Gate Throttle

[Gnuvolari]

I think these are also known as sliding plate throttles. I am trying to re-instate this arrangement on a 1950's MG sports/racer. The set up was twin SU carbs with the butterflies and spindles removed. I have the carbs but need to carry out the sliding plate throttle mod. I'm looking for any data/pics/parts.

[zac510]

Are you referring to the setup as seen in the Mikuni FCR carb?

[Gnuvolari]

I'm not sure what the Mikuni set-up is like. The arrangement I am looking for is to use on a 1930's MG which was extensively (and very effectively) modified for competition use. The sliding throttles were custom made by the guy who built the car (Don Moore of Cambridge). He showed me the original carbs about 12 years ago but unbeknown to me he sold them. The car is now toghether and I'd like to use the sliding throttle arrangement. I'm expecting to have to fabricate something, and I have a reasonable mind's eye picture of the original set up. However, any data is scarce, to say the least. I have been trying for ages to get any info' on this subject and even placed adverts in motor magazines asking for info, but you are one of the very few people who have responded.

[zac510]

I did a quick search on Google but I couldn't find a definitive picture. If you search yourself you should see and read enough to understand.
They're also known as 'flatslides' and I've seen them fitted to all sorts of cars. Just a fortnight ago I saw them on a Volvo B23 engine!

Converting carbs sounds like a nightmare but it sounds like you want authenticity aswell?

[tzei]

Are you referring to something like these?
http://e30m3performance.com/tech_art...ides/index.htm

[Gnuvolari]

Yes!

I have been doing some drawings and I don't think it will be impossible to difficult to fabricate something.

Many thanks for the assistance.

[tzei]

Originally Posted by Gnuvolari

Yes!

I have been doing some drawings and I don't think it will be impossible to difficult to fabricate something.

Many thanks for the assistance.

No it´s not impossible but moldings costs money - at least here. A friend of mine is making a copy from BMW Schnitzer slide throtle assay...

Good luck with your quest!

[thebear]

They were also fitted to the BMW engines in some Chevron sports cars.

The comments about throttle response are true. Consider that you have two identical circles, side by side. The area (not the diameter) of the circle is the air intake. As the two circles begin to overlap there is a small opening but it does not increase linearly as the overlap continues.

Perhaps if a transition piece were fabricated the openings could be rectangular and therefore linear (but VERY expensive and lots of testing required).

[zac510]

Originally Posted by thebear

Perhaps if a transition piece were fabricated the openings could be rectangular and therefore linear (but VERY expensive and lots of testing required).

The same phenomenon occurs with butterfly throttles.

[tzei]

Well i haven´t notised any problems with throttle response. What comments are those? TheBear if you will...

[VIVA GT]

I've always been told that the major advantage with slide throttles compared with butterflies is that on full throttle there are no restrictions. Wheras there is still the throttle spindle and thickness of the butterfly in the airflow with the other systems. (Plus of course, if the throttle stop gets bent allowing the butterfly to go over centre, it's effectively closing again).

Just my ten pennyworth...

[MikeBz]

And I've been told that that 'major' advantage is about 1% of JS.

Raise you 20p

Mike

[thebear]

Originally Posted by tzei

Well i haven´t notised any problems with throttle response. What comments are those? TheBear if you will...

The ones in this article:

Join Date: Aug 2003
Finland

Posts: 82
Are you referring to something like these?
http://e30m3performance.com/tech_art...ides/index.htm

[JohnD]

The bear,
In fact the area of an opening slide throttle is linear, if the slide hole and intake lumen are circular in section, or if they are both rectangular. If the intake is circular and the slide edge rectangular, or vide versa, the area does not change linearly.
I can show you the calculations if you like.

The problems of the slide throtle are:
1/ that the slide arrangement approximates to a variable orifice, rather than a tube of variable diameter, as a butterfly does. Turbulence occurs at much lower flows in an orifice than in a tube, hence the apparent non-linearity of the slide throttle.
2/ At low throttle openings, the low pressure down stream means that atmospheric pressure exerts a considerable force inwards on the slide plate. There must be careful provision in the design for free movement even under this condition, or the plate will stick.

Barrel throttles achieve the same lack of obstruction at full throttle, but approximate to variable diameter tubes, and have a much larger bearing area to combat sticking. It is of interest that early carbs had slide throttles that were cylinders, achieving the same advantages, but needing a lot of end clearance.

I think that slide throttles are a design dead-end, a motorsport dinosaur, only to be copied for originality reasons. Barrels are the way to go.

John

[surfpac]

Gentlemen,

I found your forum when I googled "slide throttle". Here's a link to pic of a slide throttle that some rotary aviation guys over here in the US is using on their aeroplanes.
http://www.rotaryeng.net/slide-throttle5.jpg

Here's a link to one on a peripheral ported Mazda 20B engine that some guys in Canada fabricated. Very nice indeed. Look at the short video of the engine idling on the page of this link. You can see the throttle plate moving when they rev engine.
http://mpsys.ca/MPSv1.htm

JohnD, I would like to know more about the barrell throttle system and how this can be fabricated. It sounds very interesting.

Thanks

[Gnuvolari]

All very interesting. Looks like the slide may be made out of PTFE/tufnol or something similar, presumably to reduce sticking?
I am going to look at a pre-war setup soon (Dixon, I think) which apparently works well and somehow incorporates rollers in the slide.

[JohnD]

surfpac,
Thank you for thinking that I am capable of making one!
I did try to make a slide throttle, but ran into the problems above (and I did take into account the points made by Gnuvolare)

AFAIK, a barrel throttle needs to be manufactured from a specially made casting, or possibly from a billet, so that the barrel is a very close sliding fit inside the throttle body. Standard engineering methods, I'm sure, but I couldn't do it in my workshop.

There are places that advertise them for sale, eg Lumenition, but they are usually quoted as "Price on application", which is another way of saying, "If you have to ask the price, you can't afford it!"

Jhn

John

[R59]

Regarding the original question, was it something like the Amal remote float chamber type jobbys (Concentric) that they used on things like the BSA A65 Spitfire, if you take the float chamber away, it's just a plain throttle body, and 1.5" seems to ring a bell. They were a barrel slide, rather than a flat slide.
Just clutching at straws... maybe Amal made a true flat slide version that was bigger, who knows!

You'll probably find Lumention's POA is due to them being virtually one-offs where they make them to fit your application, and add into that the variable price of the matierials.

Aluminium and Brass have been going up and down like yo-yo's on the commodities markets recently.

What are F1 engines using these days? Flutterbys, rotaries (barrel), slides? or something equally cunning?

Rob.

[Gnuvolari]

The original question refers specifically to SU carbs which were modified to slide throttle operation. These were originally fitted to a 1934 MG developed by Cambridge engine tuner Don Moore (as in Lister Jaguar "works" engine builder).
Before he died, about 10 years ago, Don showed me the pair of 1" SU's with his sliding throttle conversion. He'd kept them for some 50 years. Since then, I have 're-created' his old MG racer using the original bodywork which was found in a south coast lock-up.
Alhough the car is presently running quite well on standard SU butterfly throttles, I'm very keen to get the car exactly as it was when Don ran it, and the sliding throttles were something he was very proud of. He was getting over 70 brake from a 1 litre engine of 1934 origin. He was a pretty hairy driver by all accounts and won masses of events with the car. I have some of his trophies.
Herewith a period picture of the car with Don and one taken this year with yours truly.
Since starting this thread, I have managed to borrow a 6 carb set-up from a Brooke Riley which has a sliding throttle arrangement. The sliding plate has little rollers. I will probably copy this detail as it is known to work well.
Fortunately, I am able to remember quite a lot of detail about the original carbs which I held in my hands about 10 years ago, but which, unbeknown to me, were sold at a classic car auction in a box of bits. Frustaratingly, they are probably sitting under someone's bench.
I will post some pics of the Brooke set-up after the w/e when I have taken them.

[Gnuvolari]

Here are the pics