vacuum vs. mechanicals

[Frank R]

It seems the rule for carbs is vacuum secondaries for automatics and mechanicals for manual shift. Does the engine know whats behind it ? why does it matter? can I run mechanicals with an automatic? The subject is a 440 backed with a 727 at about 425 horse with 355 gears. the car is not a daily driver and I want it to run like a scalded dog in the 1/4. I am considering a 750- 850 cfm carb but want info to decide between vacs or mech. Frank R.

[Mister Fiberglass]

alot of it depends on what converter, rear end gear and the weight of the car you are running..........a 3.5 gear, a heavy car and a stock converter is about boarder-line with a mech secondary carb.

It would be a touch better with a looser converter and a bigger gear.

[Frank R]

Guess I shoulda mentioned I'm running a 2200 stall with a shift kit. The car is about 3400 lbs. is that considered a heavy car...

[Mister Fiberglass]

sticky rear tire too ?

[Frank R]

not currently, but definately after I settle on a carb and performance tuned.

[Mister Fiberglass]

I would stick with a vac 2nd-ary carb.

[Frank R]

yeah but why? what's the reasoning behind your opinion? why not Mechanicals, will it fall on its face if I use mechanicals? will I get better performance with vacuums? I am not concerned with gas milage this is a play car...

[Mister Fiberglass]

JUST as you said......it could fall-on-it's-face - without a bigger gear and converter.

If you are set on a DP ....just gear it right with a 4.30 or so.....with a decent tire and you are all set.

[mtrv8n]

Av properly set up mech will do the same thing as a vacuum secondary, but under a stricter set of conditions. The idea of the vac is to only open the secondaries enough to meet the demand for volume. This can be done wrong as well, but the range of acceptable error is greater.

What's happening is that a heavier or taller geared vehicle does not accelerate as quickly, so the motor will not wind up as quickly. When that happens, the throttle is open too wide to create the vacuum neccessary to draw fuel through the metering system.

A mech fixes this by pumping a shot of raw fuel down the throat, a vac delays the opening by sensing the conditions that warrant more air/less need for venturi effect.

YOU can fix a mech that's too touchy by not being so rough on the throttle...

[1972roadrunner]

i disagree. my bro has a 440 that we built up a little. 10:1 pistons, slightly hotter cam, but stock converter. we put a 650 holley double pumper on it, and the only problem is if you put it close to the floor without manually downshifting, it'll ping. but if you get the rpms to rise around 3,000 before you stick her down, it'll run like a raped ape. BUT...we have the ability to control the secondaries, which is nice.

hell, try the DP and see how it does. if you don't like it, i have a holley 650vac secondary we got for the 440 but didn't like it much...not enuff "oomph" in the seat of the pants that the DP provides.

[Frank R]

so if I understand this the tranny is really inconsequential what is important is how fast the motor will wind up based on weight and gearing... if the vacuum secondaries open up at a slower rate that might inhibit the engine to rev quicker. With a mechanical the problem is they might open to quickly which will cause the car to fall on its face, but it sounds like this condition can be adjusted out. Am I started to get a clearer picture.....

[Mister Fiberglass]

the combo of a lower stall converter, short gear and heavy weight are the main factors............the vac carb can be adjusted - the mech can also be BUT with a huge pump-shot.

[Frank R]

Is a huge pump shot a bad thing? can't that be adjusted also?

[Mister Fiberglass]

No it is not a bad thing BUT at the track it can be inconsistent.

Why not just go with a big gear ?

[Frank R]

The 3.55s seem to be a nice gear, I can still cruise the freeway without revving the thing to the moon, and yet their track friendly; granted, not the best for all out racing but they do offer a good compromise for both worlds.

[1972roadrunner]

with the mechanicals, you can control the rate the secondaires open by how far you stick your foot into it. granted, it won't be as consistant...

[6 packin]

Use a Double Pumper carb on the engine. It performs better in all out racing conditiion, a 440 will like it much better than say a 360 would. The 3.55 are a good street/strip gear, not as good as the 3.90's though. Run a 27 inch tall tires with the 3.90's and the shortest/ widest tire you can get for the 3.55's. The converter isnt helping much, Im not sure what kind of compression you have or the cam profile but, a 3000 stall would be minimum. Also what intake are you running? What igntion are you running? I take it its a A body car, 3400 lbs with a 440 is light in my book. Lighter than My 73 Duster with a 440!......... 3450 lbs for my car. I reccomend a 800-850 CFM carb also.

[Frank R]

Here are some particulars: 69 Roadrunner, 440 with 10.5:1 comp, Cam is a 485 lift, 232 dur, 108 centers, 64 overlap (Isky), Weiand Action Plus dual plane intake manifold, Mopar Performance electronic ignition with orange box, opened up 906 heads, Hooker headers, 727 tranny with shift kit and a 2200 stall, Dana 60/3:55.

[b-1ken]

Go with a double pumper. A 750 -800 would be just fine. I have used a 700 cfm DP carb on a 350 c.i. engine with street gears and a stock converter and it worked great. The throttle response with a DP is terrific and you have all four barrels open now! Who needs to wait for the secondaries to finally open about 5 seconds down the road.

[atoetly]

Yes who needs the secondaries to open five seconds down the road? It would be just like me using a throttle stop on my race car.

[b-1ken]

Who the hell would use one of those?

[Frank R]

I am assuming mechanical secondaries..

[DartGT66]

If you can adjust the carb so that the accelerator pumps can provide the needed gas to the spot when there is enough flow to produce a signal to the carb so that it can work on its own, it should work just fine. With a high stall, light car and higher numerical gearing that's way easier to achieve than with s stock style configuration. With street tires, when the tires break loose, it might be easier, but once you have enough traction you may start having problems. Vacuum carb is way more forgiving about the carb size and all the other factors, and with limited traction it's a little softer and makes controlling the traction easier. I have had good luck with prepped vacuum carbs with heavy car/long gearing combos. A well prepped vacuum carb isn't slower in this kind of combos, and is way more consistent than a DP.

[mtrv8n]

Either one will work just fine, both need to be tuned to do what you want.

It's just easier to tune a vac sec for street, easier to tune a mech for strip.

Best bet, if you don't want to do the work yourself, give Don your specs and the shelf price of the carb, and he'll deliver what you need.

[atoetly]

B1-KEN I have NO idea

[beepbeepsrule]

No intake style was mentioned, or I missed it. I think a DP works better with the open plenum types. I also think the gears and stall are just as big of contributing factors. I would only run a DP on an open plenum, large cam, 4.10 or better gears and 2400 or better stall. Watch the CFM, though, DP's aren't as easy to tune out a size problem. Using the toe-in secondary activating method indicates an improperly sized DP application or an application where the vac would actually be better.

I know from experience with a couple of different buddy's mostly-race cars that an open plenum large cammed higher compression (11.0 or so) 440 really likes an 800-850 DP. Mostly-race is the key term there......

[Frank R]

The intake is a Weiand action plus dual, and I have heard the same comment on carb size from guys who run 440s, they seem to like cfm but I have always been curious about what to run vacuum or mechanical secondaries

[drag-n-cuda]

Unless I missed it, no one mentioned a thermoquad. The action plus intake I recently sold was a dual pattern (square/spreadbore design) so I'm assuming the rb version is the same flange style. Demonsizzler could hook you up with one of these.

my $.02,
drag-n

[Frank R]

OH GOD !!! you didn't say TQ did you??? If Mister fiberglass or Goober44 hear you this thread won't end until Thanksgiving......

[b-1ken]

Originally posted by atoetly
B1-KEN I have NO idea

Obviously, someone who is petrified at the thought of first gear acceleration.