396 Engine build - Chevelle Tech
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post #1 of 21 (permalink) Old May 22nd, 19, 4:51 AM Thread Starter
 
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396 Engine build

Before I started the restoration on my 69 Chevelle, the 396 that was in it pinged badly with 91 octane and dieseled when shutting it off. Disassembled the engine to find out it had L2287 large domed pistons and 074 large rectangular port aluminum heads. Compression ratio was roughly 11.2:1. My question is what compression ratio should I be targeting for pump gas? I'm thinking somewhere around 9.5:1 to eliminate detonation and run on.

I am replacing the 074 large rectangular port heads with 049 large oval port heads that have the same cc's and I am having a hard time finding pistons that will give me around 9.5:1 CR. According to a CR calculator, with a 4.135 bore, 3.760 stroke, 118cc heads and a.045 quench, I would need a -30cc domed head to achieve 9.45:1 CR. Should I have someone make pistons for me or mill the heads? Or am I being to critical? I need help achieving the proper CR. Thoughts?
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post #2 of 21 (permalink) Old May 22nd, 19, 6:57 AM
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Re: 396 Engine build

Speed pro makes the 2240 piston in 30 and 60 over. But before you condemn the old combo, what cam was in it? To me, the old combo with aluminum heads and the right cam should be fine. I had a 408 with the same pistons and steel 781 heads with a 249/[email protected] solid flat tappet cam and it made just over 500 hp on 93 octane gas at 6500 rpm. The block was zero decked and the heads were milled to make them flat.
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post #3 of 21 (permalink) Old May 22nd, 19, 8:13 AM Thread Starter
 
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Re: 396 Engine build

Old cam was a Crane 10307 with 226/[email protected] New cam is Comp CL11-248-3 with 224/[email protected] Both hydraulic flat tappet. Those 2240 pistons are a 21cc dome and I am considering them if I can mil the heads from 118cc to 110cc. I want to run 91 octane max.
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post #4 of 21 (permalink) Old May 22nd, 19, 8:31 AM
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Re: 396 Engine build

Have you considered the 3993820 oval port heads. They have 113cc nominal chamber volumes.

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post #5 of 21 (permalink) Old May 22nd, 19, 8:37 AM Thread Starter
 
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Re: 396 Engine build

I did but couldn't find any locally. Machine shop had 049's so decided to go with them. didn't know I was going to have this much of a problem finding pistons to match the 049 heads.
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post #6 of 21 (permalink) Old May 22nd, 19, 9:38 AM
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Re: 396 Engine build

Quote:
Originally Posted by jlkfam69ss View Post
I did but couldn't find any locally. Machine shop had 049's so decided to go with them. didn't know I was going to have this much of a problem finding pistons to match the 049 heads.
That 396 will go pretty good with those 049 heads. I like the Peanut Port heads on those smaller BB's. I think of it as would you put a 260cc head on a 400?, probably not. So why put them on a 396, smaller runner means more velocity. PP heads are around 220 to 230 cc and I think and know they work on 396-402's. You will still be pretty happy though with those heads you picked out.
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post #7 of 21 (permalink) Old May 22nd, 19, 11:13 AM
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Re: 396 Engine build

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Originally Posted by 68Chevele View Post
I think of it as would you put a 260cc head on a 400?, probably not. So why put them on a 396
You can't compare poured runner cc between a SBC and BBC head. The port length is very different, and a BBC head with the same CSA will have a different volume.

However, I agree with what you are saying about peanut ports likely being a good match for a 396. Problem is that the chamber is too big on most peanuts, requiring a large dome on a baby BBC to get any kind of Compression.
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post #8 of 21 (permalink) Old May 22nd, 19, 5:38 PM
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Re: 396 Engine build

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You can't compare poured runner cc between a SBC and BBC head. The port length is very different, and a BBC head with the same CSA will have a different volume.

However, I agree with what you are saying about peanut ports likely being a good match for a 396. Problem is that the chamber is too big on most peanuts, requiring a large dome on a baby BBC to get any kind of Compression.
I guess my dyno guy disagrees with that and says too much port volume is bad on most smaller engines. He also says intakes and headers are extensions of the cylinder head. He is pretty good, getting 650hp out of a 165cc head. When I switched mt 396 over to a pair of 781's years ago yup the compression dropped but the power went up and that 396 gave fits to a lot of street racers in my area. After that I only used CC heads one time. I am sold on the open chamber heads.
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post #9 of 21 (permalink) Old May 22nd, 19, 6:25 PM
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Re: 396 Engine build

be interesting to try some l29 vortec heads on it. 100 cc chambers
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post #10 of 21 (permalink) Old May 24th, 19, 9:58 AM
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Re: 396 Engine build

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Originally Posted by 68Chevele View Post
I guess my dyno guy disagrees with that and says too much port volume is bad on most smaller engines.
My point was that it is CSA that matters, not volume. Volume will vary with different length ports, so a volume comparison between a BBC, SBC, Pontiac, and Olds will tell you absolutely nothing about the CSA of the heads you are comparing.
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post #11 of 21 (permalink) Old May 24th, 19, 4:39 PM
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Re: 396 Engine build

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Originally Posted by gnicholson View Post
be interesting to try some l29 vortec heads on it. 100 cc chambers
Unfortunately, you cannot use a dome piston with the L29 Vortec head, only a flat-top or dish. Even the Closed Chamber dome interferes with the quench area on the head. I know a couple of folks why tried that and even GM told them to ditch the Vortec head with a dome.

As for Peanut Port sizes mentioned above, from everything I have seen the 236 casting has the largest port with 209cc and the 156 casting has the smallest at 195cc. 049/781 Large Oval Ports measure 255-256cc while the L29 Vortec port is slightly smaller because of the swirl vanes, around 235cc.

The 236 casting flows better than what they are give credit for and can perform quite well, the 156 casting is a TBI head that like it's small block counterpart is a lousy flowing head. If you go Peanut Port look for the 236 if possible, 320 are also decent.
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post #12 of 21 (permalink) Old May 24th, 19, 5:53 PM
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Re: 396 Engine build

you can use them you just have to machine the offending small part of the chamber. simple job
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post #13 of 21 (permalink) Old May 24th, 19, 7:27 PM
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Re: 396 Engine build

Surely you can find some closed chamber 396 heads from 1966 to 1970.

They should be cheap; most people want open chamber heads.

Then you could run a small dome to get 9.5-1
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post #14 of 21 (permalink) Old May 24th, 19, 10:02 PM
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Re: 396 Engine build

jlk,

Before you get too deep into pistons, have you actually had the heads cc'd to verify the chamber size ? I have seen stock heads vary quite a bit from published specs.

I would pay the extra to get a piston made rather than mill the heads that much. You will probably have to cut them .060 or so. Then you have to mess with the intake not fitting and either milling the manifold or the intake surface on the heads to match. By the time you do all that you will probably spend more than the difference in the piston price between a shelf piston and one that is exactly what you want. Not sure what machine shop charges are where you are located but I get a minimum of $180 to mill an intake manifold.

I would personally stick with no more than 9.5 to 1 if you really want to run 91 octane.
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post #15 of 21 (permalink) Old May 26th, 19, 12:04 PM Thread Starter
 
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Bill,

I would prefer to find 30cc pistons rather the mil the heads. Don't know how much custom pistons are. I can find 21cc pistons but that requires milling. Machine shop will cc the heads before any work and hopefully they only need to take .040 or less off. I didn't realize 396 pistons were limited or I would of tried harder to get semi-open 110cc heads that matched perfectly with 21cc pistons. I have too much invested in the 049 heads to start over.
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