Hi yall so Im rebuilding a 350 and its bored .040 over, stock stroke, and the heads that came with it originally are World cast iron 58ccs off a 305. I thought they would be fine cuz Ive heard 305 heads increase the low end power of 350s. I tried putting all the info into Summits compression calculator and it seems like theres really no way to get the compression to 10, its always 10.5-12 even with .045 deck clearance and a .041 gasket. Unfortunately the engine is at the machine shop getting the bottom end put together so I cant really measure the deck clearance rn, but Im guessing its in the range of .025 to .045. It seems like the only way to make these heads work would be if they were 64cc or higher, but Im just curious if theres any way I can use em cuz I already paid for a valve job and seals on em? I havent really rebuilt an engine before so Im just not sure. Thanks!

 

0 cc pistons ?

 

Theyre flat tops so wouldnt it be zero or is that completely wrong? Lol.
Thanks.

 

Unless you have cheap pistons, I'd be willing to guess the deck height will be closer to .010 -.025 but obviously measuring is the only way to know. Without a measurement, it's a guess and a guess means NOTHING to compression.
You'll also probably have a dish of 6cc or more for valve reliefs, that'll make a difference.
I honestly would only shoot for 64cc chambers on a 350. With the right pistons and quench you'll be in the 10-1 range.

 

Awesome, thanks for the info. I wasnt sure about the dish so I just left it zero but I will try that. And true I know I have to measure but Im just trying to ballpark for now to make sure its not too high.

 

Not entirely wrong but they usually have valve relief. If you have the part number to the pistons then the manufacturer will have published the cc dish they have for that.

 

As pointed out, you'll have to measure that deck to be sure. Piston cc (valve relief volume) and piston compression height (distance from the pin centreline to the piston top) will play a part too. Some "jobber" style pistons have a lower compression height in the anticipation that the block will get decked. If you have that style of piston then they could be way down the bore. Does the piston have part number stamped into the top?
That said, one of my stock 350's, once refitted with a Speed-Pro piston with a 1.560" compression height, was anywhere from .030" to .036" down.
If you take that example, and run it through a calculator using an .040 x 4.100" gasket:



That's 10.37:1. I made a few assumptions as you can see.
FWIW, that's where I'm at with my iron headed 357 and I consider that about the upper limit given my cam spec and application.
Something to keep in mind with the heads you have is that the combustion chamber might only be the diameter of the 305 (+/-). At 3.736", this means that there should be lots of material that you could remove to bring it to a 4.040" bore. That'll remove a few cc's from the heads and get that compression down. This is something you can still do even after the valve job. You'd need to take the necessary precautions to protect the seats. If you're not comfortable doing that, I'll bet a performance shop could make short of it for not too many dollars.
That may or may not be the case for you but it's worth investigating I would think. Seeing as you already have all these parts it may pay off with a little extra work.

 

Without having a piston part # and deck clearance measurements, Skinny's numbers are about as roughly close as you're going to get.

It's usually pretty eeeeeeeezzy to unshroud the intake valve around the "end" of the chamber a little bit, and both improve flow and lose acoupla ccs that way.



Stock heads here, but the principle should be self-evident. Makes a significant difference to low-lift flow.

 

Exactly that.
I see you estimate a couple of CC's worth of material. I would think it would be more than that but that's also estimating.

 

Could be; maybe as much as 5 or so. Kinda depends on the shape the casting starts out as, and how aggressive you wanna get.

Of course it also helps in ANY high compression situation, to remove ANY AND ALL sharp edges from the chambers; which means, go around the whole "bathtub" with a sanding roll or whatever, to knock down that edge where the chamber meets the deck. Particularly if the heads have been surfaced, which often leaves a burr all the way around, which is about guaranteed to get red-hot when running. That'll knock off another few .1s of a cc as well as being just generally a good thing.