Freestyle Porting lowers compression?

[swapmeet]

Try to keep this short and sweet.

Have 61x, hammer ported by groupK.

Installed ProTec C2 head w/37cc domes.
They claim I should see 185psi.
I get 155&160.

Protec claims it's because of the porting.

Is this true? Can it change it that much?

 

[Matt_E]

Depends on Harry's porting specs, but yes - absolutely possible.
Raised exhaust ports lower compression.

 

[Polish jet pilot]

I did not rise my windows and I got a drop in compression...so yes, it does happen...

 

[Dustin Mustangs]

According to ada, you need 35cc domes to get 185 on a 701. 37 gets you 170. So that might be some of your difference. I would be more worried about the 5psi difference between holes tho.

 

[Polish jet pilot]

also the larger the bore the lower compression, e.g I have 177 psi on 82.5mm bore with 35cc domes and I am going to install 33cc domes in that engine - should get about 190psi I guess..

 

[Matt_E]

also the larger the bore the lower compression, e.g I have 177 psi on 82.5mm bore with 35cc domes and I am going to install 33cc domes in that engine - should get about 190psi I guess..

You have that backwards. All things equal, a larger bore will have higher compression.

 

[tor*p*do]

my 61x w/35cc ada domes in a sharkfab head is spot on 185
if I dropped back to 37cc domes I would expect 170
Pretty radical freestyle ported cylinder
That 5 psi difference may be random. due a series of 10 comp tests each cyl
write them down and average to see if they are really that different

 

[Freakrider]

You have that backwards. All things equal, a larger bore will have higher compression.

X2 Matt is right...35cc domes on my 83mm bore was 205/205 with a newer Craftman's tester...

 

[Waternut]

5 psi different is nothing. Once you start seeing more than about 10% difference, then you should start considering a new top end or rings. Even then, I'd probably try to finish the season before tearing into it.

 

[RoTAxGreNade]

86mm bore w/ 33cc ada domes = 230# after break in (ADA specs 215# and god help the one who runs that .015 squish as shipped ! lol)
Just to give an idea how much bore will affect things

 

[RoTAxGreNade]

Snap on gauge with new schraeders too...