Compression ratio

[gregaba]

I need someone smarter then me to figure my CR.
I just spent all morning ccing my heads and was a little surprised at the results.
The heads cced out at 68-69 cc across both of them.
Edelbrock 427 Aluminum heads with 220-173 valves.Heads have been worked but not milled.
Here is what I have 428- Bore is 4.164.
Pistons are flat top race tech sticking out of the block 0.005.
Stock stroke.
Block was square decked and blueprinted.
Crank assemble balanced to 1 gram.
I have forgotten how much of a pain it is to cc heads and it took me a lot longer then I remember having to do in the past.
Thanks
Greg

[blykins]

I need someone smarter then me to figure my CR.
I just spent all morning ccing my heads and was a little surprised at the results.
The heads cced out at 68-69 cc across both of them.
Edelbrock 427 Aluminum heads with 220-173 valves.Heads have been worked but not milled.
Here is what I have 428- Bore is 4.164.
Pistons are flat top race tech sticking out of the block 0.005.
Stock stroke.
Block was square decked and blueprinted.
Crank assemble balanced to 1 gram.
I have forgotten how much of a pain it is to cc heads and it took me a lot longer then I remember having to do in the past.
Thanks
Greg

What is the piston volume?  What head gasket?

[JimNolan]

A person needs more information to figure CR. From what you describe I'd say you'll need rocket fuel for it to run. The Static CR won't tell you much as the Cam will decide the DCR you run. The cam will decide when your valves close to enable compression in the cylinder. DCR stays with the engine through any rpm and is the most valuable tool to decide useable cubic inch of motor and what octane gas you'll have to run to keep it from pinging.

[427mach1]

Brent, the only pistons I see on their website are the Autotec version with -11.0 cc dome volume.  Looks like it will be around 11.0:1 static CR depending on gasket thickness.

[blykins]

Brent, the only pistons I see on their website are the Autotec version with -11.0 cc dome volume.  Looks like it will be around 11.0:1 static CR depending on gasket thickness.

I wouldn't assume that he's got shelf pistons.  A lot of guys run custom slugs.  Also, an 11cc dish isn't considered a flat top.  A flat top is usually around 3-6cc.

[gregaba]

Sorry for not including all info.
The pistons are Auto Tec part number 1021059 custom ordered from Brent.
They have a 5 cc dome volume.
Gaskets are Cometic 0.40 thick.
As of now I will be running compressed natural gas for fuel.
Cam will be the Ford D cam for a while and then a custom solid roller that I will let Brent decide what I need.
Greg
 

[blykins]

Sorry for not including all info.
The pistons are Auto Tec part number 1021059 custom ordered from Brent.
They have a 5 cc dome volume.
Gaskets are Cometic 0.40 thick.
As of now I will be running compressed natural gas for fuel.
Cam will be the Ford D cam for a while and then a custom solid roller that I will let Brent decide what I need.
Greg

A 4.164" bore, 3.980" stroke, 68cc chamber, .040" x 4.400" head gasket, with a 5cc dome piston sticking .005" up out of the hole would yield an approximate compression ratio of 13.36:1. 

You said flat top in the post before, so if it's a 5cc flat top instead of a dome, compression ratio would be approximately 11.85:1.

[gregaba]

Its a flat top, no dome.
11.85 I can live with, I was afraid it would be a little higher but the amount of driving time on the car will not be much.
13.36 would worry me a little but is doable with the CNG for fuel with an octane of 140.
It will just be a fun car and not a everyday driver.
I  just don't want to be disappointed when I get on it.
Thanks for taking the time to figure this out for me.
Greg

[frnkeore]

I get, 11.7, I don't think Brent added the crevice volume.

[blykins]

I said “approximate” because you can’t add crevice volume when it’s not been measured.  It varies from piston to piston.

I also based the calculation on 68cc and he said the chambers varied a little bit.  That’s also why I said “approximate”.

[Jb427]

don't think it has been mentioned here yet but have you looked at what your dynamic compression ratio will be?   

[Gregwill16]

Yes ^^ have someone that knows, calculate your DCR. I wasted a long time on a similar combination only to have Brent line me out in no time by calculating my DCR. In Brent's words it was a "no go" and I have moved on to changing the pistons.

[frnkeore]

There is always going to be crevice volume, no matter what. On his 4.164 bore, it basically will not be less than .81 (.200) nor more than 1.22 (.300), or no more variance than .41cc.

When I do DCR, if I don't want to calculate the exact crevice volume, I just add 1cc to the chamber volume, for FE bores of 4.08 or more.

[blykins]

Of course there is crevice volume, Frank, I’m not sure why we are having menial conversations considering I have caveats up above.  Crevice volumes can be all over the place and I didn’t add it for that reason, even if racetec averages about a cc.  A .1 change in compression ratio has absolutely no bearing on Greg’s questions.  If this were a super stock motor, I’d have a difference answer. 

[frnkeore]

I always add it in, it is there in all applications.

You could have also said that the CR was approx 11.7 and been a little closer to reality. As I said, it doesn't very much on a FE bore, from 4.08 - 4.26