twin hs2 vs single hs4

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Dantiri - 04 Oct 2003 10:30 GMT

Hi,
I have a stage 1 998 engine with a cooper head and a HS4 carb. I would like
to know if I would get any benefits fitting twin HS2 carbs.

Cheers
Dantiri

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-AD- - 04 Oct 2003 12:21 GMT

And Dantiri was sitting next to Elvis in the spaceship, which I thought
was kinda strange, but then they turned to me and said:

> Hi,
> I have a stage 1 998 engine with a cooper head and a HS4 carb. I would like
> to know if I would get any benefits fitting twin HS2 carbs.

Maybe a slight improvement at the top of the rev range, but nothing
dramatic.

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(-AD-) <uniqueid 'at' lineone.net>
http://website.lineone.net/~uniqueid/
There is less in this than meets the eye.

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Steve - 04 Oct 2003 15:35 GMT

> And Dantiri was sitting next to Elvis in the spaceship, which I thought
> was kinda strange, but then they turned to me and said:
[quoted text clipped - 5 lines]
> Maybe a slight improvement at the top of the rev range, but nothing
> dramatic.

That surprises me, AD. I won't disagree with you but I always thought that
the smaller carbs would increase gas speed possibly improving low to mid
revs output. Then, because of their limited maximum flow, they would
restrict top end output.

The larger carb would then be the other way round. At lower revs, the low
gas speed would reduce efficiency. Then the larger bore would allow greater
flow at high revs.

Not totally sure about this, but it is always what I have assumed.

Obviously, the other issue with twin carbs is that they need to be in
perfect condition to work well. Worn ones will be difficult to keep in
balance. I do remember playing with twin HS2's on a Midget years ago and
never managing to keep them right. Then I rebuilt them and, hey presto, they
never needed looking at again.

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Rgds
Steve
steve@dsnclassics.co.uk
www.dsnclassics.co.uk

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-AD- - 04 Oct 2003 18:24 GMT

And Steve was sitting next to Elvis in the spaceship, which I thought was
kinda strange, but then they turned to me and said:

> > And Dantiri was sitting next to Elvis in the spaceship, which I thought
> > was kinda strange, but then they turned to me and said:
[quoted text clipped - 9 lines]
> That surprises me, AD. I won't disagree with you but I always thought that
> the smaller carbs would increase gas speed possibly improving low to mid

A smaller carb would have higher gas velocity in theory, but twin SU
manifolds typically have the two passages interconnected by a large
balance pipe. Air is effectively drawn through both carbs simultaneously.

Would be kind of interesting to block off the balance pipe in a twin SU
manifold and see what the results would be.

> revs output. Then, because of their limited maximum flow, they would
> restrict top end output.

On the dyno there is often a small lift in the power curve at around 4000-
5000 RPM through to max. speed with twin SUs - I don't really have any
explanation for the phenomenon (maybe something resonant occurring?)

> The larger carb would then be the other way round. At lower revs, the low
> gas speed would reduce efficiency. Then the larger bore would allow greater
> flow at high revs.

Large single SUs seem to be pretty good at producing the goods at low
speed. There's good tolerance of quite silly carb size mismatches.
Probably all that happens is that the piston will never open up all the
way if the carb is too big, effectively making it behave like a smaller
carb anyway.

> Not totally sure about this, but it is always what I have assumed.
>
[quoted text clipped - 3 lines]
> never managing to keep them right. Then I rebuilt them and, hey presto, they
> never needed looking at again.

Twins are also somewhat more fiddly and time-consuming to set up on the
dyno, which could be a major consideration if dyno availability/cost is a
factor.

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(-AD-) <uniqueid 'at' lineone.net>
http://website.lineone.net/~uniqueid/
ereh saw icniV ad odranoeL

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