Slide Throttle

[max123]

I was reading those old post about the slide throttle in the S42 post and here's a thought on why a slide throttles plate might provide poor throttle response.  Its all math and has everything to do with the fact that the throttle body opening is circular - if it were shaped like a square it wouldn't be a problem.

When a butterfly valve starts to open it creates an equal sized opening on the top and bottom of the circular shaped throttle body intake, as the throttle opens further the gap on the top and bottom of the throttle plate increase until at full throttle they are equally sized half circles with the butterfly valve sitting parellel to the throttle body opening  (this creates very linear and smooth throttle response).  On a slide throttle the throttle plate slides one way across the circular shaped throttle body which creates a crescent shaped opening - as the throttle opens the free area allowing air to pass into the engine does not increase in a linear fashion.  The math is fairly straightforward (pythagerous - 3.14, etc).  But the short answer is that when you first touch the throttle nothing is going to happen because there will only be a very, very small cross-section for air to go through - Touch the throttle a little more and the size of that opening is going to increase exponentially - until you pass 1/2 open - then it starts to decrease.  Anyway - it would feel nothing like a butterfly valve.  If the throttle body were square on a slide throttle it should feel exactly the same.

Does this make any sense?

[fiftytakedowns]

/thread

[cecotto]

I'm not sure I ever heard that the slides should have poor throttle response.

I've always been of the understanding that the "problem", which i would prefer to call tradeoff is the throttle modulation.

But on a racecar spending it's time on full throttle this tradeoff might just be worth it.

[twright]

Max123:

Your analysis makes sense.  However, it does not take into account what type of external linkage is on the throttle body.  For example, the E30 M42 throttle body has two different locations to hook up the throttle cable.  These two locations offer a significantly different throttle response  -  all with the same throttle butterflies.

[JP 91iS]

Max123:

Your analysis makes sense.  However, it does not take into account what type of external linkage is on the throttle body.  For example, the E30 M42 throttle body has two different locations to hook up the throttle cable.  These two locations offer a significantly different throttle response  -  all with the same throttle butterflies.

one of those is supposed to be where cruise control hooks up

[papercutout]

JP is correct.

BUT, lets be honest, with a decent slide (square) throttle, we would have a larger throttle body entry, and so let more air through.