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Discussion Starter #1
I picked up a pair of #47 heads for $75 from a running motor for my 350. They are listed as 1.96/1.66 73cc or 8.6:1 compression. I got them home and cc'd them this morning. 83cc on all 8 chambers! What the heck? I have 7h1 heads with 91cc's. I was hoping to go from 7.6:1 up to 8.6:1. With 83cc's I'm looking at a little over 8:1. I drove 3 hrs to get them. That sucks. I did visit some relatives nearby that I haven't seen in a long time. Otherwise it would have been a waste of time and gas...and $$.
 

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Well, 72-73 cc's is what they -should- be, according to all the reference material I can find (and also through my calculations based on them being 8.6:1 on a stock 350).

I don't mean to insult your intelligence or anything, but did you test all the chambers? Seal the valves/seats with grease? Measure multiple times? etc.

Bear
 

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Discussion Starter #3
Lol, I'm not insulted. I've done lots of dumb things. But yes, I ran never seize around the valves and around the edge of the combustion chambers, laid the plexiglass over it and pressed down and could see that it was sealing around the edges. I used a 60cc syringe. So each one took a full 60cc's, then another 23cc's and stopped at the 37cc mark. I did each chamber 3 times. And I made sure nothing leaked out the sides. Even so, I can't imagine each chamber would leak the exact same amount. I even did 2 additional tests with a different syringe.
 

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Discussion Starter #4
Just did a little searching on the subject. I found a post on the PY forums from last summer. The person said he had a set of #17 heads. Small valve, supposed to be 72cc's. They were actually 83.5cc's. Maybe I'm not crazy.
 

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The #47 heads are the 265-horse (low hp) 350 heads that had an advertised comp ratio of 8.6:1.

73 cc heads would produce a comp ratio of about 9.1:1, not 8.6:1, so the #47's can't be 73 cc...

83 cc heads on a 350, assuming .016 deck, .045 gasket thickness, and 5cc valve reliefs, will produce 8.3:1 compression. The factory comp ratings were with all specs at minimal or nominal, so it sounds like you got the so-called 8.6:1 heads with your 47's. If you run a thin .035 head gasket and do a small cleanup mill cut off the heads to remove 3 cc's, you'll be right at 8.66:1 compression, which is what you were after. Sounds like there's no problem.

Lars
 

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Wellllll...... if I run the numbers
3.785 bore
3.750 stroke
83 cc chamber
.045 gasket
4.16 gasket bore (Fel-Pro Q8518PT)
.016 deck clearance
5 cc valve relief

I get 8.170:1

With an .042 gasket, .020 deck, and 6 cc's of valve pocket it comes out to 8.092:1

Keeping the .042 gasket, .020 deck, and 6 cc's ---- you need 76 cc's of chamber volume to get to 8.613:1. Using .045/.016/5 it takes 77 cc's to get to 8.622:1

(All figures come from using the Excel spreadsheet that's embedded in the attached Word document)

One thing you might try... instead of dumping the whole 50 cc's at once, don't use the whole syringe capacity --- work between two discreet markings on the syringe and dispense/refill it say... 10 cc's at a time or something, just in case something's not marked right.

Searching the web for reference material, it seems that there's some discrepancy out there with regard to what volume they were, nominally. I found them listed at 73 cc's, 72 cc's, and several other sites that just said "8.6:1" which was a little unusual. I do know that Pontiac heads can vary from the factory specs, and the factory specified "compression ratio" especially was always misstated by quite a bit, but the difference you're finding does seem to be excessive (depending on which, if any, web site is 'right').

Bear
 

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Discussion Starter #7
8.6:1 was not my goal. I just thought thats what the 47's would give me. I thought for $75 I could bump up my compression. I was really surprised when they were not the 73cc's I was expecting. I did also get a 69 4 bbl intake from the same guy. So even if its not exactly what I was expecting, I'll still get some improvement. I'm up a 1/2 point in compression, 2 bbl - 4 bbl swap, Chevy qjet carb, and I'm looking at cams.
 

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Wellllll...... if I run the numbers
3.785 bore
3.750 stroke
83 cc chamber
.045 gasket
4.16 gasket bore (Fel-Pro Q8518PT)
.016 deck clearance
5 cc valve relief

I get 8.170:1
Bear -
The difference is in our assumed gasket bore diameters. You ran the "big bore" Fel-Pro, and I run the numbers with the "tight bore" Flatout 921543950 gasket at 3.9". This gasket compresses at .040, and produces the 8.3:1 comp ratio stated. You also indicated a 3.785 bore, which must be a typo, because I ran your numbers, and you must have used the correct 3.875 bore size in your calculation.

When I was running Super Stock, we found that Pontiac Compression ratios were often over-stated by a full compression point. I.e., the GTO engines advertised at 10.75:1 were actually 9.75:1 if you measured everything out. The only way to get them to the advertised 10.75:1 ratio was to mill the heads to the NHRA minimum allowable chamber volume. This would "blueprint" the heads to the advertised comp ratio. You can bet that the "47" heads are the same way: they are not anywhere close to the advertised 8.6 ratio unless they are milled to minimum allowable volume.

Lars
 

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Discussion Starter #9
So you're saying its pretty common to have bigger combustion chambers than what they should be? How come I don't read more about this happening? I cc'd my heads because thats what I learned from forums like this one. People always says to check for yourself so you know exactly what comp. ratio you'll end up with.
 

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I've always "heard" that Pontica chamber sizes can vary quite a bit from the published factory specs, but I don't have hard numbers on just how much "quite a bit" is. In your case, with 47's, I'm also having a hard time findind a factory spec that I trust as being correct. Not sure why that is...

Bear
 

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Discussion Starter #11
Yeah, I went back and looked at the head listings online and I see that my 7H1's are listed as 80cc's on one site. Can't believe I never noticed that. They are actually 91cc's. Hey, the bottom line is I'm still having fun working on my Lemans.
 
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