Right (for the second time due to a PHP Critical Error
)
I had a chat with Chip on the dog and bone, and he has attempted to explain the difference between various compression ratios. I will attempt to explain to you as best as I can.
Hopefully the resident experts will correct the bits that I get wrong!
The constants and assumtions
We will assume that the engine in question had a fixed number of cylinders, as well as stroke, and the number of cylinders is likewise fixed.
What is compression ratio
If I understand it, the compression ratio is effectively dictated by the amount of space at TDC. If you lower the compression ratio, then you are effectively slightly increasing this space. One of the advantages of the lower compression ratio is that it permits a greater Volumetric Efficiency (VE) (made by the effective increased area).
Please note that the volumetric efficiency is not improved by simply lowering the compression ratio - it merely allows for an improved VE if other things are changed.
The pros and cons of different ratios
First the pros - one of the most significant improvements is that the car is now able to run more advance without leading to detonation. Chip came up with a really useful way of visualising it.
Imagine that your car in std compression with say a T34 turbo. Currently it can work satisfactorily with say 1.5 bar of boost without danger of detonation (when properly set up). As you RAISE the compression ratio, eventually you will get to a point where you can no longer advance the engine enough to run that same level of boost.
The reverse and opposite is therefore true.
The second benefit is more subtle. As I understand it, a higher compression ratio car achieves power at a higher rpm. On the cosworth engine, the safe maximum realistic RPM is approx 8000rpm. Given this, when attempting to reach a certain amount of power, the compression ratio therefore needs to be lowered to allow for the intended power level to be reached.
Second, the cons.
If you simply change the compression ratio and stuff a large turbo on the side of the engine, you will increase the power lever achieved by the engine based on the above. BUT
Because the compression ratio has been reduced, at lower speeds, the engine needs to flow more air to run properly. Now given that the cams in the engine are basically set up for the factory compression ratio, they do not optimise our new - lower compression ratio.
It is because of this that the timing of the cams needs to be amended to reflect the change in the CR.
it is THIS that tends to make the difference between a good low comp engine, and a bad low comp engine.
Lets pause here, and make sure that what I have above is correct (cue all resident experts!)
Thanks all!
PS
Gareth-T - I bought the books, and they are on their way!!!
JJ