Hey I'm just wondering if anyone knows a general rule of thumb or has personal experience bumping up compression in A series engines and how much difference it makes. Reason I ask is when I rebuilt my A15 like 7 years ago, I put in stock style dished pistons, Sterling series made by Federal Mogul p/n 12182P. These pistons almost look like they have MORE of a dish than the stock ones. As a result I think I have pretty darn low compression, probably like 8.5:1. The head is an A14 open/round comb. chamber ported/polished/rebuilt/oversize SS valves and unmilled as well. I think I am just about at my limits for performance without altering compression/cam/bottom end with the exception of roller rockers. The car started out doing 18's in the quarter mile, and slowly progressing with bolt ons has gotten me into Low 16's, possibly high 15's with more testing/tuning. I've put on the NISMO header, exhaust cutout, 40mm Dellorto's, electric fuel pump, MSD ignition, chrome moly pushrods, dual valve springs, mild cam, aluminum flywheel and NISMO clutch. I was just wondering what kind of difference would bumping up the compression to like 10:1 make??? How much horsies would that be worth in an A15 like mine??? On the other hand, it's probably a pretty good candidate for that turbo/EFI I have, but I'm sure the engine wouldn't last too long if I boosted it, it'd probably need a good rebuild to hold up well. Anyway just wondering.

-Andy
"Back in Texas again"

were you in Japan?

the change would be night and day think of it as putting an a-15 where a tired a 12 was. the comp numbers you stated would yeald a jump of simular dimensions to the swap, plus you will see a jump in torque making the car more drivable and your fuel milage will go up if you don't change anything else.

Yep I agree...A15 race engines usually run a 10:1 comp ratio. Use Shell optimax (104 RON)for high octane and less ping...unleaded is fine as A series heads have hardened valve seats from factory

Since when has Shell optimax been 104 RON !!!???

i dunno, wish it was, thatd be sweet
its 98

but synergy is better!!!,

its also 98 but i've heard optimax can vary and is incosistant, i had a mate working for shell have four people come in in one week cracking the ####s cause after they took theyre cars with ####ed motors to their mechanics the mechanics said the fuel was at fault!!!!

scary huh, anyone heard bad things bout synergy???, i havent just wondering what other peoples opions are

I haven't heard anything bad about synergy, but I don't know anyone who uses it. BP ultimate has worked well for me.
My car didn't seem to run as well on optimax.

More compression = better fuel economy, more power and more emissions. And more expensive gasoline.

But consider that the 10:1 A12GX (83ps) was only 20% more powerful than the 9:1 A12 (69ps). And only part of that was compression. The rest was better breathing (twin carbs, larger ports and bigger cam). I would guess the 1-pt compression increase added 5%

See compression ratio calculator

If you want to read the ground-breaking study, see compression ratio report

Guys sorry Optimax is 98ron...I thought it was higher.

ah but if the breathing is already there but the compression is not.... you see my point the engine will be able to use the breathing more effectively an make more power.

example I have a L-20 that is going into my 120y the stock comp is 8.5:1 and with extractors and a weber it might make 120 hp at the crank, now if you put su's on it you might see 135-140 at the crank not bad, but if you bump the comp to say 10:1 the output would jump to around 160-170 or so with the extractors and su's.