Carb flow testing

[PRH]

Some discussion involving carb flow rates for various carb configurations was brought up in Steve Cowan’s 383 dyno thread, and I figured rather than clutter up his thread, I’d start a new one…….

Let me start out by saying this is not at all meant to be a definitive set of results, but just a comparison of different carbs, tested all in one session……..and how the flow rates compare to each other under the same conditions.

I have one of the SF carb flow test adapters, which flows one hole at a time.
This is obviously dry flow, so the observed numbers are higher than what they would flow with fuel displacing some of the area in the barrel.
I take the dry flow and multiply that by 92% as an estimate for wet flow.
The actual wet flow would vary slightly depending on the jetting-A/F ratio on the running engine.

The carbs tested were:
A-4777 Holley with choke blade removed and a Proform baseplate
B-4779 Holley with choke blade removed
C-4781 Holley with choke tower removed
D-PRH built HP950 from a genuine Holley body, and a baseplate with thinner shafts
E-PRH built HP750 built using a Holley main body and a QFT baseplate
F-BG Silver Claw 750(1.40 x 1.68)
G-CFS built ported Holley 850(4781) with annular boosters and thinned shafts.

Each barrel is flowed at 20.4”, the numbers are added together for the dry flow total, then multiplied by .92 for the estimated wet flow.

Carb—————dry————wet
A-4777———735.2———676.3
B-4779———828.0———761.8
C-4781———913.3———840.2
D-HP950——914.0———840.9
E-HP750——884.5———813.7
F-BG750——904.6———832.2
G-CFS850— 973.8———895.6

[Tuner]

Did you measure booster signal?

[PRH]

I didn’t

[PRH]

Some pics of the test subjects:
4777-

IMG_3416.jpeg

4779-

IMG_3414.jpeg

4781-

IMG_3415.jpeg

HP950-

IMG_3419.jpeg

[PRH]

HP750-

IMG_3418.jpeg

BG750-

IMG_3417.jpeg

CFS850

IMG_3420.jpeg

[PRH]

The SF carb testing adapter comes with a 4 hole spacer that has 1.687” bores.
I found one that has 1.750” bores that I use for the carbs that use the 1.750” baseplates.
I hadn’t done that test in many years, so I did it again today.
On the HP950, using the adapter with the 1.687” bores dropped the dry flow number by 10cfm.

The plate that bolts to the bench has a 1.750” hole-

IMG_3421.jpeg

IMG_3422.jpeg

[PRH]

When I was doing more carb testing years ago, it was when I had a SF-110 bench, so the actual test pressure was 10”, and was converted to 20.4”.
I did a couple of tests at different pressures on the CFS carb today.
Tested at 20.4”, then 28”, then 10”.
The converted numbers are pretty close to the actual test of 20.4”

The dry/wet numbers are:
20.4”- 973.8/895.6

10”- 680.6/626.2(converts to- 972.3/894.6)

28”- 1142.6/1051.2(converts to- 974.6/896.6)

[treyrags]

Are the annular inserts .5 or .6 id?

[PRH]

They appear to be .615”

[steve cowan]

Some discussion involving carb flow rates for various carb configurations was brought up in Steve Cowan’s 383 dyno thread, and I figured rather than clutter up his thread, I’d start a new one…….

Let me start out by saying this is not at all meant to be a definitive set of results, but just a comparison of different carbs, tested all in one session……..and how the flow rates compare to each other under the same conditions.

I have one of the SF carb flow test adapters, which flows one hole at a time.
This is obviously dry flow, so the observed numbers are higher than what they would flow with fuel displacing some of the area in the barrel.
I take the dry flow and multiply that by 92% as an estimate for wet flow.
The actual wet flow would vary slightly depending on the jetting-A/F ratio on the running engine.

The carbs tested were:
A-4777 Holley with choke blade removed and a Proform baseplate
B-4779 Holley with choke blade removed
C-4781 Holley with choke tower removed
D-PRH built HP950 from a genuine Holley body, and a baseplate with thinner shafts
E-PRH built HP750 built using a Holley main body and a QFT baseplate
F-BG Silver Claw 750(1.40 x 1.68)
G-CFS built ported Holley 850(4781) with annular boosters and thinned shafts.

Each barrel is flowed at 20.4”, the numbers are added together for the dry flow total, then multiplied by .92 for the estimated wet flow.

Carb—————dry————wet
A-4777———735.2———676.3
B-4779———828.0———761.8
C-4781———913.3———840.2
D-HP950——914.0———840.9
E-HP750——884.5———813.7
F-BG750——904.6———832.2
G-CFS850— 973.8———895.6

I really appreciate you taking the time,it's safe to say my 950hp flows more than 770cfm.
I have a question about the QFT 750,a friend gave me a brand new one and I have not used.
What's your thoughts on that carb ?
I remember QFT made good quality back in the day,I was not very impressed when I bought the QFT 1050.$ 2000 AU
I could of got a custom APD carb less than that but didn't know at the time.
On the dyno first 6 pulls were with 950hp and 1" 4 hole super sucker.put on 2" 4hole 7deg tapered matched to carb pad.
I thought it would require a couple of jet sizes but AFR stayed at 12.6 - 12.7 0.86 - 0.97 Lambda.
Power went up.
That kinda tells me that carb was not maxed out at 7000rpm.

[PRH]

Steve, what you’re looking at(in my pics), thinking it’s a QFT-750……..is actually a bunch of QFT parts attached to a Holley retrofit main body……
https://www.holley.com/products/fuel_sy ... s/134-300S

I originally bought that body to do a back to back dyno test, using my 4779 as the test piece.
It just never happened……and I ended up just making a carb out of it.
That particular carb has never been run.

20-ish years ago, when the only retrofit bodies were the Proform units, I used quite a few of those.
In retrofit applications and as a foundation for a complete carb.
Back then I’d use a replacement 4779 baseplate and metering blocks…….and they worked out pretty well.
I would usually have one on hand for dyno testing.
I had an engine builder customer who was doing dirt track motors pretty regularly, and I’d end up doing the dyno testing.
Many of those customers would show up with these “special” rag-tag carbs, that often were absolutely awful.
So, I’d have the Proform HP750 on hand to try, and they would often crush their special carb.
Several were taken home after the dyno session.

As more and more somewhat affordable carb options arrived on the scene, the need for my homebuilt Proform 750’s tapered off.
Now, anyone can buy a Brawler for less than what the parts cost to build something similar.

One test where I’m still fairly clear on the results between my HP950 and one of the Proform HP750’s was on my own 383 SBC.
It was in the 600hp range.
The intake was a mildly cleaned up SV.
I tried a few different carbs and spacers on it.
My 950 trends slightly richer as the rpm goes up, and this engine didn’t really like that.
The CFS carb was just a bit too rich for it overall, and I didn’t feel like messing with it.
Also, unlike many of the dirt mod engines I’d run with that same intake, this combo didn’t gain a thing with a super sucker on it.

The Proform HP750, which didn’t trend rich with rpm, on a 1” open spacer put up the biggest number.
A bit over 600hp at close to 7k.

The old Proform carbs I did, using the replacement 4779 metering blocks, have only 2 hole emulsion.
The QFT billet blocks on the new carb have 3 hole emulsion………so I’m thinking it may not be the slam dunk I was seeing with the old style blocks.
But I’ll find out…….. someday.

[Tuner]

Are the annular inserts .5 or .6 id?

They appear to be .615”

Does the exit end of the booster insert extend below the minor diameter of the main venturi?

What is the ID of the main venturi?

Can you do this again and plot the booster signal over the total range from very low flow, 20 CFM or so, in steps up to maximum bench flow?

[Tom68]

My 950 trends slightly richer as the rpm goes up, and this engine didn’t really like that.
The CFS carb was just a bit too rich for it overall, and I didn’t feel like messing with it.
Also, unlike many of the dirt mod engines I’d run with that same intake, this combo didn’t gain a thing with a super sucker on it.

The Proform HP750, which didn’t trend rich with rpm, on a 1” open spacer put up the biggest number.
A bit over 600hp at close to 7k.

Novice question....Do you watch for pull over from the squirters and when it starts ?

[mag2555]

I would love to see someone who has a big enough capacity bench do a test of ….let’s say a 750 carb as you did with just testing one barrel then multiply that to get the total and then flow testing the carb as a whole.

I can only assume that the total flow volume would be less then the added up total, but the empirical results would be interesting.

[skinny z]

This is obviously dry flow, so the observed numbers are higher than what they would flow with fuel displacing some of the area in the barrel.
I take the dry flow and multiply that by 92% as an estimate for wet flow.
The actual wet flow would vary slightly depending on the jetting-A/F ratio on the running engine.

This brings about a question I've had since the day I received a Barry Grant Speed Demon 750 VS carb. Circa 2010.
BG carbs were purported to be wet flow rated. I thought I had documentation to support this however it's buried somewhere in my files and remains unfound.
There some internet archive information as well.
So my question is: Yes, no, maybe so?
1.4" venturi.
1-11/16" butterflies.
.016" booster leg.
Annular boosters.
Would that then be 750/.92 CFM in dry flow terms? Or do we just call it a 750?

I'm hoping this is still on topic but if it isn't feel free to pass it by.