destroyerSS

iTrader: (2)

i have noticed that some head porters use a 4.0 bore when showing there flow #'s and a few use 3.9 bore which i would assume should be the one to judge by. Now here's my ? what type of gains are added when using a 4.0 bore to the head flow?

destroyerSS

iTrader: (2)

Anyone???

Tony Mamo @ AFR

iTrader: (13)

There is no black and white answer to your question. I think the most accurate response would be to tell you that some heads will be hurt far worse than others. It all comes down to how the air happens to exit any given port design, as well as valve size and profile. You might see as little as 4-6 CFM....you might see as much as 20 (if its a large valve head that becomes too shrouded with the smaller bore fixture). I've seen more than a 20 CFM loss going from a 4.125 bore to a 3.900 on a factory ported head I happened to have the chance to evaluate recently. Speaking of which, this head was 239 cc's and still only managed 300 CFM @ .600 lift. It did flow 317 CFM on the larger 4.125 bore and the other intake port I tested almost went 322 but wouldn't hold the number (port went turbulent seconds after you reached the .600 lift mark). Not to long ago I had another "large" factory ported casting drop from 314 CFM's to 293 CFM's when placed on the smaller 3.900 bore (again, this was with a 4.125 "baseline", not a 4").

The net change between a 3.900" and the 4" bore certainly wouldn't be as dramatic as with a 4.125, but once again, some heads will be hurt a little, some heads will be hurt alot. The only real way to really learn and quantify is to have them flowed on both bore fixtures.

Hope this helps you out,
Tony Mamo

destroyerSS

iTrader: (2)

Thanks Tony for the info I guess the point i'm trying to get to is why would'nt all of the head porters on this site go off of a 3.900 bore since this is what the majority of us run and then offer flow #'s for larger bores as well but for the norm they should use our standard bore size. This way people will get a better idea of what company's heads flow vs anyone else heads instead of thinking company's Y's head flows 310 @600 w/ 3.900 bore vs company X whose heads flow 320 @600 w/ 4.125 bore. I think using larger bore size is just kinda !! Again thanks tony and sorry for my rambling on. Thanks Shawn

Ed Curtis

Flow numbers are all relative....

If one shop tests on a 3.900 and so does another shop, and one gets more "numbers".... is this saying the "bigger" numbers are better? Hell NO!!!

Besides the "obvious" differences in methods "and" equipment, there are too many variables to say one cylinder head is "better" than the other...

Port volumes...
300 CFM with 205 vs 235 cc port volume... Which would you want?

Velocity....
Get some velocity probe data too!

Quality of the airflow...
Can you say turbulence???

Application....
Supercharged, turbo or naturally aspirated???

Cam profile...
A bad cam design will kill a good head package and vice versa....

There are plenty of lower "number" CFM cylinder heads that OUTPERFORM the big number pieces on the racetrack... I see it all the time in the Ford heads up racing series...

Ed

destroyerSS

iTrader: (2)

EDC, I should of added port volume to my orignal ? Good point!! As far as the 3.900 bore i think they should use this as a standard. Then people can gather results from flow #'s & port volume size's from the head porter to use as a starting point then check for track result's and make there choice. If company's didn't think flow #'s were not inportant why would some chose to flow check with a larger bore? maybe for higher #'s Not trying to argue! but i just like when you can choose a product with equal testing