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QUOTE(Kwanza26 @ Jul 20, 2006 - 8:01 AM) [snapback]458516[/snapback]
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QUOTE(Rexasaurus @ Jul 20, 2006 - 6:15 AM) [snapback]458461[/snapback]
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actually. if i remember correctly, bluetop head mated to the 7afe yields ~10:1 compression ratio. but if you really want to build a good motor you might as well match the 4age head by using 4age pistons and get some higher compression
You remember incorrectly. The 7AFE's stock pistons are dished and yield only 9:5 compression on the narrower, smaller 7AFE combustion chamber... the stock bigport pistons are flat and yield 9:4 compression... so if you put a head designed to make 9:4 compression with flat pistons with pistons that are dished... you make significantly less compression.
Easy way to see it... the 7AFE has a much smaller compression surface on the head, so the dishes in the pistons serve also as partial combustion chambers. The 4AGE does this entirely on the head, leaving the pistons flate... so if you put the 4AG head on the 7AFE pistons... you get really low compression... cheddar?
according to tech reference on club4ag, it says "All else being done, the engine should have 1800cc and compression ratio of between 9.8 and 11 depending on which piston and 4A-GE head you used." oh, and to add, i've ridden in several 7age powered ae86's, using totally stock 7afe bottom, with a largeport 4age head, didnt seem like it had "7" compression. i would also guess that the headgasket would play a role in that. dont know the stock thickness of the 7afe, but 4age has a 1.2mm stock gasket, and you can go all the way down to a .5mm, giving .7 more compression, then shave the head and get even more.
This post has been edited by Rexasaurus: Jul 20, 2006 - 4:59 PM