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Compression ratio

PostPosted: Mon May 06, 2019 5:13 pm
by Donels
Having identified from the piston code that my engine is bored to +.040 I have now calculated the compression ratio. This would appear to be 10.56:1 assuming the combustion chamber volume is standard for a S130. I don't have the head at the moment so cannot measure it.

10.56:1 seems a bit high to me for a standard road engine and todays unleaded fuel. Should I be fitting a thicker gasket to get the CR down a bit?

Anyone got any experience.

Dave

Re: Compression ratio

PostPosted: Mon May 06, 2019 7:59 pm
by avro
Hello Dave

10.56 is a little on the high side. Dare l suggest that your calculations may not have taken into account that the piston on a standard unsurfaced block sits approximately .025” down the bore at TDC and the Powermax pistons that you are using have the larger valve pockets. I am of course assuming that your block has not been refaced.

Nick

Re: Compression ratio

PostPosted: Tue May 07, 2019 12:12 pm
by Donels
Well spotted. I hadn't taken that into account and it does drop the CR a little to 10.54 which is still high.
Has anyone with a bored engine got any experience.

Dave

Re: Compression ratio

PostPosted: Tue May 07, 2019 12:14 pm
by 2cams70
You are making a lot of assumptions on an old engine. Best to CC the combustion chambers as it's quite unlikely they will be standard.

Re: Compression ratio

PostPosted: Tue May 07, 2019 12:28 pm
by rgh0
With a short duration standard sprint road cam 10.5 to 1 is Ok on 98 Octane unleaded and you may get away with 95 Octane. Potentially you could push it a little higher on 98 Octane. If you want to get fancy with knock sensors controlling the ignition you can go to around 11 to 1 and maybe more.

cheers
Rohan

Re: Compression ratio

PostPosted: Tue May 07, 2019 12:34 pm
by Donels
Thanks Rohan that's what I was looking for.

Dave