How about triple SU's?

[Guest JAMIE T]

I have two sets of early SU's. What do you guys think of making a triple set-up? I'm cheap, but I am a welder/fabricator(TIG/aluminum). One problem I see is the choke set-up. I would need three cables. I'm open to criticism, or ideas. The engine is a F54 shortblock, N42 head(soon, I hope), and a performance cam if I can get a good deal on one. ZX 5spd, and 3.90 gears. Car is a '73 240Z.

[Drax240z]

Jaime, I really don't think the SU's are the weak link for producing power on a nearly stock engine. Yeah if ou are running a really radical cam and headers etc. it might be worth looking into, but I believe norm got in to the 12's on stock bodied SU's with some needle and other mods. If my memory serves me, he didn't pick up much (if anything) from going to a bigger bore SU except for better fuel economy.

 

Overcarbing an engine is usually a worse demon to deal with than undercarbing an engine.

 

I just think you'll be spending time and money for very minimal results, but that isn't to say it can't be done! And if you want to try it out and give us your impressions, then all the power to ya.

[Synlubes]

Hey Jamie,

 

If you can fab a manifold I think it`s doable!

I`m sure some kind of mechanism could be made to use maybe 1 cable to comtrol all 3 chokes???

I think(I may be wrong)that some of the old Jags had a triple SU type set up!

Maybe find some pics and see how they did the choke thing!

 

I have a brand new N42, but it`s not going to be cheap!!!

[DAW]

The chokes are the least of the problems and can be gotten around by a primer/squirt system.

 

The end-pairs of intake ports can be covered by L4 SSS manifolds. The inner-most ports are more of a problem and that's where your fabrication skills would come in. You would need to take the L6 manifolds and cut the intake port flanges off then cut each manifold through the centerline (vertical) of the carb mounting face and plenum, weld the two inner halves together and switch orientation and weld the flanges on. This center manifold could now be bolted to the head with the other two.

 

Alternatively, use the lower section of an LD28 intake and fabricate an upper triple SU manifold. You may need to have the center carb face the opposite direction from the other two to allow clearance for all three.

 

With these two designs you might end up with triple SUs; but they do not incorporate paired cylinders by ignition interval which would provide the optimum charge flow and low-end torque. That is, ideally, cyls 1&6, 2&5, and 3&4 would share the same SUs (and all have equal length runners); not cyls 1&2, 3&4, and 5&6.

It's easier designing an f.i. manifold because it's carrying primarily air and not air/fuel.

 

One last thought, consider removing throttle plates from the SUs, plumb their inlets together, and use a throttle body and a single linkage to control the airflow. DAW

[Guest JAMIE T]

Like I said, I'm open to all ideas. I know the dual SU's are enough for a mild motor, but I like to do custom fabrication, and thought this would be a fun excercise. The Jag E-type is what got me thinking about the triples. I think it would look bitchen with triple SU's(freshly polished) and TWM horns(velocity stacks). I've got two of those horns on my L24. They look great, but one more would fill the space better.

[johnc]

The firing order of the L6 is setup as if the motor is two 3 cylinder engines. Every cylinder gets one of the dual SUs all to itself every 240 degrees of crankshaft rotation. This results in a very even mixture distribution, high velocity, and low reversion across all 6 cylinders.

 

With a triple SU setup pairing cylinders with each carb you get an uneven mixture distribution and uneven power production. Look at the table below. The first number is the firing order and the second is degree of crankshaft rotation.

 

1 - 0

5 - 120

3 - 240

6 - 360

2 - 480

4 - 600

1 - 720 (0)

5 - 840 (120)

3 - 960 (240)

6 - 1080 (360)

2 - 1200 (480)

4 - 1320 (600)

1 - 1440 (720)

 

Using the table above you'll see below how cylinders 3 and 4 have an even distribution of intake charge but the other cylinders have uneven intake charge distributions. This causes reversion in the intake track and a very uneven mixture across both cylinders through 1440 degrees of crankshaft rotation.

 

3-4 Intake charge every 360 degrees.

1-2 Intake charge 480 degress, then 240 degrees.

5-6 Intake charge 240 degress, then 480 degress.

 

Triple SUs have been tried many, many times and they do not work as well as dual SUs. If you need more fuel/air then go to the 50mm SUs.

 

BTW... Triple SUs will probably work if they open onto a plenum before the intake runners. The larger the plenum the better. But, you might have problems with fuel metering.

[Guest JAMIE T]

Thanks John. That was way over my head. I under stand the 2-3cylinders thing, and how that would make the triples not work properly. OK, I'll go back to my corner now

[DAW]

What makes more sense than improperly paired cylinders with triple SUs is six smaller SUs (like from a small British 4cyl or a large motorcycle cyl). The small SUs should be mounted fairly close to the head (like each one onto an individual runner of a triple weber intake manifold). The inlet tubes to each carb would be paired by cyl ignition interval (1-6, 2-5, 3-4)and are equal in length. No throttle plates in the carbs, as all air regulation will be done by a single 60mm throttle body upstream. No major synchronization or throttle linkage issues and individual cylinders can be tuned for mixture if needed. So, there are long equal length runners that carry only air, not air/fuel, and these are configured as six-into-three-into-one plenum with a single throttle body and provide a high velocity, balanced flow at low and midrange rpms.

 

The system is a staged induction design as there is a short path/high flow/large plenum secondary manifold incorporated as follows: just upstream from the individual carbs the runners have a Y-junction plumbed into the primary runner tubes. On the short secondary side of the Ys sits a log-type plenum with large inlet at one end. The secondary manifold system is a dynamic unit. The plenum has an outer shell with six ports connected to the Y's. An inner insert is a slip-fit into the shell. The insert has six same-sized port-holes and when rotated, these ports align with the outer shell ports and permit flow, when insert is rotated back; flow is regulated back. Rotation/regulation is actuated by a vacuum actuator taking its signal from a flow-sensing carb venturi source (vs manifold vacuum). Actuator rod connects to lever attached to inner shell, shells have O-ring seals.

 

Secondary plenum large inlet is plumbed to main primary central plenum with an air valve proximal to it which isolates the primary airflow from the secondary. The "airvalve" is a large single SU (no throttle plate or fuel supply, just a heavy piston) this eliminates resonance into the secondary plenum, preserving low speed throttle response, and opens in response to flow when the secondary (staged) manifold system comes into play at mid to high rpm range, when insert is rotated. The transition between pri and sec systems is essentially self-modulating and directly linked to engine speed needs.

 

This staged intake design would work equally well with multiport f.i. as with the 6 SU carb model. The f.i. model with a turbo would provide excellent off boost performance This induction system may or may not be worthwhile, but functionally it is similar to the state-of-the-art staged manifolds on many new engines. DAW

[Guest JAMIE T]

Dang DAW, you sure have put alot of thought into that induction system. I'm not going to attempt the one you described(I'm too cheap), but I hope you will do it. I have priced the Motorcycle type Mikuni's before. I do think that would be cool induction. 6 in a row, big 1000cc Kawasaki Z1 carbs. That wouls look great, and give a TB per cylinder.