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Overview of Blowers (Superchargers)

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 performance.
 - 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 over performance.
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 pressure.

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 inefficient.
 - 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 performance.
 - Turbos for maximum performance.
 - 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 aspirated engine.

Roots Blowers.
 - 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 manifold side.

Centrifugal Blowers.
 - 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.

Blower Lubrication.
 - 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 circuit.

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 first place.

Boost Control.
 - 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.

Backfire Damage.
 - Not a common problem, but needs to be addressed.
 - 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.