Triple SU set up at the Nationals

[240ZGL]

Anyone know who owns the car or better yet, where he got this set up. It was parked on the side street, white with a carbon fiber hood. That set up is just too cool.

[Goose52]

Belongs to Steve from Z-Therapy - he drove the car down from Oregon. I talked to him about it - all custom with lots of fabrication and sectioning. I didn't hear if he had any production plans - you could email and ask.

Did you see the custom decal he had made up for the air cleaner housing - shows the three carbs!

[240ZGL]

The work is so good I thought it was factory Japan. Wow what a set up.

[RAZ1]

I was talking to Steve after the show and for now it is a one of a kind setup. He has no plans to manufacture them at this time. It's very unique setup and looked very OEM. He made everything from scratch from the intake manifold to the balance tube. He did a great job putting two original air cleaners together to make one longer one. Maybe if enough people ask him about it, he might think about producing a few. But I doubt it.

A few years (quite a few) back there was a discussion about a 3 SU setup never being able to work. I think Steve did this to prove that theory wrong. Are you reading this Mark?

[Carl Beck]

Hi RAZ1:

I don't think Steve has proven Mark wrong - so much as he has had fun building a conversation piece.

Getting a 3 SU set-up to work with an in-line six isn't all that difficult, indeed it was standard on some of the earlier Jags. for a brief period. Getting it to work better than, and/or add performance to, was the real question.

Jag dropped the 3 SU set up because it didn't work better and it did add a level of difficulty to the tuning process.

The problem with the triple SU set-up had to do with maintaining an even flow of air/fuel mix to each cylinder, as it was needed through the intake runners. Because of the firing order and timing of intake valve opening/closings.. the two SU set up does a better job than the three SU set up.

Because the stock two SU's will supply all the air/fuel that an L24/L28 can use - adding the third SU really adds nothing. The triple Side Drafts, give you the ability to tune each cylinder individually and feed all equal amounts of air/fuel mix... without the conflicting intake pulses present in the triple SU set up.

Mark explained it this way:

With the dual SU manifold the air / fuel distribution is poor because the runner lengths are different. Datsun worked their butts off to try and compensate for this and really did a pretty good job (you should see how bad some of the British stuff is). The advantage of the dual manifold is that the pulsations come in even periods. Let me try to put this visually;

1---\

\

2------- 103020103020

/

3---/

A "0" represents a "dead" time slice, a time when none of the cylinders fed by this manifold are in the intake phase. Any other number indicates the cylinder which is in the intake phase.

4---\

\

5------- 050604050604

/

6---/

As you can see the pulse pattern is very even, being one time slice on, then one time slice off.

This means that each cylinder is pulling on a column of air/fuel (what is contained in the manifold) with about the same velocity, because each cylinder is preceded by the same length of "dead" time in the manifold. This promotes all sorts of positive effects, and keeps air speed in the manifolds fairly constant. It limits the amount of "flow reversal" that can take place by limiting the length of "dead time" in each manifold. These effects are mitigated to some degree by the differences in individual runner lengths and by the distances from the main plenum of each manifold to each intake valve. But all in all this manifold and firing order do work fairly well together.

Now here is the pattern for a triple SU set up;

1--\

--- 100020100020

2--/

3--\

--- 003004003004

4--/

5--\

--- 050600050600

6--/

You can see from this that the front and rear manifolds have uneven pulsations. 3 time slices off, 1 on, 1 off, 1 on. This means that the cylinder that opens after the 3-time-slices-off "dead" period, will be pulling on a much slower column of air/fuel than the other cylinder will.

This will cause those cylinders (#'s 2 & 5) to run a little leaner and their counterparts (#'s 1 & 6) to run a little richer. The reason for this is that air reacts to changes in velocity more quickly than the fuel does.

Since the air/fuel mix in the manifold has 3 time slices to slow down before cylinders 2 & 5 draw on it, it will be slower for them and as it tries to speed up again, the air will respond more quickly and get to the cylinder slightly sooner that the fuel. By then the intake stroke for these cylinders will be ending, the extra fuel meant for these cylinders will stack up in the manifold and then cylinders 1 & 6 will open.

They will be opening to a fast moving, extra rich, mixture. (On some 4 cyl motors this phenomenon is so bad that the engine idles on only two of the cylinders if the mixture is not within range).

The center manifold is different, however, its pulsation pattern is even, with 2 off time slices followed by 1 on time slice. each cylinder will be pulling on a column of air/fuel with about the same velocity, but thatvelocity will be fairly low due to the fact that twice as much time is spent "off" as is spent "on".

So the triple SU manifolding has three basic problems;

First, the pulsing patterns are not the same for all manifolds and,

Second, the outer manifolds will have distribution problems caused by uneven pulse patterns and,

Third, the center manifold will suffer from reduced velocity.

The third problem is the least important of the three and is true for triple Mikuni's as well (in fact they suffer from a 5:1, off to on, ratio). I would not be concerned about this except as it relates to the behavior of the outer manifolds.

The solution for the outer manifold's distribution problems is probably going to be similar to what was done for the British 4 cyl, siamesed engines. The runners of the manifold were "aimed" at the lean running cylinders slightly. This helped to overcome the distribution bias that was created by the uneven pulsing. I am not sure how well this would work on the Z motor, but on MGB's it helped quite a bit. It didn't eliminate the problem, but it did significantly reduce it.

The last problem, that of the manifolds not all having the samepulse patterns is not soluble except by altering the firing order. But by ameliorating the negative effects of this, we can to some degree cancel the problem out.

If the manifolding is done well it will be equal to, or slightly better than the stock manifold for distribution, but will increase flow. If runner and plenum size is kept small enough to keep air velocities up, the drivability and throttle response should be better than a dual set up.

Anyway, there's something for all you triple SU freaks to chew on for a while. You could solve all these problems by going to 6 of the 1.25" (32mm) SU's you know. :-)

- - - - - end quote from Mark - - - - -

FWIW,

Carl

Carl Beck

Clearwater, FL USA

http://ZHome.com

[deadflo]

There was a feature article in the old Z car magazine about a guy who had done a similar set up. The article mostly focused on his short stroke hi revving setup.I remember the car was the Baby poop yellow color, and a unique rubber door guard strip. I recall the strip because years later I saw the same car on a Z club website. I beleive the car resides in the pacfic northwest somewhere. Anyone know the car I'm referring to?

[240ZX]

Good explanation Carl. Kinda makes you wonder about a guy thats supposedly a SU guru, who goes forth with a tripple SU set-up!!!? Hmmmmmm!

[Ron Carter]

Steve did it at a conversation piece and for fun. i think he had upwards of a couple hundred hours in the conversion. It was very well done to look like an original set up and it works well. Does it work better than the stock set up...well only Steve can answer that question. My understanding that higher RPM that it really goes well.

I would not question or fault Steve for this project, he did this because he wanted to and he could make it work. And it does. He drove that car down from Oregon.

Is he a SU guru absolutely...He is a Datsun Guru as well. 240ZX, i think you are being unfair to make such a comment. There are many people on this site that use is products and swear that they are the best you can buy.

This is an unbiased opinion from me, because all my cars use Solex or Mikuni carbs. I know Steve and his wife Pam because we travel in the same circles. He knows his stuff....

[Arne]

Add to that the fact that Steve has no plans to sell this set up to anyone, nor is he making any claims that it is better than something else. I suspect he did it simply because he could, and wanted to have something different.

[26th-Z]

I think it looks cool. Steve has my recommendation as well.

[texasz]

Carl, I must say that was quite a write up! I must also admit that I didn't read it all and yet I still have a question. This information indicates that the triple SU setup is a poor idea and does not add to the performance of the car. My question is then why are there so many racers using a triple carb setup with Webbers, Solex, Mikuni, etc? If the triple setup is no better why do the racers not know this? My guess has to do with the differences in the type of carbs we are talking about...though I'm no expert and thus my curiosity.

[Arne]

Yup, type of carbs. The SU's are a single throat, the Webers and Mikunis are dual throat, so each cylinder gets its own. Which you probably would have figured out had you read Carl's post all the way through.