[mjm421]

Has anyone done drag strip testing comparing vacuum linkage vs. mechanical linkage on their tri power?

[Tom Vaught]

About 2 tenths, Mike on a basic Tri-Power 389 engine. At least on my deal, Only did the test once. My Stock 64 Service Package Tube and Plunger 64 Mechanical Linkage vs a set of 65 Vacuum Carbs I borrowed. Most of the stuff to go to the Vacuum set-up was on the 65 carbs.
Did not plumb the temp switch and already had the Vacuum can installed on the car.

Never got out of the 14s with the tires I had but car did have 4 speed and the 3.90 gears at the time. Mid America Raceways was a 1200' dragstrip located near Wentzville, Missouri. Why I did not report 1/4th mile actual times.

Tom V.

[Dick Boneske]

With a manual transmission and vacuum Tripower linkage, as you leave the starting line the manifold vacuum drops to near zero, so the end carbs will open at WOT as long as there is sufficient vacuum in the tank. If you need to feather the throttle to minimize wheelspin,, the vacuum switch on the center carb will signal the end carbs to close and you're suddenly running on only the center carb--until you hit WOT again. It takes a fraction of a second for the end carbs to open again and unless you keep WOT, the end carbs will close again.

I remember seeing brand0new '64 GTO's the first time on the drag strip leaving the starting line like a bucking bronco as the above occurred several times until full throttle could be maintained. The vacuum tank does it's job if the lines and valves are leakproof, so there is enough vacuum to keep the end carbs open through the quarter mile run. Those same guys with their new GTO's were back next week with mechanical linkage--most of them with the infamous Ansen ball bearing linkage, since they didn't know they could get the rod & tube setup from their Pontiac dealer.

For "normal" street driving, the factory vacuum linkage is not user friendly. The end carbs are either closed or wide open. There is no way to open the end carbs partially.

Additionally, there's the fraction of a second (seems like a long time) before the end carbs close as you back off the accelerator pedal. That is really scary even after you get used to it.

[Jack Gifford]

Originally Posted by Dick Boneske

... For "normal" street driving, the factory vacuum linkage is not user friendly. The end carbs are either closed or wide open. There is no way to open the end carbs partially...

Surprised you would say that. I'm sure you realize that mechanical linkage is zero help there, with nothing but main-jet fuel systems in the end carbs (no idle nor part-throttle nor power valve systems). Truth is, mechanical linkage takes away the ability to use almost full-throttle of the center carb for most driving (which is the main reason PMD employed vacuum linkage).

You are correct about the drawbacks of the factory vacuum linkage (delayed opening and closing). If a person were to address and solve those issues, it would be a great system, with more flexibility than mechanical linkage.

[Dick Boneske]

Originally Posted by Jack Gifford

Surprised you would say that. I'm sure you realize that mechanical linkage is zero help there, with nothing but main-jet fuel systems in the end carbs (no idle nor part-throttle nor power valve systems). Truth is, mechanical linkage takes away the ability to use almost full-throttle of the center carb for most driving (which is the main reason PMD employed vacuum linkage).

You are correct about the drawbacks of the factory vacuum linkage (delayed opening and closing). If a person were to address and solve those issues, it would be a great system, with more flexibility than mechanical linkage.

Jack, I'm surprised you described the mechanical vs. vacuum linkages as you did. I've owned many Olds & Pontiacs with three carbs and the difference in drivability between the two is huge.

With mechanical linkage, set up as I believe it should be, the center carb is about 2/3 open before the end carbs begin to open. Then, you have the option of WOT or part throttle on the end carbs. As you know, there are a number of holes in the levers to allow four different ratios between the center carb lever and the rear carb lever. If set for the maximum center carb opening before end carbs begin to open, the transition is very smooth, and with the added squirt of gas from the two accelerator pumps, there is no hesitation.

With vacuum linkage, you can set the center carb vacuum switch to activate the end carbs at any throttle opening you choose. You may be able to get decent operation with the switch set slightly beyond the 2/3 throttle position that can be achieved with mechanical linkage but no matter how the switch is set, you cannot achieve WOT on the center carb before the end carbs open. When they do open, they open all the way--there's no way to get part throttle with the end carbs.

[Tom Vaught]

It is a known fact that when a throttle blade is 80% open from vertical blade position you have about 95-97% of the air flow thru the carb. Been proven many times on Holley's Airflow Test Stand. That extra 10 degrees on the WOT end does not buy you much. Also the throttle blades are actually angled (at idle) so you never see a 90 degree measured change from closed to open throttle position.
I believe the Rochester Blade angle is around 12 degrees.
So 90 degrees minus 12 degrees is 78 degrees of travel and the air flow is minimal at 10 degrees from a vertical 90 degree blade so the actual real airflow vs blade angle is more like 68 degrees of blade change to get the majority of the airflow to the engine for each carb.

Tom V.

[Jack Gifford]

Originally Posted by Dick Boneske

... ,you have the option of WOT or part throttle on the end carbs...

That is not an option- If the end carbs of a Pontiac Tri-Power were held at part throttle the engine would go very lean.

[Kenth]

I once modified the vacuum secondaries to mechanical for test, using the correct mech parts.
I found the mech linkage boring since it felt like most of the rush/whooshe at WOT gone.
Also did not act well at part throttle downshift.
Feathering the throttle for part throttle into the secondaries, w/A/T, was simply not the way to driving excitement for me.
Back to the original vacuum linkage, not looking back.

JMHO

[Dick Boneske]

Wow! I am really surprised at Jack & Kenth's responses to this thread. I've experienced both vacuum & mechanical linkage on many of my cars with the exact opposite result. Of all the people for whom I've restored their Tripowers, 100% share my opinion that mechanical linkage is far superior to the factory vacuum setup. The only exception is the handful of people who wanted to retain exact factory appearance for high dollar restorations. Pontiac engineers must have arrived at this opinion also, as they offered replacement mechanical linkage for Tripowers beginning in 1961, and by 1965 built all Tripower manual transmission cars with mechanical linkage. All three-carb Cadillacs used vacuum linkage as all those cars were automatic transmission, as did the '57, '58 Olds J2 setups, nearly all of which were automatic transmission cars. I don't know what Chevrolet did on the big block three-carb Holley setups, but I believe all the small block three-carb setups used vacuum linkage. The '66 Olds three carb setup used only mechanical linkage.

To each his own, but the above is what I've experienced. I've attached a picture of a '62 setup I restored with factory vacuum linkage and a similar '59 setup with factory over-the-counter rod & tube linkage. Note that there are far fewer components needed with the mechanical linkage.

[mjm421]

So if you are converting a 65 to progressive from vacuum, what do you do with the transmission kick down switch?

[Tom Vaught]

It is possible to make the Kickdown switch work, Mike with a Mechanical Tri-Power Set-up and Turbo 400 Trans.
A guy from Bloomfield Hills had that set-up in college in the 70s and he had someone make it work right for him.

Kickdown switch activated off the center carb extension. (If I remember correctly).
Bracket for kickdown mounted off the rear carb bolt pattern somehow.

Tom V.