TR3/4 4xSU CARBS ?

[Hamish]

Just a random muse on carbs for a Tr3/4 engine after looking at the Sabrina twin cam that used very rare 2 twin choke su’s. 

It seems nowadays there is a propensity to fit big Weber’s that as I understand it have a choke for each piston. 

So my thoughts are :- 

why not fit 4 Su carbs eg the smaller h4 Su s

has it not been done because of over fuelling and or the balancing of all 4 carbs. 

The pic is the Sabrina engine. 

[oldtuckunder]

The DSU's are acknowledged to be crap, I think even by the people who are recreating them, seems to be an exercise for those with more money than sense and a desire to have something different from in period. But you have to admit they do look the part!

As for fitting 4 individual SU's  I think space wise you would only have room for the HIF models i.e. the later ones with integral float bowl, not sure but I think that the smallest HIF is 1.5" and 4 of those would be serious overcarbing for a 2L engine,  a too big CD type carb for engine capacity tends to behave badly as they spend virtually all their time at low piston lift because there is insufficient air demand, also one CD per cyclinder means they are getting very few pulses at the jet, and its the pulses as well as the air stream that affects attomisation.  Non CD carbs like a lot of small motorcycle ones behave much better one to one, if you like they are more like a single weber barrel in some respect even if they have needles. If you want 4 carbs then I'd investigate bike carbs, however given the vast amount of fabrication work and I suspect a huge amount of set up experimentation, its probably cheaper, quicker, easier (as all the parts and data are available) to fit twin weber's.

Be a fun project though, I have often looked at  the Vitesse engine and thought it would look pretty with a row of six Amal carbs.

Alan

[Hamish]

9 minutes ago, oldtuckunder said:

Be a fun project though, I have often looked at  the Vitesse engine and thought it would look pretty with a row of six Amal carbs.

Alan

Alan

thank you. I think you have answered the reason why it’s not done. Space and pulse rate for efficiency. 

 

But I’m with you - a row of carbs would look great. 

[PaulAA]

Hamish

Less in-period, but somewhat more businesslike is this 4-carb option.

This version for the TR6 certainly got my blood pumping.  All in an idealistic, forget-the-practicalities sort of way, of course.

I recall that somebody posted a six Kehlin installation on a '6 in the Other Place, for which he had fabricated the manifolds and throttle linkages himself.  I have a small framed image of that one on my mantelpiece, garlanded with a perpetual candle and burning incense

Paul

[PeterC]

Basics of SU operation are here. The air flow stilll has to create constant depression to lift the piston, whether SU, Stromberg or Keihin. or DSU.

https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/

So...reduce the constant depression by removing the spring, as in Hamills book ??  - and it will run at part throttle like a bag of nails, as the mixture quality suffers.

Intersting that it impossible to find data on the depression created across a Weber. There will be one, even if it were a perfect venturi, which it is not.

Peter

 

[JohnD]

Like this?

 

Don't look very straight to me.

Edited February 14, 2018 by JohnD

[PaulAA]

2 hours ago, JohnD said:

Don't look very straight to me.

True, they don't.  Seems pretty difficult to get the PI manifolds out of true - I wonder if it's intentional.

This looks a bit better: