Engine help - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**68350** · 01-14-2007, 11:46 PM

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

PumiceMan · 01-15-2007, 12:47 AM

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.

Jtrux · 01-15-2007, 01:02 AM

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).

RetroSBK · 01-15-2007, 02:30 AM

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!

*Billla* · 01-15-2007, 04:11 AM

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?

**68350** · 01-15-2007, 09:19 AM

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

cdowns · 01-15-2007, 10:48 AM

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

*Billla* · 01-15-2007, 11:22 AM

Quote:

Originally Posted by 68350

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

If this is the 310 HP 396 (402) then it already has 9.0:1 CR (plus the overbore, which increases the CR ~ .13). IMHO I wouldn't spend any money upgrading the shortblock...especially not just for .5 CR! 400 HP (1HP/CR) is EASILY attainable with headers, cam upgrade, intake manifold upgrade and and stock heads. Do you have the casting number from the heads?

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.

Gray Ghost · 01-15-2007, 11:30 AM

You can run up to 9:1 on regular pump gas...been doing that for years without issues.

jacobs · 01-15-2007, 11:54 AM

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.

*Billla* · 01-15-2007, 12:01 PM

Quote:

Originally Posted by cdowns

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

I was typing at the same time you were, I think

**WorkinLonghorn** · 01-15-2007, 01:29 PM

Quote:

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.

Come on Jacobs,you're at 9000' elevation according to your profile !!!
Don't try 11-1 around here (735' elev.) on 87 octane. 8.7 is the recommended max.

**68350** · 01-15-2007, 01:38 PM

i will check the heads tonight. thanks for all the help so far.

01-14-2007, 11:46 PM	#1
68350 Senior Member Join Date: Jan 2002 Location: Williston, ND Posts: 2,999	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 __________________ 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, 12:47 AM	#2
PumiceMan Junior Member Join Date: Jan 2007 Location: Wichita, KS Posts: 47	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. __________________ 69 GMC Sierra 1500 Custom 68 Camaro SS 18 GMC Sierra SLT Living the dream

01-15-2007, 01:02 AM	#3
Jtrux I'm a poor spectator Join Date: Aug 2006 Location: San Antonio, Tx Posts: 2,287	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). __________________ 2006 Jeep Unlimited IMPACT ORANGE 1993 Chevy 2500 4x4 ExCab LWB 454/NV4500 Tow rig 1977 Ford F100 2x4 LWB 1st truck I owned, still have it!!! 1979 Ford F150 4x4 SWB Built Ford Tough!!! 1971 Chevy Blazer 350 / SM465 / NP205 UNDER CONSTRUCTION Soon to have a LQ4 6.0!!! Last edited by Jtrux; 01-15-2007 at 01:04 AM.

01-15-2007, 02:30 AM	#4
RetroSBK Registered User Join Date: Jan 2007 Location: Camarillo, CA Posts: 64	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! __________________ 1970 C10, shortened long bed, Alum Headed full roller 383 (with Victor, 750DP, stainless rockers, behive springs, CNC ported heads) TCI SSF 700R4, 4.10 Posi, Bagged on C-Notch and 2.5 spindles. Air Cond, black out tint, satin black paint, custom interior, rolling low and not so slow...

01-15-2007, 04:11 AM	#5
Billla Account Suspended Join Date: Sep 2005 Location: Monroe, WA Posts: 3,814	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?

01-15-2007, 09:19 AM	#6
68350 Senior Member Join Date: Jan 2002 Location: Williston, ND Posts: 2,999	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 __________________ 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
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	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 __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

01-15-2007, 11:30 AM	#9
Gray Ghost Senior Member Join Date: Sep 2004 Location: Terrell, TX Posts: 1,967	Re: Engine help You can run up to 9:1 on regular pump gas...been doing that for years without issues. __________________ Kelly '05 GMC Sierra SLE Z71 Bone stock except for new bed rail caps.

01-15-2007, 11:54 AM	#10
jacobs Registered User Join Date: Sep 2005 Location: 9000' Mountain Man Posts: 326	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.

01-15-2007, 01:38 PM	#13
68350 Senior Member Join Date: Jan 2002 Location: Williston, ND Posts: 2,999	Re: Engine help i will check the heads tonight. thanks for all the help so far. __________________ 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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