Exhaust Throttle

[Somers Racing]

Hey guys, I hate to have to start a new subject, but I didn't know if a lot of people are still reading the other one.

Does anyone have a picture or a better way of explaining what this is and exactly what it does.

This would be a extremely good info for me to have. I like to have big acceleration. I have an OPS .45

Thanks for any input.

Peter

 

[EatMyShortsRacing]

Peter,

Just imagine a carb barrel upsized and mounted on the exhaust so it shuts the exhaust off rather than the intake. This unit basically replaces the header on the engine. As to why or if it works better than a normal carb setup I am not sure. An advantage may be being able to have a more correct venturi replacing the carb on your engine...

EMS Racing

 

[TimD]

An advantage may be being able to have a more correct venturi replacing the carb on your engine...

EMS Racing

Exackry, That is what I've been trying to tell Dale!

 

[Ian Inverarity]

What if you put the correct venturi on the carb? When the carb is full open it is just a hole, no different to a venturi?

Ian.

 

[TimD]

Yes and no.

A correctly shaped venturi is not just a hole. A good example is a carb throat from a full - sized car like a Holley or a webber. the throat narrows down and then flares out again. Also with an r/c carb the diameters are generally much larger than what is needed with a true venturi. The other thing with R/C carbs is the barrel edges create turbulence in there. They are a compromise in terms of absolute flow performance.

 

[Propjockey]

SR--here's some pics

K&B 7.5 (notice it's water cooled)

 

[Propjockey]

Exhaust side

 

[Propjockey]

Intake (venturi) side

 

[Propjockey]

K&B 3.5 (sans the lower unit)

Simple 'Flapper Valve'

 

[Propjockey]

And it's counter-part

 

[Ian Inverarity]

Propjockey,

How well did the flapper valve type work? That looks like one of the simplest systems I have seen for an exhaust throttle. 8)

Ian.

 

[EatMyShortsRacing]

My concern is how do you design a correct venturi!!!

how do you know the one you spun up on the lathe is actually going to be better than the carb you replaced???

EMS Racing

 

[Don Templeton]

OPS 67 "Older"

 

[Somers Racing]

Hey guys, WOW. Thanks a lot for showing me how this hole thing works. I now understand it.

Thanks again.

Peter

P.S. Is there a point to putting this on? I mean is it harder to tune the engine or anything. Bassically what I am asking, is, should I bother to fabricate this system or run the motor how it is?

Thanks again for explaining it more deeply.

Peter

 

[_laxmasta2039]

the venturi acts as a velocity stack and helps to decrease the turbulence coming into the carb. a well-designed venturi will give the engine a smooth, direct flow of air into the carb inlet, while leaving a flat opening like on a Walbro gas carb will alow the air coming into the engine to be choppy and misdirected. [i just learned that a couple hours ago, cool huh?]

IMO you should get rid of the exhaust throttle and install a regular carb that regulates the engine's throttle during intake, and install a nice header/tuned pipe setup.

Joe

 

[EatMyShortsRacing]

..and the question remains...

how do you design a venturi that will work??? Does anyone have formulas on designing them? where and how the fuel is introduced as well??? or is this just a trial and error thing so far?

EMS Racing

 

[Ian Inverarity]

Craig,

I have only a limited understading of venturis, but I thought the fuel should be introduced into the smallest diameter section where the air velocity is the highest, or where it starts to expand again to utilise a vacuum effect.

As for anything else I have no idea, but I once saw Brian Callahan write that the venturi profile should have a radius of approximately the min throat diameter.

But I have never experimented with different geometries...

Ian.

 

[EatMyShortsRacing]

Ian,

There must be some mathematics in this... also how far away do things like sponson booms, tanks etc have to be away from the venturi to not affect its performance???

These questions and more... LOL ;D

EMS Racing

 

[Ian Inverarity]

Craig,

That is the sort of stuff a dyno would be good for! Maybe we should start another thread....

8)

Ian

 

[TimD]

Ian,

There must be some mathematics in this... also how far away do things like sponson booms, tanks etc have to be away from the venturi to not affect its performance???

These questions and more... LOL ;D

EMS Racing

I can answer some of that one. Optimum clearance is 2.5 x inlet diameter. As for the size of the venturi I don't know the exact specs. The i.d is generally smaller than what you would use with a carb. The venturi creates high velocity and makes up for loss of dia.

There are specific formula's for venturi design but I don't have them ( I want them). I guess there would be a certain amount of fine tuning the lengths to suit the pipe pulses and stuff like that.