Custom Cam to Flow numbers

[Jix008]

I just got my new head ported and during flow testing the best where at about .460 lift on valve opening but we are allowed up to a .500 lift so should my cam be built around .460 or should I got to .500. And why could it be falling like this.


Flow Numbers

.460. In. =. 217. Exh. =. 181
.480. In. =. 205. EXh =. 173
.500. In =. 194. EXh =. 161

[drtlvr]

With a .460 lift cam, after lash, you'll never get .460 lift. I go past the max flow numbers because max lift (say .460 for example) is only for a very short time. I would go to the max lift rule.

[youngengines]

^^^ In agree, remember, your only at max lift once. I want max flow before and after lobe centers. JMO

[84Dave]

x2............. .500" net lift........... with lash. -Dave-

[Old Rob]

You always want to use a higher lift cam than those flow numbers. What is more important is what is known as the "Area Under Curve" This is why roller cams produce more power. "Area Under Curve" is the time of valve opening that is flowing. This is why knowing the flow numbers at all lift values of a given cam lobe is best for deciding which cam. Stock cams have a very short AUC. Ideally you want an asemetric cam that opens the valve very quickly, holds it open, then softly sets it down on the seat. Look at a roller profile. Valve spring selection become more critical.

Where are they measuring max lift. Should be at the valve. Given a solid setup, if you have a .010 lash setting, which is at the junctiion of the cam and rocker, you will have approx .017 gap at the valve. If you rule is .480 lift then you woulld want to have approx .495 gross valve setting before lash is adjusted. The best way to set up a cam is with the TVL tool (for accuracy and repeatability) from www.powerbyace.com Set up the cam TVL with the lash set.
With a .480 rule you will set the total valve lift at .478 - .480 with .010 lash already in.

[84Dave]

Old Rob is spot on! Area-Under-Curve (AUC) is a big deal on a race engine, when considering Volumetric Efficiency (VE). Zora Arkus Duntov & Jim McFarland discovered many years ago that best head flow should ideally occur @ 65-75 % of max net valve lift. Both opening & closing. Why? On the inlet cycle, max piston speed down the bore happens about 70-85 degrees ATDC. Max piston speed = max pressure depression in the inlet tract, thus good/best flow should occur in that area. With the cam degreed on a 100-110 degree intake centerline, peak lift, ATDC, the piston speed is slightly slower due to the max rod angle around 90 degrees ATDC. And the piston speeds up again 70-85 degrees BBDC, with a more favorable rod angle. These varying piston speeds being a function of the rod length-to-stroke ratio. So max piston speed down the bore happens in (2) places on the induction cycle. It's desirable to have the induction ports flowing their best before/in/between those (2) areas. Induction tract flow at/near peak valve lift matters least. The above parameters present the best AUC that Rob mentions. And subsequent best TQ/HP.
-Dave-