11 replies to this topic

[coopdog]

Well I'm just thinking about doing some stuff, main thing is doing the head, I'm getting a 12g295 head for my 998 running unskimmed which will give me about a cr of 9:3:1 how would this run and how much hp would I lose compare to having it skimmed to 10cr

[ACDodd]

Not much at all actually about 1%. Across the rev range, more importantly what cam are you running?

AC

[coopdog]

Not much at all actually about 1%. Across the rev range, more importantly what cam are you running?

AC

was running a standard cam atm, but will be using a mg metro cam

[ACDodd]

Leave it as is. The engine will be less fussy an longer lived. Just set it up properly and ensure that the valves seal well and the timng and fueling are correct.

AC

[racingenglishcars]

Well I'm just thinking about doing some stuff, main thing is doing the head, I'm getting a 12g295 head for my 998 running unskimmed which will give me about a cr of 9:3:1 how would this run and how much hp would I lose compare to having it skimmed to 10cr

I calculate CR=8.3 with unskimmed 12G295 head (28.3cc), 1mm head gasket (3.43cc), 0.7mm deck height (2.29cc) and 0.25mm ring land (0.32cc). This assumes no overbore and swept volume 249.1cc. Am I missing something or do you have another way of calculating CR?

Edited by racingenglishcars, 23 February 2012 - 08:45 AM.

[mini_in_progress]

Well I'm just thinking about doing some stuff, main thing is doing the head, I'm getting a 12g295 head for my 998 running unskimmed which will give me about a cr of 9:3:1 how would this run and how much hp would I lose compare to having it skimmed to 10cr

I calculate CR=8.3 with unskimmed 12G295 head (28.3cc), 1mm head gasket (3.43cc), 0.7mm deck height (2.29cc) and 0.25mm ring land (0.32cc). This assumes no overbore and swept volume 249.1cc. Am I missing something or do you have another way of calculating CR?

stick a turbo on it

[coopdog]

Well I'm just thinking about doing some stuff, main thing is doing the head, I'm getting a 12g295 head for my 998 running unskimmed which will give me about a cr of 9:3:1 how would this run and how much hp would I lose compare to having it skimmed to 10cr

I calculate CR=8.3 with unskimmed 12G295 head (28.3cc), 1mm head gasket (3.43cc), 0.7mm deck height (2.29cc) and 0.25mm ring land (0.32cc). This assumes no overbore and swept volume 249.1cc. Am I missing something or do you have another way of calculating CR?

stick a turbo on it

thats the plan :)

Well I'm just thinking about doing some stuff, main thing is doing the head, I'm getting a 12g295 head for my 998 running unskimmed which will give me about a cr of 9:3:1 how would this run and how much hp would I lose compare to having it skimmed to 10cr

I calculate CR=8.3 with unskimmed 12G295 head (28.3cc), 1mm head gasket (3.43cc), 0.7mm deck height (2.29cc) and 0.25mm ring land (0.32cc). This assumes no overbore and swept volume 249.1cc. Am I missing something or do you have another way of calculating CR?

i got told thats how much it will be if i put the head on :/

[coopdog]

Well I'm just thinking about doing some stuff, main thing is doing the head, I'm getting a 12g295 head for my 998 running unskimmed which will give me about a cr of 9:3:1 how would this run and how much hp would I lose compare to having it skimmed to 10cr

I calculate CR=8.3 with unskimmed 12G295 head (28.3cc), 1mm head gasket (3.43cc), 0.7mm deck height (2.29cc) and 0.25mm ring land (0.32cc). This assumes no overbore and swept volume 249.1cc. Am I missing something or do you have another way of calculating CR?

is this with flat top pistons?

[Wil_h]

I calculate CR=8.3 with unskimmed 12G295 head (28.3cc), 1mm head gasket (3.43cc), 0.7mm deck height (2.29cc) and 0.25mm ring land (0.32cc). This assumes no overbore and swept volume 249.1cc. Am I missing something or do you have another way of calculating CR?

9.3:1 is closer than 8.3:1. Gasket is 2.8cc not 3.4cc. Not sure where you get the deck height from? You'd need to measure the engine I guess, but typically I use 0, which is what all the 998s I have are; I've never seen 0.7mm or even close to it.

There is nothing wrong with your calculation, just the assumptions you have made.

[racingenglishcars]

OK by me, let's assume 0 deck, head gasket 2.8cc (~0.82 thick) non-skimmed 295 head, flattops and no ring land gives closer to 9:1 cr. :)

[Wil_h]

Sounds good to me. I dug an old figure out of my head. Having now checked, 9.3:1 was for a +40 998

[Turbo Phil]

I built a 998 with +0.020" Minispares flat tops & they sat just over 0.040" down the bore at TDC so it's essential to check.

Phil.