Yeap More stroke makes more power than bore size

There are too many variables to make a blanket statement that a smaller head is as good as/better than a larger head. That 305 CC head may work great on a 540 with only 5 psi of boost. When you crank the boost up to 10 psi, the 325 head will make more power. I'm just picking numbers here for the sake of the example. You have to take everything into consideration when choosing the correct heads. This includes CI, intended rpm, boosted or NA, amount of boost, intercooler or not, etc.

I know this doesn't really answer your questions MT, but there are too many things to consider to make a blanket statement about which head will work better, the larger or the smaller one.
Eddie

No

and

No. - Separate topic that can be discussed in another thread..


We can get as technical with this as you want, but for now, like SB, I'll keep it simple. Joe, a blown motor is much more tolerant of an oversized port because of the SC's ability to force air into the engine. Just like it is more tolerant of a bad port, and even bad exhaust. However, just because it works doesn't mean it's optimized. The correct csa for your cubic inch and rpm will not change whether you run NA, or boosted. The port however does have to be good, ie localized air speeds. Some of the smaller high velocity ports have localized airspeeds (especially over the ssr) that are borderline, or a bit too high. On an engine running decent boost, I've found these ports get into trouble faster then when the same engine was NA. In a case like this, a larger slightly lazier port could make more peak power, and be easier for someone who doesn't have the ability to fix the smaller port.

On a 540 peaking at 6000rpm a good 315 head works great. If you put on a killer 357 head you will not make more power, and will likely lose average power, especially mid range.
I don't mind running a slightly larger then optimal NA head when you get over 30#'s of boost because I feel the additional fuel takes up a bit of the port volume, same with running Meth, or E85. I don't have the science to back that up, but it makes sense in my head. lol.

Forget about all that port size stuff Joe, on a BBC , combustion chamber improvements is where its at.

NA is under pressure too because Atmospheric pressure is positive pressure.

We just reference Atmospheric as 0, less than atmospheric as vacuum, and more than Atmospheric as PSI.

So, with boost, we just have more psi than atmospheric.

Soooo...................

(above is true - maybe this will change that thinking ?)

Good info guys. All valid points. Thanks for participating!

I always assumed, that a compressor helps move air thru the intake port. We've seen some impressive numbers from those small oval port modern heads, on some engines. I always assumed, that the small oval port design, was meant to improve velocity, to aid in cylinder filling when relying on atmospheric pressure. I've also always assumed, that running a small oval port, would not be the way to go, on a forced induction big block.

Ive seen alot of talk about the small oval like the AFR 265 being a great choice on the 454/496 combinations lately, in N/A form. So, is it safe to assume, based on what you guys are saying, that the same package, would also outperform a larger port on that engine, with a blower? I just cant wrap my head around the idea, that the ideal port size N/A, would translate to the ideal port size for blown. I'd have a hard time believing that a 496 with 265 ovals, would outperform, a 496, with say some 305 rectangles, with say 6-8lbs of boost.

I've got an old vintage book here, written by Jim Davis, back when he was at B&M, and they were in development with their supercharger systems.. Long before all the cool cylinder heads were available like now. They did alot of back to back testing on some stuff. They took a typical 8:1 454 chevy engine, with GM oval port heads (2.06/1.72 valves), installed a 420 Mega Blower, with around 10lbs of boost, and a blower cam. The engine made around 626HP at 6000RPM. They then removed the GM oval port heads, and installed stock GM Rectangle port heads (2.19/1.88 exhaust valves), with no other changes. The engine made 718HP at 6000RPM.

I know this is dinosaur stuff, but I thought it had some interesting information.

Yes, that is dinosaur stuff.

Just like
I $400 rebuilt a 7.8:1 flat top 454 with stock valve 781's. Ran 13.60's in that car. Mild cam...very mild. Installed larger valves in those same heads, no other changes. 12.90's and a bunch of mph.

Changed cam, still relatively mild. Ran 12.40's.

So.............what does that tell you ?

Oh man.. it's just like the day you fibd out your merc 575 BASE engines are 9.5 :1 and not the stocker 7.5:1 engine you can hold wide open all day

we must remember a few things,the above mentioned is not making boost at 3500 because the intake cycle is creating more vaccum than the supercharger can make boost because the throttle blades are barely open and restricting air flow.if you now open the blades all the way the restriction is removed and we make boost.no one has mentioned exhaust ports yet,dont forget we need to get the spent exhaust out of the cylinder before we can refill it with the next intake cycle.

i have to disagree,the pressure [or boost] is created by the restriction of flow.take a supercharged bbc with a set of good flowing heads that makes 10 lbs of boost,now put a stock chevy oval port head on it with no other changes and it will make more boost because the head creates more restriction.the supercharged engine makes power because more air/fuel enters the cylinder.