Formula or rules-of-thumb converting airflow chart to smaller bore?

[NewbVetteGuy]

I'm curious whether there's any formula / calculator or even rough rules-of-thumb for taking the typical cylinder head air bench chart results performed on one bore size and converting them into another bore size?

If you've got an airflow chart that was flowed on a 4.125" bore and you're going to run on a 4.060" bore, or you've got an airflow chart that was flowed on a 4.060" bore, but you're going to run the head on a 4.030" bore?


I understand the intake valve diameter to the bore diameter makes a difference and shrouding could make this non-linear, but is there ANYTHING out there to try and do this SOMEWHAT accurately?



Adam

[mag2555]

No such cart can be had.
Sometimes intake flow wise if the head has a very shallow chamber and with all things being equal, a smaller bore might make greater power due to better pressure recovery .

[pcnsd]

EnginePro software has an adjustment for the difference between flow test @ bore size x vs. engine model at bore size y. Projected flow=Measured flow*SQRT(Measured bore/Projected bore).
Because the typical difference between measured bore and projected bore is small. The range for error is also small, but there is still a chance for error in such things.
It is what I'm using at present. I call it an estimate. It assumes the same test pressure.

[modok]

Sometimes the distance from the valve to the cylinder wall makes a big difference and sometimes it does not.
If you want to find out then test that when flow testing.

[NewbVetteGuy]

EnginePro software has an adjustment for the difference between flow test @ bore size x vs. engine model at bore size y. Projected flow=Measured flow*SQRT(Measured bore/Projected bore).
Because the typical difference between measured bore and projected bore is small. The range for error is also small, but there is still a chance for error in such things.
It is what I'm using at present. I call it an estimate. It assumes the same test pressure.

EA Pro has this? Any idea if the $150 non-Pro version has the same feature?


Adam

[NewbVetteGuy]

Sometimes the distance from the valve to the cylinder wall makes a big difference and sometimes it does not.
If you want to find out then test that when flow testing.

Interesting.

Any insights on the major factors that might determine whether it makes a big difference vs. not a big difference?

Is this just driven by valve angles or are there differences between even heads for the same platform with the same valve angles?

Valve seat angles a significant factor? (I could see how the air flows around the valve could change with a steeper valve seat -kinda like a fire hose spray can be very wide or more narrow).



The "problem" I was trying to solve would be if you had a list of 3-5 heads you were looking at for the same engine, and let's say the air bench data was all from an SF-1020 at 28" of depression, and all had the same size intake valve, but 3 were flowed on a 4.125" bore, 1 on a 4.060" bore, and 1 on a 4.155" bore, is there a way to convert them all to the same bore to do a "close enough" apples-to-apples comparison?


-It sounds like the formula that pcnsd posted, earlier IS the rough formula to do this?

Adam

[modok]

the head should be flowed at the bore size it's designed for. .040 different bore isn't a big deal but .080 different then we may need to check some things.
Can this head use a smaller bore head gasket?
Maybe, maybe not.
I think you will just have to try and see how things line up, on a case for case basis.

There is a flow bench correction factor for using a different bore size yes, but that's just for what size the bore is where it meets the open air box of the flowbench. How fast the air should be going as it meets the plenum, and that difference should be small.
There is no correction factor for screwing with your valve shrouding or head gasket size. It could go either way.

[pcnsd]

EnginePro software has an adjustment for the difference between flow test @ bore size x vs. engine model at bore size y. Projected flow=Measured flow*SQRT(Measured bore/Projected bore).
Because the typical difference between measured bore and projected bore is small. The range for error is also small, but there is still a chance for error in such things.
It is what I'm using at present. I call it an estimate. It assumes the same test pressure.

EA Pro has this? Any idea if the $150 non-Pro version has the same feature?


Adam

Adam,
Engine Pro is not EA Pro. Engine Pro was the companion software that went with Don Terrill''s book The Horsepower Chain. It is, I believe largely the work of Patrick Hale and his company Racing System Analysis and was sold to Don some time ago along with various other programs and source code that were marketed by Don on this website.