Compression ratio question.....

black mamba1

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I see many engine builders going to higher compression ratio's for modded NA cars, but I see some actually going to lower compression ratios especially for the s/c engines. I dont quite understand the different strategies. Why would not a higher compression ratio be better for both modded NA and s/c engines? When is a lower compression ever better? :dunno:
 

Chuck 98 RT/10

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More compression means a bigger bang. But the engine can only take so much compression.

When an engine is super/turbo charged it is shoving a bunch of fuel into the combustion chamber. So builders go to a lower compression ratio so the SC can pack even more fuel into it resulting in a bigger bang.

N/A pulls the fuel mixture in. So to get a bigger bang they raise the compression ratio.

Hope this helps.
 

Joseph Dell

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Without getting into all the physics of it, a high compression high boost motor is ideal BUT the power loss from 10:1 vs. 9:1 is ~3%. Since 9:1 requires less-perfect tuning (and tolerates errors in tuning more), tuners often go 8.5:1 or 8.0:1 on a boosted motor.

But there is nothing wrong with a 10.0:1 or 10.5:1 boosted motor... you just need to run better gas (race gas over 8-12psi) and need to be more in-tune with your tune.

There is more to it than that, but that gives you an idea...

JD
 
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black mamba1

black mamba1

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Without getting into all the physics of it, a high compression high boost motor is ideal BUT the power loss from 10:1 vs. 9:1 is ~3%. Since 9:1 requires less-perfect tuning (and tolerates errors in tuning more), tuners often go 8.5:1 or 8.0:1 on a boosted motor.

But there is nothing wrong with a 10.0:1 or 10.5:1 boosted motor... you just need to run better gas (race gas over 8-12psi) and need to be more in-tune with your tune.

There is more to it than that, but that gives you an idea...

JD
Thanks Joseph! This helps trememdously.
 

Mike Dolan

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Here is an alternative explanation:

If you had an engine with 100% volumetric efficiency (unlikely N/A), before compression the air pressure in the cylinders would be at 14.7 psi (absolute, at sea level & 72 degrees). With a 10:1 compression ratio (again assuming 100% efficiency) the pressure would be at 147 psi before ignition. (A lot is assumed here for clarity, I know the real world is more complex). If you started with say, 7 lbs of boost, then the starting pressure would be 21.7 psi (absolute), and after compression 217 psi. Two things are going on here; more pressure and more molecules of fuel and air. Assuming (again) 100% efficiency, the additional pressure, fuel and air would render more power. But a problem starts cropping up: how much pressure can the engine deal with before deadly detonation becomes problematic? Detonation is our enemy and limits the absolute pressure we can deal with. The most commonly used means to reduce detonation (******** spark, overly rich mixture etc.), bring their own issues to the table.

Now what happens if we drop the compression ratio to say 8:1? Well for one thing we can cram more fuel and air into the cylinder, and end up with the same pressure after compression. More fuel and air = more power. There are limits of course, as we still need room for expansion, so starting with a compression ratio of 1:1 and 217 psi of boost wouldn't work, but there are happy compromises. By dropping the compression ratio to 8:1, we can offset that by cramming more air in - quite a lot more in, along with the requisite fuel. Of course we are assuming away a lot of critical variables for clarity here, things like dynamic compression ratio, (it's less than the ratio of cylinder volume at both extremes due to valve overlap), unsteady flow dynamics (it's actually theoretically possible to attain volumetric efficiency greater than 1 normally aspirated under certain conditions, but it's generally significantly less than 1). Also the effects of tuning, necessary to deal with the higher heat and pressure will impact things to a large extent. Also consider the effects of pressurizing the inlet air - the air becomes hotter, which would result in a less dense charge of air, offset somewhat by some cooling mechanism which will also have it's impact on the flow dynamics etc. etc. The "etc's" and "assume away's" are the things your tuner deals with, and where he earns his money. It's not so hard to pressurize the inlet tract, but doing so in a way that really works and doesn't blow your motor is the magic part. In my opinion, reliable tuners are the only way to go, the good ones earn their money.

Regards,
Mike Dolan
 
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black mamba1

black mamba1

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So it seems as if boosting a motor makes tuning more challenging, whereas going NA only gives more predictable, or should I say an NA motor is more easily tuned than a s/c, or boosted motor. So is that why we see NA guys going to higher CR's? Or are the NA guys simply milking all the power out of the NA motor as possible risking the more difficult tuning?
 
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black mamba1

black mamba1

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This site is really acting wierd tonight, I saw a very impressive response by Mike Dolan a minute ago, then it vanished...now its back! Great!
Well, anyway it seems to involve thermodynamics and the ideal gas law. It seems that under ideal (no detonation worries) the higher the compression ratio the more power generated. Now, what I am still confused about is that many of you say that lowering the compression ratio allows for cramming in more air and fuel...but I do not see the direct connection there, are you guys considering the volume that the air and fuel take up in the chamber in lowering the CR? If so, that would make sense, in that more air and fuel would create a more bang and higher temps in a given volume.
Or, will the combination of higher compression AND more air and more fuel trigger predetonation at a given flashpoint (if such a thing is an issue), which as Mike D said can be controlled by retarding spark...but I would imagine that is much trickier...
 

Joseph Dell

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So it seems as if boosting a motor makes tuning more challenging, whereas going NA only gives more predictable, or should I say an NA motor is more easily tuned than a s/c, or boosted motor.

No.. but the margin for error is much greater on a N/A motor. If you screw up the air/fuel on a N/A motor, you lose power. when the cylinder pressure is higher and you mis-tune, the affects are compounded exponentially.
 
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black mamba1

black mamba1

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No.. but the margin for error is much greater on a N/A motor. If you screw up the air/fuel on a N/A motor, you lose power. when the cylinder pressure is higher and you mis-tune, the affects are compounded exponentially.
ok, so lowering the compression ratio on a blown motor makes it safer it seems.
 

Mike Dolan

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Black Mamba,

You have the general idea, if pressure (after compression) exceeds a certain point, predetanation will occur. This is partially because of the pressure, but more because compressing the air causes it to become hotter. Once the heat exceeds the flash point of the fuel/air mixture combustion WILL occur. This is how Diesel engines work. If you compress the inlet tract it will cause the air to become hotter, proportionate to the amount of compression. Two big considerations though - this air has no fuel in it (yet), and it will also be cooled somewhat by the inlet tract, and typically some other mechanism, such as an intercooler or water/**** injection. So the heat added at this stage can be partially compensated, but the heat generated by compressing the air in the cylinder has no opportunity to be cooled, so it cannot exceed some predetermined point without predetonation (usually referred to simply as detonation).

So if you said, as an example, that this point was 200 PSI absolute, (again assuming 100% efficiency), at 10:1 compression ratio, the starting absolute cylinder pressure could not exceed 20psi absolute. Sea level absolute pressure is 14.7 psi, so at 5.3 psi pressurization, you are at the maximum before detonation will start occurring. (I know this is lower than Paxton systems currently run, the numbers are simply illustrative). If the compression ratio was dropped to 8:1, the starting pressure would be 25psi absolute, so the supercharger boost could be 10.3 psi, which would consist of 25% more molecules of fuel and air. The greater amount of fuel would result in more heat & pressure, resulting in more power.

The reason retarding timing sometimes works is because the sparkplug actually fires before the piston has reached the top of its stroke, this is to offset the time it takes for the flame to propogate. As the engine revs higher this advance is increased because there is less time for the flame to propogate before the exhaust valve opens. When you increase pressure (and therefore also heat), the flame will naturally propogate faster anyway, and if it happens too soon, you also have detonation.

An overly rich mixture helps because the evaporating fuel helps to cool the charge, but if the mixture is enriched too much, power suffers, as does drivability.

Another important issue is one of 'hot spots'. If the cylinder has some point that becomes hotter, it will become the trigger to start the detonation process. A typical source of this 'hot spot' is the sparkplug tip. This is why people use 'colder' plugs. Spark plugs need to clean themselves or they become 'fouled' and a plug that is too cold will foul, resulting in misfire, but if it is too hot, it will become the trigger of detonation.

Hopefully this has shed more light than confusion.

Mike Dolan
 

Mike Dolan

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After considering my earlier posts one thing I implied, but didn't actually state is the following: During the normal combustion process, the flame front propogates in a controlled burning process, which results in a rapid but smooth increase in cylinder pressure. In detonation, the flame front does not propagate in this manner, but more closely resembles an explosion, resulting in sudden pressure spikes. It is this sudden pressure spike that damages the engine, and therefore must be avoided. When considering the cylinder pressure vis-a-vis detonation, the pressure that triggers detonation is actually after the combustion process begins, triggering the combustion process prematurely simply causes what is referred to as 'pinging' or if it's on a British car 'pinking'. Pinging, while annoying, isn't destructive, whereas detonation is rapidly destructive. This said, the considerations of my earlier missives still hold true. Preignition is pretty much benign, but in extreme cases may trigger detonation.

Regards,
Mike Dolan
 
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black mamba1

black mamba1

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Wow Mike! Thank you very much for your well thought out and informative response. I feel like I am back in thermodynamics and combustion engines in my engineering classes! I have not read such a complete and accurate response on compression ratios anywhere! I fully understand now and I commend you on your knowledge!
 
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