Vizard SU Carb's

[Mark1965TR4aBRG]

Folks

 

As my shiny new engine is taking place, thoughts turn to fuel and air...

 

So knowing that the original bog standard HS6 SU's can be improved upon who has experience of this:

 

http://www.ebay.co.uk/itm/131635598967?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

What could I expect from the Vizard type re-work?

 

or do I just fit twin 45 Webers and be done with it?

 

Previous experiences welcome

 

Many Thanks

[saggy]

Twin 40 Dellorto DHLA's over either option.

[Peter Cobbold]

Mark,

Unless you're going circuit racing stick with standard SUs and forget Vizard, Hamill etc. SU knew what they were doing, developing their carbs since 1905.

That beveled front edge to the piston was tried by the late Badfrog and it messed up the carbs. He ditched them.

All the other mods in the photos are aimed at maximising air flow at higher max rpm than normal, eg boring out the choketube, thinning the butterfly spindle, and I guess removing the spring and damper too.

For road driving we need excellent mixture quality to get flexibility- ie the best possible atomisation and to get that a fast air velocity over the jet. Racing mods can lower air velocity over the jet and increase mean droplet size at wide open throttle. Big droplets dont burn well.

Two 2/75 inch SUs are OK for 150-175 bhp:

http://www.sw-em.com/SU_vs_disp.jpg

The pressure dropped across the piston is actually very small, less than 0.25psi and probably around 1% of atmospheric pressure. No-one ever gives that figure for Webers or Dellortos but it might well be bigger !

Unless the air flow is very high the bellmouth wont reduce that 1% pressure drop, but will need the needle refettling from standard.

Peter

 

https://supertrarged.files.wordpress.com/2015/05/the-operating-limits-of-an-su-carburettor12.pdf

page 9 ( Healey example)

page 21 Atomisation and air velocity

Edited November 20, 2015 by Peter Cobbold

[Motorsport Mickey]

"Overall the flow will be up and will rival any DCOE Weber"...Ha

 

That's it then, better tell TR4 Tony that he'd better fit SUs for that performance advantage...WHAT !

 

Granted you can expect that a tastefully (and in my opinion what is shown isn't, the radiuused or tapered front piston edge is counter intuitive and loses the fuel lift from the bridge) modified pair of SUs will perform excellently and probably deliver better torque and responsiveness than a pair of large webers through the revs up to about 5500 revs and handle power figures up to maybe 160hp (mine did) but this depends upon the rest of your engine and ancillaries but for ultimate power the webers will outperform them. It doesn't have all the junk inside butterflies etc and a carb bridge that hinders the ultimate flow.

 

If you are thinking of a plus 160hp engine then I'd go for the Webers or Dellortos (unless Mikunis etc take your fancy) but then you'll be turning the car into better than a fast road spec and likely into a not very pleasant road drive. Dreaming of driving country roads with the hood down and you and the missus enjoying the sun and burbling along at 45-50mph ? not with the spec that will come out of plus 160hp and the carbs to deliver it, how about running the engine in a gear lower with the consequent increase in revs and noise to prevent it bogging down or the plugs fouling ?

 

As for carb choice to give you bling good torque delivery and a bit better top end Saggy is the man, he's on fire with his suggestion of 40s Webers or Dellortos whichever floats your boat.

 

Mick Richards

Edited November 20, 2015 by Motorsport Mickey

[stuart]

I too have twin 40 Dellortos and they do perform very well.

Stuart.

[Mark1965TR4aBRG]

Thanks folks.

 

So I plan to run her in on the SU's and have since got myself some 40mm Dellortos.

 

Stuart - can you please share your jet/needle set up please?

 

I'll be on 89mm pistons, stage 2 head and full phoenix if that helps..

 

Many Thanks

[Mark1965TR4aBRG]

The Dellortos fell through & the geezer on ebay doesn't respond, so that's out and sounded fishy at best, so after running in I'll give it a tune up on the SU's and take it from there.

 

I do like the idea of the Webers, but thank you Mark & Peter for your inputs - the humble SU properly sorted will I'm sure deliver more than enough for most of the time, but the draw of the under bonnet bling and the inlet roar is calling for that odd occasion.

 

I've run twin webers some years back albeit on a 2.0 Zetec that revved like crazy and I managed to get them running really well, so with all the info' about nowadays what could possibly go wrong!!