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  • October 24, 2020, 04:48:50 PM

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Author Topic: Max size venturi with pressure fuel system?  (Read 205 times)

Online Dennis Toth

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Max size venturi with pressure fuel system?
« on: January 21, 2020, 11:14:35 AM »
Is there a formula for the max venturi area for a given engine displacement that allows max power assuming a positive fuel delivery is used? Can you go to big and not have the engine get enough air pulled in to run or is this limited by the shaft timing opening size?

Best,    DennisT

Online Motorman

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Re: Max size venturi with pressure fuel system?
« Reply #1 on: January 21, 2020, 04:07:40 PM »
Usually just a little bigger than the hole down the shaft and you'll be rockin'. A super big venturi also has to be tall enough so prop blast ect. doesn't blow the metered fuel out. Have fun.

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Offline John Given

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Re: Max size venturi with pressure fuel system?
« Reply #2 on: January 22, 2020, 06:09:42 AM »
With pressure there is not even a need for a Venturi. This is why combat engines have straight walled intakes the shape of  the port in the crank.

Offline Paul Smith

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Re: Max size venturi with pressure fuel system?
« Reply #3 on: January 22, 2020, 10:48:18 AM »
I don't know of an exact formula.

But in my experience, many many engines have air intakes far far bigger than they need.  An engine only generates a limited amount of suction and a bigger hole will not admit more oxygen.

Attempts to limit speed in F2D have shown that the venturi can be choked-down to 4 mm with minimal loss of speed or RPM. 

As stated by an expert, huge air intakes allow fuel to splash out and thus give an uneven run.  I would guess that if you know how big you can go on suction, an additional 1/16" of diameter on pressure will give you about all the extra power you can get.
Paul Smith

Offline phil c

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Re: Max size venturi with pressure fuel system?
« Reply #4 on: January 23, 2020, 07:46:46 PM »
For running with pressure the venturi doesn't need to help draw fuel so the size can be increased a lot.  The obvious upper limit is when the engine won't run any faster.

If you have access to a lathe you can turn out whatever size you want.  If you make a venturi it is a good idea to make the inside smoothly curved from the outside diameter around into the venturi.  The inside should have an airfoil shaped section, but large enough to flow more air than the stock venturi.  Anything to smooth the airflow and eliminate corners and edges along the way helps.

Inside the motor the front of the crankshaft  should be partially filled and faired down nearly to the entrance to the crankcase using hi-temp, filled epoxy.  Besides giving the air a smoother ride around the turn, the filler helps stiffen the shaft.  Our crankshaft designs are about the worst possible from a fatigue/strength aspect.  A hardened crank, especially on where the hole ends near the front of the cutout, with a big, chunk milled out of it with sharp corners is just saying "Break Here".  So round and and smooth the corners and the inside too.

Don't start doing this kind of thing with a brand new motor.  If you're looking for a racing type motor the ones with more than four ports in the sleeve are already tuned for max performance.  Some of the car engines have to be detuned to swing a usable size prop on a plane.
phil Cartier

Offline PaulGibeault

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Re: Max size venturi with pressure fuel system?
« Reply #5 on: February 06, 2020, 11:32:43 AM »

Motorman is correct & I've gone a tad further. My venturi size rule of thumb is to use the crankshaft hole diameter + 10%.
For example: If your crankshaft hole dia. is .300" add 10% and you end up with a venturi size of .330".

As far as how far you can go... needs some trial & error, but you know for sure the venturi is too large when you cannot richen out the motor & find it will only run at full out lean.
Good luck.




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