TQ: Compression ratio

[PaulHere]

One for the techies out there []


hi im stage rallying a 2 ltr 924 im running on carbs instead of fuel injection and have fitted a piper cam i wish to increace the compression ratio and need advice as to how high i can safely go and the amount to shave off the deck height of a block on standard bore size any help appreciated.

Trevor Groves
groves@s996605.fsnet.co.uk

 

[924nutter]

First measure the deck height with feeler gauges, and see if there is around about 0.2mm. A significant increase can be acheived by the removal of 0.1mm, as you are working with the full diameter and not the combustion chamber which typically will require 0.25mm from the head to make worthwhile gains. When inreasing the C/R, remember the effect increased cylinder pressure has on pinking, and the availability of fuel with an octane rating of over 99. The 924 has no knock sensors so I would go for no more than 10.00: 1 to give the pistons a chance of surviving, and run it on Bayfords 4* which is 99.3 if I believe my local garage's B/S. Standard the 924 is 9.3:1 so some gains should be possible. I will leave it up to you to work out how much to take off to achieve 10:1, but try and leave 0.1 mm of deck height. After all that is only 0.003937" in english money, and that's kinda thin. I know there is a gasket but you dont want the piston crown hitting the metal gasket edging do you? Any one else any thoughts? How about you Carrera 937?

 

[PaulHere]

Thanks John - will pass your advice on.

P.S. You are OK to be at FrontRunner?

 

[937carrera]

Hmmm, I don't have any specific experience of adjusting the compression ratio on a 924 as the 924GT really will not take any increase in compression ratio. Having just specced up custom pistons & rods for my 944T for a decked block I have become reasonably familiar with the issues and calculations concerned. Lets start off with the basics

Combustion chamber volume = swept volume+piston dish volume+gasket volume+head colume
Compression ratio = (piston dish volume+gasket volume+head volume) / combustion chanber volume
Deck height = distance from top of piston to top of block......if the piston stands proud, then that's a negative number

There are two targets that you will be aiming for. The first of those will be compression ratio. The higher the compression ratio, the more power and heat will be generated and the easier it is for the car to get into detonation issues - of course you can start changing ignition timing as well. The second of these is the distance from the top of the piston at TDC to the base of the head, gasket thickness is critical here. This is called compression height.

Remember that you will be performing a cold calculation here, the distance will be different at 6500 RPM when hot. A general rule of thumb I have been given is that this should not be less than 1mm. Having said that, one engine builder I know will build the engine, dyno it at 500rpm more than max, strip it, deck the block again, rebuild & dyno again until there is evidence of piston / head contact and then machine the top of the piston to suit. That's way beyond the level I operate at.

The engine I have had built did have negative deck height & there were no concerns about a piston hitting the gasket, though I am using a MLS gasket. If you don't already know it, number 4 cylinder is the one that you will experience detonation issues / head gasket problems on.

Hope that helps................ I'm writing from a position that now I at least have an idea about how little I do know.

 

[924nutter]

I am currently removing gearbox of the turbo to eradicate poor first gear selection. The top left hand bolt is proving to be a git, otherwise I would already have it in my lap. Should be done by the end of the week. If it is then I will be at F/R 2006.