I just prefer SU's over ANY carb, they're so much easier to work with because you only have one needle to mess about with, the fuelling is controlled by the vacuum, and they're DAMN SMALL!!

I never did like the Nikki or Holly carbs - too many variables to mess round with

 

No problems with that, but do you actually need seperate primary and secondary SUs because of the way they operate.

 

Are hurleys intake built so you open primary and secondary port separatly? for twin su?


svenne

 

Look at the picture I posted earlier in this thread!!! That is a Hurley intake. I got it in around 1988 I think. As you can see it is one carburettor for each rotor. For each carb, one half the air goes to the intermediate housing port and half goes to the end housing port.
Like this:
front carb=primary and secondary ports of front rotor
back carb=primary and secondary ports of back rotor

There is no way to toggle the flow left or right from each carb.

 

Which is why I don't like them - you don't have as much control over the port fuelling. I'm merely combining the best of both:

# Simplicity of the SU
# Fuelling of a twin/quad barrel carb

 

And so I respectfully repeat: there is a reason why ALL SU apps are 1 butterfly per barrel, namely that the constant pressure design of an SU means that you do not have the vacuum signal problems at low throttle opening that drive other designs to need a twin choke setup.

Effectively an SU has a totally variable choke built in. That's what it does best. I look forward to seeing the outcome of your experiments, but am hard pushed to see that there is a problem trying to be solved here.

Happy to be proved wrong.

 

Ok cosmicbang Now I understand
I thougt your intake was homemade
!`m gonna build a intake like yours.
what is the diameter on the runners?

svenne

 

I had this idea for a couple of years ago so I had a question on our swedish forum but nobody thought
it gonna work because theirs no accelerator pump on the su. then I found out that hurley had a kit with these
and I put in a question here and thanks alot for all information /svenne

 

I need to try and get a better picture, but basically the steel plate that bolts onto the engine is cut to the size of the ports and then an oval tube is rolled from sheet, brazed on and the flanges added at the top. The runner to port matching is abysmal on the stock hurleys and I spent evening after evening smoothing it out.

The hurley manifold was not 'designed'. I'll measure the runners tonight.

 

there was a post on here a while ago that gave the calculations needed to tune an intake length to give higher power/efficiency at a certain Rpm. Hurley only made the runners long enough to meet up with the carb, without using too much of a sharp angle

 

Yes that is true, I had that conversation with him about runner lengths before he made my intake. Since the end result was acceptable we did not change anything. I was experimenting with phenolic spacers but didn't get any conclusive results. Another characteristic of his intake is that it is tilted upwards to clear the steering on RHD cars. I was initially concerned about the floats being affected but never had any problems with it. If it is going in a LHD car I think the intake could come out horizontally. Let me know if you want more pictures of it.

 

Yes thanks if you have more pictures of it so can send up them or mail me them if you want

svenne

 

SU's are nearly always placed on an angle anyway. On a rover V8, they're almost at 45 degrees!!!

If anything, you want the runner to be extended AFTER the carb. when you suddenly take off the accelerator, the fuel supply drops. On a normal runner length, this doesn't cause any problems really. But on a tuned inlet, it can be fairly long, and this will cause a load of fuel-rich air to still be going into the engine, causing it be sluggish on throttle response.

 

hello again!
now is my intake almost finished and the carbs are jetted but i wonder wich float valve I should use? and fuel pressure?It was verry cheap parts to the su carburators hope its gonna run fine

svenne