Intake valve job causing high-lift stall / drop-off?

[BradH]

Four different BBM standard-port, not MW, replacement heads (Chapman Stage VI, Trick Flow, early 75cc chamber Edelbrock Victor, later 72 cc Edelbrock Victor), two of which show a pattern of the intake flow dropping off after .700" lift. All have intake valves between 2.18 - 2.20" at the stock 15* angle and placement, all flow w/in about 15 cfm at those lifts.

The two Edelbrock Victors have the .700"+ drop-off problem, and both have chambers CNC'd by the same company. The drop in flow is consistent whether the heads are tested at 28" or 35": the #s from 35" converted back to 28" are virtually identical. I suspected this might be the result of that program carving away too much between the plug-side of the chamber wall and the valve (not a gradual enough transition), especially because a lightly modified as-cast 75 cc chamber doesn't show the same issue. However, the Chapman and Trick Flow also appear to have comparable amounts of the chambers being pulled back.

This has me wondering if the intake valve jobs are (part of?) the issue. Although all of the valve jobs are 45*, the two on the Victors -- while different seat & angle configs done by two different shops -- IMO have a more abrupt transition from the valve seats into the chambers. One has a short 30* top cut; the other a short 35* top cut. I don't know the top-cut angles of the Chapman or the Trick Flow, but those two, especially the Trick Flow, appear to transition well past the seat insert and into the chamber itself.

Have others seen similar behaviors where the valve job top angle or length of the transition into the chamber has an impact on the high-lift flow curve behavior? Thanks - Brad

[BradH]

Early 75 cc and late 72 cc CNC'd chambers

HughesCNCVictor75ccchamber.jpg

PRH_chamber1_sm.jpg

[BradH]

Chapman 260 Stage VI and Trick Flow 240

Chapman 260 chamber 2.jpg

TF 240 chamber.jpg

[BradH]

Same Chapman chamber in different lighting and another early 75 cc Victor with as-cast chamber cleanup and 30*/45*/60/70* valve seat (not "out of the box" Edelbrock seat config)

Chapman 260 chamber 1.jpg

Victor 75cc chmbr PRH prep.jpg

[mag2555]

If you do a flow test with the valve loaded in the guide the other way does the flow regression still present itself at all, or does it now start at a higher lift point?
What is the difference in the low lift flow of these Heads between .100" and 250" ?

[PRH]

In the case of your heads, I don’t think the valve job is “causing” the problem, I think it’s the relationship/transition between the seat and the chamber and how the chamber has been relieved.

If the question is, “can the valve job cure this phenomenon in this head”....... there’s only one way to know.
I think it would require a fair amount of valve sinking to create enough length with a steeper top angle to facilitate a nice blend out to the chamber wall.

Since the head that has very little done to the chamber, and has nothing unusual done with the valve job....... doesn’t have the flow regression issue...... it would stand to reason it’s the combination of the way the chambers are done along with the valve job that’s allowing it to happen with those other heads.

The other question would obviously be....... if you did nothing else but fix that part of the flow curve........ how much different would the power output be?

[Erland Cox]

viewtopic.php?t=34978

Erland

[BradH]

Not looking to "fix" the Victors, but thinking more about what I'd do differently if I was to ever build another set from scratch.

And, no, I don't have another set sitting on the shelf.

The dyno #s didn't give the impression that the flow curve above .700" mattered any with a cam that only lifts the valves .650". But some "big brains" on here have preached the importance of the curve not backing up below 1" lift, even for applications with significantly less lift.

[BradH]

viewtopic.php?t=34978

Erland

I have read some of that thread in the past, but not the full thing. I also dug into some older Darin Morgan posts on pressure recovery around the same time.

[GARY C]

Can you clay up the Chapman chamber at the plug area to mimic the other and see if that makes a difference?

[hoffman900]

Does improving the chamber for flow improve it for combustion?

It’s not a one way street..

[BradH]

Can you clay up the Chapman chamber at the plug area to mimic the other and see if that makes a difference?

The only heads in my possession are the 72 cc Victors... and my flowbench is still in pieces cuz I haven't been doing any head porting for a couple of years.

[GARY C]

Can you clay up the Chapman chamber at the plug area to mimic the other and see if that makes a difference?

The only heads in my possession are the 72 cc Victors... and my flowbench is still in pieces cuz I haven't been doing any head porting for a couple of years.

I see, one thing that would be interesting is to test both heads with the intake on, most ports I have had that backed up went back to working normal with the intake installed but I prefer to fix the port if possible just for piece of mind.

[GARY C]

Does improving the chamber for flow improve it for combustion?

It’s not a one way street..

Hard to say without dyno testing but it would tell you if that part of the chamber was the cause.

[hoffman900]

Does improving the chamber for flow improve it for combustion?

It’s not a one way street..

Hard to say without dyno testing but it would tell you if that part of the chamber was the cause.

It just seems to me chasing chamber shape on the flow bench is a not seeing the forest for the trees