Carb Flowbench Testing

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RaceMechaniX

RaceMechaniX

Well-Known Member
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Apr 27, 2007
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I decided to bench test several carbs for maximum air flow rate and fuel suction draw. A friend has a Superflow SF-260 which is perfect for our little engines. I laser cut a piece of 1/4" acrylic and tapped holes for common carb bolt patterns. I have a variety of carbs for CMB 67/80's and stock 91/101 carbs. Sometime this week I will post the results. One of the tests is to compare the 9B with and without the bellmouth entry to prove its worth.

I will try and plot flow rate over vacuum given enough time. I am estimating the flow rate somewhere between 10-14CFM depending on the size and RPM of the motor. If anyone has better estimates of flowrate or vacuum behind the carb please let me know so I can narrow down the results. Some pics of the plate and carbs are attached.

TG







 
Great job Tyler!

A couple of guys on here that I know of ( Rod Geraghty & Bob Bonahom ) did bench flowing on our carbs years before we had these great internet forums.

Back in those "stone age" days we had to share our findings by phone, snail mail or at an event.
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Maybe they will chime in with some their results and findings too.
 
Interesting Tyler, looking forward to the results, especially about the 9b with the bell mouth. Did you make the bell mouth yourself or is this the one that tower sells?
 
Back many years ago I became acquainted with a flow expert that did some flow work on our carbs. He charged me a set fee for any carb and I resold the service. I would tell you that the configuration of the spray bar is really critical and can make a huge difference in performance. He worked a long time on each carb and when you got them the things he did on the spray bar were amazing.

I lost his contact info and have not had success finding him. I would love to have his service on the current carbs.
 
Marty: you sent me his contact info years ago when I bought the software, Here's the problem I have a phone book with 20 years of contacts. I think I may have it but don't remember his name? do you remember his name? Post and I will look...........jeff
 
jeff walker said:
Marty: you sent me his contact info years ago when I bought the software, Here's the problem I have a phone book with 20 years of contacts. I think I may have it but don't remember his name? do you remember his name? Post and I will look...........jeff
Click to expand...
He has changed his location. I think I might have found him, but I am waiting for a reply. Hope so, as his work was awesome. I still have 1 carb that he did and it is the best that I have ever had for the 21 engines.
 
OK guys results are in. I used a SuperFlow SF-110 to measure flowrate through each of the 5 carbs at three different test pressures (15", 12" and 10" of water). I attached a precision Vacuum gauge to the fuel inlet fitting on each carb and recorded the vacuum from the spraybar at each test pressure.

I was not surprised by the difference from adding the bellmouth to the OS 9B carb. Flow increased as did vacuum from the higher velocity air passing through. I was surprised between the difference between the Zoom 67/80+ and the stock CMB 91/101RS carb without low end needle. The Zoom carb has an oval opening with an average diameter of .515". The stock 91/101RS carb has a .500" opening to the case. One would expect the Zoom carb to flow higher, but in fact the smaller 91/101RS carb flowed more. The difference likely comes form the size and intrusion of the spaybar. The Zoom carb has a much larger spray bar extending past the port center compared to the 91/101RS carb which has a tapered spray bar that does not extend past the centerline.

The Zoom carb and stock 67/80HR carb measured the highest vacuum versus flow rate of the group. The high flow rate and high vacuum of the Zoom carb help explain its overall good performance and ability to mix and atomize the fuel compared to the other carbs.

I will say I have never tried mounting a 91/101RS carb on an 80HR, but it may be worthwhile.......

Tyler

Carb Flow Testing 2015-02-28.pdf
 

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  • Carb Flow Testing 2015-02-28.pdf
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thanks for the test results Tyler and showing the difference in results on the OS 9b with the bell mouth. About the application of this carb, when I look at the results it's comparable with the 67HR carb. I'm using it on a RS91, do you think it is too small for this engine?
 
Paul,

I believe it will depend on how high you are revving the engine, but yes I do think based on this it is too small for a 91RS motor. I ran a 91RS with the stock carb mentioned above last weekend in a Seaducer and had zero problems idling down and whacking the throttle open with zero hesitation. I have had better consistency with the older cast carbs than the newer billet bodied one.

TG
 
Hello Tyler. Yes, a lot of work and testing went into the design of the Zoom carbs.

Now for the "Real Life" numbers and their meanings, Blow air ACROSS the bell opening like is happening in the boat at speed and you will see a major difference in the consistency of the carb flow and draw. This is something that I have recently begun to play with. On one engine, a stock 26 Zenoah with an airplane propeller, after tuning the carb to maximum performance with the engine running on the test bench, I placed my hand between the propeller and the carb bell.......The engine shut off completely. So there is much to be gained with work in this area.

One other thing that I found is that the numbers will change when the carb is tested on a running engine. This has to do partially with the engine pulses with the opening and closing of the intake and the reversion of the air fuel mixture through the engine during the actual running conditions. During the reversion of the air fuel mixture, it is blown backward through the venturi then pulled back through the carb. Also as the engine comes on the pipe, the airflow changes again. Another game all together.

This makes you scratch your butt and your head at the same time.

Mr. Tyler, I am not criticizing your work at all. In fact, I like the idea of someone as sharp as you are, getting into this area of performance. I am only pointing out some of the other things that have to be considered when designing a good carburetor.

It is an absolute must to have a way of measuring the differences any changes make. This area of performance is well past using the SWAG technique to find improvements.

Charles
 
Last edited by a moderator:
Hi Charles,

No offense taken and all valid points. My initial purpose of this test was to validate whether a 9B with a bellmouth could improve flow over a stock 9B. And then to determine how close it came to other known carbs. I decided to add the vacuum measurement to see any major differences between spray bars.

As you point out there are a lot of different factors when a carb is running on an engine and in the model, but testing always help debunk one theory or another.

Tyler
 
Thanks Tyler for you info, I'll try to compare them one day when I got time (currently boating is a bit low).
 
Add wet flow to the equation. When large volumes of liquid are added to the equation thing will change.

My home made wet flow box.

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