TraviZ

iTrader: (2)

How crucial is head flow when accounting for boost, I currently have a pair of canfield heads that are rated at 260cfm at 550 lift, while the new afr heads are hitting 280cfm at the same lift. Would I seee a big impact of difference? Also, runner volume, like 195cc versus 210cc, on a 383, am I gonna lose any low end due to the bigger runner volume running a supercharger or will i still make enough torque to burn tires lol.

Gunner823

iTrader: (2)

I seriously doubt you will lose any low-end torque with bigger heads on a boosted application, because the forced induction is going to increase port velocity WAY over what you would have N/A, even at low RPM. You'll see lower referenced boost numbers but you aren't losing boost, it's just being used more efficiently. Boost is pretty much the measure of restriction through your intake/heads in a forced induction application; all else being equal, when you remove restrictions (better intake, exhaust, heads) your referenced boost numbers are going to drop and your power output is going to increase.

As for "burning tires" if you can stomp on the gas and light them up now, adding a supercharger to the mix-even with larger port heads- is going to make it much easier to do.

TraviZ

iTrader: (2)

how about the rated cfm difference, is that going to effect much?

3.8TransAM

A real rough way is what he is saying is:

If u make 500hp w/ heads that flow 260 cfm at 18psi of boost.

U will make 500hp w/ heads that flow 280 cfm at 13psi of boost.

U would see less boost but make the same power do to eliminating further intake restrictions.

There is more to it than that, but that is the most basic explanation.

later
Jeremy

TraviZ

iTrader: (2)

So I wouldn't see any horsepower gains unless I upped the boost for the better heads?

Ol'Blue

iTrader: (1)

flow at .550 really isnt all that important either.....the lift of a cam spends most of it's time closer to the .400 range. there are considerable gains over the 2 like the man above stated, you will make the same power with lower boost numbers than the other heads with higher boost numbers before...run the same 18 psi on heads that flow more and you come up with your gains....another thing, if you already own canfields just have some port work done and outflow the AFR's while saving some cash.

Gunner823

iTrader: (2)

You've got it backwards.

You'll see MORE power and you will see LESS referenced boost (meaning what your boost gauge reads). You have to think of boost as a measurement, not as a constant. 18psi on one setup does not equal 18psi on another. By adding larger port heads that flow better than what you have your power will increase and your boost will decrease because the restriction is reduced, and more of the boost is being converted into power rather than adding pressure in the manifold.

At that point, if you want to increase your boost back up to your previous level you would have to change to a smaller blower pulley.

327???

also keep in mind that lower boost equals lower intake temperatures.

TraviZ

iTrader: (2)

Does this also apply to two different heads, SAME port size, but one flows better then another?

Gunner823

iTrader: (2)

Yes.

Anything you do to increase flow/decrease restriction, lowers referenced boost and adds power. Think of it like this. If it would help you without the blower, it's going to help even more with a blower. If the heads seem a bit too big without a blower, adding the blower will make up for the lost low-end velocity.

83 Crossfire TA

iTrader: (2)

Well, you make a point but not a very realistic one… 260cfm on the intake is enough to feed 530-540 hp NA assuming that everything else is well matched, so if you’re only seeing 500 with 18psi boost something is _very_ wrong.

Second, it will only make a significant difference if the rest of the combination is really well matched and you’re turning enough rpm to see the difference in flow

Lets say you’re only turning 6000rpm on a 350 and put these 2 sets of heads on the engine… at 90% VE they’ll only be pulling about 200cfm through the ports anyway so real world, the one that is better matched to the rest of the engine will make slightly more power, add boost that will still be the case, and it is possible that the 280cfm head makes exactly the same or even less power depending on what else is going on.