1968 3976 375 hp build
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I know mine is not a 396 but my 327 L79 runs with 11 to 1 compression on 93 pump with no issues on an all iron engine. I attribute some of that to the L79 cam that bleeds off some of that compression. I will leave the rest of your questions to 396 owners
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If you run the factory cam you can get by with the factory 11:1 L78 pistons on 93 octane. The factory L78 cam doesn't close the intake valve until 80* ABDC so the dynamic compression ratio comes in at 7.62:1. The key is the cam and how much of the static compression ratio the engine actually sees. Lots of pump gas racers target no more than 8:1 DCR for iron heads and 8.5:1 for aluminum heads.
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The original rods for an L78 were the 3/8" dimple rods which are 6.135 long and they have press fit pins. I upgrade the bolts to ARP just for a little added insurance.
I have an L78 mostly built stock in my 69 Camaro. I am running L89 (i.e. 842 aluminum vs 840 iron) heads on it with roller rockers. For the rotating assembly I have the cross drilled/nitrided L78 (i.e. 6223) forged crank, 3/8" dimple rods with ARP bolts, and TRW L2242 forged pistons with total seal gapless rings. Factory intake, factory holley 780 carb, and factory exhaust manifolds with a mandrel bent 2 1/2" exhaust system. I'm currently running the compcams factory muscle solid lifter cam (i.e. 11-106-3) specs are adv dur 316/302, @.050 dur 242/242, with .520/.520 lift and 114 LSA along with their recommended spring set. It pulls up to 6500 rpms with no problem and has good performance. I generally don't run it past 6500 rpm but it feels like it has a little more in it (i.e. it doesn't roll over at 6500). At one point I had a Comp XS290 cam in it and it was still pulling at 6800 rpms but I was having trouble with the vacuum for the power brakes. I run 91 - 93 octane in it with no problems.
I hope this helps.
FWIW - Now I'm building a new version of the 396 (LS COPO 396) for my 67 Chevelle. I think it will have a little more juice than my "L78" in the Camaro.
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5,677 Posts
I built an 11:1 compression 396 for a friend for his 4200lb 3/4 ton.
Block and heads were from that 1969 truck, large oval port open chamber and I went with 2.19-1.88 valves.
Ported the heads and roughed the intake runners.
280H comp magnum cam ground on a 108LSA so intake valve actually closes earlier this is a [email protected] .050 .480" lift sbc cam lobe master but calculate that with a 1.7 rocker arm you get more duration and much more lift.
91 octane 38 total timing. Has ran excellent and seen 7000 rpm a few times.
Been running strong for over 10 years.
No sharp edges in combustion space and no issues with oil in combustion space either as that will destroy one quickly.
210 psi cranking pressure
My sbc engines with 58cc or smaller I can get by with a lot more than 11:1 and 245 psi cranking pressure on 91 octane.
These are iron heads.
I will say this I ran a milled to 54cc 186 head (double hump) on a 350" and a 64cc 041 double hump both same size valves and same port job.
I had to remove 4 degrees initial timing and delay total timing by almost 400 rpm with the larger chamber head.
More to this question of yours than just what compression or what cam.
Press fit or floating .I would not care but they need to be good forged pistons.
Good stock rods have never let me down either.
But maybe i am not making enough grunt to hurt them.
Block and heads were from that 1969 truck, large oval port open chamber and I went with 2.19-1.88 valves.
Ported the heads and roughed the intake runners.
280H comp magnum cam ground on a 108LSA so intake valve actually closes earlier this is a [email protected] .050 .480" lift sbc cam lobe master but calculate that with a 1.7 rocker arm you get more duration and much more lift.
91 octane 38 total timing. Has ran excellent and seen 7000 rpm a few times.
Been running strong for over 10 years.
No sharp edges in combustion space and no issues with oil in combustion space either as that will destroy one quickly.
210 psi cranking pressure
My sbc engines with 58cc or smaller I can get by with a lot more than 11:1 and 245 psi cranking pressure on 91 octane.
These are iron heads.
I will say this I ran a milled to 54cc 186 head (double hump) on a 350" and a 64cc 041 double hump both same size valves and same port job.
I had to remove 4 degrees initial timing and delay total timing by almost 400 rpm with the larger chamber head.
More to this question of yours than just what compression or what cam.
Press fit or floating .I would not care but they need to be good forged pistons.
Good stock rods have never let me down either.
But maybe i am not making enough grunt to hurt them.
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1,909 Posts
My '69 has "correct" L78 JD coded block and 840 heads. Original JD block, late model year 291 heads and original 163 Winters intake are stored in my shop.
My '69 L78 is running 11:1 Speedpro (press fit pins) pistons/rings, stock dimple rods with ARP Series 2000 bolts, Tufftrided/Crossdrilled/6223 L78 crank, Howards L78 Cam with .539/.539 lift (otherwise stock specs), 1.72 Comp Cams Magnum rockers (total lift is .545/.545), GM 163 Winters Intake, 800 CFM Holley Double Pumper, MSD Pro Billet Dist, 13* static timing/total timing 38* at 2,700 RPM, unknown brand 1 7/8 primary tube headers, Centerforce Dual Friction clutch set, Hays SFI billet flywheel, M22 w/Hurst, 3.73 Posi (soon to be replaced with KK coded 4.10 posi). Engine is precision machined and balanced, with blueprinted clearances.
Runs perfect on 92 octane. Car is presently pictured as July 2021 Team Chevelle "Ride of the Month"
Car is listed in Dale McIntosh's L78 Registry and pictured in Dale's latest 1969 SS Chevelle Book
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My '69 L78 is running 11:1 Speedpro (press fit pins) pistons/rings, stock dimple rods with ARP Series 2000 bolts, Tufftrided/Crossdrilled/6223 L78 crank, Howards L78 Cam with .539/.539 lift (otherwise stock specs), 1.72 Comp Cams Magnum rockers (total lift is .545/.545), GM 163 Winters Intake, 800 CFM Holley Double Pumper, MSD Pro Billet Dist, 13* static timing/total timing 38* at 2,700 RPM, unknown brand 1 7/8 primary tube headers, Centerforce Dual Friction clutch set, Hays SFI billet flywheel, M22 w/Hurst, 3.73 Posi (soon to be replaced with KK coded 4.10 posi). Engine is precision machined and balanced, with blueprinted clearances.
Runs perfect on 92 octane. Car is presently pictured as July 2021 Team Chevelle "Ride of the Month"
Car is listed in Dale McIntosh's L78 Registry and pictured in Dale's latest 1969 SS Chevelle Book
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