Maximum Port Velocity

[kennedyrd]

I am reading a great book "Engine Airflow" by Harold Bettes and there is a mention on pg 67 (in case any of you have noticed it too or the author is online) that 380 -400 fps on the inlet port is too fast measured at 38 in. H2O. I wondered if this was a typo and supposed to read 28 in H2O or not?

Clarification from someone who knows would be appreciated.

[blaktopr]

I'm thinking that it's a typo, an oversight. 320-350 is like the limit at 28". Even dynamically that is pushing close to the 700"s fps. Sounds like a cool book. Harold's a cool guy. He's registered here??

[kennedyrd]

Wow that was quick! I knew one of you guys would know. Sorry don't know if he was registered here or not, but he mentions forums so worded the post in case he was on here. I am amazed you know him, but I guess it is a close knit community in the head business.

He constantly mentions he thinks 28 inches should be the industry standard so suspected that is what he meant.

Thanks for your help.

[DaveMcLain]

It used to be that if you had a tech question about your Superflow bench and either called or e-mailed then Harold was the guy who would give you an answer. He's a really cool guy.

[jfholm]

As Chris mentioned that actual port limiting velocity of the intake port in a running engine is about 690 feet per second. When testing on a flow bench at 28" H2O that correlates to 350 fps. There have been many spread sheets and programs uploaded on this forum to help calculate those velocities.

I have attached a spread sheet I did a while back that will help you design your ports for your application. Heads have to match what you are going to be doing. Look at the bottom as there are 4 tabs for 4 different functions to help.

Thanks,
John

[kennedyrd]

Thanks very much John.
I notice you are one of the guys giving a good testimonial about the Flow Quik unit. That is what I am using, but only just got it going correctly and have got some Rover heads to play with so fun times lie ahead!

[kennedyrd]

It used to be that if you had a tech question about your Superflow bench and either called or e-mailed then Harold was the guy who would give you an answer. He's a really cool guy.

He is clearly a clever guy, both from his book and the fact he held this position. Wow!

[jfholm]

I am reading a great book "Engine Airflow" by Harold Bettes and there is a mention on pg 67 (in case any of you have noticed it too or the author is online) that 380 -400 fps on the inlet port is too fast measured at 38 in. H2O. I wondered if this was a typo and supposed to read 28 in H2O or not?

Clarification from someone who knows would be appreciated.

I downloaded the book to my Nook and I have it about 1/2 read. There is a lot of good information in there and lots of math! The discussion on port velocity is very interesting. Harold mentions the mach number of .55 mach is the area we are aiming for. His formula includes local temp and barometer if I remember correctly. I know that the speed of sound is really dependent on temperature among other things. So can we really create a port with ideal port velocity?

Will these things affect port velocity to the point that velocity will change?
1. Altitude (does that really affect velocity?)

2. Temperature

3. RPM (this is a given)

4. Barometric pressure (does this have any effect?)

any other things that can affect velocity?

John

[blaktopr]

John, I think for the kind of competition we will see, we can make a pretty competitive port. I mean, we won't be doing any PS heads for anyone in the near future.

Match the port size and shape to piston demand and .55 mach.

Work the port for correct cfm flow as close to piston demand as you can get.

Work the tract to have a good avg speed and cross section for the combo.

And as for the baro and altitude. I would figure that speeds change a little due to the different pressure differentials. Just a thought.

[jfholm]

Chris,
That is just about what I wanted to hear. Sometimes we get too focused on making the perfect port. That is what I was kind of hinting at and as you say we need to just do the best we can. You are right about one thing, I do not have the time or money to do Pro-Stock.
I feel the same we just have to do the best with what we have. It still makes you stop to think and I hope that gives us the ability to do better.

Thanks Chris,
John