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Old 01-20-2011, 08:29 PM
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Pontiac Cylinder Head Question

I have a Pontiac 350 block (pontiac not chevy) and i want to build a motor to get decent mileage and power, i have a comp cam 275DEH camshaft, edelbrock performer intake and both 1972 7j2 and 1977 6X heads. the 6X heads are the larger volume chamber from a 400 engine. and the 7j2 heads we confirmed to have 96cc combustion chambers by ccing them.
we cc'd the 6X heads and it was more than 96cc. i checked online and it said 101, which sounds about like what we found it to be.

i built the motor before (and lost all of the bearings due to the block needing align bored, but failing to have it done before assembling it) and it had great power with the 7j2 heads on it.

this time i want to get decent gas mileage (and no bearing failure) with a quadrajet carburetor instead of the holley i originally used, and i know the 6X heads have the larger valves, would i gain mileage and performance with the bigger valves or lose performance and mileage due to compression?

or should i have the 6X heads milled? I still cannot decide what to do, but i want the combination that will get the best gas mileage and most horsepower out of what i have, with the least amount of money spent, since i already lost a lot on the little bearing problem. (i will be having the block machined correctly this time)

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Old 01-20-2011, 09:06 PM
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I have 6x-8 heads on my 455 and I had them milled and gasket matched and they run very well in my lemans. First thing I would suggest since you're having the block machined is, why not try looking for a 400 block and build it up? More cubic inches will lead to more power and everything will bolt up from your 350 to it including your heads. If you're stuck with the 350 I say build for power, keep your qjet on it, and get an overdrive trans like a 700 or a 200 and some decent highway gears like 3.23s for mileage. Don't really know much about that cam but you have to match it to the rest of the components for sure. The 6x heads are 2.11 int 1.66 exhaust and the 7j2 heads are 1.96 int and 1.77 exhaust. Your goal is to have 2.11 int and 1.77 exhaust and a decent cc out of either head choice. At least the int to 2.11, 1.96 on the int just wont do it justice especially being a lil 350. Either way machining is a must on either choice.
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Old 01-20-2011, 09:22 PM
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It's going into a 1977 firebird. The reason i want to stick with the 350 is because I want a little smaller cubes for the gas mileage, also, i already have the pistons for the 350 and it's been bored .030 over and there's nothing wrong with the bore. so i was just going to have it boiled again and have them align bore it (probably crank and cam) and then rehone if it needs it. it already has 3.23s in it and i have both a pontiac th350 and a 700r4, but i know for sure the 700r4 needs rebuilt, my th350 might just need seals (the only problem it had was leaking, shifts were still firm)

I have a 400, but i'm saving that for the future, some day i want to build it the right way with forged internals and aluminum heads for high power, but not right now.

If i go with the 6X heads, how much can they be milled? and how many cc's will they drop?
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Old 01-20-2011, 09:50 PM
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Since this is a street engine, and your looking for milage, go with the 7J2 heads. The smaller valves won't make any difference on the street and you wouldn't have to mill them much to get your compression where it should be. I would have screw in rocker studs installed in them, but for a 350, I think they would be the better head.
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Old 01-20-2011, 09:54 PM
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Quote:
Originally Posted by WDCreech
Since this is a street engine, and your looking for milage, go with the 7J2 heads. The smaller valves won't make any difference on the street and you wouldn't have to mill them much to get your compression where it should be. I would have screw in rocker studs installed in them, but for a 350, I think they would be the better head.
Also, the slightly smaller ports along with the smaller valves will be (IMO) better for street due to the higher intake and exhaust velocity.

The bigger valves are a bit much for the street in a 350...i dont even run 2.02 (smaller than the 6x valves) in my I-stock.
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Old 01-20-2011, 09:58 PM
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how much would i need to mill the 7j2 heads if i choose them (and how can i fix that pesky missing exhaust bolt problem?)

what kind of mileage could i expect? i have a 1980 el camino with a chevy 350 and when i had the quadrajet on it i got 22 mpg, it has the same gears, but i read that the longer stroke pontiac motor should be more fuel efficient.
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Old 01-21-2011, 01:43 AM
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Use the CHART from Wallace Racing to see how much to remove to get how much reduction in chamber volume. I wouldn't go more than 0.050".

You may be able to use "L" brackets to mount headers if the pad is entirely missing. If the pad IS there, just not drilled and tapped- drill and tap it.


The only real way to increase the mileage of a 350 Pontiac over a similarly built 350 Chevy would be to turn it slower. Same gears = same mileage, or damn near.
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Old 01-21-2011, 08:21 PM
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I would rebuild the 700 due to the overdrive and the killer 1st gear.
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Old 01-22-2011, 11:03 AM
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i would go with a set of 72 cc heads, its a 350, you need all the compession you can get.
ive done oki rebuilds on several 350's (354) and they run alright, there just gutless. even with 72 cc heads.

ive made snappy runners with closed chamber heads also.(62-64cc)

but if you are going to do any work on a oldschool pontiac v-8, throw out the idea of getting good gas mileage.
if you want to retain fuel consumption for your wallet, go buy a honda.
v-8's just soothe the soul.

whatever heads you decide to go with, make sure you port match the intake to the heads, this includes cutting the coolant cross over off.

go spend 30 bucks on Jim hands guide on building high peformance pontiacs, i swear by it
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Old 01-22-2011, 11:39 AM
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Quote:
Originally Posted by h0trod389
go spend 30 bucks on Jim hands guide on building high peformance pontiacs, i swear by it
yes! great book!
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Old 01-22-2011, 12:39 PM
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I have Jim Hand's book, i've been looking through it some, i just haven't had the chance to sit down and read it front to back yet.

it didn't really say a whole lot about 350 motors, so that's why i'm asking here.

also, if i port match the intake to the heads, why do i need to cut off the water passage?

and i don't want gas mileage like a honda would get lol, i just want to get around 20 mpg. maybe 23. I know with my el camino i got about 9 mpg with the holley carb i used on my firebird with the 350 originally, but when it was on the firebird it got 12, so would i see a similar relationship with the quadrajet? as in 20 mpg on the el camino, so maybe 22 on the firebird?

or could i leave the heads alone and just give it more initial timing to have the same effect of more compression??
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Old 01-22-2011, 12:49 PM
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With the extra weight from the Pontiac engine it should handle like a lumber wagon.
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Old 01-22-2011, 12:59 PM
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Quote:
Originally Posted by recnepswilliams
or could i leave the heads alone and just give it more initial timing to have the same effect of more compression??
Pontiac heads don't really take that much port work to get them to flow great. Just some in the bowls, and then port matching. Its worth the extra work defiantly in my book.

It explains what to do, and what not to do porting pontiac heads in that book, its pretty much the same concept, your just bolting them onto a different engine.
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Old 01-22-2011, 01:05 PM
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Quote:
Originally Posted by BigRoy1978
Pontiac heads don't really take that much port work to get them to flow great. Just some in the bowls, and then port matching. Its worth the extra work defiantly in my book.

It explains what to do, and what not to do porting pontiac heads in that book, its pretty much the same concept, your just bolting them onto a different engine.
Why would you want to port Pontiac heads unless you are a glutton for punishment. You could just slap on a set of Vortecs if you left the Chevy in there.
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Old 01-22-2011, 02:20 PM
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A Couple Thoughts...

I would run a mild split pattern cam. Something like Summit K2800, if you're interested in mileage.

Keep the 7J2's as-is, valve size-wise, although pocket porting will help if you know what you're doing. If not, leave things alone or pay to have them done by someone who knows what's up w/Pontiac heads- they are nothing like a SBC head.

Don't let anyone talk you into changing the valve seat angles, but do get a three angle valve job. The intake seat can benefit from a 20-25 top. You can use a 30 backcut on the exhaust valves.

Keep the LSA ~112. Keep the lift moderate- you do not need to go beyond 0.460" or so. You can notch the bores to help unshroud the intakes if the block wasn't done already by the factory.

Zero deck the block if possible, but in any event use a 0.040" piston-to-head measurement as your target. This will allow some amount of leeway as to octane requirement. The static CR need not be excessive. I'd MUCH rather sacrifice a small percentage of potential output to gain detonation resistance.

Headers are a must-have, whether the idea is performance OR economy. Skip any of the "3-tube" headers. They use a pair of 1-5/8" outboard tubes w/a 1-3/4" inner pipe to service both inner cylinders; this is BS. Use a Hedman 4-tube header, IMO they're the best balance between price and function for the Pontiac engine. Run the headers into a 2-1/2" dual exhaust, place the mufflers as far rearward as is practical, use an "H" or "X" connection between the two sides.

Much of the rest is just good engine/chassis building fundamentals, like:

Pay attention to carb tune (use a Q-jet), and the ignition curve, and engine cooling system performance- use a fan clutch, shroud and an adequately sized radiator. Route outside air to the filter via whatever means necessary.

Make sure the suspension bushings, shocks, springs and various parts are fresh/tight, align the front end to have 1/16"-1/8" toe-in.

But at the end of the day, a Pontiac V8 is NOT what will make for a good hyper-miler!
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