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View Full Version : How to decipher flow bench results?



novaman_68
01-02-2005, 08:10 PM
I am confused about cylinder head flow bench results? On CHP's website they have posted all kinds of head flow results. my question is about the efficiency rating, particularly Dart Iron eagle and world sportsman II's The darts have a 77% eff. rating but flow less cfm and the SportsmanII's only have a 67% rating. I am wondering what is more important to look at when selecting the best performing head? :hammer:

Fuelie Fan
01-03-2005, 09:57 AM
That's not an efficiency, it's a ratio of Exhaust flow to Intake flow (E/I). I'll let others comment on what the "ideal" ratio is (if there really is one), as I've heard a range of values. I don't think it's a total deal breaker if the heads you want to use don't have the most desirable ratio. For example, if you used the sportsman II's, which appear to have a weak exhaust port compared to the intake, you could run a dual pattern cam with a bigger exhaust lobe to compensate. BUT, if you haven't purchased anything yet, you would probably be best advised to start out with a head that has a better ratio. Again using the dart and WP as examples, the darts have only 4 cfm less intake flow but a solid 13 cfm more exhaust flow, and would probably make more power as a result. BUT, it is very heavily cam dependant. A dual pattern with the darts may make LESS power. Also, note that the E/I is only given at peak lift. If you do not plan on running .600+ lift, things may change.
Again, I'd wait for the guys who have built 10x more engines than I have to chime in, they may have direct experience with these heads.

novaman_68
01-03-2005, 05:05 PM
That helps me out a lot, I thought I had chosen a set of heads (Pro-Topline) and tried to order them from Summit racing but they don't carry them any more. I called Scoggin-****ey and guess what? they don't have any either I was told ProAction had a problem keeping up with the demand and Summit had up to a 6-month wait so they dropped them.So now I am back to the drawing board :guilty: I want a set of iron heads with around a 200cc int. runner and the produce good flow numbers. Thanks for the help

ProdigyCustoms
01-03-2005, 06:28 PM
Fuelie, I think that was a terrific answer. Ideal I to E ratio is widely accepted at 75% to 80% on small Chevy's. But with that said, it is almost impossible to achieve, and most do not because there is always so much more available on the intake, you can always lean on the cam to help the exhaust side. This is exactly why 90% of dual pattern cams lean on the exhaust side. It is a very rare to find a dual pattern cam with more duration or lift on the intake.
As far as what to look for, look at mid lift numbers, from .250 to .500 as the cam spends twice as much time at any other number below max lift. max lift makes for good press, but, especially in a street torque motor, mid numbers are where it is at.
A good example would be my Darts on the street racer. They are 230CC Pro 1s, and have been ported closer to 235 now. Many porters can get these to 330 to 340 CFM peak numbers. Ours are only 304 at max, they were 300 stock, but the mid numbers came up 60 CFM at .350 from stock on the intake, and 75CFM on the exhaust, and flow almost 80% from .250 all the way to the top where they gradually fall to 74% or so. Six degrees more duration @ .50 in the cam evens them up just fine.
Nova, the Darts will treat you fine.

novaman_68
01-03-2005, 07:50 PM
Thanks guys, I just ordered the Iron Eagles from summit. They are supposed to be here on Friday so I guess I know what I am gonna be doing this weekend. :3gears: