351W compression question

[ThePoose 37]

New dumb question.  On a 351W 4-barrel, with the stock style flat top pistons with 4 valve reliefs, is it safe to assume (dangerous words) that it is approximately the stock 10.7 compression? I know it ultimately varies depending on the heads, but pistons would need to be dished or domed to change that ratio, right? They do come up to deck height.  I was rebuilding everything to stock, but I can't remember or find the paperwork. 

Thanks.

[1969_Mach1 333]

Without changing pistons, cylinder heads with different combustion chamber volumes will change the static compression ratio.  Cylinder bore also affects compression ratio.  With your setup, stock to 0.030" overbore, head gaskets with a 4.1" dia. bore and 0.041" compressed thickness, and cylinder heads having 60cc combustion chambers you should be close to 10.7:1. 

Static compression ratio = total volume/clearance volume

Total volume is with piston at bottom dead center.

Clearance volume is remaining volume with piston at top dead center. Clearance volume is also the total volume - swept volume

Swept volume is the volume the piston consumes as it travels from bottom dead center to top dead center.

There are many online calculators.  But, you need to know many of the volume and dimension specs.  If I were you I would use one to get a good idea of what static compression ratio the engine will have.

[barnett468 418]

what heads are they?

 

the last 69 351 heads i had a few months ago were around 60 cc's.

 

[ThePoose 37]

Sorry. I’m sure I’m being confusing. I guess what I need to know is if there’s a way to tell by looking at a piston if it’s stock or what the specs are. I want to be able to tell the cam grinder the specs of the engine to get their advice  (3.500 stroke, 4.030 bore, XX:1 pistons, 60cc combustion chambers, etc.) I only brought up 10.7:1 because that’s listed as the stock final compression for a 1969 351W 4 barrel. 

So when I look at the block and see the flat top piston with 4 valve reliefs that comes up to zero (or near zero, can’t measure at the moment) deck, can I safely assume that this is a stock ratio piston? And if stock, is that piston 10.5:1 or other? Or do I need to pull one and see if it’s stamped on the skirt or something?

I’ve never done it this way. I’ve just always rebuilt from scratch and I can’t remember what I put in this one back in 1990.

Thanks  

 

 

[RPM 1,190]

Of the ones I've seen,  every overbore piston had the .030 or .040 stamped on the piston top. You may have to clean them to  see it. 

[ThePoose 37]

The engine has never been fired. .030 is on top. 

[RPM 1,190]

3 hours ago, ThePoose said:

Sorry. I’m sure I’m being confusing. I guess what I need to know is if there’s a way to tell by looking at a piston if it’s stock or what the specs are.

Sorry, I thought you didn't know which pistons you had. 

[ThePoose 37]

Well, technically I don’t. :)

[1969_Mach1 333]

If you end up with a 60cc combustion chamber cylinder heads and 0.039-0.041 compressed thickness head gaskets, your static compression ratio is probably in the 10.5:1 to 10.7:1 range.

If you're building just a street driver car . . . I don't know that a custom grind cam will be beneficial.  I've used them in the past.  But now I stick with something from one of the major cam manufacturers.  Simply because, in my mind, the major cam manufacturers do a lot of R&D and test cam designs on motors so you have better knowledge of how it performs.

[barnett468 418]

the piston volume is probably arund 5 to 6 cc's so you add this to your compression calculation. You can lay a metal ruler across the top 1/3rd of a piston the bring it up to tdc then simply use a feeler gauge to see how far below the surface the piston is. the block decks are not always square to the crank so it is better to measure each piston.

 

are you having a custom cam ground?

 

at 10.5 you may have some pinging so it is important to know what the quench/squish clearance is.

[ThePoose 37]

Yes, this will be a street/highway driven car with maybe an occasional Friday night drag.  I was going to talk to a grinder I’ve heard good things about in my area who specializes in Fords.  Also was going was going to talk Lunati about some of their roller conversions.  

The factory stock number at 10.7 seemed high (but multiple sources list that) which is why I started all this wondering. I was just poking around Summit and the rebuild kits for .030 say the final compression is about 9.1 with 63cc combustion chambers. I’m going to have to do the math when I get home. Just away for the next 10 days and was trying to keep the project moving. I can’t install anything from a hotel room but I can order things,

[barnett468 418]

you need to have your heads cc'd. chris straub is one of the best cam grinders in the business.

straubtechnologies.com

[Bob & Sue 110]

Instead of trying to explain how & why you need to cc your heads here's a good article to read. Mondello has been building engines since the 60's he's done at least 50 sets for me when I used Keith Black to build our nitro engines.

http://www.hotrod.com/articles/ctrp-0611-cylinder-head-volumes/