Back cut valves

[Walter R. Malik]

Has anyone ever done a real world dyno test on back cut valves? We know it picks up low lift, but there is the debate that low lift air flow can hurt power. Years back I did a dyno test, 30' VS 45' This was on a L69 305 with .450 lift. On the flow bench it picked up a mile! on the dyno nothing. It acted like I retarded the camshaft. lost a small amount of low end and picked up some past peak. Average was the same.

Just to be clear ... back-cutting a valve does not always pick-up low lift flow. What matters more is the width of the seat and the ANGLE of that back-cut.

[statsystems]

I'm looking at these Porsche valves here. They are both reasonably tulip shaped (not nail head) and they don't have valve back cuts. These are expensive valves, and the basic combustion chamber design was so successful that they used it for about 20 years. It's my impression (and in this case backed up with some data) that the intake can be made flow more with a more nail shaped valve that has a back cut. I don't have any evidence that it helps power, though, and the fact that the factory that put in multi-angle valve job, hand matched the intake manifold to the port in some models, etc. chose to not do the nailhead shape or the back cut makes me think that maybe the flow bench CFM <> dyno power? Just speculating, not answers here just questions.

That nail head with back cut may look better on the flow bench, but with the correct valve job and no back cut the tulip valve will, make better power..

[In-Tech]

How much is " a certain good amount of low lift flow " ?

How do you go about deciding exactly how much that is ?

Randy

Good questions right there.

Ones I don't get replies to.

Randy

Hi Randy, that's probably because it can't be a straight forward answer.

I can give you one example of testing on a stock class 1600 cc 2tc Hemi Toyota. We cannot regrind cams, we ran stock valves at all ICL, it liked ~110. We undercut the intake and ex valves, picked up 3-4 cfm, good right? meh... it was slower regardless of tune. Went back and forth and actually cut a trough on the intake valve right before the 45 and killed the low lift flow by 3-4 cfm, it liked ~107 now and car was faster. Hmmmmmm. Ended up with a backcut intake and exhaust and ICL of 117.

What did that teach us? The reversion in that crossflow hemi during overlap was critical due to many many reasons(Stock intake, carb, ex man, etc). The flow goes both ways and increasing low lift flow doesn't mean it will make it flow IN

Once again, flowz are flowz but if at the wrong timing event what you might gain on the bench doesn't get realized on the track.

And let's please not change this into a "what if your changed the fuel ratio or ignition timing" debate. Trust me, all that and valve lash was moved around to find optimum. I would rather have a discussion about low lift flow and its' advantages/disadvantages.

[mag2555]

Everything needs to be looked at as a sperate situation and without question confirmed on some form of dyno!
I have had cases where on the flow bench a Tulip valve picked up more low lift flow than a non Tulip valve with a back cut
, and this was in terms of both a Intake and Exh flow gain, yet on the dyno the motor made less power.

My conclusion in this case was the rate of the opening of both valves with this peticular Cam was not fast enough and the Tulip back side of both valves was just blocking off flow that could have been had!