Hi all,

Just wondered if someone could tell me how to get the compression ratio of my RS engine. The head on my RS has had 0.5mm taken off it which suggests it has been skimmed once or twice so I need to know if this is the reason as to why the pistons have cracked.

Cheers
Dave

 

no that wouldnt cause it ,but over boost would ,have they cracked where the the piston rings sit [ring lands]

 

Yes but the boost was turned right down due to me wanting to run the car in properly. I did about 400 miles in the car when I noticed it wasn't quite right.

Cheers
Dave

 

did you fit new rings? or did you put pistons in and did you have it bored?

 

It was bored to 0.5 oversize and honed with new pistons and rings which were put in with a piston ring compressor.

Cheers
Dave

 

sounds like damage from detonation. a skimmed head as you know will give a higher compression ratio which generally means that you must run less ignition advance for a given boost. if the chip that you had fitted had quite an aggressive ignition curve that no longer fitted the compression ratio of the engine, or low octane fuel was used, you could've been getting detonation.

the way to measure it is just the same as any other engine. you need to measure the volume of the piston bowl, the combustion chamber in the head and the dead volume and work it all out. there some helpful stuff out there on the net that explains it in general terms.

 

If I were getting detonation would I here the engine pinking?

The ecu is from a standard S1 RS. Would this effect it?

Thanks for the advice so far.

 

To answer the question inself:
You have roughly increased the CR by 4% by the skimming and a further 2% by the rebore. Giving you a CR of ~8.6

 

unlikely that you would hear detonation as it would be happening on boost when there is normally too much noise from turbo, induction, exhaust etc.

but you say you weren't running much boost?

 

It's easy to hear detonation IF you know what to listen.

 

As far as know it sounds like a peice of metal rattling around in a tin. Never heard detonation before though so I could be wrong.

Cheers
Dave

 

your only hear bad det, slite det will not always be heard, specially in a car with a dog box, a det can set up is best.