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Shakercars's 1972 Trans Am

2019 August
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i've had a 73 ventura with a 350 sitting in the shop a couple of years and today i bought a 455 from a 70 Bonneville. everything shows this YH block to be the 360 hp rated engine with 2 bolt main and it has the #15 heads. I have a set of 6x-8 heads from a 77 400 and wondered how low the compression is going to go if i run those heads. The 6x have the 2.11 valves with dual springs and i think they will flow better, but i would like to keep this 455 pushing 400 HP. I haven't decided on a cam yet until if find more info on which heads to run

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You should measure the chambers to find out what you have.some 6x's are as high as 111cc's.also depends if the PO has milled them,here is a great site http://www.wallaceracing.com/engine1.htm

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Sorry,i posted the wrong part of his site.some reason i cant post the correct one..Look around at that site you'll find the head chart and CR charts.

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You didn't mention which heads are sitting on the 350 motor. Remember, all Pontiac heads except the 301 are interchangeable. Assuming it is stock with a stock gasket on it, the heads should be a #46/4C casting with a 1.96" intake and 1.66" exhaust valve with a 96cc combustion chamber.

The'70 #15s heads from the 455 have the exact same intake and exhaust valve sizes as the #46s, the only change is the #15s sport an 87cc combustion chamber.

The '77 6X heads do have the more desirable 2.11 intake valve and will therefore flow better. However, Pontiac made two different 6X heads in '77. Both have the 2.11/1.66 valves but different combustion chambers. The 6X-8 head was on the 180HP 400s with a 7.6 compression ratio. The 6X-4 heads were on the 200HP 400s with a 8.0 compression ratio. Now to complicate things a little more, the 6X-4/6S head was also installed on the 350 engines with a compression ratio of 7.6.

So the dilemma you have is to either use the small cc heads and sacrifice the flow gained by the 6X heads 2.11 intake or use the 6X heads and hope they can be massaged to something north of 8.0 compression through different pistons, head gasket, and perhaps milling the heads.

If it were me, I would stay with the 6X heads. Lots of machine shops have seen them and have experience with them since there so many Trans Ams built. That said, depending on your budget and timing, you might try to find some other Pontiac heads, perhaps less valuable than the desirable and elusive Ram Air I-IVs, H.O. and Super Duty performance heads but will flow very well. These have 2.11 intake and 1.77 exhaust.

'67 400s & 428s - 670/97

'68 400s - # 16/ 216/ 62

'69 400 - #48, #16/ 62/ 48

'70 400 - #12, # 13, #64

'71 400 - #66, #96

'72 400 - 7K3, 7L4, 7M4

I have a set of 7K3 on my Lemans' 455, my builder was ecstatic. These heads should be a lot cheaper that the others too.

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thanks for the feedback. i do have the #46 heads on the 350. the 455 is all original/stock and the # 15 heads may be fine for what i want. I thought the 6x-8 heads would have the hardened seats and big valves, but hate to lose that much compression i really just want a dependable streetable engine at 400 hp. I let someone talk me out of a set of 69 #16's. i may be just as well off if i just clean up the heads and intake, good valve job , decent cam and exhaust and go play. i'm going to keep the power brakes,a/c,etc. At my age, i just want to "Light 'em up" now and then.

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Regardless of which head you choose to you, I would still have the machine shop examine and install new hardened seats (even if they already have them) and new valve guides. It is not that much more to the cost and it is money well spent. Talk to your engine builder or machine shop. See what which head they recommend. I think the 6x will give you the seat of the pants kick you are looking for. My choice would by 1st-6X, 2nd-#15, 3rd-#46.

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96's can be found reasonably priced.cleaned new seats etc..on a 455 will be pump gas friendly and shouldnt break the bank either.I would fill the heat crossover either way.Just depends on where ya live type of driving.

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