Compression, dynamic/static, question...

[PQ]

So I was having a conversation with Zebra in another thread.

Below are excerpts from our conversation.

We already know that a lower compression ratio set-up is better for boost do to boost levels creating much higher compression, and a higher ratio engine getting too high of compression when boosted.

From what I've read the static CR is what the engine is NA. (the way it was built) and the dynamic CR is what the actual psi is at the bottom of the compression stroke divided into the psi at the top of the compression stroke. Boosted or not.

So basically what exactly is the reason for this?

Originally Posted by zebra

i thought with lower compression, boost means less & you need more of it to make the same power... you just have more resistance to detonation because you ain't trying to squeeze X amount of air as much

and my bone stock 55kmi LS1 handles more than 10psi just fine. i can't imagine why a 'properly built/forged' iron block engine is only warrantied up to 8

Originally Posted by zebra

somebody smart correct me if i'm wrong, but the way i understand compression is that the cylinder still moves the same volume of air at 11:1 as it does at 9:1 (because the crank & rod length is still the same) - it's just that the higher compression squeezes said volume into a smaller space, which creates more heat & explosive force, resulting in a higher potential for knock but yielding more power.

as for the flyby, i was supposed to go pretty close to my house but we got vectored direct from WV to TX, so we never went that far south. i hear we might be going down to Eglin soon... there's a little potential for ya there

Originally Posted by PQ

Well I'm not smart, lol, but I figure the same. So the naturally aspirated 9:1 would be considerably less power than the NA 11:1 because the compression would be, say, only 170 pounds for the 9:1 instead of 220 pounds for the 11:1. (to use an example AND assuming the same displacement)

Boosting that 9:1 to make 220 pounds at compression to match the 11:1s 220 pounds of compression would make more power because the 220 pounds of pressure in the 9:1 would be with more volume in the compressed/clearance area.

I just assumed that was the way it worked. I was never told that just kind of made a guess.

Now I'm curious.

Originally Posted by PQ

But wait, there's more.

Also it would also be a cooler burn because the compression does not go up?

[GretchenGotGrowl]

Not exactly right. The static compression ratio is the ratio of cylinder pressures at top dead center and with the piston at the bottom of travel. Dynamic compression ratio accounts for the loss of air via intake valve closure before TDC (and scavenging affects) that happen at lower RPMs. So there is air loss as the compression stroke occurs and the total quantity of air at TDC is less than at the bottom of the strock. You can overcome this loss by forcing air in (no backup via intake valves) with boost.

[kpwalls06]

Gretchen gave a pretty good account for static CR vs. dynamic CR. Also, when you're talking about cylinder compression, you have the right concept that an engine with 9:1 CR will have lower compression than an engine at 11:1 CR. Once you start thinking of the power, and the cylinder firing, the compression becomes cylinder pressure, e.g. because the 11:1 engine puts the same volume of air into a smaller space, there will be a higher cylinder pressure during combustion, producing higher forces acting on the piston, resulting in more power. The same idea translates to boost. A 9:1 engine with boost produces higher cylinder pressure during combustion, resulting in more power.

Sorry if you already knew any of this, but using the correct terminology will help cut down on confusion in most cases.

[PQ]

Originally Posted by GretchenGotGrowl

Not exactly right. The static compression ratio is the ratio of cylinder pressures at top dead center and with the piston at the bottom of travel. Dynamic compression ratio accounts for the loss of air via intake valve closure before TDC (and scavenging affects) that happen at lower RPMs. So there is air loss as the compression stroke occurs and the total quantity of air at TDC is less than at the bottom of the strock. You can overcome this loss by forcing air in (no backup via intake valves) with boost.

Ah. Ok. Good explanation.

So I'm not looking for static vs dynamic then but something different.

7 pounds of boost on 9:1 vs 7 pounds of boost on 11:1 (yes I know the blower will work more) is more power why?

Also does CR change when boosted?

What is this chart. It implies that CR changes when boosted.

[victoryred1ss]

Compression ratio does not change with a blower, but cyl pressure goes way up. To change CR you would have to change the design of the piston or the combustion chamber. IE smaller or larger cc heads.. and dome or recessed piston.

[GretchenGotGrowl]

The problem with higher CR is that it can lead to premature detonation because more compressed air is hotter (faster flame propogation) and the explosion is bigger. Without something to control the flame propogation (like higher octane) you will have to reduce timing to prevent early detonation. Reducing timing usually results in less power, partly because the dynamic CR is lowered. So, a 9:1 CR car may be able to run just fine on 87 octane but a 11:1 car may require 93 octane. You can add 7 psi to the 9:1 and increase the octane to 93 octane without reducting timing, so it has a higher dynamic CR. However, there are limits to that. You should be able to find a boost level for the 11:1 CR engine that is lower than the 7 psi that makes as much power as the 9:1 CR car made with 7 psi, as long as you can control the flame propogation. The static CR really will only help you figure out the overall volume of air entering the system and it's POTENTIAL compression, not what it will actually be compressed to.

[kpwalls06]

Compression ratio does not change by adding boost, it changes the peak cylinder pressure during combustion, resulting in more force on the piston.

Also, an 11:1 engine with 7# of boost will make more power than 9:1 with the same boost because the peak cylinder pressure of combustion is higher in the 11:1 engine.

[kpwalls06]

Again, Gretchen explained it very well.

[PQ]

Originally Posted by victoryred1ss

Compression ratio does not change with a blower, but cyl pressure goes way up. To change CR you would have to change the design of the piston or the combustion chamber. IE smaller or larger cc heads.. and dome or recessed piston.

Ok, I knew this, or so I thought.

I got to talking to a local guy here and he asked me what the CR would be if he boosted his car with a turbo. I told him the CR would stay the same. It is what it is. But that the compressed psi would go up. He argued saying the CR changes. His point was that CR is actual measured AIR. So when you boost it it changes. I disagreed then I thought that might be what dynamic vs static was. (GretchenGotGrowl has shown me that was something different)

Originally Posted by GretchenGotGrowl

The problem with higher CR is that it can lead to premature detonation because more compressed air is hotter (faster flame propogation) and the explosion is bigger. Without something to control the flame propogation (like higher octane) you will have to reduce timing to prevent early detonation. Reducing timing usually results in less power, partly because the dynamic CR is lowered. So, a 9:1 CR car may be able to run just fine on 87 octane but a 11:1 car may require 93 octane. You can add 7 psi to the 9:1 and increase the octane to 93 octane without reducting timing, so it has a higher dynamic CR. However, there are limits to that. You should be able to find a boost level for the 11:1 CR engine that is lower than the 7 psi that makes as much power as the 9:1 CR car made with 7 psi, as long as you can control the flame propogation. The static CR really will only help you figure out the overall volume of air entering the system and it's POTENTIAL compression, not what it will actually be compressed to.

If you had to explain why lower CR is good for a blower how would you explain it?

Originally Posted by kpwalls06

Compression ratio does not change by adding boost, it changes the peak cylinder pressure during combustion, resulting in more force on the piston.

Also, an 11:1 engine with 7# of boost will make more power than 9:1 with the same boost because the peak cylinder pressure of combustion is higher in the 11:1 engine.

Ok, same question. Why is a lower CR engine better for a blower?

I had this all wrong in my head. This is good stuff.

[GretchenGotGrowl]

It isn't always better. Up until the point where the compression of the air/fuel (this is the peak pressure kpwalls06 is talking about) gets so high it basically combusts to readily, adding boost will increase power. Think of a diesel engine...no spark plug. The fuel/air mixture combusts because it is compressed to the point where it ignites from the heat. You can do that in a gas engine as well. However, gas will explode and not burn like diesel under those conditions. So, the 11:1 CR engine has higher peak pressure than a 9:1 CR engine given similar timing of the combustion event. The 11:1 CR engine will make more power than the 9:1 CR whether NA or boosted up to the point that the combustion process is no longer "clean." At that point you have to start removing timing to avoid premature detonation and you eventually get to a point where you lose as much or more power by removing timing than you gain from the extra air/fuel you are introducting. However, for the 9:1 CR engine there is more volume between the piston tops and cylinder heads to hold the air/fuel mixture. So the actual peak pressure will be lower for the same amount of air will be less than on the 11:1 CR engine. You can continue to add more air to the 9:1 CR engine before the peak pressure reaches the same level you saw with the 11:1 CR engine when you had to pull too much timing. More air means more fuel which means more power (all other things being equal).

[PQ]

Originally Posted by GretchenGotGrowl

It isn't always better. Up until the point where the compression of the air/fuel (this is the peak pressure kpwalls06 is talking about) gets so high it basically combusts to readily, adding boost will increase power. Think of a diesel engine...no spark plug. The fuel/air mixture combusts because it is compressed to the point where it ignites from the heat. You can do that in a gas engine as well. However, gas will explode and not burn like diesel under those conditions. So, the 11:1 CR engine has higher peak pressure than a 9:1 CR engine given similar timing of the combustion event. The 11:1 CR engine will make more power than the 9:1 CR whether NA or boosted up to the point that the combustion process is no longer "clean." At that point you have to start removing timing to avoid premature detonation and you eventually get to a point where you lose as much or more power by removing timing than you gain from the extra air/fuel you are introducting. However, for the 9:1 CR engine there is more volume between the piston tops and cylinder heads to hold the air/fuel mixture. So the actual peak pressure will be lower for the same amount of air will be less than on the 11:1 CR engine. You can continue to add more air to the 9:1 CR engine before the peak pressure reaches the same level you saw with the 11:1 CR engine when you had to pull too much timing. More air means more fuel which means more power (all other things being equal).

Holy crap. Epiphany here.

It all makes perfect sense now. So if it ignites just before it reaches full top them you have a piston actually kind of going AGAINST the ignition of the mixture?

And with a TVS2300 a lower CR might not be so good then. Especially if I bore it at all. As long as I forge it.

Ok, another one.

If a 9:1 is boosted to 7 pounds and 11:1 is boosted to 7 pounds the cylinder pressure would be higher for the 11:1 obviously. If you were to add enough more boost to make it the same psi for ignition then it would make more power because of volume?

[JANNETTYRACING]

Originally Posted by PQ

Ok, I knew this, or so I thought.

I got to talking to a local guy here and he asked me what the CR would be if he boosted his car with a turbo. I told him the CR would stay the same. It is what it is. But that the compressed psi would go up. He argued saying the CR changes. His point was that CR is actual measured AIR. So when you boost it it changes. I disagreed then I thought that might be what dynamic vs static was. (GretchenGotGrowl has shown me that was something different)

If you had to explain why lower CR is good for a blower how would you explain it?

Ok, same question. Why is a lower CR engine better for a blower?

I had this all wrong in my head. This is good stuff.

Compression Ratio is a Mechanical measurement of Cylinder Volume at bottom dead center VS Top Dead Center.

Dynamic is more about how the camshaft Valve events affect the cranking cylinder Pressure..

You can have an 11:1 motor with a big cam have 150 lbs of Cranking Pressure and a 8.5:1 motor with a stock cam have 150 lbs of cranking pressure.

This would Be Dynamic Pressure.

All though Both of these affect Peak Cylinder Pressure they are ONLY a Piece of the puzzle.

Peak cylinder pressure is affected by;

How much air and fuel you put in the cylinder at bottom dead center
How much of that Air and fuel you can Trap in the cylinder.
IAT
ECT
Cam Profile
Type of fuel
Timing
Head material
Block material
Piston Material
Chamber design
Exhaust back pressure
RPM
Blower speed
Blower size

And the List goes on.

Octane is always the Limiter of how much Peak Cylinder Pressure we can go to without cause Premature Ignition of the air fuel mixture

That is when the fuel ignites before the spark occurs.

The Theory behind lower compression for boosted applications is to pack more air and fuel in the same size cylinder before reaching the Knock Threshold of Pump Fuel of 91-93 octane.

Does it Pan out? is the big question.

In our experience Not very often on this platform.

With Iron Head and block engines it works better.

Then there is the Question of How low it Too Low?

Ted.

[GretchenGotGrowl]

Originally Posted by PQ

Holy crap. Epiphany here.

It all makes perfect sense now. So if it ignites just before it reaches full top them you have a piston actually kind of going AGAINST the ignition of the mixture?

And with a TVS2300 a lower CR might not be so good then. Especially if I bore it at all. As long as I forge it.

Ok, another one.

If a 9:1 is boosted to 7 pounds and 11:1 is boosted to 7 pounds the cylinder pressure would be higher for the 11:1 obviously. If you were to add enough more boost to make it the same psi for ignition then it would make more power because of volume?

Yes, if you can keep the combustion process in check with enough fuel AND octane then it will make more power. Think about it this way. One of Robert's monster turbos can push as much (actually more) CFM of air than my two little ones at 20 psi. So, theoretically I could make as much power as I do now with just 1 of his turbos at 9-10 psi. The benefits would be lower air temperatures (less likely to ignite prematurely) and more timing. The disadvantage is that that lower psi won't overcome the loss to intake valve opening as much as my 18-19 psi, so I would probably end up with a little more air in the mixture. All in all it would be about a wash. But bump one of his turbos up to 20 psi and I would have to bump up the smaller turbos to about 40 psi. At that point the IATs get so high I would have to pull more timing that it would be worth (even if I ran C16). So the psi added to the overall cylinder pressure by boosting is inconsequental to the overall dynamic CR and peak pressure, it is the VOLUME of air that matters. The psi is just allowing you to get the volume in quickly enough and with less loss back through the intake valves to be effective.

[kpwalls06]

Theoretically, and I must stress theoretically, with all other things being equal (e.g. cam specs, cylinder heads, etc.) two engines with different CR that have the same cylinder pressure would make the same power. Meaning if you added boost to a 9:1 engine to give it the same peak cylinder pressure as the 11:1 engine with 7#, you should get the same power output. Again, this is in theory. The slightest change to the cam, heads, exhaust, intake, tune, etc., will produce a noticeable power difference in real application.