Compression - How much is too much?

[NoHachi]

I understand what you said, but just disagree with you

Higher compression ratio's, all other things left stock, do not consume more air or fuel. The ECU does measure the amount of air going in,but it stays exactly the same.
If you read the link and some other references online, you will find decent (better then I can give for sure) explanations of why higher combustion pressures lead to a more efficient burn and more power.

All this is valid for a stock engine, with increased CR. Change anything else (affecting Volumetric-efficiency) and the amount of air changes.

[XJS]

NoHachi Wrote:I understand what you said, but just disagree with you

Higher compression ratio's, all other things left stock, do not consume more air or fuel. The ECU does measure the amount of air going in,but it stays exactly the same.
If you read the link and some other references online, you will find decent (better then I can give for sure) explanations of why higher combustion pressures lead to a more efficient burn and more power.

All this is valid for a stock engine, with increased CR. Change anything else (affecting Volumetric-efficiency) and the amount of air changes.

Disagreement is good, generates agreement at the end.
So it's crystal clear now: you think change in CR doesn't effect on VE, I think it does. That's ok, everybody could have different opinions.

[NoHachi]

Ah, ok, for arguments sake:
From Wikipedia:

Quote:A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.

[XJS]

NoHachi Wrote:Ah, ok, for arguments sake:
From Wikipedia:

Quote:A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.

I don't want to argue, you're right, more comression will generate more efficiency hence more power, I was wrong. Just search for me an other article where says higher compression doesn't consume more air.

[mcm95403]

This is why I am switching over to 91 small port electronics. that way I have a block with a knock sensor and a computer designed to work with it. I think it will be a much better start for a hi-comp engine.

I think you'd be fine bumping the CR up by a full point with the stock ECU. If you want, I have a set of standard bore 10:3 hi-comp pistons and rods I was about to put on E-bay. You can e-mail me through my website if you'd like to buy just the pistons. If you can, get your hands on a small port intake and epoxy it up like this (assuming you're not in a RWD car - if so you also need to relocate the TB of course):

http://www.marcmedina.com/intake.html

It made a significant improvement. Also, the longer tube from a small port that goes between the TB and AFM (assuming you're not MAP equipped) was also a nice improvement. I didn't believe it either till I tried it.

[adam]

Thanks for the offer, but 10.3:1 CR 4AGE pistons are readily available here in Australia.

I'm just considering raising compression on my motor because I blew a head gasket. I thought while I'm in need of machining the head, why not shave some off to gain a little compression?

And after wanting to raise compression, I thought why not do some minor head work as well to support the higher compression.

Can't really afford to swapover to a small port loom and ECU set up or replace my block at the moment. Just some 'cheap power' for now. Maybe once I install some camshafts and an ECU I'll start modifying my bottom end.

[oldeskewltoy]

I'm by no means the end all expert... but higher compression does NOT allow MORE air and fuel(intake charge). Cam lift and duration control the intake charge.

Higher compression allows for higher cylinder pressures which can, in of itself, produce more power from the same intake charge.

The problem becomes at one point does the added compression overheat the intake charge igniting it BEFORE the ignition ignites it.

Now... we need to add dynamic compression ratio to the equation. DCR takes into account cam duration. The longer the duration on the intake lobe, the higher the compression the engine can run. This is because the intake valve closes later in the compression cycle. This later closing compresses the intake charge to a similar pressure before iginition, as the lower compression engine did with shorter duration camshafts.

Ideally we want to maximize the intake charge and keep it cool enough to maximize the compression allowing a VERY high cylinder pressure. We can accomplish this a few ways...
1) remove the heat....
1a) lower the temp of the intake charge (intercooler?)
1b) remove hotspots inside the combustion area to remove points of pre-ignition
2) raise the temp the gas can ignite without a spark - higher octane


Books can, and already have been written on this subject. In reality it just takes someone with experience to know what to do... and more importantly what not to do.