Thread: unsure of head choice

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Well, it's time for me to look around for some different heads as I've got
stock "624's" on my 350 . Am looking at a couple from World prod. and was wondering just how much it would change comp. ratio going from 76cc to 67cc with everything else staying pretty much the same? Would I be better off staying with 76cc as this is a fairly stock build still? Crank is stock but only a couple months old and cam is also fairly new. It is a summit house cam with 204* in. and 214* ex. with lift at 442 in. and .471 ex. with 1.6 rockers. Oh, forgot something, 67cc chambers has an intake runner size of 165cc and the 76cc chamber has 171cc runner. Also, holley dual plane intake and holley 600cfm. with vac. sec. I welcome all thoughts as I'm one of the more un-knowledgeable ones around this site. Thanks

What is your current comp. ratio?

I can't really say. It's a stock rebuild with a 1973 block but the heads are from 1976. Had nothing special done to them when I had the rebuild so I'm guessing it's not real high, probably somewhere in the low 8-1 area?

O.K., I'll step up and take a shot.....
If we assume a 0.030" overbore, then the cylinder is 727 cc's. Assuming a 0.040" gasket, the volume there is 8 cc's. Assuming the piston is down in the bore by 0.040" at TDC, that's also 8 cc's. The average Chevy piston has 7 cc's worth of valve relief and we know the chambers are 76 cc's. This computes to 8.34:1 static compression ratio. I'll take a guess that the cam has an intake closing point of 29 degrees ABDC. This computes to a dynamic compression ratio of 7.583:1. Normally, you would shoot for a DCR of 8.00 to 8.25 if you had a tight (0.035" to 0.040") squish in the motor, so there may be a little left on the table with this motor. The problem is, we are not sure of the deck height. I'm assuming 0.040" and with another 0.040" in the gasket, that puts the squish at 0.080", not optimum for more compression with pump gas.

If you left everything else the same and swapped to 67 cc heads, the static compression ratio would be 9.07:1 and the dynamic compression ratio would be 8.238:1. Now, again, if the squish was tight on the motor (which we don't know at this point, this DCR would be perfect for optimum performance from the motor on pump gas.

I don't think you can make a decision about the heads until you pull your heads off and measure the deck height. If the motor was built with zero deck (highly unlikely), then you are "in like Flynn". You could also use a thinner head gasket with the 67 chambers to tighten up the squish, but you wouldn't be able to go much thinner or the DCR would go out of sight and you'd start detonating the motor on pump gas in my opinion.

Originally Posted by burrbrian

Well, it's time for me to look around for some different heads as I've got
stock "624's" on my 350 . Am looking at a couple from World prod. and was wondering just how much it would change comp. ratio going from 76cc to 67cc with everything else staying pretty much the same? Would I be better off staying with 76cc as this is a fairly stock build still? Crank is stock but only a couple months old and cam is also fairly new. It is a summit house cam with 204* in. and 214* ex. with lift at 442 in. and .471 ex. with 1.6 rockers. Oh, forgot something, 67cc chambers has an intake runner size of 165cc and the 76cc chamber has 171cc runner. Also, holley dual plane intake and holley 600cfm. with vac. sec. I welcome all thoughts as I'm one of the more un-knowledgeable ones around this site. Thanks

Both heads will work great.
I would not say you are un-knowledgeable at all.
You have made a good choice on the heads,intake and carb.
To many people think they need 200+ cc for a street car.

Thanks, guys. It seems like I could go with the 67cc's and a little bit thinner head gasket and come out ok? Providing that the pistons and gasket are what we assume them to be. If I were to step up to a larger cam in the spring such as 219* in. and 227* ex. @ .050 with .468 in. & .489 in. ex. lift
would this all work good together still? Just trying to keep from having to change too much when I try for future improvements. Thanks again- allways wanted to build my own instead of paying out the to have it done and am having a blast learning the technical side of it all!

Nothing wrong in doing it on your own,rock on!

Get the heads off and let's see what it looks like....

Okay, will let you know what I find when I get the heads off but it's gonna be hard as I don't get too many days in a row that I can put her down but will figure out something, somehow.