Don Terrill’s Speed-Talk

Larry,

Is there math to make the 240 FPS/300 FPS figures meaningful when testing at less than 28"?

I understand don't exceed flow pressure with pitot pressure.

If I was flowing 100 cfm @ 12" , that would be 153 cfm @ 28". If my pitot was reading 8.5" @ 12", that's 194 fps.

How can I convert that 194 fps @12" to something at 28".

Thanks

I think this would work

V * SQRT(newDep/oldDep)

194 * SQRT(28/12)

194 * SQRT(2.333)

194 * 1.528

296.34

Thanks Mouse,I almost had it.

At the valve seat ( Cam lift is .600" ) With the valve open say .500" what kind of velocities should I be seeing with the Pitot Tube.

Thanks

when it comes to measuring Velocities at the Curtain Area and Valve seat area..i just use or look at the "computer calculated velocities",..
its hard to measure because at that point the Pitot Probe will disturb/affect readings too much .

you could drill thru valve stem and interconnect passages to face/seat areas, but i think the calculated velocities are accurate enough indication to make decisions.

you could Blow thru Intake ports and measure velocity around valve and in Chamber easy with Pitot Probe and use a moveable Cylinder Bore/Liner to see effects easily

Larry,

Those posts are some of the best written, .. clear discussions on
port velocity I have ever read!!!

Have you concidered writing a book???

You ARE the MAN !!

Another question, .. since you are Mr. SS and comp, .. do you remember
a Camaro SS run by John Bray ??? Do you know John ???
I used to share a shop with him & do head for him when he had a shop
in Chesepeake VA behind the Log house.

Have some REALLY funny stories from those days!

Curtis

do you remember
a Camaro SS run by John Bray ???

memory's getting shot

Silver color ?? maybe with Black stripe ??
1968 ? or so ?

maxracesoftware wrote:

do you remember
a Camaro SS run by John Bray ???

memory's getting shot

Silver color ?? maybe with Black stripe ??
1968 ? or so ?

Nope, .. yellow, . maybe had blck stripes. I don't have much of a memory
left either, .. I forget what class the car was, .. 330 ci maybe ???

Just thought as long as you've been doing this you might have known John.
The car was in several magazines & held a bunch of records in it's day.

Know Tommy Sands ?? cylinder head guy, .. I think he worked on
it with John.

Curtis

what is meant by the phrase "pressure recovery" ?

what is meant by the phrase "pressure recovery" ?

My understanding is that Larry was talking about the way in which air pressures "recover" to near or above manifold pressure on the intake stroke.

In the book "Scientific Design of Exhaust and Intake Systems" there are some interesting charts in Chapter 10 of the air pressure profile during the intake stroke on a test engine.

In general there is a sharp drop in cylinder pressure during the first 1/3 of the intake stroke as the air demand of the rapidly accelerating piston out draws the air flow capability of the the opening intake valve. Peak depression pressures of about -3 to -5 psi, occured at around 30-40 deg ATDC. Then a short period at or near that low pressure until about 90 deg ATDC (depending on RPM) then a rapid recovery in air pressure from about 100 - 110 deg ATDC to manifold pressure or slightly above in the period near BDC depending on engine rpm, cam timing etc.

In the higher RPM plots you can also clearly see the effects of acoustic resonance in the intake system as it effects cylinder filling shortly after the period of maximum piston speed.

LOTS of info in that chapter, I have re-read it many times and still am not sure I understand all the subtle details of whats going on.

There is also some interesting notes in that chapter on the effect of higher cylinder pressures due to the exhaust system residual pressure on intake filling. Poor flowing exhaust systems create a reversion effect, delaying effective intake flow, by causing early flow through the intake valve to want to move in the wrong direction. This leads to lower max depression in cylinder pressure at the 30-40 deg ATDC point (higher pumping losses) and more pressure recovery must occur in the remaining time to get back to manifold pressure or above as the intake valve closes.

As I interpret this, the poor exhaust blow down and scavenging of a restrictive exhaust system, puts a greater flow demand on the intake side during the last 90 degrees or so of the intake stroke.

Larry (hotrod)

hi y'all

highly interesting this all. i'm learning more stuff than i should

i plugged in my values for a small 4 banger (80 isch CDI/1330cc) in the calculator on wallace racing to get a limiting port velocity.

according to that i would get 486ft/s@6800rpm. port choke point would be 9100 rpm



1st:
how would that relate to the flow bench numbers posted earlier?

2nd:
In my case the port area is dictated by the narrowest part at the pushrods, after that the port volume/surface goes up about four fold as it is a head with siamesed intake ports (much like an MGB 5 port.. two in three out... brits had a few lagers too many i guess) .

so you have pit of port runner, widening into two ports (not unlike a 4v setup but only valve opens at a time)

velocity at the ssr would be ??? ( much lower?..i dunno)

the port area near the valve would give more like 371 ft/s..



the siamese ports makes it a bit of a guess as to what applies to my situation.

anyone have a clue/idea?

thanx

Joost

Larry,

That just about proves it will you write a book and fill it full of pictures and text. This is a great thread. The next time you find yourself looking at a book shelf take a look for How to Rebuild & Modify Porsche 911 Engines, Wayne R. Dempsey, ISBN 0-7603-1087-4. Wayne is a nice guy, knows a bit about Porsches but what he did for the book was awesome, he went to a PRO engine builder, and lived there with a nice digital camera (no film cost) and shot HUNDREDS of pictures, cherry picked them, picked the builders brain, and then made one of the best darn engine assembly books I have ever read.

The point you have a huge amount of experience, good writing skills, great computer skills, get a high school aged gear head on the year book staff that can shoot some good pictures and make us all a great book on a subject few have ever been written on.

I'll put a deposit in now if that helps.

VWRacer,

Not sure if I know you or not but if you really want to see the port velocity profiles go nuts add that Weber IDA manifold on top and the whole thing goes upside down. Try Jim Mc Farland's thin intake manifold gasket with the four corner pressure probes with a nice radius inlet then a manifold, I bet you get a surprise out of that one.

Regards,

PFM

Has any one used pitot probes in the intake ports in a engine on the dyno? Installed in the heads stratigictally placed and sealed up thru the valve covers to read air speed and other parameters. Brent

Brent,

The big boys have done it all, tied it to major high speed data AQ systems, analyzed the data every way you can imagine. What it does for you and I is not all that much. Listen to the Speed Talk interview with Jim Mc Farland about the days with Edelbrock. There is some interesting info to chew on.

Regards,

PFM

PFM, Thanks for the tip, I've been wanting to buy some more of the CD's. I guess I need more sensory overload! Any others you recomend, It is easier for me to retain what works for some things and not on other things when I hear it several times,to beat in my pea brain Brent

Brent,

I picked up the "special package" last year on air flow, Don did a great job on that deal. I need to look and see what's new myself.

Yes it will put you into overload.

Regards,

PFM