Mechanical vs vacuum secondaries?

[Super Blue 72]

What is best?  I ask as I may be getting a used carb, a Holley 4165, a spread bore, 650cfm manual choke, double pumper with mechanical secondaries.

Will it work on my application?

The Holley carb selector says that mechanical secondaries are for cars lighter than 3100lbs, manual trannies.

The seller thinks it should work, just that the mechanical secondaries won't bog when you get on the gas like vacuum secondaries.

My stock Thermoquad has mechanical secondaries though?   

This is going on a bone stock '72 340, 8.5:1 compression, auto tranny.

Any input appreciated as I may buy it tomorrow if I can swing by the place.

Thanks!

Phil   

[Chryco Psycho]

It should be fine just do expect to floor the throttle from Idle without a stumble , you may have to 1/2 open it until you hit 1800 rpm or so .
It just takes a bit of practice to learn to drive it .
The TQ has mechanical secondaries but has an spring loaded air door prevents full air flow even with the throttle plates wide open .

[Super Blue 72]

It should be fine just do expect to floor the throttle from Idle without a stumble , you may have to 1.2 open it until you hit 1800 rpm or so .
It just takes a bit of practice to learn to drive it .
The TQ has mechanical secondaries but has an spring loaded air door prevents full air flow even with the throttle plates wide open .

Thanks for the reply! 

Sorry, Chryco, I'm easily confused.  So no stumble?  That good, yes?

What do you mean "have to 1.2 open it until you hit 1800rpm or so? 

So much to learn... 

Thanks! 

[RzeroB]

Generally speaking, for a mild mannered street car with an automatic transmission, a vacuum secondary carb is better than a mechanical secondary carb. A mechanical secondary carb will usually cause the car to stumble if you floor it at a low rpm because of the stock torque converter limiting the "flash" rpm of the engine. The stock converter doesn't have enough "slip" in it to allow the rpms to come up quickly enough to where the engine can draw the gas from the secondary main jet circuit. With all four barrels of the carb open the engine will consume the shot of gas from the accelerator pump shot before the engine can build enough revs to start drawing gas through the secondary main jets and the result will be an over lean condition that you will notice as a "stumble".

A spread-bore vacuum secondary carb will be better for your application. Vacuum secondary carbs are a lot less "stumble" prone - the secondaries will only open in proportion to the engines demand for more air thus preventing the over lean stumble. Plus with the spread-bore design, the small primaries will give you increased throttle response because of the increased air velocity thru the venturi and better gas economy (relatively speaking of course).

[bad440]

Had that carb on my 74 360 Challenger with a Torker manifold, pulled like a bear; it has a 50 cc accelerator pump(secondary), you should have no bog. I tried a 750 vacuum Holley after this and there was a definite change in power and 1/4 mile time. Try it think you will like it.

[cudabob496]

Had that carb on my 74 360 Challenger with a Torker manifold, pulled like a bear; it has a 50 cc accelerator pump(secondary), you should have no bog. I tried a 750 vacuum Holley after this and there was a definite change in power and 1/4 mile time. Try it think you will like it.

change in power as in more, or less, with the Holley?

[Super Blue 72]

I think he means like more power...I think.  That's what I have heard so far about this carb, it's more "performance" oriented.  Uses more gas too.

I picked it up for $40.  Wanted a cheap carb to throw on there if my Thermoquad drama has an unhappy ending... 

Thanks for the replies guys! 

[cudabob496]

I had a new 750 EB on my built 440 Cuda, and a mechanic sold me a Holley 750 DP that he had
adjusted for my car. Man, what a difference. Felt like an honest 30 hp gain over the new EB.

[Super Blue 72]

I guess on a built engine it'll benefit from the DP. 

Mine is a wheezer 340 with 8.5:1 compression.  I'll just have to feather the carb if I use this carb.  Just wanted something that I wouldn't have to chage the intake thus the spread bore Holley. 

[Chryco Psycho]

With a bit of tuming you will not miss the TQ , I had one of these carbs on a 360 Power Wagon I used to own
ran great with it even in a heavy truck with low compression 

[Super Blue 72]

Sounds good, Chryco!  Your words are like gold. 

[BIGSHCLUNK]

Give 'er he11 Phil!   

[Super Blue 72]