Could Someone Please Explain...

[Propjockey]

...the difference between a slide carb and a barrel type carb?

Maybe some pics?

 

[rcboatlover]

With the slide carb the throttle body is pulled in and out to adjust engine speed. I think they are more for car use and hard to hook up in a boat. The barrel type rotates the throttle body to open and close the carb. Hope that makes sence!

 

[Ian Inverarity]

As rc said.

When the barrel carb is fully open they have a smoother flow path than a slide carb. But the slide carbs work much better at partial throttle, which is much more important on a car. Also they are much easier to hook up in many installations in cars.

The only boat engine maufacturer making slide carbs for motors larger than 21's that I know of is OPS. The carb fitted to the 67's and 80's is a slide carb, and this also works well on the OPS 45.

Ian.

 

[GTR]

Hi

Nitrocrazed has covered most of the points - but a slide carbs major benefit is that it does not cause the air travelling through the carby to change direction, like a rotary barrel carb does. I don't understand the better flow path of a rotary carb fully open, as fully open is fully open. Slide carbs can be smaller than rotary carbs because the slide can be the diameter of the venturi opening. The nature of barrel carbs means that the barrel must be considerably larger than the venturi path or when you ratate the barrel it will expose the other side of the opening.

There is no reason a slide carb cannot be used in a mono but hooking up in a tunnel or rigger would be almost impossible with cumbersome linkages involved.

Horses for courses I guess. The slide carb found favour in RC Cars because the mixture from idle throttle to WFO must be carefully metered - otherwise you can't get progressive drive out of corners. Much less of an issue for boats when idle is used to return to shore.

GT

 

[Ian Inverarity]

GTR,

I have used the slide carb on my Nova 21 in a few riggers. I built a bell-crank setup which hung off the side of the tub for one (not very neat, but it worked). In another I rotated the carb as far as I could and used a cable to connect directly to it.

With a barrel carb (a properly designed one at least) at full open the carb is simply a straight thru hole with a spray bar. A fully open slide carb varies a bit with design, but for the nova carb there is a hemisperical projection with the fuel outlet, the other side is formed by the funny shape on the end of the slide, and that area is also interrupted by the bore for the slide. Not an optimal shape, but considering the large bore used, probably not that important.

I saw an article in a race car mag about a system built the supposedly had teh advantages of both systems. It was a barrel carb with a sleeve over the barrel that opens in the opposite direction. Thus the carb begins opening in the middle so the air flow is smoother than a normal barrel carb. At full open it is like a normal barrel carb. I have been thinking about trying it, but the design and linkages are more complicated!

Ian.

 

[GTR]

Hi Nitrocrazed,

Sounds like a very interesting design and may combine the best attributes of both designs. Ultimately for the best of power and mixture consistency we need a design that uses an adjustable aperture (slr camera style) combined with an annular discharge fuel inlet. This should maintain uniform flow in a straight line along with no obstructions to the airflow. In a true venturi shape inlet tract off course.

I suspect that carbs are an as yet untapped source of additional - or more effective- power making. Maybe we need to look at the ability to vary the length of the inlet tract as well related to RPM for the ultimate effectiveness.

Time will tell.

GT

 

[TimD]

I suspect that carbs are an as yet untapped source of additional - or more effective- power making. Maybe we need to look at the ability to vary the length of the inlet tract as well related to RPM for the ultimate effectiveness.

Time will tell.

GT

Carbs are compromised in terms of performance - no question. However if you look at our particular application where we are mostly at WOT then variable lenth inlets are not needed. Variable length inlets are used to shift the torque curve through the rev range. This is the sort of thing a load type dyno is perfect for. You can tune the inlet tract to WOT at the correct amount of revs.

Tim.

 

[GTR]

Hi TimD,

I take your point about the variable length intake tract. I think we compromise our cars way to much - even given they are WOT most of the time. As an example of this I have seen few (any?) barrel carbs or slide that are actually a venturi shape the all seem to be straight through the barrel/needle area. I guess machining an actual venturi in a steel barrel that disrupts the airflow at lower openings doesn't warrant the diffculty from a manufacturing point of view. However in search of optimal (rather than ultimate) power I think we could do a bit more development.

GT

 

[TimD]

You are 100% correct in that our carbs are not a true venturi mainly due to manufacturing costs etc.

Perhaps a slide plate carb with the correct shape and length venturi in front of it would work.

Ian and I did a little research a month or so ago into venturi shapes in regards to optimum airflow on a flowbench and there are surprising gains to be made with minor changes - up to 12% in terms of flow thru the same sized orifice.

 

[Ian Inverarity]

Some carbs do have close to a venturi shape. The actual barrel section is parallel, but the rest of the profile is approximately venturi shaped. One of the better ones I have seen is the A-90 carbs.

GT, I am not sure quite what shape you are describing with the camera apeture type design, but these would have quite a bit of turbulence at partial throttle.

Ian.

 

[GTR]

Hi Nitrocrazed,

I meant something along the lines of multiple overlapping blades - would probably have turbulance issues at lower openings but this could be an advantage to fuel atomisation at slow lower airflows. Don't know.

Maybe some kind of slide carb where the barrel/slide travels ito and across the opening at a tangent. Really hard to describe what I mean. Would be interested in your flow results if you guy's are happy to share.

GT