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PostPosted: Thu Jul 23, 2020 10:57 pm 
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a few years back me and my dad bought a 69 mach 1 mustang with a 351 cleveland in it. the guy said it only had a few hundred miles on the rebuild of the engine and i was kinda curious what kind of compression it has. i run 87 octane in it and i have never once heard it spark knock or anything.

so here's what i know. the guy said it was stock stroke and it has 2v heads with 4v valves in it. i put a camera down the spark plug hole and looked at the top of the pistons to see if they were dished, domed, or flat top. they are flat top with 2 valve reliefs. they also have .030 and h555p on them. i looked them up and they are speed pro hyper pistons which give 8.83 compression with 76 cc heads. i am assuming thats a stock block and stock heads.

so my question is what would be the most compression this combo would make if the block was zero decked and heads milled? 9.2? 9.3? 9.5?


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PostPosted: Fri Jul 24, 2020 2:32 am 
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Location: Texas
So I typed that piston number in and the only site that gave any info was Ebay. I saw where you got the 8.83 compression. So I went on the Summit web to research. Are you sure this isn't the number https://www.summitracing.com/tx/parts/s ... ocation=tx

So I used their compression calculator. Stock compressed gasket and +4 on piston volume for the 2 valve reliefs. I'm not sure how far down the stock Cleveland pistons are but I put .005 in the hole for this. Just remember, depending on how much you mill the heads, will determine how much you have to mill the intake. If its compression your after, I would do what I did and find you some Aussie 2v heads. If not that then some Cleveland 4vcc heads or just save your money and get some Trick Flow or CHI heads. Back in like 2002 or so I went to the Pates Swap Meet and picked up my Aussie 2v heads still assembled with all valve train for $245 for both. Those got heavy after carrying them for almost a mile to my truck. I put new valves, hardened exhaust seats, 3 angle valve job, matching springs for my Comp 282s cam, 10 degree locks, the Crane rocker conversion and Crane 1.7 Energizer rockers. I still have the engine waiting to go back in something.

Imagecompression calculatorr by 359daddy, on Flickr


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PostPosted: Fri Jul 24, 2020 6:13 am 
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Location: Orange County, NC
I've seen factory pistons (and aftermarket ones) be as far as .035 in the hole with an uncut block.
You can get the open chamber heads down to around 67 cc's by taking .055 off them. I've done it. It still leaves a little material for future clean ups, but not much.
Most flat tops I have seen specs on show right around 2 cc's for the valve reliefs.

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PostPosted: Fri Jul 24, 2020 8:32 am 
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kv racing
If he cuts .055 off them to 67cc then will he have to take that much off the intake too?


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PostPosted: Fri Jul 24, 2020 8:41 am 
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The OC 2V I ran had 74cc heads and with a true zero deck it was 9.56:1. You say .055 off some heads you're going to likely need to mod the intake also. 67cc would give 10.3:1 and almost guarantee you'll be on 91 minimum with a careful tuneup and advance limits. The good thing about cutting the hell out of OC heads is that the smaller chamber gets more turbulence and that reduces their tendency to get into detonation. Also, if the heads are good, take care with them. 2V heads are real bad about cracking around the exhaust seat, especially later ones in the 72 up years.

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PostPosted: Fri Jul 24, 2020 10:23 am 
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nova467spanker wrote:
kv racing
If he cuts .055 off them to 67cc then will he have to take that much off the intake too?


No. He'll have to take about .039 off of the intake side on each head. Asquared + Bsquared = C squared / 2.


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PostPosted: Fri Jul 24, 2020 10:32 am 
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I found one place that told the compression but it wasn’t eBay. Can’t remember where off hand. Found plenty of places that had h555cp pistons instead of h555p. They look the same and maybe just an updated number. I was really just curious as like I said it runs great on 87 octane. It’s got an old accel dual point distributor and I got the initial timing set at about 18. Not sure on total timing or when it all comes in. But it cranks over smooth with no grunting and runs on 87 with no pinging so I always wandered what kind of compression it was.


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PostPosted: Fri Jul 24, 2020 10:33 am 
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The pistons could say h555cp. Hard to tell with a camera down the hole


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PostPosted: Fri Jul 24, 2020 4:30 pm 
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Every "stocker" I've built with stock rods and aftermarket (usually TRW/SPeedpro) ended up around .016 in the hole more or less. That would put it - with a Felpro blue gasket, 76cc chamber - at around 9:1.

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PostPosted: Fri Jul 24, 2020 6:25 pm 
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if i found a set of closed chamber 4v heads and put on it i would probably be looking at around 10.5 to 1 then. give or take. would that be be too much for 93 octane?


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