Compression ratio

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Compression ratio

Postby Jersey imp » Mon Jan 18, 2016 8:40 pm

Hello, I am new to the forum and need some technical advice please.
I am currently building my 998 bored to 73mm giving 1010cc. The car is for road and occasional hill and sprints, I'm using an R20 cam, chessman race head, webers etc. I have seen a figure of 11.5 to 1 but would like to confirm if this is suitable for pump fuel.
Also can anyone confirm the compressed thickness or cc of a wills ring for volume calculations.
I will of course cc the engine when assembled but these figures would help.
Thanks in advance. Imp007
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Re: Compression ratio

Postby Turboimp » Mon Jan 18, 2016 9:38 pm

The water gasket is usually 0.019 inch thick , so substitute that in your headgasket thickness calculation ... Imp009
1967 Imp Sport turbo 328000 Imp Miles , in the same car
1972 Clan Crusader 232000 clan Miles in the same car
must be some kind of record !
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Re: Compression ratio

Postby Lotus-e-Clan » Mon Jan 18, 2016 10:53 pm

Yes you want the high 11.6- 11.8 comp ratio for the R20 and it will work no probs on Shell V max Ron 98 etc. Mine is 1040 with a hotter cam gb72 on 11.8SCR using shell Vmax.
The high comp is needed at the bottom end (low mid revs) because the late inlet closing leaks away compression until you get to high revs (4K plus) where the gasses have less time to escape. You will only have trouble with high comp /detonation using cams giving early inlet closing (Sport cams) that are loaded (WOT) at low revs and are run too hot (and with poor ignition).

You will need a 19 cc combustion chamber volume, Pistons 7 thou down bore, 19 thou WR gasket and large valve piston cutouts (2.1cc) on a 1010 (73 mm bore) to get 11.6 SCR - you wont easily get 11.8 without going to 1040 cc or by skimming the head a lot (maybe too much) to get below 19cc. A std sport head has a 19cc volume btw . if you flare the chamber (I haven't done mine and it's very torquey) you will have to skim a lot to get back to 19cc or use intruder pistons.
73mmcomprat.jpg

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Re: Compression ratio

Postby Jersey imp » Thu Jan 21, 2016 9:37 pm

Hello and thank you for the detailed reply.
The information provided will be very useful over the next week or two. I was close as I have calculated my head to be 19cc for the required ratio. The head must have been skimmed as the chamber is releaved around both valves. I think it was at 18.2 when fitted previously to my 998 using bhp Pistons. I am now using Maynard with large cut outs.
Thanks again for your time and advice.
Regards
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Re: Compression ratio

Postby Lotus-e-Clan » Thu Jan 21, 2016 9:46 pm

Well with an 18.2 combustion chmber volume you will have a mighty SCR of 11.97:1 8)

73mmcom182.jpg

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