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

[Erland Cox]

You have never thought of it in veliocity-pressure terms?
If you slow the flow down over a turn velocity goes down, the boundary layer thickens and the possibility for separation increases.
If you go down in area over the turn the velocity increases, the boundary layer gets thinner and the chance of separation gets smaller.
When you cut down the floor or lay back the SSR you are effectively making the port bigger before the throat.
So you are going towards making the flow area smaller over the turn by making it bigger before the turn.
What I found was that the flow does not separate at the SSR.
Insead it separates from the SSR side valve seat and turns at the valve at higher lifts.
That gives a wake above the valve seat on the SSR side.
This problem was described in the Chevrolet Power book when Chevrolet came out with the then new iron Bowtie sbc head.
They sad, I guess it was Ken Sperling that you should not take to much out of the chamber on the spark plug side as this would make the head stall.

Erland

[JoePorting]

AFR and Tony Mamo I believe is the one that really perfected this idea of lowering the floor and pulling back the SSR. I remember seeing there heads back in the late 1990's and thinking that was a radical concept. Now everyone does it so it's no big deal.

[BradH]

I don't think it's a short-turn issue, based on the following additional background info. Sorry, but this turned into a very lengthy post...

Dwayne, from Porter Racing Heads (member "PRH"), and I worked on a number of different castings before the 72 cc Victors shown ended up going on my engine. I had (have) an untouched out-of-the-box 72 cc Victor that was tested, as well. All the heads that I and/or he worked on had the identical valve seat config cut by another shop (long story; doesn't matter now). It's 35/45/60/70, IIRC.

To save the time w/ the chamber prep work the Victors needed, I had a shop I'd dealt with previously do their CNC chamber program on the Victors. They also have a full CNC porting program, too, but I wanted to get comparable flow w/o ending up with as large an intake runner volume as their program. What I refer to as my "crash test dummy", a Procomp Victor clone, was the guinea pig for a couple of different approaches I wanted to try out, as well as Dwayne giving one port his basic "full port" job. Part of my testing was to clay-in the chamber of the Procomp to more closely match the CNC'd Victor.

72 cc CNC 77929_ 75 cc Pc clayed.jpg

With nothing but the above-mentioned valve job, this was the results:

.100---71.9
.200--147.6
.300--204.2
.400--257.8
.450--277.7
.500--292.2
.550--298.7
.600--307.5
.650--313.2
.700--313.2
.750--314.0

The first port I did wasn't horrible, but I realized that I had started doing "something" wrong w/ the short turn and stopped messing with it at that point:

.100---71.9
.200--152.9
.300--218.3
.400--275.8
.450--299.8
.500--317.8
.550--328.5
.600--338.0
.650--345.7
.700--356.0
.750--355.2

I took a different approach with my second test port, trying to keep the high-lift #s from backing up. It never had the lower-lift flow of the first port, but seemed to pull pretty well from .500" up, though.

.100---71.4
.200--150.0
.300--209.0
.400--265.4
.450--294.9
.500--317.8
.550--336.1
.600--338.8
.650--348.4
.700--355.2
.750--359.0

75 cc Pc SSR.JPG

Dwayne took one of the remaining untouched ports and carved out something pretty solid, especially given he left a completely as-machined stock-sized port entry with this one (both of mine were slightly larger than stock). This ended up being the "prototype" for the porting to be done on my 72 cc Victors.

.100---73.7
.200--154.9
.300--222.0
.400--277.7
.450--303.7
.500--321.6
.550--332.3
.600--340.7
.650--345.7
.700--349.5
.750--350.3

My original plan was to do all the porting myself. However, I found myself squeezed for time and sent the heads to Dwayne finish the work once that realization hit home.

This is a pic of the standard 72 cc Victor chamber with the previously-mentioned valve job, but prior to the CNC work:

72 cc 77929 valve job.JPG

This is a pic of the head with the CNC chambers & valve job, but before any porting:

72 cc 77929 CNCchmbr.jpg

Here's the #7 cylinder test results before any porting:

Lift-------- I / E
.100---70.8 / 59.0
.200--142.7 / 119.0
.300--212.2 / 155.0
.400--266.5 / 172.8
.500--303.2 / 179.5
.550--311.8 / 179.8
.600--302.4 / 181.0
.650--304.2 / 184.7
.700--312.6 / 183.5

These are the results from what Dwayne referred to as his "template" ports (no exhaust back-cut & no flow tube during test). The heads required a LOT of grinding, primarily because the 72 cc Victors come with the as-cast bowl far smaller than the earlier 75 cc versions did:

Lift-------- I / E
.100--75.5 / 53.5
.200-152.9 / 119.0
.300-220.8 / 157.4
.400-279.7 / 195.1
.500-325.0 / 221.9
.550-336.4 / 230.6
.600-346.6 / 236.8
.650-349.6 / 241.4
.700-359.1 / 246.4
.750-343.9 / xxx.x

So... for the first time during this effort, the intake flow was dropping off significantly above .700". Here are Dwayne's intake tests at 35” H20 while checking how this seat/port/chamber combination responded to higher test pressure -- i.e., to see if it backed up worse -- now that it dropped off above .700”:

.600--387.6
.650--391.5
.700--399.0
.750--384.7

The original 28” H20 results compared with the 35" H20 converted back to 28” [ SQRT (28/35) = .8944 ] were consistent, regardless of the test pressure:

H2O – 28" / 28" conv from 35"
.600--346.6 / 346.7 < +.1 >
.650--349.6 / 350.2 < +.6 >
.700--359.1 / 356.9 < -2.2 >
.750--343.9 / 344.8 < +.9 >

There was some testing of different intake back-cut angles and even with no back-cut, but the drop-off seen above .700" still occurred. At that point, the "It is what it is" perspective prevailed and Dwayne finished the heads. Here is the data for the average of four (4) different ports, two from each head:

Intake (Average of 4 ports)
0.100 -- 75.0
0.200 -- 151.3
0.300 -- 220.1
0.400 -- 280.3
0.500 -- 326.1
0.550 -- 338.2
0.600 -- 346.0
0.650 -- 350.5
0.700 -- 355.5
0.750 -- 346.7

At this point, I believe -- as Dwayne suggested earlier this thread -- that there is some combination of factors where THAT valve job matched with THAT CNC chamber profile simply becomes "unhappy" beyond .700". My net cam lift is about .650", so it's possible that this doesn't affect my particular engine combination, or that it's not obvious. But the fact that it does it is what got me to post about it in the first place.

[PRH]

Another thing to add is that with the “prototype” port I did in the procomp head, I didn’t use any clay in the chamber.
So the top angle of short turn side of the valve seat is all the transition into the chamber there is, then you’re into that flat open area.
It’s the 6th picture on page 1.

I think if you took that procomp port I did, and scalloped out some of the chamber on the plug side, down near the seat, so it was similar to how the cnc’d chambers are done...... you’d find that port would also have the same problem at high lift.

In hindsight, I should have tried filling that area on the plug side of the cnc’d chamber, adjacent to the seat, and tested it like that to see if that’s really where the problem is.

I also feel that concave area in the chamber roof adjacent to the plug doesn’t help matters any.
You’ll notice the Chapman stage 6 has a nice little convex area there instead, and the Trick Flow has a nice little damn in that area to help the air steam transition past the seat into the chamber.

The intake bowl diameter as delivered was 1.650(at a point centered on and even with the end of the guide) for a 2.200 valve.
Not the ID of the seat ring, but down in the bowl area itself.
I dont know who thought that was a good size for a starting point....... but to me, it’s just dumb to have to cut that much out of what’s supposed to be somewhat of a “race” head.

[BradH]

Well, I suppose it's just the Brad & Dwayne Show at this point.

At least the Pro DV / Anti DV debate that derailed the topic stopped.

[n2xlr8n]

Great info, thank you.

[RevTheory]

^^^Agreed!^^^

[Powertrip]

Thanks for posting all of this info, Brad.
In my own BBM head experiments, I have found the following:
- Carving out the chamber on the plug side too far (too concave) will hurt high lift flow.
- Stock (open) style chamber shape on the quench side allows the intake charge to flow along the spark plug, across the exhaust valve, and swirl back to the bore wall side of the intake valve. The clayed in shape of yours, and the Trick-Flow chamber shape on the quench side help alleviate that.
- The short turn shape and height is EXTREMELY sensitive. I have found .040" to make dramatic differences in flow!

Any more info you can share would be greatly appreciated, I am open to sharing my findings also.

[BradH]

Any more info you can share would be greatly appreciated...

Such as?

[Powertrip]

You mentioned that you "started doing something wrong" with the SSR, I am curious if you have learned anything from that. I have found the BBM SSR extremely touchy, I am always looking for more info and pictures of that area.
There's not much you can do with stock castings, or even RPM's for that matter, not enough meat in the SSR to get really radical.
I have no experience with the Victor castings.

[BradH]

It may take a few days, but I can dig up some pics that you might find interesting...