Why do the experts say to use a vacuum secondary carb on a car with a automatic tranny?What performance problems would occur if a mechanical carb was installed on a auto?

 

Not sure what experts you are talking to, I'm no expert but I've used both vacuum secondaries and mechanical secondaries with automatic transmissions and there are no problems. The vacuum secondaries are a little easier on gas. IMHO, If you aren't out racing all the time, the vacuum secondaries are more than adequate for most applications.

 

On most any street machine, particularly those above about 3200#, the vac sec carb is typically the right call. By the nature of their on-demand operation, they are more forgiving of oversizing errors while being easier for most people to fine tune. It isn't hard to figure out that this allows a bigger carb to be utilized for top end flow without suffering as many drivability/tunability issues.

IMHO, proper fine tuning of the right vac sec carb can pretty much eliminate most response complaints, which aren't as critical on the street in the first place. As evidence I practice what I preach, I'm putting a vac sec 995-1095 King Demon RS on my 496 redux, which is mating up with a Muncie 4-speed.

My $0.02...

 

Well,I dont care about gas mileage!I want performance!! But,I dont want to have driveability problems.

 

A P-40 and a C3 make an awesome combination.

Depending on the carb. design, purely mechanical secondaries with no additional air metering can cause a stumble by opening before the engine can use the extra charge. The Quadrajet on my '73 BB has an air valve over the mechanical secondaries that, even when the mechanical secondaries are open, do not allow air in until the engine is "ready". I'm not sure if the entire line of Quadrajets had that air valve or not. This setup eliminates the need for a vacuum actuator while providing the same on demand metering.

Bottom line is that the only difference between mechanical and vacuum secondaries is when/how they will open. WOT should see no performance difference.


--Outlaw.

 

Just wish the Warhawk were mine. If interested in more...

http://forums.corvetteforum.com/show...post1564015739


bozzman3, I should have mentioned that I've run mechanicals on both auto and manual C3's without issues, too. However the one with the auto was only a 650 on a mildly modded L82 350, so I'd call that a rather conservative selection as that looks a bit undersized in hindsight. The other examples with manuals weren't on mild builds, one having been a progressive 1050 Dominator on a 427 which was the hottest BB I'd run on the street up to that point. That Dom could dump a ton of fuel...

 

I love mechanical secondaries. I run them on all my old cars. No questions about "are the secondaries opening". The answer is yes. I have a double pump I just put in my 68 L36 with Turbo 400. There is no stumblimg when I get into the secondaries. Just a nice roar from the big block and it just goes I have them in my 61 and 65 both with 4 speeds. No stumblimg.

In theory, mechanical secondaries allow fuel to be dumped in before the engine really needs it all. Vacuum secondaries rely on engine vacuum to tell the secondaries when to open. Regardless, I like mechanical secondaries and have never experienced any performance problems other than wanting to put my right foot down too often.

 

I'm definitely a mechanical secondary lover. If it stumbles, something is wrong. You have two accelerator pumps with plenty of room to play around. I've run vacuum carbs too. Actually, they seem to work best to me on a car with drag slicks etc so that you can get the secondaries opening consistent and smooth. On anything that spins it's like driving with a rubber band on the pedal. Just when you have traction under control they want to open and add more power. I feel much better with a mechanical secondary I can control with my foot.

I don't know how many times I've seen dyno testing when all that cool *airflow on demand* theory just didn't work. Engine at WOT on a chassis or engine and they only open as much as they want to even with lighter springs etc. I've yet to see one that I made open much quicker that didn't add a lot more power. Don't get me wrong, it is possible to get a stumble since you don't really have a sec pump to overcome the quick opening,,,,but engines generally like the extra airflow. That's where DP's come in.

I literally used the same 750 DP with minimal changes on a stock 82 Mustang GT converted to a 4 bbl, a 396 Nova with a T-400 and 3.08 gears, a 440 roadrunner...and before that, same car with a 383, a 10 sec nitroused 427 Camaro, a '67, a 66, a 65 and a 64 Vette with small blocks of varying HP levels, a 427 '57 Chevy with 4.88's, a 454 '68 Camaro and who knows how many others. That same carb is still running on that Roadrunner or the '65 Vette..not sure what my buddy has it on. But I can tell you that it never stumbles and drives great with next to no work.

All that said, it does take a little tuning work to make a DP work on a stock motor and a stock converter and say 3.08 gears. But if you ever plan on moving up....it's worth getting one up front.

Skunkworks...you're REALLY going to use one of those vacuum Dominators??? I've got a feeling that as quick as that new motor is going to rev, the secondaries better be working fast if they are ever going to get open before the next shift point!


JIM

 

Some other questions...Are you using this car in the winter? Do you want an automatic choke or manual choke? Is this for a street car or race only? For a colder weather driven street car I'd go with the vacuum secondaries with an automatic choke...like a Holley Street Avenger...very tunable and better mileage and still very good response. For a car thats more for racing or summer driving, I'd go for the mechanical secondaries with a manual choke.

 

Bozz,
I put a Holley 'Spreadbore' with mechanical secondaries on my 71, 350/270, in 1971. The Quadrajet is still on the shelf in the garage, and the Holley has been rebuilt twice. You learn pretty quickly how to use the mechanical secondaries with your foot.
Regards,
Alan

 

Thanks guys!! A mechanical secondarie carb is in my near future.

 

Stop, you're confusing me!

I've been planning to do so since not long after the Road King Demon vac sec came out, and the BG guys have been urging me to stay with it. Assuming I'll tune it right, it sure looks good on paper. But, I must admit that I never had issues with the 1050 progressive on that 427, other than the ~9mpg vs. ~14mpg w/Tri-power (which was significantly less potent, too - hmmm).

It will be a while before I commit $$, and I promise to put your comment into the mixer, but between this and recently influencing my decision to go ahead and step up to a bigger cam, you're really trying to send me back at the drawing board again, aren't you...

 

 

I agree too, good choice!
Cheers,
Pete

 

The only reason mechanical secondaries get bad mileage is because they are so much fun to play with! If you keep your foot out of them they will get as good as mileage as a vacuum.

Skunkworks.....step away from the light! Come back to us! I mean really...a vacuum Dominator? BG just found a cool way to market stuff.

The same 1050 I'm running now ran for years on my 427. Too much? I dunno...but it sure ran faster with it! So...I guess not.

Now...tell us about that cam again??


JIM

 

Jim, I've posted a new thread...

http://forums.corvetteforum.com/show....php?t=1937539

 

Finally, an informative post on mechanical secondaries.