WD-40(or soapy water) over the entire block, looking for air
find a crack that is small and not in a critical area then it can be repaired
easily. Otherwise you will have to junk the block and get another one.
However, some blocks routinely crack in certain areas and its better to have a
repaired block rather than having one that hasn’t cracked, but probably will
as soon as its subjected to competition use. If you are aware of a point on
the crank that does give trouble during competition use then it’s a good idea
to a pre-emptive fix by reinforcing this area.
successful pressure testing, clean the head stud and main bearing cap threads
of dirt and burrs using a plug tap. This is essential so that you get a
correct tension reading when rebuilding. Chamfer any holes that need it so
that threads don’t pull up, causing head gasket sealing problems. A pulled
main bearing cap thread can let a bearing spin. Check the depth of each hole
to make sure the bolt won’t bottom out. Grind away any casting slag,
especially around the main bearing webs, the sump pan deck. This will minimise
the risk of localised expansion and contraction and therefore, cracks.
the block must be squared. The main bearing holes should be perfectly aligned
otherwise the bearings and even the crank will be destroyed, and at best it
will rob the engine of power due to frictional losses. So not assume that the
alignment is correct, especially if you plan to exceed the previous rpm limit.Any
miss-alignment can be corrected by line boring. This will also be necessary if
the main bearing caps are replaced by heavy duty items. The main bearing caps
should be numbered and have the front marked. This will help to correctly
align them every time the engine is stripped.
this check that the top of the block (deck) and the crankshaft centreline are
identically spaced from left to right (that they are parallel). If the block
is out, then milling will be required.
the cylinder walls must be exactly 90deg to the centreline of the crank and
must not be canted to the front or the read, to keep frictional losses at a
minimum and to maintain good ring sealing. There cant be any steps of tapers
at the top or bottom of the bore.
and honing to 0.0003in tolerance must be carried out. Small boring machines
that bolt onto the deck are not acceptable. The machinist cannot setup the
boring bar based on the existing position of the cylinders. A bore index plate
must be bolted the block as a reference to locate the bore centres correctly.
This ensures that the cylinders are centred over the crank throws. This also
allows different heads to be swapped between blocks with the combustion
chambers correctly matching the bores.
0.01in should be removed with each cut. This will cost more in labour, but it
will be easier to accurately hone because the boring tool marks won’t be as
and heat will warp the cylinder walls if they are overbored too much. This
will lead to blow-by past the rings. At high power outputs the main bearing
webs can break away from the bottom of the cylinders.
we only want to bore the cylinders to rectify damage or sloppy factory boring.
We are not seeking to increase the capacity of the engine. There is no point
in gaining a few CC’s if it means the power will be robbed through wall
flexing and warping. Most modern cast blocks only have a wall of 0.1-0.15in
when finished with a standard bore. However, due to casting the wall thickness
can be down to 0.07in in places. Added to this the fact that most
manufacturers will use soft cast iron in their blocks, such thin walls cant
afford to be bored any wider.
good ring seal a wall of 0.1in is needed if making 70bhp per cylinder. This is
especially true at the top half of the cylinder, where ring seal is critical,
first to make compression and then to retain cylinder pressure during
combustion. Towards the bottom of the cylinder we can afford to drop 0.02in of
thickness if necessary. But there are other considerations such as operating
rpm, crank stroke, main bearing web strength, detonation inclination.
per cylinder wall thickness should be 0.11in to 0.13in.
per cylinder wall thickness should be 0.15 to 0.18in.
per cylinder wall thickness should be 0.22 to 0.25in.
want an engine to seal well at 8500rmp for long periods of hard use then these
thicknesses should be used.
tuners get away with less, but they tend to scrap blocks more frequently and
the risk of main bearing web falling away is higher.
Measuring Cylinder Wall Thickness. Checking
for wall thickness and core shift (due to casting) can be difficult. You can
measure the thickness of the web between cylinders and then measure the
thickness of the water passage between the cylinders using feeler guages, but
this won’t allow for cast bore shift so it is meaningless.
testing can be used to give a reasonably accurate estimate of wall thickness.
Care and good judgement must be used to measure at the right spot. Also, this
method wont detect porous areas or areas of dirty metal. The measurements
taken may also be 0.02in less than indicated.
to so pressure crack testing along with the sonic test. It is also wise to
backup your measurements with basic measuring tools.
Strengthening Open-Deck Blocks.
blocks have more problems than wall thickness. Open deck designs with the top
of the cylinder unsupported or lightly supported will experience problems
handling extra horsepower and rpms. Cylinder and block flexing can become
Hondas, you can fit a block guard by inserting into the water jacket to fit
between the block and the cylinder.
Subaru’s with an open deck can require expensive mods to switch to a closed
K-series has a different problem. Even before supercharging, the block can
experience problems at 200bhp and above. The cylinder liners can split. The
solution is to shrink a steel sleeve around the outside of the liner up
towards the top.
task on a block is to hone the cylinder bores to their finished size. A 2in
honing plate should be fitted and torqued to normal head torque levels to the
block with a head gasket in place. This is needed to ensure perfect honing
(get it made when you are getting the bores bored). The block is then
distorted to the same level as when the head is fitted. The main bearing caps
should also be bolted on with the correct torque because these also distort
the block a bit.
should be honed to 0.004in of the finished diameter and then matched to each
piston. Keep piston clearance to a tolerance of 0.0003in.
method varies from block to block but the following general procedure should
220 grit stones, remove 0.003in of material to bring the bore to 0.001in of
final size. Use 280 grit stones, remove 0.0005in. Use 400 grit stones to bring
the bore to its final diameter. The result is a very accurate bore with little
break-in time required.
important to leave a cross-hatch pattern on the walls. A 45deg cross-hatch
pattern must be left with a finish of 10-12 micron. This type of finish means
you have to run the rings in, but they will last a long time and will seldom
leak. A smoother finish will cause a glaze to form on the ring face and bore
wall due to lack of lubrication. Oil consumption and power loss will result. A
rougher finish will lessen ring bed-in time, but they won’t last as long and
glazing might again be a problem because the rings grind off the walls and
raise temperatures which contribute to glaze forming.
and lower lip of each cylinder should be lightly chamfered to remove sharp
edges. Lightly dull the sharp edges of the main bearing caps and webs as well.
camshaft bearing bores must be in line and each tappet bore must be of the
correct bore and perpendicular to the centreline of the crankshaft. A tappet
tipped off centre may dig into the lobe of a racing cam causing wear or
breakage (pushrod engines).
available use high quality high tensile bearing studs. If standard main
bearing studs are used they must be new. Never reuse. Usually the standard
caps are OK, but for very high power engines use steep caps for reliability
and peace of mind. In very hot engines use a main bearing support saddle. This
is a one-piece item that supports all main bearings and sometimes replaces the
individual main bearing caps. It may extend to the oil pan deck and transfer
the bearing load so that it is shared by the outside of the block as well as
the main bearing webs.
the machining, the block needs another cleaning with warm soapy water. Blow
the block dry with compressed air and spray all the cylinders, tappet bores,
and bearing bores with WD-40.
engines use cast nodular iron crankshafts. These are OK is low rpm limits and
light internals are used. Some of them come from the factory with rolled
fillets which adds fatigue resistance. Some are also heat treated by using
power and rpm limits increase so does the need for a forged steel crankshaft.
increases the density of the metal-it is squeezed into shape and compacted to
give a stronger core and a better fatigue resistance. But it is the type of
steel used and the heat treatment received that determines a cranks
suitability for competition use. The steel must have a high tensile strength
(ability to resist breaking under high loads) and a high fatigue resistance
(ability to resist repeated bending and twisting).
production forged cranks made by Japanese and Euro makers are generally good
enough for high output engines. Some will feature rolled fillets and some will
be heat treated by the nitriding process. However, steel cranks made from 1053
grade steel need to be avoided, even if nitrided. Also note that some steel
cranks are twisted during the manufacturing process (more common on v8
cranks). The front and rear throws are twisted 90deg to give the required
crankpin orientation. But often the throws will not be at exactly 90deg and
will be 2 or 3deg out. They can still be balanced, but the counterweights
required to do this will place undesirable high loads in the high rpm engine.
Therefore, non-twisted forged cranks should be used, where the rod journals
are forged in place.
of these bad steel cranks are American (e.g. small block Chevy v8 steel).
Preparation and Balancing.
standard crank is to be used it must be crack tested before working on it. If
a magnetic type crack test is done, then it must be fully demagnetised. Next
check its straightness. If its not perfectly straight then it must be
discarded. It can be straightened but will likely revert once high loads are
applied to it. Measure each main bearing and crank pin journal. Crank journals
wear oval, so check the diameter at several angles. Measure at each end and in
the middle of every journal. Ovality and taper should be less than 0.0003in
out. If the wear is greater, the journals will have to be ground undersize.
But this will weaken the crank so it may have to be replaced.
casting slag should also be ground off to avoid cracks.
bearing inertia load the crank should be dynamically balanced. This will
reduce the shock loading and vibration that any imbalance will cause. This is
only necessary if planning to run the engine above 6000rpm for extended
periods, or if heavier than standard internals are to be used.
Group A Crankshafts.
Competition cranks are often forged from EN40B steel. In the US 4340 and the
inferior 5140 steel is used.
because high grade steel is used it is no guarantee of quality of strength.
The steel may be dirty or the alloy might not be upto spec. Some heat
treatments are better than others. The manufacturer may use an inferior heat
treatment system so that more batches of cranks survive the process. Such
cranks may be cheaper to buy, but they wont be cheaper in the long run if they
fail under load.
cranks will also be more expensive, but they will pay dividends by reducing
reciprocating weight and improving acceleration out of the corners and in
reduced bearing load.
expensive and best cranks are machined from a billet of high grade steel that
has been hammer forged. After this it is shot-peened and heat treated to
further enhance its strength and fatigue resistance.
should be fully counterbalanced – overall and at each crankpin. Balancing each
crankpin doesn’t modify the overall balance, but achieves an internal balance
for each throw. But if bearings can be replaced frequently and a crack test
can be performed regularly, then it might be a good idea not to fully balance
the crank when it involves adding counterweights. This will ensure maximum
modifications are undertaken to prevent bearing failure.
purely performance standpoint, the crank throws should be of equal lengths and
correctly indexed. Slight differences in length from cylinder to cylinder wont
make much difference, but incorrect phasing will knock power. If a throw is
out by 5deg this will have the same effect on performance as an ignition or
camshaft timing error of 5deg. Most cranks will have perfect phasing or
indexing, but check to be sure.
rods experience alternating compression and tensile loads. They have a tougher
job than any other internal part. A large number of failures are experienced
in competition engines due to rod malfunction or failure.
highest load is experienced when the piston is at TDC on the exhaust stroke.
The tension can reach 15,000lbs. This maximum load is experienced on the
non-firing stroke and is caused by the inertia of the reciprocating assembly
(piston, pin, small-end). At TDC the piston is suddenly stopped and reversedproducing
the high tensile load. On the compression stroke the load is not as severe
because pressure builds up slowly and because soon after TDC the load changes
to a light compression load.
loads are applied and removed on every stroke. Which is much more severe than
continually applying the same load. A rod has to survive millions of stress
cycles in its lifetime.
are made from carbon steel. Some are still cast and some are made from
titanium or aluminium. Cast iron is to be avoided; titanium can be used if the
budget is big. Aluminium rods can be used on drag cars and hill-climbers where
they can be replaced regularly. They are light and reasonable strong and
deliver less of a shock to the bearings when detonation occurs.
rods are made from 4340 or E4340 steel. The standard rod is called an I beam
rod and it is very strong. It is manufactured by respected rod manufacturers
such as Oliver and Crower. H beam rods are made by Carillo who are the masters
in making rods. Their success lies in their use
of the best E4340 steel and in
their heating and shot-peening techniques
(at great expense).
rod ratio is the rod length vs. the crank stroke. Most rod ratios range from
1.5 to 2.1. The average is 1.7.
A long rod
will cause the piston to dwell at TDC longer and to move away from TDC more
slowly. This does not affect the power output of engines with flat top or dish
top pistons. It is only on nitrous and NA high compression engines with very
high top pistons that combustion will be upset at high power and rpm range.
shorter rods, engine wear is a problem because increased cylinder wall and
piston loadings occur. For this reason, a ratio lower than 1.65:1 should be
avoided. A ratio of around 1.8:1 is better. Fitting a long stroke crank will
lower the ratio and will increase the tendency of the piston to rock in the
cylinder. Pistons with a reduced compression height and shorter skirt must be
used with a stroker crank. This aggravates wear and ring seal problems.
tensile strength of the bolt used must be 185,000psi minimum. High quality
boron bolts can be reused several times, but standard bolts must never be
reused. If the bolts are over tensioned they will fail. Of course they will
loosen if they are not tensioned enough. Reaching this preload is not just a
matter of using a torque wrench. The bolts and rod threads must be lubricated.
The under head area needs to be lubed. Only use the oil recommended by the
If engine oil is recommended and you use diff oil you can end up
over tensioning by up to 10lbs.
the bolt tension of the bolt manufacturer even if this is different to the
Use a good
torque wrench and tighten the bolts to the correct torque and loosen the bolts
twice before finally tightening the bolts…
never reuse standard bolts and only reuse boron steel bolts three or four
have to provide a low-friction wearing surface while absorbing tremendous
shock loads. Only top quality trimetal bearings should be used for the main
and bigends. White metal bearings can be used for the camshafts. Vandervall
lead-indium and Clevite 77 bearings are best and can stand up to the worst
conditions. These bearings are steel backed, with a copper-lead intermediate
layer that gives the bearing good fatigue strength and load carrying capacity.
The running surface of the Vandervall bearing is a precision overlay of
lead-indium, the Clevite 77 overlay is lead-tin.
clearance is important. Too much clearance causes knocking and pounding and
higher frictional losses and increased oil consumption. Excessive clearance at
the big ends will lead to oil starvation at the main bearings. Too little
clearance will cause rapid bearing failure due to high temps and too thin oil
films forming. Generally we should have 0.0009in clearance for each inch of
shaft diameter for main bearings and 0.0012in clearance for each inch of shaft
diameter of rod bearings when running 15W-50 fully synthetic. With 0W-30
clearance can be a little tighter.
following is a good guide for trimetal copper lead bearings.
Clearance side of bearing
0.004 to 0.006
0.005 to 0.007
0.005 to 0.007
0.005 to 0.007
0.006 to 0.008
0.007 to 0.009
0.007 to 0.009
aluminium alloy block, more expansion takes place so modified clearances must
be used. A clearance of 0.002in cold will increase to 0.004in when hot. The
engine should be assembled with tighter main bearing clearances when cold.
With such tight clearances, the oil and water should be preheated before
start-up – on race engines obviously.
procedure must be followed when fitting bearings.
the inside diameter of the bearings and big-end housing without the bearing.
Do the same with the crankshaft main and crankpin journals.
the bearings in solvent to remove the protective film. Do not damage the
overlay, do not use any abrasive material.
worry about a rough overlay – it will be flattened after installation.
the bearing shell thickness and double it. The
difference between the shaft and housing diameters and the doubled shell
measurement is the space left for running clearance.
you have calculated the running clearance for each bearing, some might have a
little more clearance and others might have a little less clearance. If this
is the case, the manufacturer might have 0.001in oversize and undersize
bearings available. If you want even less you can mix one bearing with another
and get a difference or 0.0005in. If this is the case use the smaller bearing
on the top for a main bearing and on the bottom for a big-end.
shells and check oil hole alignment, any misalignment should be corrected with
a small file.
all the bearings with engine oil and fit the crankshaft.
main bearing caps in the right order and orientation
Gradually tighten the bolts.
final tightening, tap the crank to each side with a soft hammer to line up the
the crankshaft end float, it should be about 0.005in with a cast iron block.
If its bigger than this fit bigger thrust washers.
the same procedure for the big-end bearings.
shells are pushed into their housings they should feel springy and snap into
place. If not the bearing housing is distorted or oversize bearings are
required to match housings that have been bored oversize.
Loctite on the threads to avoid problems with loose bolts.
states of tune demand unslotted cast or unslotted forged pistons.
recognise the piston type… cast pistons have intricate under crown shapes
around the gudgeon pin boss, while forged pistons are smooth inside and lack
intricate struts and braces.
hypereutectic pistons can be a good choice for highly tuned NA engines because
they are lighter than forged pistons, but for forced induction engines, forged
pistons are the only choice.
pistons are much denser, leading to high tensile strength. The can withstand
higher pressure and heat loads and their higher density improves their
thermal efficiency to the extent that they run 30degC cooler.
appear round, but the skirt is actually ground oval and it tapers from bottom
to top. Both of these features prevent seizure. In operation the top of the
piston is twice as hot as the bottom of the skirt, so it expands more. Also,
because of the heat in the pin bosses, the piston is elongated across the
piston pin axis. Most pistons are 0.005 to 0.012in less in diameter across the
pin axis. Only measure piston clearance across the thrust axis (at the bottom of the skirt or near the pin – check with
performance engines need more clearance than standard engines. Check with
supplier for the correct clearance but in general use a min of 0.002in.
with cast pistons and bores of 80-110mm use 0.003in.
with forged pistons and use 0.0012 to 0.0015in per inch of bore.
with forged pistons and use 0.0015 to 0.0018in per inch of bore.
some JE Forged pistons use just 0.003in clearance.
should be a max of 0.0005in difference between the tightest and loosest
cylinders. If its more than this, try swapping around the pistons in different
cylinders. Then mark the position of each piston. Do not mark the crown incase
it is machined later.
to Piston Clearance.
0.06mm vertical clearance. For push rod engines use 0.08in for the inlets and
0.01in for the exhausts. The cut-out diameter for safety is 0.12in greater
than the valve head diameter. This can be reduced to 0.05in for engines with
no carbon build-up on valves or pistons.
to Cylinder Head Clearance.
of each piston must rise to the same point in each cylinder (deck height of the piston). A good engine with steel rods
can run with 0.04in clearance between the piston and squish area of the head.
If the head gasket is 0.03in when compressed, then we can run a deck clearance
of 0.01in – all the pistons should then be machined to reach this clearance.
aluminium rods, allow 0.07in clearance to allow for the additional expansion
and stretch. Engines with steel rods, but with block flex or crankshaft whip
will require 0.06in clearance.
The top of
the piston is a part of the combustion chamber and the profile of the top of
the piston influences combustion as well as maximising compression.
profiles simply maximise compression without regard for compression,
combustion and exhaust. Tuners worry about modifying the shape because of lost
compression but the gain in flow and better combustion can far exceed the
slight drop in compression ratio.
the flame to travel smoothly across the piston dome and back towards the
squish area. Abrupt edges on the compression lump will disrupt the flame
leaving pockets of unburnt mixture. Rounding the edges of the compression lump
helps alleviate this.
exhaust valve cut-out must not be laid back. The sharpness of the exhaust lump
also affects flame propagation, but cylinder scavenging and exhaust flow are
usually better with the sharp exhaust lump.
completeness can be assessed by examining the coloration of the piston after a
few hundred miles of use. The piston dome colour will indicate where flame
propagation is being stifled (this method is
the compression domes lightly until the carbon build-up is more even in
quantity and colour. It will not be necessary to machine the entire uncoloured
area of the lump. Just machine the area in front of and around the start of
the uncoloured area – relative to the direction of the flame travel.
and turbos may require a lower compression ratio to avoid detonation.
Conversion from NA will definitely need lowered compression.
pistons should be used for best combustion control. They have a flat band
around the outside of the crown that will come into close contact with the
head and provide the required squish for good burning.
low boost, high rev engines use two rings, but most engines use three rings. The first
compression ring contains the combustion pressure and dissipates most of the
compression ring backs up the first, but its primary purpose is to carry more
heat from the piston. It also assists the oil ring in scraping off excess oil
from the cylinder wall.
oil control ring scrapes oil from the wall and ensures enough oil remains
behind to lubricate the upper rings and assist in sealing.
rings cause a good deal of friction and therefore cost power. To give maximum
flexibility in choosing rings, the manufacturers supply three grades of rings.
Standard tension for normal engines, low tension for competition engines where
almost the same control is retained but the frictional losses are greatly
reduced, and high tension for the odd engine running at very high rpm and poor
length to stroke ratio.
lots of ways to get heat out of the pistons, but the most effective way is by
using the piston rings.
What we do
with the piston rings has a big effect on piston heat dissipation.
ring land is the thickest and is 9mm from the crown. The second ring is 6mm
down from the first and the oil ring is 3mm below the second ring. The further
down the piston crown the cooler it gets, that’s why the rings are closer
inherent radial tension holds it against the cylinder wall to a small extent,
but it is the gas pressure behind the back of the ring that really forces it
against the wall. Ring float or flutter can occur in high rpm engines which
limits heat transfer from the piston and can lead to detonation. This occurs
because the weight of the ring causes it to break contact with the walls at
TDC. Radial tension is unable to prevent blow-by caused by the ring flutter.
The soln is to replace the rings with new ones.
narrower the ring the higher the rpm that flutter will occur, however,
reducing the ring width will also reduce the heat transfer from the piston.
The top ring land may distort and at worst the piston will melt.
rings also have reduced service life.
majority of performance engines running to 8000rpm the top and second ring
should be 1.5mm thick. For 9500rpm engines use 1.2mm top ring and a 1.5mm
second ring. Above 10500rpm use a 1mm top ring and a 1.2mm middle ring.
reducing the width of the rings you can also drill some lateral gas ports
above the top ring to maintain the pressure behind the ring. These ports help
an ordinary ring work like a Dykes L ring by ensuring there is always an open
path for gas to travel behind the ring and push it out against the bore wall.
ring can also be pushed up by pressure building between the top ring and the
second ring. Gas escaping past the first ring cant easily get past the second
ring and a pressure build up occurs. The top ring then breaks contact with the
bottom of the top groove and combustion pressure is lost. We can combat this
by increasing the end gap of the second ring or increase the space between the
have an engine running extremely rich especially on methanol or nitro,
cylinder lubrication is marginal. Also a huge volume of fuel can escape into
the oil sump and dilute the oil.
rings appear to stem the flow of fuel into the oil. The rings and cylinder
bore then bed-in better. The lube oil isn’t diluted as much and the engine
retains more horsepower by keeping the bearings and cam lobes healthy.
However, a better solution is to lean up the mixture. If the mixture is super
rich to provide cooling then maybe another area is deficient and the rich mix
is masking this problem.
We have to
be careful to fit each ring perfectly square in its bore and then measure the
end gap and increase it if necessary by filing the ring ends. The min gap per
inch of bore for supercharged engines is 0.006in. This is also suitable for
light nitrous engines. Competition nitrous engines need 0.0065in per inch of
bore. Oil ring rails must be 0.004in per inch of bore, but more wont hurt.
engine is running, the gap of the top ring will reduce by 50%. If the engine
gets close to detonation, the gap will be almost closed. Check your old rings
to see if the ends are polished. If they are then they were touching and the
gap should be increased by 0.002in. If you don’t the rings could break.
engine should have groove clearance of 0.001 to 0.0015in.
breakage is rare and can usually be contributed to worn piston ring grooves
which allow the rings to move around and break, or excessive taper of the
bore, causing radial flutter and ring breakage. If new rings are fitted to the
old work grooves they will break again.
nodular cast iron is normally used to make high performance rings. Normal cast
iron is used to make standard rings.
top ring use moly-filled nodular cast iron rings. It has three times the
strength of standard rings. It is ductile rather than brittle and can be bent
without breaking. Molybdenum belongs to the same chemical family as chrome. It
has a lower coefficient of friction and high resistance to abrasion. Its
thermal conductivity is several times higher than that of cast iron or
chrome-plated cast iron. Its porosity acts as an oil reservoir, reducing
scuffing and cylinder wear.
control use the multi-plate pressure back type. This minimises frictional
losses to the min while at the same time maintaining adequate oil control.
Only low tension oil control rings should be used for competition engines.
fitting rings the wrong way up and avoid damage by incorrect installation.
They should be expanded sufficiently to fit over the top of the piston and be
allowed to drop into the groove. Expander tools are available for this, but a
better way is to use two feeler guage blades placed between the ring and
piston which prevents scraping.
rings are marked TOP or have a dimple mark to indicate the top. The bottom of
the ring should be wider than the top.
torsional twist ring is fitted with the inner chamfered edge uppermost.
seal type second ring is fitted with the serrations in the ring face
position the gap ends to minimise oil and gas leakage(although this doesn’t
really matter because of eventual ring rotation).
entire piston in engine oil before fitting into the engine.
initial bed-in is done be giving the engine a full throttle burst for a few
seconds(this forces the rings out against the bore walls), followed by
snapping the throttle shut(this causes a vacuum in the cylinders drawing oil
up) and coasting for a few seconds. This is repeated 12-15 times with the
engine at normal operating temperatures. Accelerate in top gear from the
slowest speed it will pull in that gear. This minimises the risk of glazing
and allows the ring face and cylinder wall to cool.
initial bed in, the engine can be driven at 80% of power, but constantly vary
the engine speed, otherwise, the rings may still glaze. After a half hour of
this on the dyno the rings are bedded in.
engine with chrome rings need 200 miles preferably in one session to bed in.
Avoid constant high speed until 700 miles are completed.
should really be initially run in on the dyno. When the engine cools, the
tappets are adjusted and the head retensioned.
warm up, full load tests are done to determine ignition advance, fuelling etc.
By the end of the tests the engine is fully run-in having spent four hours on
guage should be connected to the crankcase and tested at full power. This will
give a quick overall indication of the ring seal. If the power is down and the
blow-by guage is reading high then the rings aren’t sealing properly.
leak-down tester will reveal which piston rings are leaking. Run the engine to
normal operating temp, remove all the sparkplugs and bring piston 1 upto TDC
on the compression stroke, connect the leak-down tester to the spark –plug
hole and compressed air to the tester. Ideally we want 2-3% leakage. 5% or
more and there is a problem. Do the same test on each piston at TDC.
leak-test should be carried out regularly. In a competition engine it should
be carried out after every event. If there is a problem then the engine should
be stripped, the walls lightly honed and a new set of rings used, touch up the
valve seats and replace the valve springs and bearings also.
harmonic balancer is attached to the nose of the crankshaft and extends the
durability of the engine. Most engines use a pressed metal drive pulley spot
welded or bolted onto the crank. These are liable to break-up at much above
6000rpm so should be replaced with a cast or machined pulley.
balancer dampens crank torsional vibrations.
fitting the harmonic balancer, spray it matt black and file a large groove at
TDC to match the engine timing case. Paint the groove white or silver. Also
paint the area around the timing marks black and paint the timing marks
themselves silver. This will make timing adjustment and checking much easier.
flywheel is attached to the other end of the crank. Two good dowels and
retaining bolts coated with Loctite are used to hold the flywheel in place.
reduction in the engines reciprocating mass or rotating mass will improve
acceleration. Lightweight pistons, rods, cranks, clutches and flywheels all
the flywheel also increases crank service life by reducing the twisting load
on the end of the crank. There is less risk of flywheel explosion due to less
use, the flywheel should only be marginally lightened, for full performance
the flywheel should be lightened as much as possible while retaining the
required strength to function safely.
weight of a road flywheel minimises lumpy idling and low speed surging. A
heavy flywheel better absorbs the uneven torsional impulses coming through the
crank and keep the engine turning smoothly at low engine revs. A highly tuned
engine with a heavier flywheel will be down a little on overall acceleration
and response, but will be much more pleasant to drive.
the flywheel you can machine your existing flywheel or choose an aftermarket
lightened flywheel. There may also be a lighter flywheel available from the
same type of engine fitted to a smaller car, or an engine of smaller capacity
made by the same manufacturer, but fitted with a lighter, compatible flywheel.
lightening your own flywheel, only remove metal from the outside. Never remove
metal from the centre or close to the centre. You will improve the overall
flywheel strength and durability by removing some metal from the very outside
of the flywheel. This reduces stress on the centre. To prevent distortion,
don’t overdo it. Don’t remove so much metal on the clutch side, such that
clutch slip happens due to overheating.
Cylinder Block Deck.
needs to be perfectly smooth, free from rust, grime or any foreign matter.
Take care not to allow dislodged material fall into the oil or water passages.
Never use emery paper or light sandpaper. Abrasive material will lodge in the
cylinders and the cam and lifters. Compressed air will only drive the grit
further into the engine.
gasket will have to withstand pressures up to 1800psi. The stock gasket will
not do on an up rated engine. Even a light rise in boost will require a better
head gasket. Its not he boost in itself that wrecks the gasket, it is
detonation. No matter what the boost level, the gasket will be OK if
detonation never occurs(excluding head lifting etc due to boost).
asbestos gaskets are too soft. Stainless steel sheet gaskets are too hard to
deform into minute imperfections. Steel/copper/asbestos types work a bit
better. High performance composite gaskets use a cylinder fire ring which can
better withstand detonation. A round wire steel ring is OK for cast iron
heads, but for aluminium heads they are too aggressive, so a pre-flattened
steel ring gasket or a round wire hard copper ring gasket may work better.
specialised sealing methods a stock composite gasket or a thick full copper
gasket can be used with special O rings fitted around the cylinders. With
softer composite gaskets, the W ring is preferred, followed by the O ring, the
mild steel round wire O ring may also be preferred with solid copper type
gaskets. The usual setup is to machine the block to accept 0.041in mild steel
wire so that is stands proud of the deck by 0.007in. It therefore exerts a
high compressive force against the gasket and head.
big destroyer of head gaskets is insufficient clamping force, allowing the
gasket firing ring to flap during violent combustion. Poor design of the head
or block or ineffective clamping (read, head or stud
bolts) is usually the cause.
the bolts with head studs will improve the clamping force and reduce
distortion and pulling of the block deck. When initially fitting the studs,
torque them to 30ftlb. Note that after fitting studs, it may be more difficult
to remove the head when its in the engine bay. Some factory blocks have bolts
removed to make manufacturing easier. Re-inserting these bolts and torquing
them to 20ftlbs can eliminate leakage problems.
still have a problem, then check the integrity of the deck of the head. If the
deck of the head is so thin and lacking in internal bracing then it will bow
under pressure and not clamp tight against the head gasket. An aftermarket
head might have stronger, thicker deck. Otherwise, try to strengthen the
existing head. The best bet here is to find a race team that uses this head
and ask them how they deal with the problem.
deck of the block is bowing then we have to switch to a dry deck block. All
water passages and oil return pipes in the block should be blocked with
threaded plugs. Any holes that cannot be plugged must be welded shut. This
strengthens and braces the block. An external water tube must then be devised
to carry the coolant into the head and an oil return to the sump (wet sump).
was to strengthen the deck is to glue and screw a 8-12mm steel plate to the
deck. After bonding and curing, the deck plate is surface ground and the
cylinder bores honed. Longer con-rods can then be used and/or more crank
stroke can be used. We have to use a piston design which keeps the top of the
top ring away from the new deck plate.
A lot of
engines was stretch-to-yield head bolts. These can never be reused. If they
are reused they will continue to stretch and not provide the clamping force
necessary. If sticking with a factory setup, then follow the manufacturer’s
instructions to the letter when fitting the head.
bolts should have their threads buffed and oiled(along with under the head of
the bolt) and be tightened according to the right sequence. Reverse sequence
to remove the head. Use four or five progressive steps to avoid head warpage.
After the gasket has time to bed-in(15-30min) tighten again to the final
torque. if the head is alloy that’s it. If it’s a cast iron head then warm-up
the engine and retension. Do 300 miles and retension again. If the
head has long bolts on one side and short on the other then tighten the longer
bolts 10% more.
Cylinder Head Heat Treatment.
have an aluminium head from an engine that has previously overheated then it
might be annealed and no gasket, studs or good decks will give a good seal.
The hot gases destroy the heat treatment on the head and it goes soft and cant
hold its shape and allows leakage on the deck. It also gets shorter and
narrower. If the studs don’t fit well, or the bolts are hard to get started
then the head has been annealed and needs to be replaced.
It can be
checked with a Brinell or Rockwell hardness tester. On a Brinell scale the
head should read 95 or more, anything under 75 is too soft. On the Rockwell-B
scale the hardness should be between 48-60. Anything under 38 is too soft. The
tests must be carried out in several places close to where the head gasket
contacts the head… upto 20 tests.