Tubulence and Valve Lift

[RGL]

Just a question , and I know this is a band aid at best, and the real answer is to fix the heads etc. but......

A head that goes crazy at say 0.675 lift , ( 40 cfm flow loss) that currently runs a camshaft at .900 lift ----- Could it be helped by decreasing valve lift , and possibly increasing duration till the heads can be fixed ( if they are able to be fixed, may be a better way to put it) , the head hangs on to 0.775 lift with the manifold on it ( single plane cast), but still takes a big dump afterwards ) any ideas , suggestions ??? thanks for any help.

[cboggs]

In my experience a head that "takes a dump" like yours is
very bad for power and can't be helped with cam.

Even if the cam lift is much less then the point where the head
goes turbulent on the flow bench, .. in dynamic running conditions
I'm convinced the turbulence happens much sooner.

I'd get the head fixed. And it's just a guess without seeing it, ..
but if the manifold helps it I'd bet the short turn is not correct.

What type of head is it???

Curtis

[Darin Morgan]

In my experience a head that "takes a dump" like yours is
very bad for power and can't be helped with cam.

Even if the cam lift is much less then the point where the head
goes turbulent on the flow bench, .. in dynamic running conditions
I'm convinced the turbulence happens much sooner.

I'd get the head fixed. And it's just a guess without seeing it, ..
but if the manifold helps it I'd bet the short turn is not correct.

What type of head is it???

Curtis

ditto

[Bos's5.0]

Is the short turn THE reason a head may become turbulent?
What is it about the short turn that makes the air decide to go all weird and turbulent?

[Darin Morgan]

Is the short turn THE reason a head may become turbulent?
What is it about the short turn that makes the air decide to go all weird and turbulent?

The SSR is the point at which the air has to turn the most degrees in the shortest amount of time and it has to do so at a fairly high rate of speed. If the speed is to fast the boundary layer shears and the air can no longer follow the surface curvature. The idea is to have an SSR that is not to high, not to steep and has a gradual enough radius so it turns the air efficiently at all lifts and over a range of velocity. Your SSR or seat contouring is ( highly scientific term here) GOOFED UP. When the port goes backwards its a 90% chance the SSR and/or seat contouring is to blame. If you ask us to explain seat contouring I am going to have to bow out because that's a book all by itself. I am sure that my colleges here have as much if not more flow bench time as I do ( 8000hours+) and we can tell you that the SSR area in conjunction with the seat valve will give you more headaches than any other area of the port.

[cboggs]

SSR area in conjunction with the seat valve will give you more headaches than any other area of the port.

Ditto, ..

Curtis

[Boport]

Your SSR shape needs some work...

[bill jones]

-it would really be nice if we knew what the head was you guys are talking about.
-Some heads may be easy to explain the problem if we know what it is but not everybody is always talking about a small block or Big block Chevy, and even then there are about 100 variations of each that do different tricks.
-Even the valve sizes can affect the stall if the valve is right up against the shroud.

[RGL]

First let me say thanks for taking your time to reply guys , what your saying is what I had been thinking, but you know how the questions keep popping up in your mind , what if....

the head in question is a SBF Edelbrock Victor 7721 , 2.18 valve ( 32 deg backcut .030" wide , .075" seat) , 1.96 Bowl Diameter , 50 deg seat . Yes, the manifold helps it to hang on longer, though the manifold does not pick up the head -- about 10 cfm loss across the board. The head was also extremely sensitive to the backcut , it fell off @ 0.600" lift with too wide a cut till the small cut described above helped it out a little.

The chamber is cut for 4.140" bore , the head was flowed on a 4.185 fixture.

The motor is down bigtime on power, everything else seems mechanically correct , so i am blaming the heads , track times back up about only 690-700 HP

thanks again guys, i will be jerking the heads off this week.

[cboggs]

I just went through this with another set of Edelbrock heads, ..
really awfull short turn.

had me scratching my head, .. Darin & Larry helped me out, ..
it was, .. .. you guessed it, .. the short turn.
I'm not so sure that head will like the 50º seat, .. ???

You may want to make a rubber port mold to get a "outside"
view of the port.
It helps a bunch and if you get stuck it's easy to show others the port.

Curtis

[Bos's5.0]

are you able to tell any of us what it took to fix that edelbrock head? I've got some issues with a performer head.

[bill jones]

-BOS--The Edelbrock street head has a terrible short turn to start with because the entire port floor is so low there is only so much you can do with it, especially if you want to maintain any decent low lift numbers.
-When you lay the short turn radius back on these low floor ports to get the upper lift numbers you usually kill off the low and midlift flow.
-What is the flow numbers of the street head that are bothering you?
--------------------------------
-RGL's head must be one of the Edebrocks racing heads but I haven't seen one so I can't comment but I'd say the port isn't large enough or wide enough at the crest of the floor, or the shape from the crest isn't right.
-When the airflow is too fast it just can't make that corner, has no grip to the metal, sort of like a nascar car that has "push" in the corner.
-If you slow the air (or the car) it can make the turn a whole lot easier.
--------------------------------------------
-It might be something as simple on both sets of of these Ford heads as the port floor being too slick and shiney.
-I have found that quite often there needs to be some texture in the drop from the crest of the floor nearly to the seat and the demarcations of the seat angle and at least one bowl angle needs to have a nice defined demarcation.
----------------------------------------
-When I get a problem like you guys are talking about, especially when I've never seen a particular head, I flow test every port and if I get lucky I'll find one port that does something even a little bit right so that I can make templates off that floor contour and compare it back to the worst port.
----------------------------------------
-You guys think you have problems with your SB Ford's, try using a set of those puny little Edelbrock heads like what BOS has, on a 408 Ford with a turbo charger and try to figure out how to make it run just to 5800rpm with respectability for a daily street driven car.

[cboggs]

-BOS--The Edelbrock street head has a terrible short turn to start with because the entire port floor is so low there is only so much you can do with it, especially if you want to maintain any decent low lift numbers.
-When you lay the short turn radius back on these low floor ports to get the upper lift numbers you usually kill off the low and midlift flow.

Bill, .. I've found with a TON of time on the Eddy's that flow through
the entire range can go up with proper short turn work, .. it's very
sensitive tho'.

On the last set of Eddy's I did, .. different head but the short turn design
is the same, . .. The short turn had too sharp of a "crest" or apex.
I made sure the short turn coming off the bottom angle on the seat
was less then 15º from the valve centerline, .. 12º I think.
Then from my port mold I could see the radius of the the turn was too
sharp and where I had to work in the port to fix it.

I left a small area off the bottom seat angle in the center of
the short turn flat for a very short area, .. .125" maybe, .. then
rolled the top of the short turn or "crest" till I had a nice radius from
the floor to the flat at the seat.

This thing drove me nuts, .. and looking at the short turn in the head
and using the old "finger test" I couldn't get it right.
It wasn't till I made a mold of the port, .. and even had Darin & Larry
look at it that it became clear what the problem was.

Just looking in the port it's hard to see where the real "crest" of the short
turn is and where to work the radius. This port mold shows the "before"
short turn and the "apex" that got reworked for a better radius.



Hope this helps,

Curtis

[Darin Morgan]

--When I get a problem like you guys are talking about, especially when I've never seen a particular head, I flow test every port and if I get lucky I'll find one port that does something even a little bit right so that I can make templates off that floor contour and compare it back to the worst port.

Thats some good advice. Thanks Bill.

On anouther note,
Do any of you guys know about Logitec mold compound? Its used in dentistry to make mouth molds and sets in five minutes. The down side is that it only lasts an hour then breaks down and shrinks to nothing over night. Its just for quick molds. I will get some info on the distributor and let you know. Its cool stuff especially when your stuck on a SSR or a prototype port and need some visual help. I couldn't live without the stuff.

[Bos's5.0]

-. This port mold shows the "before"
short turn and the "apex" that got reworked for a better radius.



Hope this helps,

Curtis

Would you happen to have an AFTER shot of the SSR?