Triple SU's for L series engines

By: Marc Sayer


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---/



4---\

\

5------- 050604050604

/

6---/

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. 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--\

--- 400300400300

4--/



5--\

--- 506000506000

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 second problem, 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 same pulse 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 SU's you know. :-)

Marc Sayer,


Copyright Marc Sayer 1999-2003