Need engine builder help again
Thread Starter
Racer
Joined: Sep 2014
Posts: 425
Likes: 32
From: richmond virginia
Need engine builder help again
I'm in the process of doing a complete rebuild on my L78 396. I want it to make as much HP as possible but still be fun and drivable. I am boring it ,030 so I'm buying new pistons. The question I have for you is, what is the maximum/optimum compression ratio that I need. I obviously want it to be as high as possible but I will run pump gas and I don't need a valve rattle problem. As usual, your expertise will be greatly appreciated.
Le Mans Master
Joined: Nov 2015
Posts: 7,429
Likes: 2,599
From: Really Central IL Illinois
You need to calculate the final compression ratio with the combustion chamber of your heads, the head gasket thickness, piston to deck height, etc. to come up with the actual compression ratio.
I would stay around 10:1 or less if I wanted to run ethanol pump fuel.
The Pros may have better advice. I am scared to death of detonation, but you can retard the timing and make concessions to go higher compression, but I don't think you will be able to use the maximum performance of the high compression engine on pump gas without computer controls.
Just my $.02.
I would stay around 10:1 or less if I wanted to run ethanol pump fuel.
The Pros may have better advice. I am scared to death of detonation, but you can retard the timing and make concessions to go higher compression, but I don't think you will be able to use the maximum performance of the high compression engine on pump gas without computer controls.
Just my $.02.
Safety Car
Joined: Mar 2009
Posts: 4,191
Likes: 951
From: Corpus Christi, TX
I would stay around 10:1 or less if I wanted to run ethanol pump fuel.
The Pros may have better advice. I am scared to death of detonation, but you can retard the timing and make concessions to go higher compression, but I don't think you will be able to use the maximum performance of the high compression engine on pump gas without computer controls.
Just my $.02.
The Pros may have better advice. I am scared to death of detonation, but you can retard the timing and make concessions to go higher compression, but I don't think you will be able to use the maximum performance of the high compression engine on pump gas without computer controls.
Just my $.02.
I'm using Speed Pro L2300 pistons in my 427. These are forged versions of the factory cast 10.25 compression pistons. Speed Pro makes a forged 396 low compression piston. .030 over version is number L2240NF30.If you can afford it, a modern hydraulic roller cam would help, especially if you don't plan to use factory exhaust manifolds. Comp Cams #11-451-8 would be a good choice. Otherwise, the factory solid lifter cam works really well, even with 10.2 compression.
Burning Brakes
Joined: Feb 2005
Posts: 894
Likes: 91
We've done a number of pump-gas 396's with 500+ HP.
Some were iron headed, some aluminum (G.M./Winters-077's). The aluminum platform used 10.5:1 with some "heavy" TRW pistons with a 5/64" ring pack and non-roller cam and made 510.
If you go "iron-heads" I would keep it at about 9.75:1 or so.
With a "hyd-retro-roller" in play, some lighter-weight pistons, a "floated wrist pin", and a 1/16" ring pack you should be able get very close to that 500 number! I would not consider any more 5/64" ring packs if looking for any type of power improvements!
Thanks, Gary in N.Y.
P.S. Need a bit more info on ALL the components you are planning to use? We recently put in a custom piston/ring/bushed rod pkge for these 396's BUT only for one o'size, 4.145". This covers both the 396" and the 402" units for us!
Some were iron headed, some aluminum (G.M./Winters-077's). The aluminum platform used 10.5:1 with some "heavy" TRW pistons with a 5/64" ring pack and non-roller cam and made 510.
If you go "iron-heads" I would keep it at about 9.75:1 or so.
With a "hyd-retro-roller" in play, some lighter-weight pistons, a "floated wrist pin", and a 1/16" ring pack you should be able get very close to that 500 number! I would not consider any more 5/64" ring packs if looking for any type of power improvements!
Thanks, Gary in N.Y.
P.S. Need a bit more info on ALL the components you are planning to use? We recently put in a custom piston/ring/bushed rod pkge for these 396's BUT only for one o'size, 4.145". This covers both the 396" and the 402" units for us!
Thread Starter
Racer
Joined: Sep 2014
Posts: 425
Likes: 32
From: richmond virginia
The engine is basically an L78 with s.p. heads, Winters intake, Lunati roller cam/lifters/rockers, balanced crank, stock Corvette headers, 750 d.p./d.f. carb. My rear is 3.55. The heads are closed port 109 cc. The pistons I plan on using are Speed Pro. The dome is .319". The bowls have had some minor work done so they are probably more than 109 cc. We'll check the compression ratio when the heads are bolted on. Based on your advise, I'll work to be at 10-1. The builder will break it in and dyno it. I'll report back on the h.p. results. It'll be Sept. when the build is complete. Thanks to you all for your input.
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Safety Car
Joined: Mar 2009
Posts: 4,191
Likes: 951
From: Corpus Christi, TX
Taking R66''s advice a bit further, make sure to the piston to deck height and use that to figure what your total head gasket thickness should be to get close to .040 head to piston quench clearance . You'll get better results with quench clearance set correctly than worrying about your static compression being a quarter point too high.
Spend the extra money for a torque plate finish hone and Total Seal gapless rings. They made a noticeable difference on my motor in cutting blow by.
Spend the extra money for a torque plate finish hone and Total Seal gapless rings. They made a noticeable difference on my motor in cutting blow by.
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
It all depends on the cam you plan on using. At what degree does the intake valve "close" at advertised duration (.006 valve lift)?
Last edited by 65tripleblack; Jul 2, 2017 at 08:03 AM.
Thread Starter
Racer
Joined: Sep 2014
Posts: 425
Likes: 32
From: richmond virginia
Intake opens: 9.5 BTDC
Intake closes: 41.5 ABDC
Exhaust opens: 53.5 BBDC
Exhaust closes: 5.5 ATDC
Intake centerline 106
Lobe separation 110
Duration at .050 231 intake
Duration at .050 239 exhaust
Advertised Duration 283 intake
Advertised Duration 291 exhaust
Intake lift at valve .601
Exhaust lift at valve .601
Lunati part # 20110712
Rocker ratio 1.700
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
Here's the specs on my cam:
Intake opens: 9.5 BTDC
Intake closes: 41.5 ABDC
Exhaust opens: 53.5 BBDC
Exhaust closes: 5.5 ATDC
Intake centerline 106
Lobe separation 110
Duration at .050 231 intake
Duration at .050 239 exhaust
Advertised Duration 283 intake
Advertised Duration 291 exhaust
Intake lift at valve .601
Exhaust lift at valve .601
Lunati part # 20110712
Rocker ratio 1.700
Intake opens: 9.5 BTDC
Intake closes: 41.5 ABDC
Exhaust opens: 53.5 BBDC
Exhaust closes: 5.5 ATDC
Intake centerline 106
Lobe separation 110
Duration at .050 231 intake
Duration at .050 239 exhaust
Advertised Duration 283 intake
Advertised Duration 291 exhaust
Intake lift at valve .601
Exhaust lift at valve .601
Lunati part # 20110712
Rocker ratio 1.700
Make it no more than 10.6:1 if you use stock cooling capacity, which is marginal at best. Because you use iron heads and large quench, you cannot take full advantage of SCR boost and must be conservative. With this SCR, you have some margin to have spark advance come in more aggressively.
Thread Starter
Racer
Joined: Sep 2014
Posts: 425
Likes: 32
From: richmond virginia
Make your static compression ratio maximum of 11.0:1 if you have better than stock cooling capacity and keep your intake isolated from heat with a moderate rate of spark advance.
Make it no more than 10.6:1 if you use stock cooling capacity, which is marginal at best. Because you use iron heads and large quench, you cannot take full advantage of SCR boost and must be conservative. With this SCR, you have some margin to have spark advance come in more aggressively.
Make it no more than 10.6:1 if you use stock cooling capacity, which is marginal at best. Because you use iron heads and large quench, you cannot take full advantage of SCR boost and must be conservative. With this SCR, you have some margin to have spark advance come in more aggressively.
From a performance perspective, how do you feel about my cam selection?
Race Director
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From: Corsicana, Tx
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An L-78 is a fun one if you twist it a little. Not much happening until 5500+ with the big heads...so need to make things happy valvetrain wise up there.
Unless you swap heads to some nice ovals or aftermarket.
If for whatever reason the crank needs to be replaced...I'd be looking at sneaking at least a 4" crank out of a 454 or even a 4.25" one in there for 450+ inches. That would wake up those heads and no one would ever know the difference!
I wouldn't be scared of 10.0-10.5 compression at all if you tune it a little. I've run much higher than that (11.2) with iron heads on my 427 on 93 octane with no issues in 100*+ temps.
JIM
Unless you swap heads to some nice ovals or aftermarket.
If for whatever reason the crank needs to be replaced...I'd be looking at sneaking at least a 4" crank out of a 454 or even a 4.25" one in there for 450+ inches. That would wake up those heads and no one would ever know the difference!
I wouldn't be scared of 10.0-10.5 compression at all if you tune it a little. I've run much higher than that (11.2) with iron heads on my 427 on 93 octane with no issues in 100*+ temps.
JIM
Last edited by 427Hotrod; Jul 3, 2017 at 12:49 AM.
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
As far as the cam................how can you go wrong with a hydraulic roller. A very worthwhile upgrade from a flat tappet and you'll see a big improvement in performance across the board. The cam specs look like a good cam for street/strip duty. Strong low/midrange and good top end power. I like it for a moderate/not radical build.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Unless you swap heads to some nice ovals or aftermarket.
best thing you can do is get a good set of modern alum cyl heads nice power gain, weight loss more ping resistant.
if the cam is right you can get away with running more compression
Last edited by cv67; Jul 3, 2017 at 09:24 AM.
Burning Brakes
Joined: Feb 2005
Posts: 894
Likes: 91
If your budget is not all that limited, would definitely recommend the 4.000" stroke (mentioned above by Jim), the Race-Tec pistons (pictured above), a retro-hyd-roller cam, and a set of inexpensive aluminum heads. This will give you a 427" (or more depending on the o'bore) pump-gas platform. With the kits we have on the shelf independent (recommended) or dependent balanced you get a 432", 9.8:1 (nominal), unit!
With this setup you will be in the 550 HP range AND you can still keep it all 100% stock-appearing at the same time. We do these all the time.
On a side note, with the BB castings you could do the roller cam without needing any cam button or "chasing" end play. Stock timing covers work fine.
Thanks, Gary in N.Y.
P.S. This type build would be a real "sleeper" on the stock-appearing platform.
With this setup you will be in the 550 HP range AND you can still keep it all 100% stock-appearing at the same time. We do these all the time.
On a side note, with the BB castings you could do the roller cam without needing any cam button or "chasing" end play. Stock timing covers work fine.
Thanks, Gary in N.Y.
P.S. This type build would be a real "sleeper" on the stock-appearing platform.
Last edited by GOSFAST; Jul 3, 2017 at 11:20 AM. Reason: Add