Questions about ports that stall, and cam lift.

[gregsdart]

I have a set of 572-13 Indy mopar bb heads that were just flowed with back cut, and not back cut intake valves. The back cut helps midrange, but flow goes flat at .700 lift with a back cut, keeps on going without a back cut. My cam has a lot of duration at 284/296 at .050, 205/213 at .200. NET lift is .848 intake. The heads were done with no back cut on the intakes. My question is, with a port going flat at .700 or less, does the headflow have turbulance, and does that cause poor flow as the valve closes at speed? The numbers from the heads i used to run were flowed on the same bench and i included them for comparing to the new, bigger heads.
These heads are going on a 580 cu in. 15/1 compression shortblock.

[steve cowan]

As a starting point I would look at how much cross section area and cfm you require for your cubes and the rpm you want at peak demand.
EG - CID X RPM X 0.0009785
DIVIDED BY # OF CYLINDERS.
EG - 580ci X 6000rpm X 0.0009785 ( constant) divided by 8 cylinders = 425 cfm at peak piston speed @ aprox 76* ATDC.

Minimum cross sectional area
Bore X bore X stroke X rpm X 0.00353 (constant) = ?? Divide by
690 (mach index) = Minimum cross sectional area.
These are just starting points
Hope it helps.
I would talk to PRH on this forum as well because he done heaps of mopar work

[mag2555]

There are two big reasons for using a valve with a back cut and both are very important!

1) a huge part of getting the VE up well above 100% comes from the overlap phase of the Cam.

This is when a tuned Exh system with a merge collector produces a large depression/ vacuum in the cylinder, and many times this depression is grater then what was used to flow test the heads in the first place, so the vacuum of the out gong Exh is sucking in fresh Intake charge.

At this point a with a racing duration Cam the Intake valve during overlap is open .200" or more.

Now look at the difference in flow numbers you have down there at .200", now which valve would you use to get a early start at filling up your cylinders?

This VE gain far more then offsets the high lift flow loss seen between the two valve set ups!

The reason that the back cut picks up air flow is because high speed air will not follow around anything tighter then a 15 degree angle with out just sheering away, and the highest air speed is always where the valve bowl turns into the chamber.

Now with the back cut the port side of your valve is 12 degree, 30 degree and then the main seat of 45.
It's now has a shape far more friendly to turning air.

Also any Intake flow testing done for a race motor needs to be conducted with the Intake bolted on.



A slight reduction of peak flow levels produced by the Manifold might be just enough to slow down the air speed and not see the high lift back up that your seeing.

2) the back cut also greatly helps to re-atomize blobs of wet flow fuel that would otherwise not burn, but also be a starting point for detonation when it lights off late, not to mention the added upper cylinder ware from oil diluting.

Long story shot keep the back cut and don't fret about the high lift flow loss.

[ClassAct]

Flow the head backwards with and without the back cut and see what you get. I’m betting the back cut is flowing too much in reverse. That’s a power killer.

[CamKing]

Since the flow levels out, and does not go backwards, there's no turbulence. Running .900" lift won't hurt the power. It will help, because you are spending more time at max flow.

[skinny z]

Since the flow levels out, and does not go backwards, there's no turbulence.

Having said that then does this mean that if flow decreases with added lift then the port is turbulent?
And other than revisiting the intake porting job, what does that mean for maximum lift? Keep it at peak flow and no further? Compensate with added duration?

[CamKing]

Since the flow levels out, and does not go backwards, there's no turbulence.

Having said that then does this mean that if flow decreases with added lift then the port is turbulent?
And other than revisiting the intake porting job, what does that mean for maximum lift? Keep it at peak flow and no further? Compensate with added duration?

Depends on how much the flow drops off.
If I port gets turbulent beyond a given lift(you can normally hear it on the flow bench), and the flow drops off a bunch, you don't want to go above that point, because for the amount of time you spend above where the port gets turbulent, it will take a give amount of time for the port to recover after you get back below that point. In cases like that, you want to keep the lift below the point it goes turbulent, and if the engine needs more cylinder fill you increase the area without increasing the lift.

[skinny z]

Since the flow levels out, and does not go backwards, there's no turbulence.

Having said that then does this mean that if flow decreases with added lift then the port is turbulent?
And other than revisiting the intake porting job, what does that mean for maximum lift? Keep it at peak flow and no further? Compensate with added duration?

Depends on how much the flow drops off.
If I port gets turbulent beyond a given lift(you can normally hear it on the flow bench), and the flow drops off a bunch, you don't want to go above that point, because for the amount of time you spend above where the port gets turbulent, it will take a give amount of time for the port to recover after you get back below that point. In cases like that, you want to keep the lift below the point it goes turbulent, and if the engine needs more cylinder fill you increase the area without increasing the lift.

I'm seeing about a 3% drop from .500" to .600" (254 to 246) .
It does rebound approaching .700" though but that's far and beyond the .575" target suggested.
Might not be worth thinking about.
Thanks.

[CamKing]

I'm seeing about a 3% drop from .500" to .600" (254 to 246) .
It does rebound approaching .700" though but that's far and beyond the .575" target suggested.
Might not be worth thinking about.
Thanks.

Is this a stock GM head by chance?
This is something we run into with classes that have to run stock, unported heads. Many of the Chevy heads would back up after .500". The more successful engine builders would work on the valve job, until they moved the point the port flow started decreasing, to above ,550" lift.

[skinny z]

I'm seeing about a 3% drop from .500" to .600" (254 to 246) .
It does rebound approaching .700" though but that's far and beyond the .575" target suggested.
Might not be worth thinking about.
Thanks.

Is this a stock GM head by chance?
This is something we run into with classes that have to run stock, unported heads. Many of the Chevy heads would back up after .500". The more successful engine builders would work on the valve job, until they moved the point the port flow started decreasing, to above ,550" lift.

Not stock but RHS' version of a Vortec replacement. 2.02/1.60, 170cc. Since bought new there's been some bowl work and cleaned up the exhaust but nothing major. 5 angle valve job all done by a top end shop.
I had one cylinder flowed (4.03" fixture) and am working with those numbers.
There's obviously more that could be done but my investment in these heads is about at an end. A little chamber work, measure everything up and then order a cam.

It's the subject of a back cut on the intake that caught my attention (and thanks to gregsdart for the loan of the thread). Something I had done when I was running Vortecs although I understand that's more to do with what happens below .200" rather than where this port stalls. That said, I'm pretty sure my new intake valves are as not back cut. Something I'll have to address.

Thanks again.

[Stan Weiss]

Having said that then does this mean that if flow decreases with added lift then the port is turbulent?
And other than revisiting the intake porting job, what does that mean for maximum lift? Keep it at peak flow and no further? Compensate with added duration?

Depends on how much the flow drops off.
If I port gets turbulent beyond a given lift(you can normally hear it on the flow bench), and the flow drops off a bunch, you don't want to go above that point, because for the amount of time you spend above where the port gets turbulent, it will take a give amount of time for the port to recover after you get back below that point. In cases like that, you want to keep the lift below the point it goes turbulent, and if the engine needs more cylinder fill you increase the area without increasing the lift.

I'm seeing about a 3% drop from .500" to .600" (254 to 246) .
It does rebound approaching .700" though but that's far and beyond the .575" target suggested.
Might not be worth thinking about.
Thanks.

Kevin,
What did your head do from .6 to .7? Did it continue to drop or did it level off?

Stan

[skinny z]

Hi Stan
Here's a screenshot of the flow chart.

RHS AMS.jpg

[PRH]

Scroll down to “notes on airflow”

https://jonkaaseracingengines.com/shop/ ... der-heads/

[skinny z]

Scroll down to “notes about flow testing”

https://jonkaaseracingengines.com/shop/ ... der-heads/

That's a good read. Thanks for that PRH.

Interesting point made in the article with respect to the short side turn. Similar to the discussion I had with the shop that rebuilt these heads. Beyond what was done, which was nothing more than cleaning up below the valve job and a little beyond plus a decent valve job (pics available), the next step would have been the short side. This would address the dip beyond .500". Or so I was told but is also my understanding. But I had to draw a line somewhere.
Anyway, these are done aside cleaning up the chambers from some previous, previous work, cc'ing the chambers and ports plus measuring the port itself. Then on they go again.

[mag2555]

So Classact. I take it that I should then just listen to you and not what I was taught at a Darin Morgan / BES class that I attended ?