Technical Hemi compression

Just wondering what sort of compression people are running in early Hemis on the street? I have a set of 11:1 pistons here, I know I have to measure the deck height, cc the heads etc to get the real figure .Just be interested in what anyone else is running.

 

9.5 to 1

 

Early Hemis can tolerate 10:1 with hi-test. The Hemi combustion chamber is why.


Ago

 

I have the 9.5:1 Ross pistons. Seriously think 10:1 or more would be fine and wish I had gone higher. So if you are unhappy with those 11:1's let me know.

 

The usual comment is...a Hemi can run 1 point higher C/R with the same octane as a wedge engine. So a 11:1 hemi would need the same octane as a 10:1 brand X. Unless your building for racing or planning on alloy heads I'd stop @ 10:1.

 

7:1 with a 671

 

You maybe consider machining the domes of the 11:1 pistons down to 10:1 if the dome is thick enough. Then re balance the pistons.


Ago

 

Sounds good, this will be my first rod without a blower. Usually build my engines around 8:1

 

I was thinking along these lines, once everything is measured and I know for sure what the final compression is.

 

I run 8.5:1 on my 6-71 blown Hemi.


Ago

 

What oversize on the pistons? Cast or forged?
I will suggest that you try first to determine if the dome is thick enough to make any serious cuts. Once you have a hole bored out to the piston size in order to check compression it is hard to back...
It might be easier to sell the 11's and buy a new set of 10's.

 

They are 0.020 over and they are Venolia forged pistons.

 

No mention of which engine... It is possible to use a fat copper head gasket and if you measure enough heads you may find a pair with big chambers, then, with some very careful trimming on the domes you might get the number down, but be careful as forged pieces are not very thick...
Either that or I'll sell you a new set for $700...

 

My guess is it's nowhere near 11 to 1. Is it a stock stroke 3.75 ? Regardless it's easy to fix.
Thicker head gaskets "tulip" the valves or sink the valves deeper in the heads are just a few. We always had trouble getting enough compression not getting rid of it.

 

325 Dodge with 9 to 1. Could've gone a little higher.

 

...bad idea...really screws up the air flow and changes the rocker arm geometry...

 

What he said.