vacuum secondaries vs mechanical

[Brightyellow69rtse]

hey guys i know the performance guys seem to prefer mechanical. ive pretty much always run a vacuum carb. except fpr a short time when i bought my camaro when i was a kid. that had a 650 d.p. on it that backfired as much as it burned gas. i just replaced it as i knew nothing about them. i still have it in the cellar but i never really tried to rebuild it.

one thing that i question is how do you know the secondaries actually open all the way in a vacuum setup?  do you guys think im leaving power on the table by not using a mechanical setup?  im using a 750 holley with a proform mainbody.

heres my motor build its got maybe 150 miles on her

1972 440 .055 over, 0 deck

height bored and honed with torque plates

steel crank ground .010/.010

scat h-beam rods

keith black icon forged pistons flat top w/valve relief

everything was balanced

holley 750 vac sec carb with a 750 proform main body

lunati cam  60303lk
Intake Duration at 050 inch Lift      226
Exhaust Duration at 050 inch Lift      234
Duration at 050 inch Lift      226 int./234 exh.
Advertised Intake Duration      268
Advertised Exhaust Duration      276
Advertised Duration      268 int./276 exh.
Intake Valve Lift with Factory Rocker Arm Ratio      0.494 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio      0.513 in.
Valve Lift with Factory Rocker Arm Ratio      0.494 int./0.513 exh.
Lobe Separation (degrees)      110

street dominator intake & 1/4" 4 hole phenolic spacer

stealth aluminum heads no work done but they were checked out by my shop i did upgrade the locks

msd 6al and 8.5 super conductor wires blaster 2 coil, ngk plugs

mopar perf electronic distributor w/ vacuum plugged off

carter street fuel pump, 3/8 fuel line

hemi 6q oil pan, milodon windage tray

melling hv oil pump

stock rockers w/new 5/16 pushrods

crappy hedman headers for now

3" exhaust with x pipe, ultraflows and dougs electric cutouts

4.10's in the dana and a 4 speed

[c00nhunterjoe]

You should still feel them open as you go to full throttle. That drivetrain sounds like a blast to drive!

I don't think you are losing any peak hp with your carb,If you do decide to swap, go to an 850 on that engine. You'll pick up a good bit of power with it and still be extremely cruise friendly. A 950 would make even more power. I prefer mech secondaries but its just personal preference

[XH29N0G]

I think you are supposed to stick something like a paperclip on the rod from the vacuum canister to the secondary butterflies and then see how far it is pushed when you get into the secondaries when driving.

Curious if others have another way to check this.

As far as power on the table is concerned - that is for someone else to answer.  I am using vac secondary and thinking about trying a dp to see if it makes a difference for my setup.

[Brightyellow69rtse]

ron turned me onto a plate that changes the fixed orifices in the secondary to jets like in the mains. im going to order that tonight or tomorrow. as long as ive had that carb even on my old 400 there was never a kick in the ass when i stomped on it. its very smooth like a fuel injected car that dosent downshift when you stomp it. the old thermoquad was a blast to drive! you stomped that and you knew it!

im wondering if my secondaries have never really been opening as i have never really felt a good kick ion the ass like the old thermoquad.

coon, yeah its pretty sweet

[firefighter3931]

Mike, the properly sized double pumper is allways going to feel stronger/sharper than a vacuum secondary. Those mechanical secondaries will open faster than the vacuum pod can open them. 

That being said ; a vac secondary carb can be tuned to perform pretty good.  Yours needs more jet on the secondary side with the new build so installing the Quickfuel plate with adjustable jets will allow for some experimentation. The fuel metering for a 750 vacuum secondary carb is inadequate for the new build which is probably pushing in the 475hp range. 

A 750 vac (with std jetting) is a good carb for a stock 440 making 375-380hp.



Ron

[Brightyellow69rtse]

Aww I was hoping to crack the 500hp mark. Maybe nex :icon_smile_big:t year I'll get her there

[firefighter3931]

Aww I was hoping to crack the 500hp mark. Maybe nex :icon_smile_big:t year I'll get her there

Mike, you're only a cam swap and some head porting to (substantially) exceeding that mark 



Ron

[Cooter]

I've run both and prefer the mech. secondaries.

[Brightyellow69rtse]

Aww I was hoping to crack the 500hp mark. Maybe nex :icon_smile_big:t year I'll get her there

Mike, you're only a cam swap and some head porting to (substantially) exceeding that mark 



Ron

oh yeah!!!!