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Jim87 Offline
#1 Posted : Monday, 29 May 2023 5:01:34 PM(UTC)
Jim87

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Hi has anyone had much to do with 396/402 bbc just bought a hq one tonner with 402 and doesn’t seem to have as much power as I thought flat out spinning the tyres when giving it a bit it has rpm air gap intake,isky 292/292 hydraulic cam, flattop pistons,standard gm Crank,standard iron head 3975950 casting number,1.7 crow cam roller rockers,750holley,hei ignition,so just really wondering is it worth porting these head and valve job to liven it up or what heads would be the best for this set up I’ve read so many thing on heads and what the best I just get lost with it all not looking for huge power just a bit more wouldn’t mind around the 400 at the wheels cheer

Edited by user Monday, 29 May 2023 8:28:19 PM(UTC)  | Reason: Not specified

HK1837 Offline
#2 Posted : Monday, 29 May 2023 6:38:25 PM(UTC)
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Those engines are weapons if built right. They are no different to any other engine, just need the right balance of heads, cam, and carb to suit the transmission, diff ratio and expected use. These came factory rated as high as 425hp as an L78 engine, but these were 11:1, solid lifter, 800cfm carb and rectangular port heads.

My guess is you have really low compression. With those large chamber truck heads and flat tops you'll have something like 8:1 compression. The cam is probably too big for that static compression. I'll go and have a look at what I have to help you.
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Jim87 Offline
#3 Posted : Monday, 29 May 2023 7:07:56 PM(UTC)
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Thanks that would be great it has Turbo350 3k stall and diff gears are around the 3.7 ratio
HK1837 Offline
#4 Posted : Monday, 29 May 2023 7:48:37 PM(UTC)
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Had a look at a few books and looked at a few sites. Those heads are oval port but seems to be conflict as to whether open or closed chamber. On one site they measured them as 116cc which is open chamber.
If you look at quality piston manufacturer sites like Wiseco, they tell you that a 402 with 15cc piston dome and 118cc heads gives you 8.2:1 at zero deck and 40 thou head gasket. So with flat tops you are going to be near 7.5:1. With that cam no wonder it is making zero power!

You’d have to look at the cam specs but my guess is you’d want at least 9.5:1 static with that cam.

Edited by user Monday, 29 May 2023 7:58:44 PM(UTC)  | Reason: Not specified

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If we all had the same (good) taste, who would buy all the Fords?
Jim87 Offline
#5 Posted : Monday, 29 May 2023 8:45:33 PM(UTC)
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Yeah cam spec say 10-11.1 comp
So look like I need closed chamber head to get comp up or would just massive dome pistons what would you recommend thanks for all the info you have given me
HK1837 Offline
#6 Posted : Monday, 29 May 2023 9:13:12 PM(UTC)
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Check your piston are actually flat top. Most BBC are dome top. Once you know what you have work out what chamber size you need. And make sure it’s the right cam for your needs. If you only have small dome pistons, with factory closed chamber heads you might be able to get to 9.5:1 and with the right cam it’ll still make good power. What I’m getting at is a different cam will be a lot cheaper than a piston change as well. There are a lot of good books on these to read. Do a bit of planning before jumping in!

I’ll have a look in my shed, there is a pair of large oval port heads in there somewhere I picked up at a swap meet years ago. I think they are closed chamber but have been ported and had larger valves fitted. Will see what they are. I remember the ports are opened up a fair bit and no idea who did them.
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If we all had the same (good) taste, who would buy all the Fords?
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