What makes a good head.

CptanPanic2 · Post by **CptanPanic2** » Mon Jul 23, 2001 8:47 pm

Talking about a bare head, what makes a good performing head. Is it CFMs, or a combination of something else.
Thanks,
brian

Matt Harris · Post by **Matt Harris** » Tue Jul 24, 2001 3:27 pm

See replies on CLF

CptanPanic2 · Post by **CptanPanic2** » Wed Jul 25, 2001 9:27 pm

For those that missed the response, basically it is not the volume of air that matters, it is the speed of air. So basically the flow CFMs divided by the volume of the intake ports. Since this is what matters, it is too bad that no one does this calculation.
bri

JohnConnolly · Post by **JohnConnolly** » Wed Jul 25, 2001 9:50 pm

close, but not quite. The volume of the ports doesn't matter; this rumor is a carryover to the "american" way of describing port sizes on V-8s, by volume. In reality what matters is the cross-sectional area.

Think about it; if port volume was huge, how does that affect how much air is flowing thru one POINT in the port? It doesn't.

Volume/area = velocity, and that matters a lot. not everything, but a lot. volume is important too, but a little gain in volume coupled with a lot of velocity loss is a bad trade off.

There is an article coming up in VWT that is a head-shootout of sorts. When you look at the flow #s, be sure to also look at the head ports themselves. There were only two "honest" heads in the shootout, the rest were ringers built soley to win the contest, and this is VERY CLEAR looking at the pictures of the ports. It's important to note that you could never buy these "ringer" heads since there was too much time involved in them (to win the contest) for them to be practical to produce if someone actually wanted to buy a set.

Look at the 2 honest sets of heads and look at the flow #s, and decide for yourself where the value and honesty is.

Trebor · Post by **Trebor** » Tue Jul 31, 2001 7:13 pm

John, could you explain in more detail how to calculate the velocity of a given cross section.

Robert

JohnConnolly · Post by **JohnConnolly** » Tue Jul 31, 2001 10:32 pm

((Volume/time)/Area)

basically, more displacement OR RPMs gives more velocity.

A smaller port gives more velocity, until the flow drops off a certain amount.

It's a balancing act, not enough velocity and you'll have a pig, too much velocity MIGHT indicate too small a port.

John
Aircooled.Net Inc.

Trebor · Post by **Trebor** » Wed Aug 01, 2001 8:55 am

So if the compression is raised the velocity should go up right? I have a software program that I got online the gives the optimum rpm for a given port area. How does it come up with that number?

Robert

Spyke · Post by **Spyke** » Wed Aug 01, 2001 9:49 am

Assuming you don't change the displacement (i.e. stroke or bore), raising the compression ratio will not raise the intake velocity.

The program can calculate an optimum rpm by assuming an optimum intake velocity. Look at the math. If you take the flow value (cubic feet/minute) and divide it by the port area (square feet) you end up with feet/minute, which is velocity. If your port area is known, and you've assumed an optimum intake velocity, it's pretty simple to find the rpm that is "optimum".

-Craig
www.teamyikes.com

[This message has been edited by Spyke (edited 08-01-2001).]

Trebor · Post by **Trebor** » Wed Aug 01, 2001 10:30 am

does anyone have charts that show actual flow rates based on port conditions? (eg. flow is actually 75 to 80% if manifolds and ports are matched). How do I know what a set of 44 idf's flow with 36mm venturies?

Robert

Tony Z · Post by **Tony Z** » Sun Aug 05, 2001 9:27 am

What makes good head? Well, a blonde would be a good place to start!!! LOL