13 replies to this topic

[VAR1016]

In one of DCN's BRM books (Vol 2) Tony Rudd is quoted as saying (roughly) that when a throttle butterfly is opened there is a huge increase in airflow (this to illustrate the difference between butterflies and slides and also appropos of the Lucas P.I. system).

I should like to know if anyone is aware of published curves showing the airflow characteristic of a butterfly throttle.

Thanks

PdeRL

[VAR1016]

Please chaps! Can anyone help?
Thanks

pdeRL

[Greg Locock]

If you want a sensible reply I think you'll have to put the exact quote in from the book, what you have written makes little sense.

[VAR1016]

Originally posted by Greg Locock
If you want a sensible reply I think you'll have to put the exact quote in from the book, what you have written makes little sense.

Whoops I'll do it today

PdeRL

[VAR1016]

O.K.
Here's the quotation:

"Sadly the amount of air passed by a throttle butterfly is not in straight-line proportion to its angular movement. The first five degrees from closed would produce a massive increase in airflow and power. Then although the last five (out of 80 degrees) below the torque peak might not produce any more power, the [Lucas] system still sent it six per cent more fuel to burn."

PdeRL

[Greg Locock]

That's just geometry. Calculate the plot of open area vs throttle angle for a disc. For a slide carb it is linear. The fuel management system needs to account for this difference in characteristic. As he says, the last 5 degrees of a disc opening makes very little difference to the airflow, whereas the last 5% of a slide valve increases the throat area by 5%.

[VAR1016]

Originally posted by Greg Locock
That's just geometry. Calculate the plot of open area vs throttle angle for a disc. For a slide carb it is linear. The fuel management system needs to account for this difference in characteristic. As he says, the last 5 degrees of a disc opening makes very little difference to the airflow, whereas the last 5% of a slide valve increases the throat area by 5%.

Thank you Greg; now I shall ask my brother (he's not motoring-aware but at least he can probably do the geometry!)

PdeRL

[Powersteer]

www.lumenition.com/roller.pdf

Some stats there for flow. My curiosity is will the perfect shape of barrel and slide throttle under full throttle increase horsepower compared to butterfly type because by design they seem to provide undisturbed flow. I also read before on two strokes that a bridge exaust ports has 30% flow resistence than an open port but the bridge helps increase overall port size. Thats 30%, i wonder how much flow does the butterfly disrupts.

[turboteener]

Is it just me or does that flow information make the roller barrels look really bad? They lost flow over every other test except up at WOT. I don't know many road race cars that spend all their time at WOT. Part throttle transiition is more important.

[Powersteer]

Seems the barrel type has a more consistent flow, a straight flow line through out the flow chart. Reason why the graph makes the butterfly look good is because at say 45 degree the buttlerfly throttle opens two holes with the barrel opening one.

[VAR1016]

Originally posted by Powersteer
Seems the barrel type has a more consistent flow, a straight flow line through out the flow chart. Reason why the graph makes the butterfly look good is because at say 45 degree the buttlerfly throttle opens two holes with the barrel opening one.

First I should say thanks to Powersteer for posting the link, although it did not help me. I am stuck with butterflies since any alternative is too expensive and the manufactured throttle bodies fit only standard Weber spacing.

The reson I wanted to know about airflow through butterflies was that I run Lucas mechanical fuel injection. The fuel cam on the car came from a FVA engine and as such would have been intended for slide throttles. I can't afford to develop a fuel cam either; mind you the car is going very well at present!

PdeRL

[J. Edlund]

There is a way to calculate the massflow through a butterfly throttle based on its angle, I have some info about this, the problem is that it isn't in english. However, this means that it should be possible to calculate a curve for massflow through the butterfly throttle, compare with slide throttles and a compensation for this can be made.

[ZoRG]

Ive played around with modelling barrels and slides, slides are difficult to package, barrels are fairly simply, there is a SAE article studying the tumble a barrel system produces, which seems impressive, but then your injectors need to be on the outside of the barrel, the problem here is that when you let off from WOT the fuel will pool in the barrel opening unless you tune it with a tps. bit desireable on FI cars.

I am in the process of making 2 sets of barrel tb's, one for tubo, one for NA, kind of like the lumenition system, will let you know how it turns out.

[Patrice L'Rodent]

Hi All from Australia on ANZAC day, the special day when we remember our war dead.

Several posters have stated one way or another that a slide throttle gives linear opening. This is not correct with a circular throttle body. Slide movement gives linearly increased area up to half throttle and then decreases linearly until the throttle is closed. For a matched travel/area solution, one would need a square or rectangular throttle body.
Pat D'Rat