Here we go into the detail of blowers and discuss the different designs that
were tried and the designs that are in use on competition and road cars today.
Choosing Cams for Blown Engines
- Major role in blower
- Standard cams induce inlet
back pressure. Promote high temperatures.
- Race cams offers more time
to expel exhaust gases.
- Race cams produce lower
overall boost, but more usable boost.
Cam duration and valve overlap have a lot of
influence over how a blown engine performs. In fact, apart from the boost
delivered by the blower, the cam setup has the next most important influence
At low engine speeds, there is a lot of leakage between the rotors and the
casing of the blower.
A stock cam has a small amount of overlap, so at low rpm only a minor amount of
air charge will be lost through the still open exhaust valve.
A race cam with say 100deg overlap and the inlet and exhaust valve 0.3in open at
TDC, will lose a large amount of air (boost) straight through the exhaust at low
rpm. At high rpm, the situation is reversed. There is still leakage in the blower
itself, but compared to the volume of air involved its minimal. During the valve
overlap period, the pressure waves generated in the inlet and exhaust tract now
restrict charge loss into the exhaust. Less loss means more boost – power. As mentioned, cam lobe design will play a major role in determining boost
pressure after the blower drive ratio has been set.
Despite the above facts, milder cam lobes on blown engines is not always best.
Boost pressure will be maximised but this wont necessarily mean more power. When a blower is driven at high engine rpms its efficiency quickly drops off. It
starts to put a lot more heat into the air. More heat means more boost pressure,
but it also means less air density (less oxygen in the air). With a mild cam,
the inlet valve closes early so the air being forced out of the cylinder has
nowhere to go. It gets forced back towards the blower and therefore registers a
high boost value, despite the fact that its not contributing to higher power.
This lobe design reduces blower efficiency, pushes up temperature, and increases
the risk of detonation. With a race cam on the other hand boost is reduced but the cylinders have time
to procedd output from the blower. More air flows into the cylinder rather than
getting trapped in the inlet tract against a closed inlet valve. At the time the
inlet valve closes, cylinder pressure closely mirrors boost pressure. On the
other hand, with a mild cam, cylinder pressure is lower than registered boost
So charge loss with a race cam is a mixed blessing. At low rpm, fuel is wasted
and power is lost because some of the blower output has gone out the exhaust
valve. Coupled to this, the blower has consumed engine power to produce this
boost in the first place so the power loss is compounded. However, at higher engine speed, pressure waves reduce through flow. And loss
out of the exhaust valve becomes a net gain because it purges the cylinders of
excess exhaust gas. Additionally, this flow takes a lot of cylinder heat with it
which promotes det free operation. A cooler charge is ingested which increases
oxygen content and increases power output.
Blower Size and Effeciency.
- Match the blower to the boost.
- Small blower with big boost is inefficient.
- Big blower with low boost is leaky and
- Lysholm blower is most efficient.
Different size and design of blowers have a band
of flow and pressure at which they work most efficiently. Outside of this,
excessive heat is introduced to the charge flow. The further outside the
efficiency band, the more heat is introduced. The Lysholm screw and the centrifugal blower have reduced parasitic power drain
and greater operating efficiency producing a much cooler charge. In addition to
this they are also the most compact designs available.
Blowers or Turbos.
- Blowers for low to medium
- Turbos for maximum
- Blowers if retaining NA
compression and pistons.
- Turbos for competition and
highly modded blocks.
Because of the rapid increase in availability of
blower models from manufacturers and from aftermarket suppliers, the blower has
become a real alternative to the turbo for big power road cars.
Turbos are not always the best way to go anymore.
A blower can be a much better option for a road car. High top end boost is often
unusable on a road car without making extensive changes. So if you are planning
to keep your stock gearbox, retain your existing compression ratio (retain your
pistons), and not go nuts on strengthening your engine internals then a blower
can be a much more suitable and satisfying option when adding to your naturally
- Low cost, very popular.
- Good low to mid range.
- Can be inefficient.
For low cost and low boost a Roots blower can’t
be beaten. However, they are the most power hungry and the least efficient of
all the major blowers.
The Roots blower can be made more efficient by injecting water or fuel into the blower. This closes the gap between the rotor and the casing, which improves pumping efficiency.
Boost goes up, so a lower ratio gear can be used, cutting parasitic losses a little. The evaporation of the water or fuel takes heat out
of the charge and provides an intercooling effect. Methenol is particularly
effective in this regard, but water is also very effective. Needless to say, if intercooling is employed, then the water injection should be kept on the inlet
- Similar characteristics to a turbo.
- Better mid to high performance than a Roots.
- More responsive than turbo.
Centrifugal blowers look a lot like turbos and
are efficient like turbos. But they also share the turbos reluctance to produce
low down boost. I they are geared for low down boost, then they produce
excessive boost at high rpms. Some see this as a good thing but in general it’s
to be avoided or worked around. Being belt driven they will always be more
responsive than a turbo. On front wheel drive cars, a lack of low down boost can
be less of a problem because the car will probably suffer uncontrollable wheel
spin at low speeds and low gears.
Lysholm Screw Blowers.
- Best but expensive.
- Good low, mid and high performance.
- Excellent efficiency.
- Not used very much outside of competition.
There are the daddy of supercharger blowers. They
are as efficient (or more efficient) as the Centrifugal blower so they don’t
require a lot of charge cooling. They are suitable in high boost applications
working upto 50psi. At low rpm they make even more boost than a roots blower,
with boost starting to come on stream at 1000rpm.
So when the widest possible power range is required with high compressor
efficiency and high top end boost, the Lysholm Screw is the only option to go
for. The reason all blowers aren't Lysholm blowers is down to cost. They are easily
the most expensive option.
- Easier lubrication arrangement than turbo.
- Fully synthetic is a must.
Blowers are generally very reliable and arguably
more reliable than turbos.
Lubrication is usually provided from an internal oil reserve. Some centrifugal
blowers have to be connected in a similar way to a turbo – via the engines oil
The main area for concern is the drive belt and the mounting brackets.
Consideration must be given to the power that the drive must carry to operate
the blower. In hard driving or competition these loads must be retained for long
periods of time. Ordinary cam type belts that last for 1000s of miles on NA cars
might not last 5minutes on a blown car. The mounting brackets must be sufficient
to house the blower and pulleys in a way that keeps the belts properly aligned
and don’t flex under sustained full power operations. The mounts and idler
pulleys must allow for the correct tension to be dialled into the belts in the
- Drive ratio determines boost limit.
- If drive ratio allows over max boost, then a
blow valve is used – inefficient.
Boost control for blowers is usually determined
by choosing the correct drive ratio. Different drive wheels are often referred
to in terms of psi(eg, the 8psi wheel and the 11psi wheel etc). However, with
centrifugal blowers being driven at high speeds to allow good low down boost, it
might be necessary to limit boost at the top end by venting to the atmosphere.
An ideal setup is to use a blow off valve which is controlled using the EMU or
an add-on electronic controller. If an intercooler is used then the valve should
be placed before the intercooler to improve charge cooling efficiency. If the blower has on bypass valve then a dump valve will have to be used.
- Not a common problem, but needs to be
- Centrifugal blowers not affected.
- Avoided thru good design.
An often overlooked area to consider when fitting
blowers is the risk of backfire or flashback damage. Centrifugal blowers aren’t
at risk but the other types are. EFI systems which spray fuel at the back of the
inlet valve are also pretty safe. However, if the cam has long duration and big
overlap, and if fuel is introduced into the blower or the plenum chamber, there
will be danger or a backfire. When the flame hits the fuel vapour it explodes
and causes serious damage. Rotor and gear damage is the usual damage done, but
in extreme cases the blower will explode.
This can be protected against by placing two vents in each end of the plenum
chamber. They should be set to open at 1.5 times max boost. The situation can be avoided altogether by elimination valve bounce and by
investigating any tendency for an inlet valve to hang in the open position.
Valve bounce is caused by the driver overspeeding the engine or due to weak
valves. An inlet valve sticking open can be due to valve gear binding problems
or a bent valve stem or insufficient valve stem clearance.
Hydraulic lifters are to be avoided when using blowers but if you have no choice
but to use them be sure to avoid over revving, test the lifters often, use oil
which doesn’t foam easily and control oil drain back to the sump, keeping it
away from the spinning crank and rods as much as possible. In wet sump setups,
keep the oil level well below the crank by not overfilling and by using adequate
baffling to avoid surge. In a dry sump, use a properly designed oil tank with
adequate volume, height and plates.
Waste spark ignition must also be avoided.