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Engine basics => Engine set up tips => Topic started by: phil myers on November 03, 2009, 04:05:32 PM

Title: venturi verses r/c carb
Post by: phil myers on November 03, 2009, 04:05:32 PM
My ASP .21 has had 1hrs bench run time now with a r/c carb turning a 9x4 about 11500. If I installed a venturi instead, how much more revs should I expect or would there be no difference? Just curious.... Thanks
Phil
Title: Re: venturi verses r/c carb
Post by: ray copeland on November 03, 2009, 04:25:54 PM
Great topic Phil, i will be watching this one with you! The only time i ever really checked the difference on my plane was a 25 fsr for slow combat on bladder pressure , lost about 800 rpm using a control line venturi over the r/c carb. I still run several fp's and a 46la with the r/c carb. It is expensive to change over, I ordered two venturi and needle valve assembly's to fit the os 40fp today and spent $53.00.
Title: Re: venturi verses r/c carb
Post by: Alan Hahn on November 03, 2009, 05:58:03 PM
Of course it depends on the venturi size---just like the rpm with the rc carb depends on the rotation of the barrel.

One difference is that the carb probably depends on having muffler pressure, so it can stand being a larger diameter opening. Most of us DON'T use muffler pressure on CL venturi's so to get a good fuel draw, that usually means a smaller diameter venturi opening than that of a wide open RC carb.

Also don't forget to take the spraybar into account. A lot od CL spraybars are wider than the rc carb nozzles. This also cuts down on the CL venturi size if you don't compensate.

So as normal, it depends! #^
Title: Re: venturi verses r/c carb
Post by: phil c on November 04, 2009, 06:46:58 PM
The R/C carb makes a remarkably good venturi setup.  The OS LA series carbs are pretty light and work well.  About the only negative is occasionally the barrel fit is too loose and it may leak air sporadically.  It's an excellent way to tune the motor to a particular plane and prop, since you can easily adjust the venturi size to find the correct opening for a specific combination.  Once you know what rpm you want, you can make/buy a venturi to hit that speed if you want.  The only real negative is the half-one ounce extra weight.  Especially on motors with large two needle carbs the throttle barrel alone can weigh an ounce.
Title: Re: venturi verses r/c carb
Post by: phil myers on November 06, 2009, 02:24:57 AM
Thanks to all for the replys, again I've learned something from these pages...
Phil (Norfolk, England)
Title: Re: venturi verses r/c carb
Post by: Clancy Arnold on November 10, 2009, 04:19:35 PM
A few comments on RC carbs.

I have been experimenting with Different Carbs on OS 25 engines.

I wondered why the plane bearing (FP) engine ran slower than a Ball Bearing (FSR) of the same size on the same prop and fuel.  The first thing I noticed was the difference in throat area.  The FP carb (2A) had an opening of .180 dia while the FSR carb (2B had an opening  of .216.  I have asked a friend to open the throttle hole in a couple of 2A throttles to the .216 diameter.  The only problem is that the barrel has been hardened and can only be ground to enlarge the hole.  An alternate to the 2B carb is get a 3A carb from an OS 40 FP that measures .205 dia.  That will get you a few hundred more revs.

Experimenting is no problem as all three carbs are interchangeable.
Clancy
Title: Re: venturi verses r/c carb
Post by: Douglas Ames on November 11, 2009, 10:10:06 AM
A few comments on RC carbs.

I have been experimenting with Different Carbs on OS 25 engines.

I wondered why the plane bearing (FP) engine ran slower than a Ball Bearing (FSR) of the same size on the same prop and fuel. 
Clancy

FRICTION... The ball bearings have considerably less resistance than the bushed engines. The down-side is the extra weight and cost.
Title: Re: venturi verses r/c carb
Post by: 50+AirYears on November 18, 2009, 11:03:40 PM
I'd kind of guess today's carbs are also better and less restrictive than they used to be.  That might make a difference in whether or not it's worthwhile to replace the carb with a venturi on a given engine.

I have an OS Max .30 RC engine that is a mild easy handling engine that doesn't match my surviving Fox .19 baffle engine on a 9-4 prop for power, let alone a .25 Schnurle.  However, a few years ago, for SAM competition, I pulled the RC carb and turned a 0.200 venturi, and used an old thin Kap-Pak needle assembly.  The engine went from about 11000 max to over 14600 on 10% fuel.