79corvetteTPI

i am thinking about getting a demon 750, but not quite sure which one i should get. i am scrapping my whole tpi project.

what are the differences and benefits of the vacuum and mechanical secondaries and should i go with an electric choke?


thanks in advance!!!

GrandSportC3

mech. secondaries = more power
vacuum secondaries = better fuel economy and less power

Avette4me

I dunno GS ...I've always been taught that vacuum works best with auto trans and machanical is the one for a stick...

JohnD77

Well I just ordered a brandy new ZZ4 and Demon carb yesterday from Jim Pace. Their salesperson suggested that I go with the vacuum secondaries for regular driving. So that's what I did. We'll see. JD

Cali,68,L-79

79corvetteTPI

that is what i thought also

79corvetteTPI

so which is easier to tune?

Vetterodder

Not necessarily. If the carbs are set up the same otherwise mileage and (peak) power should be the same. Keep in mind that in "normal" driving the secondaries aren't used much, if at all. Unless the secondaries are constantly being opened and closed, the small amount of fuel used by the secondary pump would have little effect on overall economy.

Once the secondaries are fully opened on a vacuum secondary carb(again assuming the same setup otherwise and that they are in fact opening fully) there should be no difference in peak power. The main benefit of the mechanical secondary carb, besides giving the driver total control, is that the secondary pump shot allows more throttle sooner. It's not a huge difference but making more power sooner results in faster accelleration. I've been real happy with some vacuum secondary carbs once properly set up but even happier with a mechanical carb.

bobs77vet

mine is a 650 speed demon,vac sec,electric choke to go with my auto trans....

GrandSportC3

On the dyno and on the race track - you'll always get better numbers with mech. secondaries carbs... (if both carbs are equally well tuned)
The reason why people say that vacuum seconaried are better with AT cars is because the rpm drop between gears is higher on automatics because there is usually 1 less gear and when often the torque curve is a little better with vacuum secondaries.. but the HP is just about always lower with vacuum secondaries.. I gained 20 RWHP when I switched from vacuum secondaries Holley 750 to mech. secondaries Holley 750 HP series.. I have the dyno sheets with both carbs somewhere..

Chuck-75

tks for the post,Ive just recieved a ZZ383 and am pondering the same question?, Iv also been told that Vac goes better with auto, and Mech goes with manual trans.?
because Im just a shade tree mechanic ( and dont know very much) Im been told that vac.secondarys are easier for the not so experienced?(thats ME), so Ive not made a commentiment yet but I think Ill go with the BG speed demon vac. secondarys with elec choke...if Im wrong some one pls chime in and give me advice also? PLS. .... save the wave and Happy new years

GrandSportC3

The transmission doesn't have any effect on engine power...
There's a reason why all the serious drag racers (most of them with automatics) are using mech. secondaries carbs... They just make more power - period!! You get less power but more bottom end torque with a vacuum secondaries carb..

406shark

Others have told you the benefits of the different carbs.

Do yourself a favor and call BG tech line and give them your cars information...they can/will tell you the best carb for your application.

I have a Mighty Demon on my Vette...high camshaft duration numbers, hi-compression, steeper gears, hi-stall, etc.

Demon designs different carbs based on cam, comp, gearing, info.

If you check out this web site http://www.barrygrant.com/default.aspx it should give you an idea on what info they use to determine which carb works for which application.

Don't make a mistake on a $500.00 part.

Jeff

427Hotrod

First, I prefer mechanical secondaries on just about anything....auto...stick...tight converter or loose.....any gear...whatever. Just works better.

*IF* you have absolutely perfect traction, then a vacuum works OK...it just opens as needed. The trick is getting it to open quick enough and not bog. Most all are set up WAY too tight so they won't bog and what you find is most never open all the way. Even on a dyno that holds it back a lot...you ahve to change springs to get them to open. But then when you do get them open all the way, often you don't have enough pump shot to cove the flat spot..then you bog again.

That's why double pumpers are made. They open all the way and the shoot some gas in all four barrels so it won't bog. If set up right, they will flat pin you back in the seat even with highway gears from a dead stop.

The other thing I hate about vacuum carbs, is that when traction is not perfect and it's spinning..like most of these cars dowith some HP..it's like driving with a rubber band attached to the linkage. It opens when RPM startes climbing...even though you haven't moved your foot anymore while feathering throttle to get traction. I prefer to have total control of wheelspin with my foot.

So, the vacuum is simpler to install and forget about...but forget about some power too unless you do some work. The DP will do well out of the box too and it's easier to tune in to me. Mechanical control Vs vacuum stuff. Too many variables with vacuum.

And one last thing...I would personally move over to the Holley line of carbs. Not a big fan of the BG's. If you can afford it the Holley HP's are nice, but a good 'ole 4779 750 will run on anything. Don't believe the hype....Holley can still build a carb that they designed!


JIM

OkeeGuy

i like less moving parts...less to screw up and not work right etc...
i am running mech sec and NO choke for that reason...

by the way...Lars built my set up and i bought the stuff i bought as per his recomendations (for my car and engine)

where is Lars when you need him!??!

GrandSportC3

Jim, great post!!

71roadster

If any of you guys are interested, I have a 750 Vac Secondary Speed Demon that I ran for several months for sale. $300 flat. I'll pay shipping. Box, guide, and Tuning video included.

Chris

gkull

Chris, Sold! Do you take VISA from my company? Custom Choppers and Corvette's.

bobs77vet

the reason i went with the vac secondaries and electric choke were purely laziness and ease of maintenance....i wanted to set it up initially and walk away from it.... i didn't mind losing some performance edge since this is just a cruiser

427Hotrod

BTW...I forgot to mention....I have a good friend that has a nice 750 vacuum Holley for sale very reasonable. He bought it new and then added the metering block setup to rear to allow jet changes, added quick change vacuum can, then later had it blueprinted by a shop very popular around Houston. Not sure what all was done to it, but he swore by the work when he got it back. He can provide all details on mods. Carb was setup and dialed in on dyno (engine and chassis) on a '69 Shelby Mustang (show/street car that made a few 1/4 mile passes) with an aluminum head 410 hp 351 Windsor. It was an automatic car and did very well.

Seriously, I do know that it worked very well and he made those changes so it could be tuned easier. I gotta confess I talked him into using a DP later and that's what he has now. But if you want a real nice vac 750, this could be the one for you.

Shoot me an e-mail if interested and I'll put you together with him.




JIM