Originally Posted by JjCoDeX75
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.
Yes, thats correct for a given bottom end, the way to calculate it is:
(Swept + Unswept) / Unswept
Unswept is the bit above the piston (including piston dish and the gasket as well as the volume of the head)
Swept is the volume displaced by the piston as it moves from bottom to top, so 500cc in the case of a 2000cc engine (YB is actually a touch under 2000cc)
Swept is calculated as Stroke * Bore * Bore * Pi
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.
Thats correct, just to add a little more detail though, the reason that a lower CR will lead to a greater permissable VE is that if you are squeezing the mixture less hard, you can have more mixture to squeeze essentially.
So the CR does not in any way effect the VE (slightly untrue actually, there is a TINY reduction in VE when you drop the CR, which is most effected at the top of the rev range)
So your comment of allowing other things to be changed would on a turbo engine typically mean that more boost would be safe without risking det, and its from this increased boost that the VE improvement comes.
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.
Its RETARD the ignition, ie fire it sooner. (easy to see why doing it sooner sounds like you have advanced it though, quite confusing that really!)
The reverse and opposite is therefore true.
Braodly speaking yes, but as with most things in life its an "X squared" type of curve in terms of power versus compression ratio, ie the power will go up and up until you get to a point and then start to come down again, this point is the optimum CR for your setup at those particular revs.
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.
Yes, thats correct, there is a direct relationship between air consumed per cylce and torque, so as we have learnt that we cant over fill the cylinder too much in one cylce for risk of detonation, it means that we are effectively capping the amount of torque we can ask the engine to reliabley produce for any given fuel.
So if we want big power figures within a fixed amount of rpm (ie we cant get it by carrying the torque on till 10,000 rpm as there are mechanical limits) then it means that we need to alter the CR to allow us to generate more torque reliabley per cycle so that we can get the power we want within the fixed number of cycles per minute we are limited to.
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.
After a remap of course to take account of the new airflow characteristsics
BUT
Because the compression ratio has been reduced, at lower speeds, the engine needs to flow more air to run properly.
"run properly" is a misleading term, the correct way of wording it would be more like "in order to make the same amount of torque as before" or similar
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.
Yes altering the cams can help you to increase the low speed VE of the engine to get back the torque we lost by going lower CR potentially, but it will of course have a negative impact further up the rev range if you do this, so great care is needed so as not to end up choking the engine at high rpm just to get back the (from a performance point of view) off boost drivability of the engine, so again, its a compromise sadly.
It is because of this that the timing of the cams needs to be amended to reflect the change in the CR.
Its because of the increased boost, the fact that the engine has got a new turbo on the side, its because of all of it, including the CR
it is THIS that tends to make the difference between a good low comp engine, and a bad low comp engine.
THIS is one of the most important factors, but its not by any means the only important factor, getting the right cylinder head modifications can be equally as important for example, particuarly at higher power levels, and the correct selection of a turbo unrestrictive enough to flow well at high rpm and still small enough to make boost readily is also vital.
Lets pause here, and make sure that what I have above is correct (cue all resident experts!)
Im quite surpised how much you seemed to have picked up in the last week, you obviously arent as thick as GarethT looks