Power is such a terrible thing to waste!

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Slant Six... out of the ordinary - just like me!

Well Red, I intend to get that kinda snap goin', and I'll let you know how it goes with keeping my foot out of it...

I know at least with my low 14s beast I can rein it in, after I get done "testing" everything out.

Lou

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Home of Slant6-powered fun machines since 1988

Here's some edited commentary by a guy named Marc Sayer about the relative virtues and vices of dual vs. trible carburation on inline sixes. He was talking about Japanese and Brit engines, but the principles also apply to our slants. Thought this info might be useful to any of you thinking seriously about designing and fabricating a triple-carb intake.

Note that the triple-carb intake problems outlined below are a direct result of firing order...this is no doubt why Offy chose a dual rather than a triple when they designed their slant, multi-carb intake back in the 60s.

While reading this, keep in mind that the slant's firing order is 1-5-3-6-2-4.

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With the dual carb manifold the air / feul distribution is poor because the runner lengths are different. 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 carb 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 carb 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 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 slant, 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.

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---Red

Triple Weber DCOE's would be the best setup for even fuel distribution, however, the cost is pretty high. If cost is no object, then I have seen EFI throttle bodies that fit the WEBER manifold for a proprietary EFI system. TWD I think it was.......anyway, they have ads in Grassroots Motorsports magazine. That would be the ultmate setup.

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"That thing gotta HEMI in it??"

TWM is the company...and "not cheap" is the price. Would be slick, but I think you'd end up dropping at least $3-5K into that set-up before the smoke cleared. Not for the "budget minded".

A setup from Rance:

http://www.rancefi.com/

would probably be a little cheaper ($2-3K?), but that's not multi-port.

---Red

For $3k Rance can set you up with a complete multiport system including intake fabrication. Last I checked with them they said they don't do TBI systems anymore.

If you want to get crafty and cheap(relatively, each is about $600 new, I'm sure you can get cheaper ones if you looked), you can make a 1 barrel to GM TBI adapter, although that is going to be a bit restrictive, and run two of them on the offy manifold. Well actually you don't need to make any adapters, there are already 1 to 2 barrel plates by mr gasket etc and howell makes a 2 barrel holley to 2 barrel gm tbi plate. Either way it might be a bit high for your hood. A dual GM tbi setup would be nice, maybe a bit big though at 500cfm for each unit.

I recently saw some Weber 40 DCOEs for sale on ebay, from some guy in Italy. He was selling several configurations, a few of them were three sets of two barrel side drafts. That would keep costs down on the three Weber configuration.

One advantage of doing three Webers is that the tuned length of the inlet tract goes all the way through the carb, all the way to the end of the intake trumpet. This would contribute to better torque in the 3,000 - 4,500 rpm range. Most of the other carburetion configurations have a plenum under the carb so the tuned length terminates at the plenum. Unless you have a Hyper-Pak manifold, which I understand contributes to torque at around 4,500 rpm the manifold runners are so short that the tuned length is best at over 5,000 rpm. Many V-8s, by virtue of their shorter cranks, can spin to 7,000 - 8000 and make real use of the shorter tuned length ports.

Regarding peak torque vs peak horsepower: horsepower = torque (in ft.-lbs.) x rpm /5252; --- the torque curve multiplied by the rpm gives the horsepower curve. The horsepower peak always occurs at a higher speed than the torque peak. In the old days the rule of thumb was that the peak horsepower was at twice the rpm of the peak torque, but nowadays they are closer together - say maybe 1.5x

My point is that it is important to have as long of a tuned length inlet as possible in a slant 6. Three side draft deuces is one way to do this.

Would the air pulse problem be less if the distance from carb to head were reduced?

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Slant Six... out of the ordinary - just like me!

The carb is usually separated from the head by a reasonable distance. Because pressure pulses bounce back and forth there can be periods of reverse flow through the carb and it is not uncommon to have a fog of fuel just outside the carb, so a velocity stack is usually a good idea, maybe 6 inches or so. I have an old 40 hp. VW that needs to have an orange juice can on the carburetor in order for it to run right. A filter housing can also serve to contain any fuel fog, should it occur. Having said that, the carb is usually the next thing in the inlet tract.

If an engine designer had his choice, he would probably design the inlet tract with a two to seven degree taper, wider at the atmospheric end. If you've ever had the opportunity to see a cross section of a motorcycle cylinder head in a fairly recent magazine you have probably noticed this. Placing the carburetor near the atmospheric end of the tract puts it at the larger end of the tract and it is therefore easier to have a larger throttle bore which blends smoothly with the port. But as with anything else in an automotive design, there are tradeoffs to be evaluated.

I have been working on a design which the intake sweeps up 90 degrees to the carbs about 6 1/2". The head side is standard and the carb side is 3" wide by the length of the track. I don't worry about hood clearance, so if I need a stack, then I'll slap one on I guess.

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Slant Six... out of the ordinary - just like me!

Ok. Has anyone ever thought of running 6 1bbl's? That would be a bit of work, but?

I was thinking of that as another option, but thought it would suck too much air? I was tossing around a design for 3 primary carbs with 3 more as secondaries kinda like what a 4 barrel would do. It would look like a 6 pack. the linkage would turn out interesting...

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Slant Six... out of the ordinary - just like me!