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Old 04-11-2006, 12:15 PM
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curtis73 curtis73 is offline
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I would personally skip these heads. The money you will spend to get them ported could cost more than a set that flows this well already. Just my opinion. There are plenty of 58cc heads that flow much better than the old-school L98s.

I agree that 235 cfm is a good target flow. If you go with newer heads, I might bump that up to 250 or so. Many newer heads take advantage of port profiles that give more flow with less sacrifice to velocity. That means you can make more power in the heads with less cam and not sacrifice low RPM velocity.

Here's what I do:
-I find what my target intake flow should be at sites like this. flow calculator I like to use 28 inches of water as my test data in that calculator since that's how most heads are measured. Some trial and error there will get you a target flow. Keep in mind that those numbers are theoretical. Thats the power you COULD produce if the engine ingests all of that air. On an actual engine, 235 CFMs would require 6500 rpm to produce 500 hp and a whopper cam to get there. If you bumped it up to say 250 cfm, you could probably make that power at 6000 with a milder cam. It sounds backwards I know, but its the benefit of modern head design.
-Then I visit sites like this... head flow numbers and find chevy heads that flow those numbers (or close enough that I can port them to get it.) Disregard valve size for now. They can be an effective tool to boost flow if you need it, but putting 2" valves in a stock L98 head won't do much because its not the limiting factor. Once you find a group of acceptable heads with the right range of flow at about .500" lift, start comparing intake port volume, E/I ratio, and chamber volume. The smaller ports almost always win. If they flow the same with smaller ports, its a winner. Since this is an 11.6:1 motor, I doubt its as big a deal for you. You'll be burning a steady diet of 110 fuel, so I don't picture it as a cross-country cruiser. E/I ratio should be on the order of 75-80%. Chamber volume can be increased or decreased with milling, valve jobs, and chamber grinding, but the closer the better.
-Talk to a cam manufacturer and tell them everything you know so far. They'll help you pick a cam.
-Then I plug all my stuff into Desktop Dyno. Its not perfect, but what a wonderfully informative program. It at least helps to identify mismatches and little things. On DD you can "match" things up. Play with net valve lifts to see what rocker arm ratio you need, play with port flows, valve sizes, etc.
-Then I visit forums like this one and ask questions like, "how much can I mill off a set of xxx heads?" or "should I use 2" or 1.96" valves?"
-Then I bloody build it.

Have fun.
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