A small, semi-tech thought on multi-carb setups

[C9]

Rudy's (Roadstar) post on swapping his triple 2 setup for dual quads got me to thinking.

My preference is straight linkage on multi-carb setups.

As far as progressive linkage goes, I think there's a better way to do it.

Thinking back to the good old daze where some guys ran two 2 bbls on an intake that had the carbs sitting about where the end carbs on a triple setup would be, most reported no real change in gas mileage and some reported an improvement in mileage. Slight, granted, but for those who paid attention there was an improvement.
Said improvement due to better fuel distribution within the intake.

That said, I'm wondering if we're not going about the triple 2 progressive linkages all wrong?
All wrong being the center carb feeds the engine until approx half throttle and then the others kick in for the performance gain.

Makes sense that a single carb would give good mileage, but I'd bet that blocking off the center carb and running straight linkage to the outside carbs would improve the mileage a small amount as well as improve throttle response with two accell pumps getting the engine over the momentary lean spot until the main jets start pulling fuel.

To that end, why not have all three carbs on a triple setup running a functioning accell pump and have the end carbs doing the low speed duty until half throttle and then the middle carb comes in for added flow and HP?

I believe the middle carb should still have its accell pump operational to help the engine over the lean spot when the middle carb is cracked open.

Seems too that the progressive linkage would be a little more fool proof.
At least it would if built with quality components.
Some of the commercially available shit (can I say shit?) for triple 2's I see are just disasters waiting to happen.
Best one of those seen so far - and this on a show quality late 50's Chevy pickup - was nothing more than bent wire.
Wire about .120 in diameter, but it was still bent wire.
Including the slider that had a simple round spot bent in the end.
I won't be surprised to learn that this cool little truck has rear-ended someone or worse.

I know the guy who owns the truck, but he thinks the linkage is just fine.
Maybe so . . . it is chrome plated.... http://www.jalopyjournal.com/ubbthreads/images/graemlins/tongue.gif

[Scotch]

Lots to think about, but I feel it's relative to everything under the intake, and also to the engine design architecture in general. The intake plenum is also a major consideration.

On the sbc and Pontiac with their four pairs of teamed intake ports, I feel the needs are distinctly different than somehting like a sbf Windsor or Mopar Hemi with evenly-spaced and spread out intake ports. Modern intake manifold designs have proven that plenum volume and shape are critical to proper distribution, and many of the "early" tri-power or multi-carb setups had none of this knowledge behind their design. It seems to be a "hit or miss" engineering exercise with many of them.

I feel the paired intake ports of the Poncho and sbc sem to prefer the center carb active/outboard carb progressive style, and this may be due to the proper plenum sizing on even the factory tri-power units (from Pontiac anyway). I think teh Chevy stuff borrowed much from the Poncho and works well because of it.

I also think the sbf/hemi style of spread out intake ports likes a larger plenum (relatively speaking) and may respond well to the linkage design you've outlined.

I also feel the further the carbs are from the intake valve, the more of a pump shot they require to maintain aolid off-idle response. I ran a cross-ram on the street (a Max Wedge part on a 440 Mopar) and I couldn't give it enough pump shot. The larger the pump shot, the more it yanked. I maxed out the adjustments as an experiment at the dragstrip once and was rewarded with the best launch, best 60-ft time, and best ET the car had posted to date. Big plenum intakes like big pump shots.

So, to summarize, I feel a tri-power setup will work fine with progressive linkage on a small-plenum intake with small ports and a small cam. The acceleration should be linear and smooth, and not too much pump shot should be required.

For a large plenum intake, more pump shot is required and less progressive carb linkage is necessary...

Just my opinions...!!

Scotch!~

[DrJ]

Along C9's thought progression of sinchronized pairs of carbs, I was looking through the Hot Rods by Ed "Big Daddy" Roth book a short while back and noticed that the center two carb's air horns on Tweedy Pie's 6x97's set up were clean while the four on the corners were doscolored. Then I noticed the center two carbs didn't have the external enrichening rods on them, because they're dummies!
It looks like Ed ran it on the four carbs sorta like a Man-a-fre set up.

Having run a lot of different British 4 bangers with dual SU's I think it's a good way to go too.

Especially since they usually have the paired intakes in the heads, like Scotch was talking about, with a carb over each, (and a balance tube, so they can "cheat" off each other) but they don't really have any plenum area to speak of but are very efficient when tuned right.

( I'm convinced that most British sports car "tune" problems are poor valve adjustment, not the SUs, but people try to tune the carbs before adjusting the valves and that just doesn't work!)

[Boones]

I was planning to run the four outers on my 6x2 intake..and keep the two inners as dummy's

Thanks for the "food for thought"

[C9]

[ QUOTE ]
people try to tune the carbs before adjusting the valves and that just doesn't work!)



[/ QUOTE ]

Saw a lot of that on dirt bikes.
I think cuz the knobs et al are out there in the open and they're easy to screw with.
Ran a few of these twiddled with bike carbs back to baseline settings and that usually cleared them up.
And if we were at altitude in the dez or mountains, lowering the needle one step lean usually did the trick.