Forced induction? Compression?

Ok I got 2 questions that have really puzzled me:

1) How do I lower the compression of an engine? Stroking and boring will only increase horsepower and torque right?

2) What does F/I do? I know it shoves more air into the cylinders, but what's the point when you are going to lower the compression just so the engine doesn't detonate? Doesn't compression = power? An N/A engine has high compression, but no blower, a turbo'd engine has low compression but the turbo will blow air into it, how does a turbo have SOOO much more power if it cannot squeeze anymore air into a cylinder than a high compression engine would?

Oh and one more question:

Why do superchargers have blow-off valves? I don't see much of a reason to have one

1) compression really depends on a number of things. You could lower compression with a thicker headgasket, new pistons, larger bore. But it all really works hand in hand.

2) Power is also gained with VE (volumetric efficiency). Turboed cars can reach a higher volumetric efficiency since air is being forced into the heads. It's always a big choice to make when deciding on compression. some say 8.0:1 is good for top end turbo power and others say 8.5 - 9.0:1 is good for topend and nice for some bottom torque. With lower compression the engine will be able to flow a lot more air, more flow = more top end, but the trade off is loss of low end torque since you're relying more on the turbo for hp.

i hope this makes some sense, if there are any errors, someone correct me please. t hanks

turbos compress air to get more air into the same size area. the more air, the more fuel. lower compress normally does mean higher top end.turbos have more power because they have a bigger top end. they dont have ass much as n/as at low end power. off the line a n/a will beat it, then later the turbo will catch up when the turbo starts spooling.

superchargers have bovs to make sure that it doesnt put too much air into teh engine. if there is more air than it can put in, the bov will open to let the extra air out.
pistons and headgaskets are the best way to lower compression.

Well, thanks guys, it was good to know :)

I have quoted Corky Bell on this topic before and I will do it again:
"CHANGING A COMPRESSION RATIO. A variety of methods exist to change a compression ratio. Almost all are unacceptable. The crux of the matter is upsetting the "squish volume" around the rim of the chamber. A chamber is designed so that the charge is pushed toward its center as the piston achieves top dead center. This is perhaps the strongest deterrent to detonation designed into the system, as it tends to either eliminate end gas or keep charge turbulence high. This squish volume is a rim about .3 to .4 inch wide around the chamber, an approximately .04 thick-a big, washer-shaped volume between piston and head. Consider "squish volume" sacred and do not tamper. It is possible to err so badly in removing the squish that a resulting 7-to-l compression ratio may ping worse than a 9-to-l with proper squish. Clearly then, choices for reducing compression ratio are limited to opening up selected parts of the head side on the chamber, installing a new piston with a dish in the center, or remachining the original piston to create a dish. It is perhaps a little risky to undertake remachining a combustion chamber, because the thickness of the material is usually unknown. Furthermore, chamber shapes are closely controlled features of most modern engines. If the chamber must be recut, ultrasonic inspection can determine the material thickness. Commercial inspection service companies frequently offer this service. An entirely new piston, with the required dish that maintains the squish volume, is a proper approach. Machining a dish into the original piston is sound, provided the top thickness is adequate. A reasonable rule would require the top thickness to be at least 6% of the bore. Approaches to lowering the compression ratio that do not work are thicker head gaskets and shorter connecting rods."

Boring a cylinder actually increases the compression ratio, since you are increasing the displacement of the cylinders but not the cylinder head into which the air fuel mixture is compressed.

Lowering the compression ratio is a bad thing to do since it does kill power and it is by no means necessary in order to avoid detonation. Only high compression ratio's may need to be cut in order to avoid detonation, but many people have had success even in the 9:1 range.

The compression ratio of an engine doesn't affect air mass flow through the engine, however, using both high compression and high boost pressure will give very high in cylinder pressures. For an otto engine the pressure can't be too high since that will cause detonation after the air fuel mixture is ignited when the pressure and temperature are increasing even further.
So when using high compression ratios in turbocharged engines this often means that the ignition must be retarded and the fuel mixture must be rich to supress knock. This causes temperature problems due to the later ignition and also an increased fuel consumption during boost due to the ignition and richer fuel mixture. Most modern roadgoing turbocharged engines are built this way which mean that they can run with a better efficiency of boost while still have the power that is needed sometimes.

Turbocharged racing cars are often built with a low compression ratio, this allows the to use high boost pressures, a leaner mixture when boosted and a more advanced ignition timing. This allows the cars to run more efficient during boost with increased power and a lower fuel consumption. The downside is that the won't run that well when off boost.

The compression ratio is a volume ratio, that is the difference in volume between when the piston is at top dead center and at bottom dead center.
The volume at TDC is the volume of the combustion chamber and the volume at BDC is the displacement of a cylinder + the combustion chamber volume; the compression ratio is therefore (combustion chamber volume + displacement)/combustion chamber volume.

But wait, there is more to it. In an engine bore, stroke, con-rod length, squish zones and so on affect the engine in many ways when thay are changed. For example, if we use a thicker head gasket to lower the compression ratio this will also increase the clearence between the piston and cylinder head. Those surfaces that are near eachother at TDC are called squish zones and the cause turbulence and also help to cool the air fuel mixture between them, this can affect combustion speed, engine knock and so on. Usually a little more distance between the surfaces are wanted for full throttle (racing engines) but a smaller distance are wanted for part throttle (road cars), so there are many things to think of when something is altered.

Superchargers can both use blow off valves and pop off valves. The blow off valve is used like on a turbocharged engine and it's only used if the supercharger is placed before the throttle. A pop off valve will open when a certain boost pressure has been reached, and it can therby limit the boost pressure.

Before the turbocharger starts to deliver boost the engine will basicly have the power curve like an NA engine, and then when the turbo can deliver boost the pressure in the intake manifold will be higher than for a NA engine. When the pressure is higher the density of the air is also higher so one liter of air will at 2 atm (about 28 psi) have the same denisty as 2 litres of air at 1 atm (about 14 psi).

The larger the turbo is the more air it can deliver and the more power can be made with it, however, when a larger turbocharger is used it will start to deliver boost at a higher engine speed.

Gasoline engines are what is called unstable at boost, this since the fuel is regulated to match the airflow. So when the turbo starts to delivre air more fuel will be added with then cuases the turbocharger to deliver even more air until a boost pressure where the increase in power consumption from the compressor will be higher than the extra power delivered from the turbine (usually something has broken before this).
In diesel engines the fuel isn't regulated to the air, they are therefore stable at boost and of this reason they don't need any wastegate if the turbocharger is matched for the engine.
Of there simple reasons it can be easy to understand that the boost pressure in a gasoline engine is very dependant on the amount the throttle is open.

When the engine is at a speed where the turbocharger can deliver boost it will take some time until the turbocharger delivers full boost, called "turbo lag". The lag is dependant on the turbocharger and on the engine which the turbocharger is connected to, but it is also dependant on the amount of boost, the higher the boost used are the longer it will take for the turbocharger to reach full boost.

Since you don't want any lag there are several methods to make the lag as short as possibly, among these I can mention ball bearings and variable geometry. But in racing cars there are also other methods so that boost, full or partial can be delivered then the driver gives full throttle. Those "anti lag" methods are often used in racing like drag racing and rally, which for example means that the turbochargers are already delivering boost while a dragracing car is standning still at the line.

I have quoted Corky Bell on this topic before and I will do it again:
"CHANGING A COMPRESSION RATIO. A variety of methods exist to change a compression ratio. Almost all are unacceptable. The crux of the matter is upsetting the "squish volume" around the rim of the chamber. A chamber is designed so that the charge is pushed toward its center as the piston achieves top dead center. This is perhaps the strongest deterrent to detonation designed into the system, as it tends to either eliminate end gas or keep charge turbulence high. This squish volume is a rim about .3 to .4 inch wide around the chamber, an approximately .04 thick-a big, washer-shaped volume between piston and head. Consider "squish volume" sacred and do not tamper. It is possible to err so badly in removing the squish that a resulting 7-to-l compression ratio may ping worse than a 9-to-l with proper squish. Clearly then, choices for reducing compression ratio are limited to opening up selected parts of the head side on the chamber, installing a new piston with a dish in the center, or remachining the original piston to create a dish. It is perhaps a little risky to undertake remachining a combustion chamber, because the thickness of the material is usually unknown. Furthermore, chamber shapes are closely controlled features of most modern engines. If the chamber must be recut, ultrasonic inspection can determine the material thickness. Commercial inspection service companies frequently offer this service. An entirely new piston, with the required dish that maintains the squish volume, is a proper approach. Machining a dish into the original piston is sound, provided the top thickness is adequate. A reasonable rule would require the top thickness to be at least 6% of the bore. Approaches to lowering the compression ratio that do not work are thicker head gaskets and shorter connecting rods."

Lowering the compression ratio is a bad thing to do since it does kill power and it is by no means necessary in order to avoid detonation. Only high compression ratio's may need to be cut in order to avoid detonation, but many people have had success even in the 9:1 range.
I know ratios are proportioned, but what is "squish volume", or this "dish"?

Why are you saying that lowering compression doesn't help stop detonation in a turbo car? Are you implying that a high compression turbo car can be less prone to detonation than a low compression turbo car? Doesn't air heat up when it is being compressed?

So when using high compression ratios in turbocharged engines this often means that the ignition must be retarded and the fuel mixture must be rich to supress knock. This causes temperature problems due to the later ignition and also an increased fuel consumption during boost due to the ignition and richer fuel mixture. Most modern roadgoing turbocharged engines are built this way which mean that they can run with a better efficiency of boost while still have the power that is needed sometimes.

Are you saying that as long as the ignition is retarded and the the fuel mixture is more rich, I can make the compression as high as I want to?


Turbocharged racing cars are often built with a low compression ratio, this allows the to use high boost pressures, a leaner mixture when boosted and a more advanced ignition timing. This allows the cars to run more efficient during boost with increased power and a lower fuel consumption. The downside is that the won't run that well when off boost.


So by lowering compression, I can run leaner and advance the ignition timing? Isn't running lean a bad thing? Does it not cause detonation?


But wait, there is more to it. In an engine bore, stroke, con-rod length, squish zones and so on affect the engine in many ways when thay are changed. For example, if we use a thicker head gasket to lower the compression ratio this will also increase the clearence between the piston and cylinder head. Those surfaces that are near eachother at TDC are called squish zones and the cause turbulence and also help to cool the air fuel mixture between them, this can affect combustion speed, engine knock and so on. Usually a little more distance between the surfaces are wanted for full throttle (racing engines) but a smaller distance are wanted for part throttle (road cars), so there are many things to think of when something is altered.

I thought that lowering compression with thicker gaskets, and bigger piston rods were bad? Why would having a thicker gasket be better for full throttle?


Superchargers can both use blow off valves and pop off valves. The blow off valve is used like on a turbocharged engine and it's only used if the supercharger is placed before the throttle. A pop off valve will open when a certain boost pressure has been reached, and it can therby limit the boost pressure.

So a popoff valve will open when there's too much pressure, and the blow-off valve opens up when there's too much pressure, but keeps the supercharger spinning? Now I am confused :confused:.

Those weren't my words, I merely quoted a respected author because he has said it so much better than I can without taking a long time to write it out my self. But yes, if the compression ratio is improperly changed, it is possible that it will detonate worse. This is fairly uncommon unless you really fudged up the job, however it is possible.

You are correct that the more a gas is compressed, the hotter it gets, however there are more forces at work inside the combustion chamber than just compression, these also help to reduce detonation. SaabJohan touched on them in his post.

So... We don't know what squish volume, or what dish means?

What other forces inside help stop detonation? Retarding ignition ? running rich? I know running rich will help stop detonation, but retarding timing only helps reduce hotspots. Don't cylinders detonate due to the air being hot enough from compression to set off the fuel?

There is a whole new set of problems incorporated with retarding the ignition timing, so while retarding ignition timing can be a valuable tool when tuning a motor, one must keep it within reason. Also keep in mind that retarding the ignition timing kills power.

Running lean is bad for a few reasons, but good for one main reason. Running lean can cause detonation in motor that isn't set up to safely do so, however motors actually make the most power when they run a little leaner than 14.7:1, somewhere around 12:1. The reason 14.7:1 is used on road cars is because that is where vehicles make the lowest emissions, and it gives better detonation resistance.

As for lowering the compression ratio with rods and gaskets, it is bad if you don't know what you are doing. I never said it was impossible to do, just that if you don't know what you are doing you can mess up the squish area and actually make your engine more prone to detonation. SaabJohan was a little more specific in that area, I believe he was saying that the relationship between the parts in the combustion chamber is complicated and have a very synergetic effect, changing one thing may change the whole dynamics of the combustion chamber.

Remember that on turbocharged cars, blow off valves are not used to control boost levels, just vent the pressure that builds up at the throttle plate when it is closed, and prevent compressor surge. However, pop off valves are like radiator caps, they open when the pressure reaches a preset level, these can be used to control boost to the engine, as once the boost pressure reaches a certain level they will open and vent excess boost to the atmosphere.

A dish is just that, a concave curve in the piston, designed to lower the compression ratio.

Squish volume is described in the quote I posted by Corky Bell: "The crux of the matter is upsetting the "squish volume" around the rim of the chamber. A chamber is designed so that the charge is pushed toward its center as the piston achieves top dead center. This is perhaps the strongest deterrent to detonation designed into the system, as it tends to either eliminate end gas or keep charge turbulence high. This squish volume is a rim about .3 to .4 inch wide around the chamber, an approximately .04 thick-a big, washer-shaped volume between piston and head."

By the way, I edited my first post, I was wrong about the bore's effect on compression ratio because I forgot about the unchanged combustion chamber volume.

Remember that on turbocharged cars, blow off valves are not used to control boost levels, just vent the pressure that builds up at the throttle plate when it is closed, and prevent compressor surge. However, pop off valves are like radiator caps, they open when the pressure reaches a preset level, these can be used to control boost to the engine, as once the boost pressure reaches a certain level they will open and vent excess boost to the atmosphere.

So can pop off valves double as a blow off valve, controlling both boost pressure and excess pressure? Why would a supercharger need a pop-off valve anyway?

Isn't the boost already regulated by the engine RPMs so that x amount of boost will be made from x amount of RPMS?

I don't see a need for a blow off valve either, when you release the pedal, the engine starts trying to go idle speed, the super charger goes as fast as the engine will spin it, so if the engine goes to idle speed, won't the supercharger follow along?

A turbo, will keep spinning the compressor for a while after the gas pedal has been let off, this builds up pressure on the throttle plate, a blowoff/bypass valve releases this harmful excess pressure, this is why they have a BOV right?

What other things could I do to stop detonation besides lowering compression, retard timing, and running rich?

BTW, this might help me, what is a crux? :D (checked the dictionary, too many meanings)

A crux is a focal point, in this case crux means most important point.

Think of it this way, the motor slows down because the throttle plate closes, allowing less air to enter the combustion chamber. So the compressor is still producing boost until the throttle plate closes, and the moment the throttle plate has closed, the remaining pressurised air will not be able to exit the piping. Also remember that boost pressure created by a turbocharger is actually more related to exhaust volume and energy than RPM's, although there is a correlation between RPM's and exhaust. So when the throttle plate closes, although the engine may still be at high RPM, there is less fuel being burned (and less exhaust being produced as a result) so the turbocharger will not really produce more boost. The problem the blow off valve avoids is the already pressurised air trapped by the throttle plate.

As for the pop off valve issue, I was merely clairifying what SaabJohan said, I have no experience using them to control boost beyond what I have read, and very little experience with superchargers for that matter.

When the ignition is retarded and/or a richer fuel mixture is used this can supress knock, to some degree that is.

A pop off valves open when the boost pressure is high. A blow off valve open becasue of the pressure difference before and after the throttle, that means that it will open if the throttle is closed after boost preventing the turbocharger, or supercharger from surge. Note that blow off valves are only used when the turbo or supercharger is placed before the throttle.

An engine will deliver maximum power with an air fuel ratio, by weight, of about 12-13:1, this is rich, but to supress knock a even richer mixture must sometimes be used and this will cause power to drop. For maximum efficiency the engine should be runned lean, leaner than stoichiometric.

When the engine knocks, it is a detonation that happends after the spark has ignited the fuel (if it happens before it's a pre ignition). The more the ignition is advanced (up to the point of maximum torque setting) the power output will increase and so will the combustion pressure which increase the possbility of knock.

Positive displacement superchargers will give a constant boost pressure but not dynamic compressors like the centrifugal compressor, it will increase boost with rpm. However, using pop off valves are not a good way to limit boost (it can however be used as a safety device or in racing so a higher boost pressure than what is allowed isn't used, like in CART).

Using a high octane fuel is a good way to stop detonation, water injection can also be used but it has downsides.

Caught my mistake, 12:1 is richer not leaner...apparently I'm not on the ball today.

When the ignition is retarded and/or a richer fuel mixture is used this can supress knock, to some degree that is.

A pop off valves open when the boost pressure is high. A blow off valve open becasue of the pressure difference before and after the throttle, that means that it will open if the throttle is closed after boost preventing the turbocharger, or supercharger from surge. Note that blow off valves are only used when the turbo or supercharger is placed before the throttle.


pop-off valves work only when the throttle is open, and blow off valves only work when throttle is closed?

An engine will deliver maximum power with an air fuel ratio, by weight, of about 12-13:1, this is rich, but to supress knock a even richer mixture must sometimes be used and this will cause power to drop. For maximum efficiency the engine should be runned lean, leaner than stoichiometric.

Leaner than 14.7:1?


When the engine knocks, it is a detonation that happends after the spark has ignited the fuel (if it happens before it's a pre ignition). The more the ignition is advanced (up to the point of maximum torque setting) the power output will increase and so will the combustion pressure which increase the possbility of knock.

Umm, doesn't the spark plug ignite the fuel? Wouldn't that burn pretty much all of it?
How is detonation so bad when it only happens during the power stroke?
Does the spark plug NOT burn all the fuel?


Positive displacement superchargers will give a constant boost pressure but not dynamic compressors like the centrifugal compressor, it will increase boost with rpm. However, using pop off valves are not a good way to limit boost (it can however be used as a safety device or in racing so a higher boost pressure than what is allowed isn't used, like in CART).

Using a high octane fuel is a good way to stop detonation, water injection can also be used but it has downsides.

Postive displacement: Constantly giving a set amount of PSI regardless of engine RPMS?

Why is it bad to use pop-off valves to control boost? Isn't it like a wastegate?

What's wrong with water injection? Doesn't it cool down the hotspots?

Pop off valves work when the boost pressure overcomes the spring load, opening the valve and venting boost to the atmosphere, this could be when the throttle is open or closed, just as long as the pressure is enough to overcome the spring. Blow off valves work by manifold vacuum; the blow off valve has a line connected to the manifold (usually), when the thottle plate is open the boost pressure is equal to the manifold pressure, however when the throttle plate closes the manifold is under vacuum and this vacuum is transferred through the line to the BOV where it overcomes the spring force that normally keeps the BOV closed and the BOV opens, venting trapped boost pressure to the atmosphere. This is the reason that many BOV's have filters on them, at idle the manifold is under vacuum so the BOV is open, however there is no boost being vented since usual turbocharged vehicles don't make boost at idle, so outside air is pulled through the BOV and into the engine.

The reason for spark advance and retard is because fuel is not instantly combusted the moment the plug fires, it is a controlled combustion. If for whatever reason the fuel ignites at a spot other than where the spark plug ignited it, there will be opposing flame fronts, which will cause pressure and temperature spikes when they meet. The pressure and temperature spikes reached by detonation are bad for the motor for three main reasons; they may scour away the oil film on the cylinder walls causing undue wear to occur, they may impose undue load on the piston which can cause extreme damage to it, and they can scour away the protective gas that normally helps protect the combustion chamber from the full temperatures reached by combustion.

Positive displacement superchargers move a fixed amount of air for each revolution. This means that the airflow increases linearly with engine RPM, resulting in a constant air pressure. A roots type supercharger is positive displacement. If a roots type supercharger is moving at x rpm is will deliver y amount of air, if it moves at 2x rpm it will deliver 2y air. Positive displacement superchargers give constant pressure since engine air consumption increases linearly with air flow through the supercharger.

Pop off valves are a poor method of boost control because they tend to be inaccurate, noisey (some people may like that), and environmentally unfriendly on draw through systems. They do however have some merit as safety devices, just in case the wastegate fails.

I have never used water injection, so SaabJohan would be better at explaining his beef with it.

The largest problem with the pop off valves is that that they blow off air that could be used, and the compressor will need to pump that air anyway.
With a wastegate the turbine power is controlled, therefore the compressor will not do unneccesary work.

Stoichiometric air fuel ratio is for gasoline about 14.7:1.

The spark plug will give a spark, then with a short delay the flame front will start to travel, first with the laminar flame speed (usually 0.3-0.5 m/s for normal fuel). The combustion products will have a different density and will therefore push the flame outwards at a much higher speed, say 20 to 50 m/s or even higher at high engine speeds. All this will take some time, usually we want the combustion to be done some after TDC, perhaps 15 degrees or so, lets say that we ignited the fuel at 20 degrees before TDC the combustion had a duration of 35 crankshaft degrees, this can be recalculated to time if the engine speed is known.

When the engine knocks this is usually a detonation around TDC, after the fuel is ignited. The preoblem is usually largest on mid engine speed.

For info about water injection look in the thread about water injection.

Oh so there IS a wastegate for superchargers... Didn't know that. But if pop off valves blow off air that could be used, what's so different about what a wastegate does?

BTW, I still don't know what a BOV does, maybe on turbos, not sure on superchargers


The pressure and temperature spikes reached by detonation are bad for the motor for three main reasons; they may scour away the oil film on the cylinder walls causing undue wear to occur, they may impose undue load on the piston which can cause extreme damage to it, and they can scour away the protective gas that normally helps protect the combustion chamber from the full temperatures reached by combustion.


How does it scour away the oil films?

Does detonation give the crankshaft a sort of "shock"?

What is the "protective gasses"? :confused:

no wastegate for superchargers...at least not in the same respect as a turbocharger. The turbo wastegate regulates how much Psi is run by diverting exhaust gases to the exhaust thereby limiting the amount of air going back to the turbocharger.

the supercharger 'wastegate' is like a valve on the intake side of things that will regulate boost if it goes over a certain amount by bleeding it out of the intake tract.

so a wastegate on a supercharger bleeds pressure out of the intake, and the turbo wastegates bleed out of the exhaust?

as far as i know, yes

I'm not actually 100% sure on how exactly detonation removes the oil film, as I have never seen it happen or any research on the subject, this is just what I have learned from personal research. I would assume, however, that it has to do with the increased temperatures, pressures, and vibratory load imposed when the pressure front meets the cylinder walls.

Detonation certainly does give the crankshaft a shock, however, this is not the most concerning thing. The real issue is the shock it imposes on the piston, this can be severely damaging, especially to cast pistons.

The protective gasses are stagnant combustion products and unburnt air/fuel that remain relatively motionless lining the combustion chamber, I believe this is due to the surface friction between the gasses and the combustion chamber (but I may be wrong). This motionless gas acts quite effectively in protecting the combustion chamber from the full heat produced by combustion because it isolates the combusting gasses from the combustion chamber. As soon as detonation occurs it quickly removes this barrier gas and exposes the combustion chamber to the full temperatures attained by combustion, piston temperatures rise quickly, followed by everything else. This rise in temperature then promotes preignition.

turbos compress air to get more air into the same size area. the more air, the more fuel. lower compress normally does mean higher top end.turbos have more power because they have a bigger top end. they dont have ass much as n/as at low end power. off the line a n/a will beat it, then later the turbo will catch up when the turbo starts spooling.

superchargers have bovs to make sure that it doesnt put too much air into teh engine. if there is more air than it can put in, the bov will open to let the extra air out.
pistons and headgaskets are the best way to lower compression.
its the wastegate that controls the boost the turbo makes, excess boost is let out thru the wastegate. the blow off valve lets excess air created by the spinning turbine escape when you let off the gas, thats what makes that psssssst sound.
*superchargers dont need blow off valves because they're belt driven. and dont be so sure that a n/a engine will beat a turbo off the line, that depends on the setup and if the turbo lags.

technically u dont need to lower compression, thats just for added safety, if u run higher octane, or lower boost u can avoid lowering compression. higher boost needs lower compression to keep the engine from detonating. dish pistons can lower compression, dome pistons can raise compression.

Superchargers will need a blow off valve if they have the throttle after the compressor.

As mor most turbo engines on the market they give more more power at mid engine speed for a given peak output compared to a NA engine so often the stock turbocharged engines are very fast. As for racing cars these can often have boost before they even start to accelerate. If turbochargers was allowed in the faster dragracing classes they would likely beat the compressor engines, due to their higher power output.