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Custom Triple Su Carbs


DatsunZman04

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I had a crazy thought rolling around in my head and was wanting to get all of your opinions on the subject.

 

Why hasn't anyone built a custom intake and linkage to run 3 SU carbs? I know its easier to buy a set of triple mukinis or solerex for around $1,500 to $1,000. So my question is what would be the downfall of running 3 round top su carbs??

 

I can make a template and plasma cam a flat flange and custom make a intake to bolt on 3 SU's. Now I know that the linkage would have to be redone but it's doable. .

 

The Benifit

1. Everyone has a set or two of Su carbs laying around.

2. It's a different look.

3. performance might possible be the same.

4. I can make a intake for less than a couple hundred at my shop, plus I can do my own powdercoating.

 

Downfalls????

 

I leave this all open for your guys suggestions, you wont hurt my feelings any just let me hear what you have to say!!

 

DatsunZman04 :2thumbs:

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It has been done a few times already. There are actually threads on this site that explain why it isn't done more commonly.

 

Look around, maybe check out the place that reconditions SU carbs for Z cars....He might have the setup on his own car.

 

The difference is in firing order.

 

Datsun 4-cylinder:

 

Front carb sees 1 miss 2 miss 1 miss 2 miss

 

Rear carb sees 4 miss 3 miss 4 miss 3 miss

 

Nice, even, consistant vacuum pulses through each carburator.

 

Datsun 6-cylinder:

 

Front carb sees 1 miss miss miss 2 miss 1 miss miss miss 2 miss 1

 

Mid carb sees miss miss 3 miss miss 4 miss miss 3 miss miss 4 miss

 

Rear carb sees miss 5 miss 6 miss miss miss 5 miss 6 miss miss miss

 

The firing order results in very uneven vacuum pulses across the different carburators; making fueling less predictable across all the cylinders. A balance tube normally would balance this out, and would defintitely be REQUIRED to make it work well.

 

No, performance will not be equivalent to the triple-two barrels. The Weber/Mikuni/Solex/OER/SK two-barrel sidedraft carb setups are independent throttling setups. One cylinder, one throttle plate. Individual per-cylinder tuning can be done, although it's rarely required. The big benefit comes from the cumulative throttle plate areas.

 

A single 50mm throttle body is enough to feed a 350HP L28ET. A single 44MM SU carb is enough to feed the L24....two were used SOLELY for mixture distribution. You can, and there are other Nissan built intakes, to install a single downdraft carb in the middle. But, let's look at the advantage of the Triple sidedrafts...

 

A pair of 44mm throttle plates (stock Z carbs) is 304cm^2 of throttle plate area.

 

A set of 44mm Mikunis has a throttle plate area of 912cm^2...three times the throttle plate area.

 

A set of 44mm Mikunis will flow a HELL of a lot more are than the stock carbs, at WOT. You would need SIX stock carburators (so one SU per cylinder) to get the same benefit as running the triple sidedraft carbs.

 

So, the reason it's not done more often is that it's MUCH easier and simpler to run the three triple sidedrafts on a commonly available manifold for DOUBLE the potential throttle area, than it is to try to cram three SU's into place on a fully customized, one-off manifold that will need to be very well designed in order to work properly.

 

That, and a set of three matching, reconditioned, properly jetted SU's, plus the fuel rail, plus the linkage, plus the manifold, PLUS the air cleaners, PLUS the PCV system, brake booster, distributor, ect....

 

You are going to come out cheaper buying a set of used triples and rebuilding them.

Edited by Xnke
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Triple SUs can work if they dump into a plenum before going into the intake runners.  Jaguar is one example of that working.  But individual runner triples have the firing order issue listed above.  Its been tried many, many times.

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Yes, and the balance tube (the bar running between all the runners) is probably not big enough to serve as a plenum...run all the runners into a plenum as Johnc points out above, and you can get away from the intake pulsation problem.

 

Looks like a TR6 manifold there, eh?

 

I have a buddy here in town who built his own manifold like that for the Zenith-Stromberg carbs, and his runs "ok" around town but once opened up it sounds pretty off and his plugs are all over the place for mixture. O2 sensor says he's running a clean 13.5:1 AFR, but the plugs don't lie...

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  • 2 weeks later...

I met up with a guy locally today to have some joint help work done on my car. While wandering through his parts pile, I saw a custom triple SU manifold. I asked him if thats what it was, and indeed it was he said. He said he might consider parting with it, but it was custom made, its all chromed out, and he said he wouldnt part with it for less than 2500$... first time I saw one, and reminded me of this post...

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I had a crazy thought rolling around in my head and was wanting to get all of your opinions on the subject.

 

Why hasn't anyone built a custom intake and linkage to run 3 SU carbs? I know its easier to buy a set of triple mukinis or solerex for around $1,500 to $1,000. So my question is what would be the downfall of running 3 round top su carbs??

 

I can make a template and plasma cam a flat flange and custom make a intake to bolt on 3 SU's. Now I know that the linkage would have to be redone but it's doable. .

 

The Benifit

1. Everyone has a set or two of Su carbs laying around.

2. It's a different look.

3. performance might possible be the same.

4. I can make a intake for less than a couple hundred at my shop, plus I can do my own powdercoating.

 

Downfalls????

 

I leave this all open for your guys suggestions, you wont hurt my feelings any just let me hear what you have to say!!

 

DatsunZman04 :2thumbs:

 

 

Been done. I took this of a local vintage racer. Not pretty but well sorted

and very fast car. Photo taken about 8 years ago.

post-188-0-26747100-1382844295_thumb.jpg

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  • 5 years later...

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