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Old 10-28-2004, 10:21   #1 (permalink)
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Compression question

I have just bought GT40P's and i'm going to get them ported..... and possibly shaved for more compression BUT

i already have 9.5:1 comp pistons, have had the block decked, and so i;'ve read GT40P's give a half a point more compression over E7's, so that would give a bit over 10:1......

My question is, if i get them shaved will it be too much compression?
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Old 10-28-2004, 16:55   #2 (permalink)
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Re: Compression question

I think one point is from 9.5 to 9.6?? But cc the combustion chamber.I wouldn't go higher than 9.5 / 9.7 max or you'll need to retard timing etc and start to loose power..Ultra fuel is not always available either..You'r chambers should be around 60 to 62 cc.Is the piston flush with deck or out of hole alittle? As it has diffrent hi comp pistons you have to do your own theory as you have changed the goal posts somewhat...
Here is a link to work out comp once you fill in bore stroke head gasget thickness,chamber cc etc..

http://www.rosspistons.com/calculator.php
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Old 10-28-2004, 18:09   #3 (permalink)
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Re: Compression question

The choice is entirely yours - if you can live with premium unleaded and the possibility of a bottle of 104+ octane booster every time you fill your tank then go for the higher compression. Compression is a good way to make power but power cost money - how much can you afford to continually outlay?
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Old 10-28-2004, 19:41   #4 (permalink)
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Re: Compression question

i think i may avoid getting the heads shaved. I'm not too fussed about premium as i run it now but octane booster might be a pain...

I know this is a completely different motor but, the boss 290 runs 10.5:1 comp, are they retarding the timing a lot to run it?
And the new gen4 has some really high comp as well, and they are saying it can run on ulp (although not recommended)
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Old 10-29-2004, 00:23   #5 (permalink)
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Re: Compression question

Longer stroke motors can get away with more comp.."In general"...
Notice latter motors have undersquare bores, the smaller the head area the LESS chance of detonation,in general...The V.E works there too,obviously the lower the V.E the more comp an engine can run.Though staic comp is high, the actual working comp is alittle lower...
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Old 10-29-2004, 04:32   #6 (permalink)
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Re: Compression question

Quote:
Originally Posted by SiKEDXR8
I know this is a completely different motor but, the boss 290 runs 10.5:1 comp, are they retarding the timing a lot to run it?
And the new gen4 has some really high comp as well, and they are saying it can run on ulp (although not recommended)
they can run the high comp because of better designed/more efficient combustion chambers and possibly better designed coolant passages

id say the ecu would prob have much better fuel/ignition maps aswell
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Old 10-29-2004, 06:16   #7 (permalink)
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Re: Compression question

4 valve motors can as they scavenge exhaust much better...
The Cobra dohc 4.6 motors can take heaps of boost when blown with std compression too...
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