If you're fighting Weber Gremlins...check this out concerning linkages

[traindriver]

I'm in the process of switching over to a 4x 48IDA setup for my 351W. I wasn't happy with the pre-done kits so I'm going to build my own. In designing the linkage I was looking the typical side pull systems. Typical of webercarbsdirect.com kits and some but not all others.

For back to back or front to front carb mounting, these systems typically use a side mounted bellcrank and then a rod to the topside mounted lever of the left carb bank and then a second rod to a bottomside mounted lever on the right bank carbs.

I built a kinematic computer model of the linkage and measured its ability to move the left and right bank carb levers equally. The results were fasinating. Although it looks as though the linkage would move the levers identically, they don't, in fact the error at half throttle can be over 7 degrees. that works out to a 25% cross sectional area difference in the throat!! Here's a graph of the results, the dashed line is a perfect linkage, the solid line is the side pull linkage. Bottom line, for a front to front or back to back set up, use a centrally mounted bell crank, which BTW needs to be very accurately centered particularly down the axis of the motor...but that's another analysis.

A graph of the results is attached along with the linkage model.

The reson for the big error in the side pull, look at the approach angle of the linakge rod to each lever, one is acute the other obtuse. The tangential component of each displacement vector when the lever is moved is different.

If you have Webers you know Cal Metal's favorite point, it's usually not the carbs but rather the linkage.

Food for thought if you're battling your Webers.

Regards,

Chuck
SP1591

[RedBarchetta]

Originally posted by traindriver


"I built a kinematic computer model of the linkage.... The tangential component of each displacement vector when the lever is moved is different."

All I know is that the optimal angle for achieving the longest home run is 35 degrees, and that the actual break (deflection from a straight line) of a curve ball is more like 3-4 inches. The rest is a combination of optical illusion and the force of gravity.

I'm going to stay close to this thread....eventually I'll dwelve into Webers, too.

-Deano

[tblackgt350]

You are exactly right concerning this linkage; I knew the carbs weren't opening at the same rate, I just couldn't figure why. I do not think it is possible to mount the carbs back to back(all fuel lines on the outside) and use aside pull linkage with optimum results. Once I got a set of reversing plates that allowed me to mount the carbs facing the same way, with all throttle levers pointing up, the synchronizing became pretty easy.

[traindriver]

gt350,

You can do a side pull using 2 bellcranks, but then you have to start worrying about slop.

Here's a pic of a Pierce center bellcrank on a front to front FE set up, if you reverse the bellcrank and have it pull from the left, the rod should end up close enough to you next link at the firewall. You may have to extend your actuator a bit but it should work...It better, that's my plan for my SPF.

Chuck

[DonC]

Just my opinion but I'd want the throttles to be pulled open rather than pushed. Also might change the position of the bell crank a bit forward. Looks like the onside linkage is at a greater angle than the off side. Also try to "balance" or center the amount of pivot that the bell crank has to make so it's even from stop to wot.
Just thoughts.
DonC

[tblackgt350]

Traindriver-

what I should have said is that you cannot get the linkage right on a 289-302 with the carbs mounted back to back on a GT350 or mustang. You can get it right pretty easily on a big block because you have all that room to mount a bellcrank in between the carbs.

If I were going to run IDAs on a small block in a Cobra, I would definately run a shaft down the center of the carbs mounted on two stanchions with each carb having its' own hex link running to the shaft. The shaft twists and pulls each throttle lever equally; also, that way there are no interconnecting tabs between carbs eliminating slop.

Tim

[traindriver]

GT,

I saw CSX 2327?? at a car show saturday. It had the longitudinal shaft, but the carbs were facing the same way. With a 351 I'm hoping to have enough room for the bellcrank method. I just ordered the carbs from redline tonight, should begin working on it this weekend.

Can you post a pic of your setup?

Chuck

[tblackgt350]

I don't have a digital camera yet, but I would be happy to send photos by snail mail if you like. By the way, if you saw my set up in the current Mustang Monthly magazine that is not how it looks or works now, so disregard. I'll see if I can get my brother to shoot a couple of pix and post on here tommorrow.

Thanks, Tim

[marcocsx3121]

A general question about Webers: while the visual impact is undeniable, are they worth the hassle and cost vs. a well tuned four barrel? I saw a dyno comparison somewhere recently that did not indicate significant gains anywhere in the rev range.

[tblackgt350]

If the car is tuned properly there is no doubt that 48 IDAs produce more power than a four barrel; you have a venturi spraying cool emulsified fuel right into the intake port from about two inches away. For all practical purposes you have a fuel injection system. I wondered if it would make a big difference too, so I checked. Using the same chassis dyno before and after, I gained 44 HP.

[tblackgt350]

Traindriver-

I am shooting digital pix of my carb setup tommorrow, will try to post Sunday night. Also-you should check out our new website. tnsaac.com and look at the slide show for SAAC 28. There is an image of Jim Maxwell's perfect 289 cobra system. No throttle interconnects-just a hex link to each carb.

Tim

[marcocsx3121]

Quote: "I wondered if it would make a big difference too, so I checked. Using the same chassis dyno before and after, I gained 44 HP."

To my point, I don't know what your baseline was, but $4K+ and the necessity for a lot of tuning time may not be merited by 44 RWHP. I gained 60 on a blown LT1 Camaro by removing the cats.

[ToyCollector]

Awesome analysis and insight Chuck!

[tblackgt350]

It just depends on what you're looking for, really. If all you want is the most horsepower you can get for the dollar, webers are not the way to go. You can get a supercharger for less. It is just a nostalgia thing; some of us like a cool induction set up that is period correct for the car, that's all.

[Bluedog]

The tnsaac.com link doesn't work. Is this the correct URL?

Thx,
Bluedog

[tblackgt350]

You are correct- www.tnsaac.com should work. I just tried it and it came up okay. Thanks,

Tim

[Ford of France]

My kudos to Chuck. He is light years a head of me in understanding the physics of all of this. Very good analysis. I wish my parents had invested in a math tutor for me at an early age. My time would have been better served.

Tim is correct, too. The best way to make these work is to eliminate the throttle inter connections between the carbs. It really leaves to much room for distortion.

The cleanest set up I have seen using the system Tim has described was on Rob Walton's Daytona Coupe. Absolutely, no room for slop and very clean.

Thanks for the discussion, guys

Cal Metal

[BERNIE]

My own linkage work perfectly at any stage closed, mid open, open with no variation between right and left carb.
Bernie.

[priobe]

Has anybody tried the linkage from Pierce Manifold. Also does anybody know what length rods are used from center to carb?

[priobe]

Has anybody tried the linkage from Pierce Manifold. Also does anybody know what length rods are used from center to carb?