#13 or #11 heads? - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 07-16-2012, 01:31 PM Thread Starter
 
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#13 or #11 heads?

I have a .030" over 400 with TRW flat tops. Currently have some 4x heads 98cc chambers (I measured them) and 2.11/1.77 valves installed, in my car. Block was not decked and using felpro gaskets. My calculations I am currently at 7.9:1.

I have 2 other sets of heads. #13's which measured to be 75-76cc's and the #11's which I measured to be 84cc. So I guess I am looking at 8.9:1 with the #11's or 9.5:1 with the #13's. 13's have 2.11 and 1.77 valves, the #11's have a 2.11 and 1.66 installed.

The guy I got the heads from said he felt the #11's had more power, he had both sets on the same car. Seems odd as they were rated at much less hp than the #13's.

Cam I am running is a lunati 00071 cam. Specs are 230/230 at .050" 292/292 .490"/.490" lift and a 110lsa. Running 1.5 rockers. Another big reason I want to put either set of heads on is the 7/16" screw in studs and the adjustable nuts/locks I have for the rockers. They don't fit my 3/8" press in studs. Am I going to notice a difference in power changing out my 4x's? Also using ram air exhaust manifolds and a ram air 3 intake manifold. The guy had run 1.65 rockers and he gave me them as well, but I haven't pulled his motor apart to verify what cam he had in it. He told me it was a RAIV but I have no idea if that is true until I pull what is left of it out.

What should I do? Car has 3.42 gears and 215/60/15 tires soon to be changed to 275/60/15. I do have a 2600 stall I could throw in which would be a little better with my cam.
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post #2 of 5 (permalink) Old 07-16-2012, 04:38 PM
 
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13's are definitely the better head. Not sure why/how the 11's could have been better. The 11's should also be small valve heads unless he had bigger valves installed. I've been doing a lot of research on 400 builds since I just bought one. A very common build is using heads like your 13's - big valve 72-75cc chamber - and dished pistons to get compression in the low 9's and a cam to match. A dual pattern with a little less duration than the one you have might work better. But, I'm not an expert. I'm still researching and learning. I'm sure others will put their 2 cents in.
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post #3 of 5 (permalink) Old 07-16-2012, 07:45 PM
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I ran the numbers through my spreadsheet and assumed .020 piston deck clearance, .042 compressed gasket thickness, 4.16 gasket bore, and 6 cc's in the piston valve reliefs.
76 cc's figured out at 9.686:1, 84 cc's figured out at 9.017:1

Its taking a chance, but with those gears, 2600 stall converter, a top-notch cooling system, and an accurate tune up (you sure don't want to be lean), you might be ok with the smaller heads. Understand there's never a guarantee on these things. You'll want to monitor your spark plugs really closely for any signs of detonation (shiny metallic specs) regardless of whether you can hear it ping or not. You've got 'enough' cam that it should also help. Make sure you degree it and -don't- advance it any. Advancing the cam causes the intake valve to close earlier and thus build more cylinder pressure.

Or... you could go with the larger heads and "fer sure" not have to worry about it. I set up a quick model in my Engine Analyzer just to compare the difference between them. All other things being equal, the difference between 9.017:1 and 9.686:1 compression was worth about 9 peak HP.

Your choice.

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post #4 of 5 (permalink) Old 07-16-2012, 11:59 PM Thread Starter
 
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I will buy a degree wheel for the weekend if I can find one in town, the rest of the stuff I have.
Quick question, I've never done it before but just watched a few videos. You say not to advance the cam. My cam card shows:
IN open 11 BTDC, IN closes 39 ABDC, EX opens 51 bbdc and EX closes -1 atdc. 110 - the above centerline will make this cam 6 degree advanced. What does that last part mean?

The 9hp peak hp loss doesn't sound like much. But that just takes into account the compression difference and all other things being equal, right? I'm wondering what the difference will be because of the valve size. Intakes are both 2.11 I measured, but the exhaust is 1.66 on the lower compression heads. The 11's were rated at nearly 100hp less in 1970. Is that likely because a smaller cam on the 350/400 with 2bbrl intake and carb or are the heads going to flow less?

I have been working on a 1970 GTO for over 2 years, but just picked up a 72 Lemans in original paint this weekend. Threw the motor I had in the GTO into the lemans and drove all weekend, so much fun! I know I could probably be a little better off and more happy picking a different cam to better suit my needs, but this is what I have for the time being.
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post #5 of 5 (permalink) Old 07-17-2012, 08:46 AM
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Lunati's specs say to install it at 106 degrees ICL (Bracket Master II Cam - Pontiac V8 292/292 - Lunati Power) , so that's where I'd put it. Cam manufacturers usually have a pretty good idea of how best to use their products, so unless one has direct experience with doing it differently and understands all the whys/wherefores, it's best to start with their recommendations in my opinion. If you put it there an *IF* you find you've got detonation problems, then one possible approach might be to retard the cam a degree or two at a time until they go away. That's going to both reduce cylinder pressure at low rpm and also tend to shift peak VE/torque higher in the rpm range.

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