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Old 08-20-2008, 01:03 PM
oldbogie oldbogie is online now
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Quote:
Originally Posted by Sixtyninemercury
Why have I heard that it's not worth it to have Edelbrock RPM heads ported? I have the #60259 heads. I have an old tappet 351W block with dished .060" pistons. A shop here in town milled the heads at their "max" (?) to try and give me 9.5CR because the owner of the shop says my motor had 8.5CR pistons. I have the Comp XE274H cam with RPM air-gap intake and Holley 750Vac carb. I also have a modified manual C4, 11" - 2500 converter and a 4.56 rear. Because of my dished pistons - which that shop also flycut so the 2.02" valve reliefs "worked" - and since the current heads are already milled - rather than spend a couple of thousand $ to get better heads and then get them milled - I was hoping it would be feasible to have these Edelbrock RPM heads seriously ported so that they flow better than 251CFM at .500" lift if I'm quoting Edelbrock's flow chart correctly. I am looking for more top-end HP while keeping it under 6000RPM.
You really need better pistons. You can mill the heads over dished pistons till the cows come home and not gain anything worth chasing. The problem with circular dish pistons is a lack of squish/quench in the combustion space. This results in a less than ideal burn with a tendency toward detonation and preignition problems. You can port and cam forever but you'll never get out of this engine what it should deliver because you've got a fundamental engineering flaw in the combustion chamber because of the piston crown shape. To get the compression up and get some squish/quench going you need to run a piston with as much flat space as possible to get under the flat portion of the head as close as reasonable somewhere in the zone of .050 to .060 inch including the distance the piston sits below the head deck, and the head gasket thickness. Starting with the combustion chamber volume, you need to add the head gasket volume and the volume the piston has from it's crown to the head deck. These will establish the base volumes of the combustion space. The compression ratio is then trimmed with the piston crown, which can be a dome, flat, or if a dish is need a D shaped dish under the valve's and sparkplug side of the chamber. As the cam gets into longer durations and or shorter LSAs, the compression should go up, an XE274 with an aluminum (Edlebrock) head will easily support 10 to one on 92 octane unleaded for the street.

Also when computing head gasket volume with an aluminum head, remember that you can't use thin steel shim gaskets, you'll be looking at composition gaskets that will have about .050 inch thickness already, so it doesn't have much allowance for the piston to be down the hole very far at TDC. This can be a problem with replacement pistons as they often come with a shoter pin to crown distance as the manufacturers assume the block will be decked, so they're trying to build in some cushion. You need to talk directly to them to insure you get the properly sized stuff or all the work going into getting the chamber right goes out the window.

Bogie
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