01-14-2007, 11:46 PM | #1 |
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Engine help
I am wanting to put higher compression pistons in my big block 402. It is bored .060 over and is stock right now. What is the highest I can run on 87,90,91,93 octanes? Is 9.50:1 about it? thanks
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01-15-2007, 12:47 AM | #2 |
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Re: Engine help
I've only built a few engines, but my 350 is a shade over 10 to 1. It's complete, but on a stand (the truck is NOT complete), so I can't tell you what octane it needs. My camaro's got basically the same engine, but with aluminum heads, so it could go higher CRs and still use pump gas. Someone else may chime in differently, but I'd think you'd be okay to at least 10 to 1 if using 91 octane, maybe 11 to 1 if using aluminum heads. Cam profile and timing required will factor in too, though.
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01-15-2007, 01:02 AM | #3 |
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Re: Engine help
Alrighty, here we go...
Low octane (87) you can go with 8:1 with cast iron heads or 9:1 with aluminum heads. Mid-grade fuel (89) will allow 8.5:1 with cast iron heads or 9.5:1 with aluminum heads. Top-grade (91-93) with allow 9:1 with cast iron heads or 10:1 with aluminum heads. Pre-detonation occurs when the air/fuel mixture reaches too high of a temperature and predetonates in the cylinder before the piston reaches the top of the cylinder. Aluminum dissapates heat quicker therefore a higher compression ratio can be sustained with aluminum heads. If you run an EFI setup you can even go higher with the compression ratio because the computer can accomodate it (i'm not sure exactly) and make it work. Also, if you're into race stuff and your cam is huge you'll have to run more compression because the overlap of the exhaust and intake lobes will cause a loss in compression (if i'm not mistaken).
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01-15-2007, 02:30 AM | #4 |
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Re: Engine help
A lot depends on the head and the combustion chanber shape.
Remember that the lower the octane, the higher the hp for a given fuel. If you have the engine tuned on a dyno with Lambda, and its got a carb, you can run upwards of 9.8 -10 to 1. 10.5 with aluminum heads. thats with 87, running a 170deg thermostat. Also remember that running with FI will somewhat lower the atomization of the fuel, but that is why most all systems are fitted with knock sensors, and can self adjust for Lambda The real bummer is that there is a LOT of torque to be made with more CR. Our Motorcycles run upwards of 14:1, and make over 225hp on gas, with only 1 liter to do it! Figure on being limited at about 10:1 no matter what all combinations you run. Good luck!
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01-15-2007, 04:11 AM | #5 |
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Re: Engine help
That's a reasonably safe number. The biggest single factor is the dynamic compression ratio as determined by the cam. How much HP do you want to make?
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01-15-2007, 09:19 AM | #6 |
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Re: Engine help
I'm not sure yet, it has 310 factory horse. So, I was thinking 400 shouldn't be too unreasonable. So if I ran the stock 402 heads with 9.5:1 pistons, I should run 93 octane? thanks
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68 Chevy CST/10, 50th Anniversary, 327/th400, 373 posi, buckets 68 GMC Shortbox Stepside, 402/th400, Tilt, Tach, Bucket Seats, 3.08 Positraction 69 Chevy Camaro SS, 383, 4 speed, positraction 70 Chevy El Camino SS396, Triple Black, 454/th400, Tach 70 Chevy Shortbox Stepside 4x4, 350/th350, Tilt, Tach, Cab Lights, Bumper Guards, Tow Hooks 70 Chevy 3/4 Longbox, 350/auto, 70k, SURVIVOR 71 Chevy Blazer, CST, 350/auto 71 GMC Sierra K2500, 454, 4 speed, 4.10 posi, 37s 71 Custom/30, Open Road, 350/th400, tilt 72 Chevy Shortbox Fleetside, 496 Stroker/th400, Tilt, Tach, Bucket Seats, 3.42 Posi 72 GMC Sierra Grande 2500. 427/th400, tilt, a/c, tool and stowage, buckets/console, 3.54 posi |
01-15-2007, 10:48 AM | #7 |
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Re: Engine help
i'd just do some work on the heads cleaning them up and a performance manifold dual exhaust with a mild cam and you'd be near the 400hp range on reg gas //screw hi-test
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01-15-2007, 11:22 AM | #8 | |
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Re: Engine help
Quote:
I expect the 310HP factory number was a bit optimistic and was gross vs. SAE net HP, especially given the stock cam lift of .434/.452. With cast-iron manifolds and mufflers I expect it was closer to 275 or so. Last edited by Billla; 01-15-2007 at 11:58 AM. |
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01-15-2007, 11:30 AM | #9 |
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Re: Engine help
You can run up to 9:1 on regular pump gas...been doing that for years without issues.
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01-15-2007, 11:54 AM | #10 |
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Re: Engine help
I'm running 11:1 CR with basically an otherwise stock 350 (cast iron heads) on 87 octane gas. Yes it does rattle a little IF I lug it down but I built it for economy. MPG is directly proportional to compression ratio. I built it in 2000 and have 50,000 miles on the engine with no ill effects even at lower elevations down to 3000ft. It's been a good mod for me.
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01-15-2007, 12:01 PM | #11 |
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Re: Engine help
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01-15-2007, 01:29 PM | #12 | |
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Re: Engine help
Quote:
Don't try 11-1 around here (735' elev.) on 87 octane. 8.7 is the recommended max.
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01-15-2007, 01:38 PM | #13 |
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Re: Engine help
i will check the heads tonight. thanks for all the help so far.
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68 Chevy CST/10, 50th Anniversary, 327/th400, 373 posi, buckets 68 GMC Shortbox Stepside, 402/th400, Tilt, Tach, Bucket Seats, 3.08 Positraction 69 Chevy Camaro SS, 383, 4 speed, positraction 70 Chevy El Camino SS396, Triple Black, 454/th400, Tach 70 Chevy Shortbox Stepside 4x4, 350/th350, Tilt, Tach, Cab Lights, Bumper Guards, Tow Hooks 70 Chevy 3/4 Longbox, 350/auto, 70k, SURVIVOR 71 Chevy Blazer, CST, 350/auto 71 GMC Sierra K2500, 454, 4 speed, 4.10 posi, 37s 71 Custom/30, Open Road, 350/th400, tilt 72 Chevy Shortbox Fleetside, 496 Stroker/th400, Tilt, Tach, Bucket Seats, 3.42 Posi 72 GMC Sierra Grande 2500. 427/th400, tilt, a/c, tool and stowage, buckets/console, 3.54 posi |
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