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Old 10-27-2012, 02:12 PM
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what piston for higher compression in a 351C

I have a 351C with 2V heads in a 72 gran torino. I got helpful info from the forum not too long ago about a question of using racing gas. My question is, if I stick with the 2V heads I already have, what piston do I use to achieve a 10.5:1 compression ratio? I plan on boring the cylinders .030" over. Also, the stock crankshaft and rods should be good enough for being a some what serious street bruiser and also seeing a little time on the strip shouldn't they?

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Old 10-27-2012, 08:38 PM
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Do a simple search here or google for an online c.r. calculator, figure out what you have for associated parts to calculate c.r.'s for different available pistons, flat, dished, domed.
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Old 10-28-2012, 10:32 AM
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I believe Icon makes a flat-top for the Cleveland that will get you close. I'm not at the shop today, so no catelogs. I'll look tomorrow to be certain.

I'm not sure the gains from upping to 10.5:1 would be worth the added cost and "hassle" of race gas. If you're going to run high octane fuel anyway, make it 11.5:1. We've learned, that's the "line" where wear factors start to increase significantly. Perhaps why most factory performance engines from the "old days" were left there. You will gain about 3.5-4% horsepower for every full point of increase in compression. We've also learned through necessity, "flow trumps compression". That is, a head that flows well will make more power at 9:1 than a "stock" head on the same engine at 10.5:1 (using the proper octane).

As for static compression ratio, do NOT take "published data" as gospel regarding chamber volume. You must do the actual measurements. Particluarly on engines this "old", the history of the head may not be known. If it's been milled a couple times, volume will be less than original. Also, orginal "numbers" are nominal, not necessarily exact. If your goal is to run on "pump gas" (recommmended), I would keep static ratio under 9.5:1. The large open chambers are prone to detonation. There's an Australian 2-bbl. head with "quench pads", not unlike the "good" 4-bbl. heads. Rare. Expensive. THAT head can be used to press the limit of compression with less detonation.

I don't recommend 4-bbl. heads for a street engine. Adding 4-bbl. valves to the 2-bbl. heads, combined with a nice solid lifter cam, REALLY wakes up the C. It still maintains a semblance of low-end power, so driving isn't unpleasant like the 4-bbl. heads. It will rev nicely to about 7,000. We have built several of these. Port work isn't really needed, just a good "blend" after the larger valves are installed. The Comp XS series is ideal.

The stock rods are quite good. The nodular crank is "safe" to about 7,500. If it's the least bit "edgy", grind it. Since it's cast, going .020 or .030 "under" is no big deal. Be sure the type of rod bearing you want to use is available in the undersize. I would add ARP bolts (resize the rods after installing the new bolts) and a good balance job. An SFI-approved balancer would be prudent (cheaper is NOT better, but no need for a "high end" unit).

Jim

Last edited by Mr. P-Body; 10-28-2012 at 10:38 AM.
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