boost/compression relationship

[focusedrage]

i assum that incressing the compression requires lowering the boost? like alot?

I heard that higher compression pistons would led to a faster engine spin up.

wouldent it also alow more umph from a turbo since less boost is required to get the same end result? im thinking about 9.5 or 10 to 1, this is just thinking thougth.

Cart champ cars have an obsene compression and turbo. but i know a champ car is not a dsm...

 

[pneumo]

There is a limit to how much boost/ compression can be run, and it's the final pressure in the combustion chamber with the piston at TDC. Too much of either boost or high compression ratio, or both combined, will result in detonation. Just so you know, that is the main thing limiting the power in all engines. Think of the size of the combustion chamber when the piston is at TDC. With lower compression there is more volume, and the turbo fills up that volume with air/ fuel so that the overall pressure is just as high as a NT with higher compression. But with a turbo there is more air/ fuel, hence more power. Going too much higher would lead to detonation. But there are ways of putting off detonation, such as more efficient intercoolers, good tuning, proper sized turbo, etc. In the early stages of my education about turbos I wondered what if I build an engine with a 6:1 compression ratio and ran the boost up to 50? Could I get 600 HP on pump gas? !

 

[GSX4LIFE]

This is a very good explanation. Detonation is the enemy. 9:1 is the highest I would go in an engine running serious boost. I actually run very low(7.8:1). High compression will make more power at any given boost level with the right tuning. However, it gets more difficult to control detonation, the higher compression you run.

 

[focusedrage]

thats cool ill keep that in mind

 

[nine5raptor]

Originally posted by pneumo
There is a limit to how much boost/ compression can be run, and it's the final pressure in the combustion chamber with the piston at TDC. Too much of either boost or high compression ratio, or both combined, will result in detonation. Just so you know, that is the main thing limiting the power in all engines. Think of the size of the combustion chamber when the piston is at TDC. With lower compression there is more volume, and the turbo fills up that volume with air/ fuel so that the overall pressure is just as high as a NT with higher compression. But with a turbo there is more air/ fuel, hence more power. Going too much higher would lead to detonation. But there are ways of putting off detonation, such as more efficient intercoolers, good tuning, proper sized turbo, etc. In the early stages of my education about turbos I wondered what if I build an engine with a 6:1 compression ratio and ran the boost up to 50? Could I get 600 HP on pump gas? !

i've often wondered the same thing...problem is getting any power before the turbo spools!

 

[focusedrage]

that is why i think peopel go to high compression pistons, more low end, but still room for boost... the ultimate thing would be variable compression buistions, that as some sort of internal piston that is raised and lowered

 

[GonePostal]

Both boost and higher compression create heat and heat is the enemy. So either way if you lower boost and raise compression or lower compression and raise boost, you'll still have the same enemy. The main benifit raising compression is that it doesn't require anymore fuel where as raising the boost means having injecting more fuel. However, if you can successfully cool the incoming air enough to allow you to run more boost, that is also an option. Depending on the turbo/intercooler setup, there will be different effieciency levels for boost. If boost was raised and the turbo became horribly inefficient, it would probably be better to raise compression with that setup.

 

[Black95_TURBO]

maybe this is off topic but since you are all talking about compression I was wondering what this formula meant. It came with the installation guide for my mbc. Instructions states:

A little helpful formula to calculat your final compression ration (FCR):
( ( boost / 14.7 ) + 1 ) x FCR
EX. ( ( 14 / 14.7 ) + 1 ) x 8.0:1 = 15.6:1

My question is what are we finding with this, it seems to state that we are finding the FCR but yet it is used in the equation. So what does this formula do?

 

[crankbender]

whoa raising the dead.

( ( boost / 14.7 ) + 1 ) x FCR

replace the FCR with CR and ya got it basically.

 

[kpt4321]

Right on guys, that formula is pretty good for making basic comparisons. However, there are a couple other things to consider.

For the following example, let's invent two motors. Motor A has lower compression an runs higher boost, and motor B has higher compression and runs lower boost.

First, let's consider temperature. Since both motor A and B have to arrive at the same "final compression ratio," the total amount of pressure increase will be the same. However, the total amount of temp rise will not be the same.

If you do most of the pressurization with the turbo, then you will gain a little more temperature because a turbo is not as efficient as a stright compression. HOWEVER, you have the ability to intercool air that is compressed with a turbo! With this ability, you can bring the total efficieny of the turbo and intercooler system well above 100%l basically, you heat the air up less than the ideal gas law would say that you would.

On the other hand, all of the heat generated by the compression in the motor has to stay there; there is no way to cool this.

Therefore, the total heat rise is going to be greater in motor B (higher compression ratio, less boost). That means a couple of things, mainly less density and a higher chance of detonation.

Also, you can consider the mass per cycle of the motor. Assuming that both of these motors have the same displacement, then you have effectively the same volume at bottom dead center (in reality, the lower compression motor will have less volume here as well, but it's not a huge difference. On the order of 1.5% bigger for 7:8 to 8.8.). So, if the volume is the same, then whichever system has the air of higher density entering it, will have a greater mass of air as well.

The motor with lower compression and higher boost will have denser air. The temperature and pressure will both be higher, but the temperature will be relatively lower, compared to the pressure rise. That means that you have a more violent combustion in motor A.

I'd be willing to bet that if you plotted out two motors with the same FCR, you would find that there are points of diminishing returns in both ways.

Of course, the problem is that like every other system in a car, you can't just optimize the compression ratio for best peak power. Doing so would give you a car that was a dog out of boost, and required a crap ton of airflow and fuel to support.

 

[candela]

Very well said KPT

The main benifit raising compression is that it doesn't require anymore fuel where as raising the boost means having injecting more fuel. However, if you can successfully cool the incoming air enough to allow you to run more boost, that is also an option. Depending on the turbo/intercooler setup, there will be different effieciency levels for boost. If boost was raised and the turbo became horribly inefficient, it would probably be better to raise compression with that setup.

That benefit of NOT having to add mroe fuel also means warmer temps as well since fuel also cools the mixture. A good intercooling system/heat ransfer is always key and can permit VERY high boost levels. You also have to keep i mind that raising compression and also wack a turbo out of its efficiency range in a case as you described. Imagine what a t-25 would be like on a 9.5:1 4G63 compared to stock CR....

 

[Mangold]

a good data logger can be very helpful in this situation... sometimes you can never really calculate how much boost is too much

there are a lot of variables ie. temperature, octane level (i get 93 in tx) and many other things that go into it

if you have a good data logger just watch for knock... if you detect any drop your psi a little...

high compression is nice for off the line but if you know how to launch you can usually avoid being stuck at 2800 rpm's waiting for turbo so...