SUs v Triple Twin Choke

[Rob Gaskin]

Ok guys, rather than hijack Vipergts's Thread lets discuss on here.

Why can you get more power from a triple twin choke set-up than twin SUs? If a good L series will give 250bhp on triples what would be lost on SUs?

Surely both types of carb can be tuned to give the correct mixture throughout the rev-range so is it just the air-flow restrictions of 2 chokes rather than 6?

Why can't 3 SUs work, thus giving 3 chokes? Is it to do with firing order and which cylinders are sharing each carb? After all Jags and Healeys use 3 carbs.

Who on here wouldn't fit SUs because they don't look as sporty as triples?

Who would prefer to fit SUs if they were available 'off the shelf'?

From a personal viewpoint I have stuck with my 2" SUs because Kevin Irons (who built my engine) suggested I stick with them. He took them away with manifolds (for opening up to match the carbs) and had them reconditioned for me. They have been great but are 45 years old now.

If I could guarantee getting another 10 - 15 bhp by fitting triples I might consider it but that would cost about a grand as far as I know. For comparison I would love to try my carbs on a similar spec engine currently using triples (or visa-versa) . Yes I know they wouldn't be perfectly matched but it would be very interesting.

 

[status]

Triple SU s have been done before on a zed,cant remember who but it was many moons ago

 

[SKiddell]

Triple SU's why
The pulse order is wrong, they require custom fabrication and complex linkages for no extra gain

A little bit of searching on this forum will answer all of these questions and more

 

[rallymanDP]

Slightly coming 'out of left field' to your topic, I know, Rob, but perhaps you could widen the question to discuss the third option of a set of ( 6) Bike Carbs at the same time.

In theory, could be even better than triples for a number of reasons, but I would like to hear from someone who has really made them work on the 'L' series of Engines and why they are considered to be superior in throttle response, etc.

 

[MikeB]

Duncan

I think bike carbs work because they are a flat slide configuration, so when the throttle is full open there is no butterfly in the middle of the venturi mucking up the air stream.

Had a set of Keihns on my 220hp GSXR hillclimb engine back in the late 90s, they make a clicking noise on tick over as the slides rattle in their slides. Very simple and easy to tune carbs, only one jet per carb from memory. One cautionary tale is make sure they don't suck in any water spray, as the water emulsifies on the slides and they stick open !!


Cheers

Michael

 

[Rob Gaskin]

A little bit of searching on this forum will answer all of these questions and more

No it wont, e.g.

Who on here wouldn't fit SUs because they don't look as sporty as triples?

Who would prefer to fit SUs if they were available 'off the shelf'?

 

[zedhead260]

Isolated runner manifold - this makes a difference too, it's not just Webers (etc)

 

[SeanDezart]

No it wont, e.g.

Who on here wouldn't fit SUs because they don't look as sporty as triples? Really ?

Who would prefer to fit SUs if they were available 'off the shelf'?

Depends upon the application.

How much power are the HSCC cars getting on standard size 240Z SUs ?

I like Duncans' suggestion and "Who would prefer to fit them if they were available 'off the shelf'?

 

[Sylv1]

Rebello advertises his 2.7 config with 240hp sae in twin SU compared to 270hp with triple
he has two combinations of camshaft... I assume it is with same setup

so? about 15% extra power with triple?

there is also probably to consider an asymptote for twin SU at a certain range of power




question could be also milage comparaison... because triple is also triple ration of petrol

Would you consider 5 or 10 extra HP if it costs 2 or 3 liters per 100km? that is a question nowadays

 

[Rob Gaskin]

Isolated runner manifold - this makes a difference too, it's not just Webers (etc)

Which is what I think Rob. As I said surely both systems can be tuned to give optimum fuel/air ratio and if that's the case they should both be as economical as each other.

Sylvain - interesting figures from Rebello, if they are the same engine spec. thanks.

Duncan, what would be the main reason for using bike carbs over triples?

I'm trying to determine if there is a 'market' for new SUs, mainly 2" HD8s. These are the only SUs Burlen would consider because they have a Jag 'market' for converting US spec E Types from Strombergs. Now there's a point Strombergs! (had those on an Imp Sport and Avenger GT), but then the Avenger was modified with twin-chokes !

 

[amocrace]

no one has mentioned the holley conversion. in theory no balance problems. i have not seen any power or consumption figures for these. i do know an engineering shop that has used bike carbs on a couple of non Z conversions and he has had problems with the manifolds with the mounting flanges being too thin and distorting

 

[SKiddell]

No it wont, e.g.

Who on here wouldn't fit SUs because they don't look as sporty as triples?

Who would prefer to fit SUs if they were available 'off the shelf'?

No but it is sufficient enough to provid those interested with enough information to make a well informed choice, and "sporty look" should have been eliminated in the first 5 minutes, however we all know better than that

 

[SKiddell]

no one has mentioned the holley conversion. in theory no balance problems. i have not seen any power or consumption figures for these. i do know an engineering shop that has used bike carbs on a couple of non Z conversions and he has had problems with the manifolds with the mounting flanges being too thin and distorting

Are you refering to that Swan neck manifold conversion that turns the air/fuel mixture through 90 degreees robbing it of all its velocity and inertia.......nuff said

No one has demonstrated any setup that delivers more power than either a set of triple twin chokes or throttle bodies, maybe theres a reason why considering that every possible permatation would have been tried over the years

 

[bigblock]

I really wish i could have played with these engines years ago,from what i remember with su's it's extra critical to get the correct weight spring and damper oil and manifold length/volume when there is a fair delay/spread in the induction pulses or cam on certain engines,especially triples on inline sixes(makes you wonder how harleys manage so well) after all its a cv carb,extra messing is why i imagine most go to other carbs.Plus if chasing power apply some of the carb flowing mods in many books to keep the velocity up but increase your airflow,you could also play about with a nominal size of balance tube,i also seem to remember webers and the like meter a larger fuel droplet size than su's possibly giving more max power.I've seen a lot of people just give up when tuning su's for other engines,they're my favourite carb,cfm is cfm,so cant really see well modded su carbs giving up that much max power to others.Still cant beat fuel injection for overall spread of power/metering though,note i didn't say maximum power lol.long live su's.Ps someone mentioned bike carbs, little info,straighter shot to chamber plus less volume of air to accelerate(quicker response) link to mikuni's/bike carbs usage in cars,loads of sites too choose from,info about half way down link page.3 modded hif 6 su carbs on decent intakes would look nice and i reckon would be very close to the usual webers and co. http://www.v-performance.com/products/printable_catalog.html

 

[bigblock]

Or another way of looking at it,modified SINGLE hif 6 13/4 inch su with extra plenum spacers/21/2 ram on a 1380 na road biased a series good enough for 120 odd horsepower plus excellent driveability with no hiccups anywhere.Modded su carb is like moving up to next size of carb without loss of velocity.We set up a weber 45 just for the top end on same engine/same day with decent manifold and gained 4hp.Oh and everyone at time said i was daft using the su and would be **** and lose loads of power.

 

[Mr.F]

HSCC cars ... on standard size 240Z SUs