Weber 32/36 Vs Twin Su Carb

[phantom]

I have tried to have a search and have had no success!

 

I have a 4k that I am about to bolt back into my ke70 and wanted to know what carb I will get more power out of?

 

I have the option of a 32/36 weber jetted down to suit a 4k, rebuilt and all.

or a twin su setup to suit 4k.

 

The only other mod I have done to the car is extractors, everything else is standard.

 

I prefer the look of the twin SU's but I am after power not cosmetics.

 

any help greatly appreciated.

 

Cheers

[Taz_Rx]

Reading this thread would be a good start...

 

http://www.rollaclub.com/board/?showtopic=34252&st=15

 

:jamie:

[altezzaclub]

I can't really speak for downdraught Webers, I've only dealt with sidedraughts & SUs on Datsuns-

 

However twin DCOEs will outperform SUs in instant acceleration as they have pumps jets (usually massive ones!) and SUs have a fraction of a second's worth of lag before they get going. You just get used to driving SUs, a slightly different style.

 

When it comes to paying at the pump, SUs blow Webers out of the water completely, especially on trips when the slide carb system comes to the fore. SUs will run your car at the minimum fuel/air to suit the throttle opening.

 

I expect the downdraught Weber will have the advantages & the disadvantages of the DCOEs, but SUs may give better topend power if you're talking inch & 1/2, as that's two 37mm chokes when wide open, compared to your 32 & 36. In reality other factors will probably overwhelm that difference- manifold design, airbox design, air filters used, cold air box & heat shields...

 

I'm going SUs, with an airbox, panel filter, & cold air intake. I'll fit heat-wrapped 4-2-1 extractors if I can get them, or the 4-1 set I have in the garage.

[Felix]

However twin DCOEs will outperform SUs in instant acceleration as they have pumps jets (usually massive ones!) and SUs have a fraction of a second's worth of lag before they get going. You just get used to driving SUs, a slightly different style.

 

Not quite. Su's have an accelerator pump effect. Read up on what the dashpot oil does.

 

but SUs may give better topend power if you're talking inch & 1/2, as that's two 37mm chokes when wide open, compared to your 32 & 36.

 

This seems to be a common misconception.

 

With twin 1.5" SU's having a divided manifold, where each carb can only serve two cylinders, each cylinder can only see one carb throttle plate at a time. Area of one 37mm butterfly = 1075 mm^2

 

With a 32/36 DGV each cylinder can see both throttle plates of the carb.... Area of DGV's butterflies = 804+1018=1822 mm^2

 

Considering the 4k is stock with only extractors, you'd probably get better results by retaining the stock carb. Fit a sports airfilter and rejet it.

[kickn5k]

Not quite. Su's have an accelerator pump effect. Read up on what the dashpot oil does.

 

Considering the 4k is stock with only extractors, you'd probably get better results by retaining the stock carb. Fit a sports airfilter and rejet it.

 

 

Yes my car was toatally different car to drive depending on what dashpot oil i used.

None = snappy throttle response but poor economy.

Auto trans fluid = good throttle response and reasonable economy.

Penrite dashpot oil = best of all!!!!!

 

Back to topic su`s sound awesome and you will get really good economy for a set of twins.

But I'm with felix get it tuned properly with a good air filter, will save you a bit of cashola untill you can mod the old 4k to get the full benefit of the twins.

p.s. don't i repeat don't use the datto su knockoffs they are more headache than they are worth!!!

I ran a set of english su`s for over 2 years and only needed the setup tune and that was it. A mate has a set of jap ones on his datto and seems to need a tune about once a month.

 

stu.

[philbey]

Yeh I've heard nothing but bad words about the Hitachi's off the Datto's. Funny that, the english making something better than the japs.

 

They're the ones with the really square edges.

[Redwarf]

I had a pair of 1.5 SU's, dyno'd them once and they were great. Never touched them until I pulled them off for DCOE's.

 

Hitachi's make great paper weights. A lot like Solex's. :jamie:

[altezzaclub]

The bit that I can't copy/paste about a cylinder 'seeing' one or two throttle plates :jamie:

 

I think that works if the cylinder is very small compared to the throttle size. However at what I think the drag of the chokes becomes more important and you still have a 4 cylinder motor feeding off either 2150 or 1822 sq mm.

 

Some dyno testing will tell!