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Compression ratio
KC built me an engine, which may not have much power below 4500rpm, this nascar based engine is 373 cuin and is dynoed at 675bhp at 7200rpm, the compression ratio is 10.3.1, with Yates alloy heads, on our BP 98 octane (your 92octane) and the timing back 2-3 degrees it should run fine on the road for cruising semi hard use etc.
The best thing to do is run some race fuel, 60/40 or 50/50 we have an airport 5 mins drive and they are happy to fill 44 gallon drums, as a friend has a fuel card, same price as normal fuel, plus the Porsche GT3 race guys run the Aussie NULON PRO fuel additive, you may have that available in the US, apparently this additive has been tested with other additives (and their crap claims), Nulon actually does improve the octane rating, so throw a good octane booster in with premium pump gas, the timing back slightly and there should be no problems.........food (fuel) for thought!!!!!!!! |
10:1 on my stroked 460 (545 inch) with E Performer RPM heads. No problems with 91 octane, best that's currently available in Des Moines, Iowa :3DSMILE:
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I'm running 10.8 - 1 with Aluminum Heads on my Keith Craft built 427 genesis engine. Runs great on pump gas.
:D :D :D |
I am right at 11-1, maybe a just a little bit less, 68 427 SO block stroked with a Scat 428 crank, bored 16 thousands over, Rpm performer heads ported and polished using flat top Diamond pistons (with valve relief), comp cams 282s cam with flat tappet roller rockers. I use 93 octane and xylene as a fuel additive and the engine loves it.
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jdean, your onto something big with that piston quench area thing. You should not underestimate the quench area effect on detonation. In many cases THAT is the reason one guy can run 10.5 or 11 and 'get away with it' and another guy can't! As well as the timing curve (base and total advance) has a BIG impact on what you can and can't 'get away with'. There is more to this than a simple answer of C.R. will work well with any given motor.
Of course 'rules of thumb' are a good place to start. A little higher C.R. with alloy heads, a little lower C.R. with iron heads, for instance. Consideration must also be given to the cam profile and it's effect on the C.R. (dynamic C.R.). My FIRST rule of thumb is: I want as much C.R. as I can get away with using PUMP GAS. Additives are a pain in the butt. |
I'm running 9.5:1 with iron heads. I would have run 10.5:1 if I had built the motor from scratch, but I went with a nice already built motor. It's been flawless thus far.
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Add a gallon of tuolene to a near full tank of 93 and you'll get about 96 octane. Get it at a paint store, a few bucks a gallon. Or same with Xylene at Home Depot. There is a lengthy post about this on forum. Search "toulene".
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After getting tired of hauling av-gas, I dropped the compression in my 427 s/o from 12:1 to 10.5:1. No problem with 93 octane pump gas now. Engine is fitted with ci tunnel port heads, which don't soak up as much heat as uncoated aluminum heads. Other that head material, another huge consideration not mentioned is camshaft events and timing. The more overlap you're running allows more compression, due to pressure loss during the overlap period. You can play with camshaft advance, too. Don't forget spark lead, i.e., advance versus retard. We can get into combustion-chamber efficiency and air/fuel mixture, too, so there is no pat answer on the compression one engine can tolerate versus another with a given octane petrol rating.:JEKYLHYDE
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10.5 :1 and I run 93 around town with some 110 added in for safety. On the track I use 110 only. No problems with detonation (427 w. Edel heads (worked a little) and MR w Demon 4V)
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Roush built 434 C.I. stroked Windsor with Aluminum AFR ported heads running 10.25 compression. Likes 91 octane.
I was running a mix of F&L 111 octane and 91 octane and the car started running bad on the top end. Took the car to my mechanic thinking it was a fuel issue and after calling the person in charge of engine developement for Roush was told that the gas was to high octane and therefor was burning to slow for the motor. Got rid of the mix and went back to the 91 and the car runs perfect now. Almost makes me want to bump the compression and run straight race fuel (which is easily available here) and see how much more hp can make, but I think at almost 600 I should probably leave it alone.:cool: |
418W with aluminum Edelbrock Victor Jr. heads, polished/ported. 10.83:1 compression with 91 octane. 20 degrees initial timing, 32-34 full advance. Runs like a top!
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10.5:1 w/ no detonation problems on 91 pump gas. 516 C.I., port injected, aluminum head, Boss 429
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Hi,
I would think it's Impossible to hear any detonation at all in a noisy cobra. Probably the best way is to check the spark plugs. Perry. |
I'm running 11:1 with Edelbrock heads and Probe pistons. Flat top pistons with valve reliefs. DCR is 8.27:1. Runs on 93 octane.
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Thanks for the replies. So it seems that especially with aluminum heads, 9.5:1 is somewhat conservative. Thinking back, my Mustang Mach 1 (351C) had TRW forged pistons- supposedly between 10.5 and 10.7:1, but leaded gas was still available then. %/ It has been 15 years since I sold it, but I don't recall any detonation or pinging problems. I religiously ran Sunoco 94 Ultra.
I am still having trouble finding pistons, which is what motivated this post. I guess I knew going into it that FE parts are still scarce, but theres still less available that I had thought. Diamond quoted me $685.00 (41002's), Probe said 800-900, Egge machine said 300.00 for KB's, and Summit tried to talk me into 9.5:1 TRW's for 295.00...but nothing seems to work out CR wise. I am still looking, but based on your advice and info, I would'nt be afraid to consider something 10.5:1 or so. Theres also more variables than I had realized which makes this thread even more interesting. Greg |
It is tricky specing the right pistons. I went with Arias, custom slugs. Had to do a LOT of measuring and calculating to arrive at my specs. I moved the piston 'up' in the hole to gain quench, but then had to 'dish' it to keep compression at my target 9.6 (or 9.8, I forget now). It cost a bunch!
B.T.W. typical 427 specs leave the piston .030 below deck, this means virtually no 'quench'. Back in the day high octane was readily available and quench wasn't a serious consideration like it is now. If you run 'off the shelf' pistons it is a big question what your real C.R. will wind up being (and no quench either). |
Greg: I had to pop for custom machined JE's for my tunnel-port 427. Got all I wanted, i.e., compression ratio, quench, size and quality for just a bit over $900. I cried once.
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Im running 11.9 to 1 on 93 octane pump gas. No detonation. It's all in combustion chamber design. I'm using Edelbrock Victor junior heads and Keith Black flat top pistons in a 393.
http://www.theoldone.com/articles/Th...5FHead%5F1999/ I'm sure this has been hashed out here before but the link above gives a lot of information relative to combustion chamber design in performance vehicles. It works! h dog |
Hound Dog, while your engine is at the 'extreme' end of things your point is a good one. It IS all about combustion chamber design (quench being a big part of that) as to what C.R. you can get away with.
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My smallblock is 12:5 to 1, solid cam, dual 4's, 1 3/4 headers into 3 1/2 collectors. have to run 95+ octane with a manual tranny. TOOOOOOMUCH FUN.
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