670 heads on a 350???? - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 11-12-2013, 07:58 AM Thread Starter
 
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670 heads on a 350????

Hey guys, newbie here. I am a 20+ yr Chevy guy with my 1st Pontiac. I just bought a 70 Lemans with the original 350 short block and 670 heads. It is an A/T with point dst,performer intake w/ and edelbrock carb (650 cfm?) and headers. I search this combo and have seen many differen opinions. The previous owner has driven this combo for 6 yrs. I will be pulling the motor to re-seal and paint shortly, so this would be my opportunity to make any changes if necessary. BTW, it has the original open diff w/ highway gears. S my question is what should I look for and/or change once I have it out. My pl ns were to send heads out but I don't want to put money into heads ta won't work for me. On more thing, I am changing it over to a 4spd. Any ideas or opinions would be much appreciate! Thanks.
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post #2 of 5 (permalink) Old 11-13-2013, 10:33 AM
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670's were GTO-only heads in 1967. The flow pretty well, but they're closed chamber heads and usually need quite a bit of timing. 400 heads on a 350 can work, but you need to make sure there's no interference between the valves and the block. Even 400's had chamfers at the tops of the bores for valve clearance. My handy-dandy compression ratio tool tells me that 670's on a +0.030 Pontiac 350 that's in all other respects "factory nominal" (always measure everything first!!!) will put you in the neighborhood of 9.4:1 static compression. With the right cam, it should live ok on 93 octane as long as your cooling system and fuel system/mixture are spot on. One concern I have is that the smaller 350 bore with the "usual" .020 Pontiac deck clearance is going to have some "prime" hot-spot, detonation-inducing real estate in the chambers due to the top edge of the cylinder bore. The closed chamber 670's may help with that some if the flat part of the head decks completely cover the cylinder bore edges.

Also, forget your 20+ years working with chevys. Very little of that detailed knowledge applies. Pontiacs make torque and they make it down low. You don't NEED to spin them fast to get things moving, so you're not going to need a lot of rear gear. My 69 GTO weighs 4000 pounds or so with me in it, has 3.50 rear gears, and has run a best so far of 11.86/113 at the track. Engine dyno says it's north of 500 lb.ft. by 2800 rpm.

There are two good reference books out the for you that will help:
Both have the same title: "How to Build Max-Performance Pontiac V8's"
One is by Jim Hand and has been "the standard" for years, the other is more recent and is by Rocky Rotella. I recommend you get both.

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post #3 of 5 (permalink) Old 11-13-2013, 07:09 PM Thread Starter
 
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Thanks Bear,
The previous owner has been driving this combo for approx six years, so I,guess piston to valve clearance is ok. You lost me a bit on the hot spot detonation, can you simplify that for a "Chevy" guy? In a perfect world I would like to keep these heads and just have a reliable healthy engine. I will send them out to machine shop to get check out, replace cam and carb and replace points dist with an hei.
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post #4 of 5 (permalink) Old 11-14-2013, 01:53 AM
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Exposed sharp edges in the combustion chamber tend to get hot - hotter than the surrounding areas - making them prime candidates for causing detonation. It's very common for factory nominal Pontiacs to have the pistons .020 "down the hole" when they're at TDC. This leaves the top edge of the cylinder exposed to the full force of combustion temps, and those edges tend to get hot --- leading to detonation problems if the engine is already borderline on compression. "zero-decking" the block (shaving enough off the decks so that the piston tops at tdc are exactly even with the tops of the cylinders, lets the piston crowns "cover" those sharp edges. It also improves the "quench" characteristics: having the pistons now coming closer to the heads induces more turbulence in the chamber, and that's good for keeping things nice and mixed, and also helps prevent hot spots. However, that operation also reduces total combustion volume which increases compression ratio, so you've got to allow for that.


And it wasn't piston to valve clearance I was talking about, it was valve to cylinder bore clearance. Even if the valves aren't touching the bores, if they're very close it has an adverse effect on air flow around the valve which hurts torque/power production.

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post #5 of 5 (permalink) Old 11-19-2013, 02:28 PM
 
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Bear knows his stuff and hes right about the valve to cylinder wall clearence. You are using 400 heads on a 350, larger bore head on a small bore engine. Bear are those 2.11 intake valves or the smaller? 670s are goat heads i believe they'd have the 2.11s so hes right youre close to the walls.

My .02 is if you are doing ANYTHING to that short block just dump it and find a 400. Those 670s may be a little high on compression for pump gas on a 400 but a guy with a '67 goat will be appreciative of you selling them. 6x-4 heads are plentiful (i sold my complete set for $300) and on a 400 with a 60919 cam, alum manifold, headers, and a HEI it should run really good for ya on the cheap.

You outta see her on a road course, or a quarter mile
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

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