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Did you tell the Summit guy you were taking off 75cc heads, which may be the more likely ones you have? Unless you otherwise confirmed what heads you have by looking up the casting numbers, which is what you needed to do before determining what replacement heads to go with from there. You need to start with a baseline of what's known for sure. Throwing on a thicker gasket for the sake of decreasing your c.r. increases your quench area (the space between the flat head portion and the piston at TDC), which hurts performance. This article explains it: Piston Head Clearance Guide - Tech Article - Chevy High Performance Magazine
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Without knowing the previous CC of the combustion chamber/piston, it's a little tough to determine what your CR will be for sure. If the 307 used a flat top to get at 8.5cr with a 75cc head, then you will definitely be over 10:1 cr. If it was a dished top, then it depends on what the dish depth is, but I'm pretty sure they were flat tops. Not sure I'd want a 56cc chamber regardless, as it will still bump the engine's CR up to premium gas either way. The calculator at Eagle's web site says the flat tops in a std. bore, with 56cc heads puts your engine at 10.77 cr. The aluminum heads will help that a bit, but not enough to run lower grade gas. Better get the thickest head gasket you can find! Felpro sells one that's .051" compressed.
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hm..of course you are right. Without knowing exactly what heads are on, ist difficult to plan. As the car is 100% stock, I supposed the guy at Summit would know what heads are on. Maybe wrong....
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I used random factors (since what's inside is unknown), in conjunction with having 75cc heads, to come up with a baseline of 8.5:1 c.r (how his 307 came from the factory). With everything else being the same, the 56cc heads will give him the 10.69 as I showed before. Doesn't matter what type of pistons he really has, as the head volume is the only variable that changed.
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Originally Posted by 1971BB427
(Post 686796)
These are the head numbers found on 307 Chevys.
3911032 (1968, 70cc) 3917290 (1968, also used on some 327s) 3917293 (1968, 75cc) 3931633 (1968-1973) 3986388 (Used from 1968-1976 on 307 and 350 engines) 3927185 (1969-1976 307/327/350, 70cc) 3932454 (1969-1973) 3927188 (1970, 74cc) 3986339 (1971 307/350) 3998991 (1972-1973 307/350, 75cc) 3998993 (1972-1973 307/350, 75cc) None of them are what we would consider "great" heads, or even good. If you have a set of Chevy performance heads, or good aftermarket heads, either will make more HP than any stock 307 head. These are not terrible engines, but with their reverse bore/stroke, they tend to be a little tougher to get big HP numbers. The right heads, cam, and intake will really help, so if you're going to replace the heads consider doing a cam and intake, plus headers also. |
Originally Posted by Bamaro
(Post 686836)
What are you referring to with "but with their reverse bore/stroke" (3.875/3.25)
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Found the time to check the head casting: 3998993. So this would mean a 75 cc chamber. Now calculating the stock CR I am a bit lost, as I get quite lower results than 8.5. What stock deck clearance should I use? (I will of course measure it later when I have the heads off). If I add some 2 - 4 cc for the valve reliefs (calculate as dish?) in the piston, I even get lower.... maybe I am wrong somewhere?
Bore: (diameter) in. mm Stroke: in. mm Cylinder Head Volume: cc in. Effective Dome Volume: Use (-) for Dome and (+) for Dish. cc in. Deck Clearance: in. mm Compressed Gasket Thickness: in. mm Number of Cylinders: 4 6 8 10 12 Compression Ratio :7.67 : 1Total Displacement (in.3) : 306.63Total Displacement cc's :5026.72 |
The compression calculator didn't make it into my previous post, so here it is ;)
Found it on summit racing. Bore: (diameter) 3.875 in. Stroke: 3.25 in. Cylinder Head Volume: 75 cc Effective Dome Volume: Use (-) for Dome and (+) for Dish. 4 cc Deck Clearance: 0.06 in. Compressed Gasket Thickness: 0.039 in. Number of Cylinders: 8 Compression Ratio : 7.4 : 1 Total Displacement (in.3) : 306.63 Total Displacement cc's : 5026.72 Calculate |
From 1969-71, from the factory the 307 had a 9:1 cr. Starting in 1972, c.r. dropped to 8.5:1. Flat top pistons generally have valve reliefs, that is not a dished piston. A dished piston has a definite drop to them, and figuring it as if you have dished is what's throwing your calculations off. Google dished pistons and you'll see what they look like. In my previous post, I already figured your c.r. using the stock 8.5 c.r. as a baseline (with 75 cc heads), and calculated the difference the new heads are going to make.
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Finally I found the time to take the engine apart. I found the heads are 76cc. Calculating all again, after measuring the heads, the volume of the chamber from piston to deck (at TDC), I came out with a CR of 7.6! Quite lower than the 8.5 stated in the books.. With the new heads (56cc) I should end with a CR of 8.9, just fine for my Project.
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