17 replies to this topic

[grizzler73]

Hi knowledgeable types
I am wondering if it is worth filling in the balance pipe on a twin Su manifold. I have to use twin 1 1/4" on my race GTM (regs say so, bah) and will be doing the Vizard tweaks to them, will blocking the pipe off at either end with quicksteel epoxy or similar and then flowing it make much difference?
Cheers in advance!

[tiger99]

Why do you think it will make any improvement, and what do you expect it to improve?

[nicklouse]

As above. What are you looking for? Vizard did a test showing that it reduced flow on the BL casting. But the BL casting is really really bad as it is.

[Spider]

There are improvements to be had, but leave a small opening in there, about 1/4 - 3/8" does it.

[carbon]

Grizzler,

 

Do the regs restrict you to a specific BMC/BL twin HS2 inlet manifold? There are quite a number of variations, and some are likely to flow better than others.

[Dusky]

Wonder if you'd be allowed something like this?

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[nicklouse]

As the GTM does not have the bulkhead like the Mini any of the other BL manifolds from the inline configurations could work and they flow better.

[grizzler73]

I have a MG midget one and an early cooper one as well, I have to use a standard "looking" one for the regs, I was wondering if filling it in would improve flow significantly or not.

[timmy850]

I can't recall ever seeing a manifold without them, even all the performance ones made by aftermarket companies have the balance tube. I'd guess that for a Siamese port engine the small amount of cross flow would be beneficial, though as above improving the transition between the balance tube and inlet ports may improve the overall performance. 

[grizzler73]

From what I understand the balance pipe is there to compensate for the carbs going out of balance, so if you have a set up your carbs really accurately then it is not needed so much. Maybe the answer is to fill it but bond in a small alloy tube at each end, that way you still get a balance effect but with no massive holes to the side messing up the flow.

[carbon]

The balance pipe is there to steady out the flow though the two individual SUs.

 

It will be much trickier to get mixture correct throughout the range without it.

[grizzler73]

Hmmm. I think I need to look at all the a series manifolds and pick the best of a bad bunch! Thanks for the input everybody.

[nicklouse]

Also re read the rules and see exactly how far you think you could push them.

Could you fit an inlet for bigger carbs and modify to suit the smaller carbs?

Don't see why not. Not that I have read the rules but not that hard to modify.

As with any Motorsport read the rules. Read what is no written and if that helps use it.

Typical example, F1 front wings are not allowed to flex. Sounds simple.

So how are the wings tested. They are tested by applying load to the ends and if they do not defect more than 10mm they pass.

So front wings are allowed to flex 10mm. Big difference.

[grizzler73]

The regulations state:

 

"Carburettors must be of the same size, manufacture, number as original specification and located directly on to the original manifold which can be polished and ported.

BMC A Series 1275cc engines to use 1¼” maximum SU Carburetors"

 

Now, given it is a kit car there is scope for a bit of "interpretation" shall we say! I think I could fit a inlet for twin H4 SU's as it would still have casting numbers etc so look like the standard one, like the minispares HS4/6 one for instance? Or do you think there is a better option?

Is there a big advantage? I think I would put some spacers on as well. 

I really need to get this to breath well as I am only on H2 set up, I am really focused on getting the top end to work well as on these carbs it will top out at 6500 I suspect.

G

[carbon]

Some of the Midget 1275 hillclimb cars use twin HS2s and get very respectable power curves, will rev well past 6,500 but not sure at what point they get peak power..

 

Just takes a lot of patient fettling, it's all detailed in Vizard's magnum opus.