Pontiac heads #142 or 6X8? - Pontiac GTO Forum
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post #1 of 23 (permalink) Old 06-11-2012, 07:02 PM Thread Starter
 
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Pontiac heads #142 or 6X8?

I'm trying to build up a Pontiac 400 and these head are my choices. The #142 cc'd at 90cc and the 6X8 came in at 98cc. The obvious advantage of the 142 is the lower cc and thus compression while the 6X8 have the larger valves. Both are screw in studs. The 142 could be milled a bit.
The 400 is .030 over, forged pistons flattop, lunati voodoo cam
Does the 142s compression potential outweigh the 6X8s valves size?



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post #2 of 23 (permalink) Old 06-12-2012, 08:22 AM
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Neither set of heads is going to work very well for you. 90cc's on a +.030 is going to put you somewhere between 8.6:1 and 8.9:1, depending on whether or not you zero deck the block. That combination will run on even crappy 87 octane fuel, but it's not going to make much power. 98 cc's would yield 8.0:1 to 8.3:1 which would be even worse. If you both zero-deck the block AND mill the heads close to the max (which is about .050) you can get the 142's to around 9.5:1 which will be enough compression, but cutting them that much is going to leave the heads pretty thin and also will guarantee that you have to mill the intake surfaces to get the intake manifold to line up correctly, plus you'll want to go ahead and have these heads re-done for the larger valves.
If those are your only two viable choices, then go with the 142's but just understand you're going to have to spend a pretty good amount on machining and modification (big valves) to make them work for you.

Which specific Voodoo cam are you using? What are the specs on it?

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post #3 of 23 (permalink) Old 06-12-2012, 11:22 AM Thread Starter
 
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http://www.lunatipower.com/Product.aspx?id=1775&gid=287

There's the cam I have.
I ran through a couple compression calcs and found milling to 85cc's and using a .021 gasket will get me low 9s compression. The machinist seems pretty convinced the valves can't be enlarged. Eventually I'll get better heads down the road once the finances allow it.


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post #4 of 23 (permalink) Old 06-12-2012, 12:17 PM
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There are more considerations that just raw compression though. For example, running a gasket with a compressed thickness of .021 will put the pistons too close to the head, especially if you're running forged pistons and have zero decked the block. They'll be hitting the head at rpm and that ain't good. You generally don't want the pistons to be set up any closer than .035" from the head "cold".

Consideration two: you can't just "mill to 85 cc's" without first thinking about how much material there is in the head deck that can be safely removed. On Pontiac heads, .050 is about the max (and if you do that there's nothing left for "later" if you need to rebuild the heads to square up/re-flatten the decks). .050 will remove about 10 cc's from the chamber --- so on those 142's you can take them from 90 cc's down to 80 cc's, but on those 6x-8's you can only get them down to about 88 cc's.

I still don't know why your machinist says he can't install larger valves. 142's should be 1967 heads, and in 67 Pontiac changed the center to center valve spacing on all the heads specifically to accommodate the larger 2.11/1.77 valves. I could measure the center to center distance on a pair of my heads that for certain have the larger valves in them, and if the 142's you have are the same dimension, they'll fit.

If you cut those 142's "to the max" so that they have 80 cc chambers, run standard .040 gaskets, don't zero deck the block (Pontiacs are usually .020 "down" if the block is untouched so I'm making that assumption), then at +.030 with flat top pistons (that usually have 6 cc's in the valve reliefs) you'll be at 9.375:1 compression. That's workable. That Lunati cam you're planning is actually a little on the mild side (despite them calling it a "torque monster"). It should give you good idle vacuum, and the 112 degree LSA means you'll have a slightly later intake closing event that should help manage cylinder pressure and thus give you some protection from detonation, if you install it at their recommended 108 degree ICL. (By way of comparison, the 'advertised' duration on that cam is 262/268 - compare that to the Pontiac factory Ram Air IV which is advertised at 308/320).

With that "small" of a cam personally I would be hesitant to push compression any higher than 9.3:1.

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post #5 of 23 (permalink) Old 06-12-2012, 01:00 PM Thread Starter
 
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The block deck height wasn't changed. With it milled to the max, could I still run 1.65 rockers?

I suppose having it milled down max would be less expensive than buying a pricey thin gasket in conjunction with 85cc milling. The pistons are valve relieved.
I guess the main question is, what is the wisest thing to do? Is there anything wrong with combining some milling with a thinner gasket, seems to me to be best since its leaving metal on the heads yet accomplishing the same effect. I could afford either way

Looking at the valves, I may have to concur with the machinist, the valves edges are very close to each other.

These won't be the permanent heads for this engine. Eventually something truly better, so having under-par heads for now isn't real dissapointing.


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post #6 of 23 (permalink) Old 06-12-2012, 03:50 PM
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Milling the heads has no effect on which rockers you can run. If you do go with 1.65's on "normal" Pontiac heads you will however need to elongate the pushrod passages in the heads to make them wider top to bottom, otherwise the pushrods will rub.

It's your choice really as to whether to use the thinner gaskets or to "get it all" out of the head. Just be careful with that minimum piston crown to head surface distance (sometimes called 'quench distance'). .035 is ideal --- more will reduce turbulence in the chamber and that starts to negatively affect combustion efficiency as it gets wider and in extreme cases will make the motor more prone to detonation, less puts you in danger of having the pistons hit the heads at rpm.

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post #7 of 23 (permalink) Old 06-12-2012, 03:59 PM Thread Starter
 
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How does that work? I thought the 1.65 would push the valve down more to keep it open longer?

I'll probably go with the head gasket method to save some metal on the heads.

I appreciate your info, I was a little surprised you call that lunati a mild cam. What constitutes a moderate or aggressive cam?


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post #8 of 23 (permalink) Old 06-12-2012, 06:32 PM
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The difference between 1.65 rockers and 1.5 rockers is that on the 1.65's the pushrod cup is closer to the rocker pivot point. That makes the pushrod pass through the head at a "lower" angle, so it will rub on the head if those passages aren't enlarged. Pushrod passages on Ram Air IV heads were already larger to accomodate that - RA IV's also ran 1.65 rockers.

A cam's "personality" depends in part on the engine displacement. A cam that's so aggressive and rowdy in a 326 that it's definitely 'race only' might be downright sedate in a 461. The reason has to do with displacement (pumping capacity) and the volume and velocity of air flow that the engine is able to maintain.
Lunati also makes some cams that are Ram Air IV and Ram Air III equivalent grinds, with these specs:
Duration @ .050": 231/240 (IV) 220/228 (III)
Gross lift (1.5 rockers): .470/.470 (IV) .424/.424 (III)
LSA: 113 (IV) 110 (III)

Compare those numbers to the one you're planning, which has a duration at .050 of 219/227 and gross lift of .468/.489 on a 112 degree LSA, and you can see it's sort of a hybrid - the duration is closer to a Ram Air III cam, lift is closer to a Ram Air IV. It's primarily the duration specs that give a cam it's "attitude" because duration combined with LSA determine the length of the overlap period --- the time when both valves are open. As overlap period gets "longer", the rpm where the engine will develop peak torque and peak horsepower gets higher - and also the more "attitude"/lope the engine is going to have at idle (along with decreased idle manifold vacuum).

My 461 has a "moderate" solid roller cam in it. It has a duration of 236/242 @ .050, .622/.629 gross lift (with 1.65 rockers), and a 110 degree LSA. It definitely has some attitude, but it's still very streetable in my engine. It'd might not be streetable in a 400 though.

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post #9 of 23 (permalink) Old 06-13-2012, 06:46 PM Thread Starter
 
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Thank you for that, gonna have to read that a couple times through to understand the cam aggressiveness.

Bringing the cc's to 85 would be .025 milling, combined with a .021 gasket would I need shorter push-rods??
I don't know what stock pushrods length is for a 400 77'.



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You might, especially since you'll most likely need an adjustable valve train and different springs to work with that cam. The right way to do it is to get a "checking spring" and an adjustable pushrod, install one head with those components and using the same came, lifters, rocker, etc. that you'll be running on the engine. Color in the end of a valve with a marker, adjust the pushrod length until the contact patch on the valve stem is centered. Then "read" the pushrod to find out what length pushrods you'll need to order.

here are some examples:
COMP Cams: Low Tension Checking Springs - Pair
COMP Cams: Hi-Tech Checking Pushrod 8.800"-9.800"
COMP Cams Quick Tech Video: Achieving Proper Rocker Arm Geometry - CPG Nation Forum

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