400 v8 compression with 72cc heads - Pontiac GTO Forum
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post #1 of 8 (permalink) Old 05-06-2012, 06:11 PM Thread Starter
 
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400 v8 compression with 72cc heads

Me again, I was contemplating giving up on using my #13 heads for my 40 bored .030 over. A: because they'd need a lot of work and B; I was under the impression they'd create too high a CR for pump gas.
Granted I could not enter the data for sealed power speed pro forged pistons so used the 6cc valve relief value, and two CR calcs came up with 9.96 and 9.88 cr.
Both of these seem pump gas friendly or am I doing something wrong?

The other option was to bid on a pair of 6X4 heads and mill them down to 88cc(as per a suggestion) to make a max pump has cr.

I hope this ain't confusing, my goal is to spend money wisely and have heads that max out the comp. these #13 heads are bad, one valve still rusted in place despite soakings and hammering. Even then it had lots of rusty gunk build up inside the passages and needs seats all around.
Theres a pair of 6X4s on eBay all set to go that end tomorrow. I want these assuming that #13 would indeed make a too high compression ratio anyway and make dumping money into pointless for this job.
By no means would I cast the #13s into the dump, I'd just not use them for my build.


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post #2 of 8 (permalink) Old 05-06-2012, 09:00 PM
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Hi Marty,

#13's with 72 cc chambers on a +.030 400 - if everthing else is "factory nominal" (pistons .020 "down the hole", .042 head gaskets, 6 cc's in the piston valve reliefs) you'll be at 10.065:1 compression. There are people running that much compression successfully on pump gas (you need a pretty radical cam to get away with it), but personally - I wouldn't try it.

Leave everything else the same, but instead run pistons with 12 cc's of dish (D-shaped, not round) puts it at 9.509:1 --- much safer --- and the D-shaped dishes mean you don't sacrifice your quench pad area - this is a good thing.

If you have 6x-4's (98 cc's nominal) you'd have to cut them down to 85 cc's, AND zero deck the block, AND run thinner .035" gaskets to get to 9.417:1 compression on a +.030 400. I'm not sure how much they can be safely milled, and then too you start getting into issues where you have to do additional machining to make the intake manifold fit right, and get shorter pushrods to make the valve geometry right... it adds up. However, those thin gaskets make for a very tight quench area which is heap big goodness.

HOWEVER - build that 400 into a stroker (461), run those 6x-4's untouched at 98 cc's, run flat top pistons, zero deck the block, and run .035 gaskets - that puts you at 9.430:1 with very good quench --- and what's not to like about 461 cubic inches?

From what you describe, chances are by the time you spend the money to get those #13's into useable shape you're going to have spent enough coin to get a good ready to run set of 6x-4's...

Bear

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post #3 of 8 (permalink) Old 05-06-2012, 10:53 PM Thread Starter
 
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I have a Lunati voodoo cam
http://www.lunatipower.com/Product.aspx?id=1775&gid=287
And Rhoads lifters. How does that affect the result?
A Stroker would be fun but it is a 557 block,
and it's already been rebuilt and assembled awaiting heads.

I am searching eBay and found rebuilt #142 heads at 90cc

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Last edited by SPCMarty; 05-07-2012 at 12:43 AM.
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post #4 of 8 (permalink) Old 05-07-2012, 02:02 PM
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Listen to Bear, Marty. He's right. Rhoads lifters are great, but if you have a pinging problem, they will make it even worse due to increased cylinder pressures off idle. I've been in the game for a long time, and I have two GTO's these days: one with stock compression that I never drive because it HAS to have race gas, and one with 9 to 1 compression that I drive all the time because I can on pump gas. The only way I'd use 72cc heads is with custom pistons, period. JMHO....I have never been able to
"get away" with high compression on the street with these cars with CA 91 octane. I thought I did once, but I was wrong....I broke 4 pistons.
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post #5 of 8 (permalink) Old 05-07-2012, 05:54 PM Thread Starter
 
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Well I figure I'll work on those #13 heads myself and maybe use them with my extra 400 block.
I picked up a pair of #142 heads already rebuilt, I'll have them shaved some though. Research says the 142 is 90cc chamber.
I'd prefer to use rhoads lifters


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post #6 of 8 (permalink) Old 05-07-2012, 07:50 PM
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Quote:
Originally Posted by SPCMarty View Post
I picked up a pair of #142 heads already rebuilt, I'll have them shaved some though. Research says the 142 is 90cc chamber.
Don't take anyone's word for that, always measure them yourself and make sure. Here's one reason why: If you have a head that has the "big valve" 2.11" intakes and 1.77" exhausts, and those heads have seen a few valve jobs, enough to "sink" the seat into the head by a mere .020", that will increase the chamber volume by 1.146 cc's - and that's just the intake valve. If the exhaust valve is likewise "sunk" by the same .020", that's another .80 cc's of volume. Taken together that's an additional 2 cc's just because of a few valve jobs.

2 cc's is enough volume on a +.030 400 to make about 2 points difference in compression ratio (9.5 vs. 9.3). Admittedly, that's not a lot --- unless you're riding the ragged edge of detonation already then it could be quite literally a "life and death" difference as far as your engine is concerned.

Add into the mix the fact that chamber volumes are known to vary from factory specs on Pontiacs, it's easy to see why it's important to measure your heads to be sure.

Quote:
I'd prefer to use rhoads lifters
Mind if I ask why?

Bear

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post #7 of 8 (permalink) Old 05-07-2012, 09:33 PM
 
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I'll throw some opinion into this discussion, too...

Having raced Pontiacs for 35 years, a few comments that support and vary from the above opinions:

Pontiac "advertised" compression ratios were generally a full point higher than what was actually delivered off the assembly line. I.e., the GTO 10.75:1 advertised comp ratio was actually 9.75:1 The only way you could achieve the advertised comp ratio was to mill the heads down to the minimum allowable chamber volume, which was the NHRA minimum volume. This is what Royal Pontiac was doing on the Royal Bobcat GTOs: They were "blueprinting" the engines down to max compression specs and running minimum chamber volume to get an actual 10.75:1.

A .030" over 400 with 6cc valve reliefs in the pistons, .010" block deck height, a .045" compressed gasket thickness, and 72 cc heads will produce about 10.1:1 compression. If you don't get too crazy on the aggressiveness of the ignition advance curve, you can easily run this compression on premium pump gas, and if you measure your actual chamber volume with valves and spark plugs installed, you may find that you have somthing closer to 75 ccs... I just built a 400 with this comp ratio and published all the specs and the dyno results here on this Forum - there was no detonation at all, even hammering this engine on the dyno from 2000 rpm on up using 91 octane fuel. No problems at all with a modest street cam.

But here's part of the key: As Bear and geeteeohguy have noted, you better measure and check everything, because a slight change in numbers will alter comp ratio dramatically. If you get it too high, detonation will destroy an engine in a heartbeat.

Rhoads lifters will not gain you much, if anything. I've run the Rhoads and standard lifters in two identical engines with the old H-O Racing "744HL" cams, and I can't tell the difference between the two other than the noise at idle. If I were you, I'd scrap the entire flat-tappet idea and fork out another $500 for a hydraulic roller setup... it'll blow the doors off a Rhoads flat tappet engine.

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post #8 of 8 (permalink) Old 05-10-2012, 06:06 PM Thread Starter
 
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So the valve reliefs in most pistons have a 6cc value.
I bought the 142 cast heads, my research said they are 90cc/8.4cr.
I plan on having them shaved some, and modified accept the large valves, threaded studs put in too.
How much can an un-cut head be shaved?
I went with standard lunati lifters to go with the lunati cam and timing set.
I already have the cam so making the switch to roller would be too expensive, I'm on a budget anyway.
Can I re-use valves from my other heads?

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