painterman52

I`m putting a 750 dp with mechanical secondarys on my 383, what are the pro`s & cons of mechanical verses vacuum secondaries?

StickShiftCorvette

Mechanical secondaries can give you a more transient power IF the throttle is opened at the right time. I have seen, on a ZZ4, where if the secondaries are mechanically actuated a car will break traction (spin the tires) from a rolling start, but NOT do so with vacuum actuation.

Vacuum secondaries virturally eliminate bogging due to over agressive throttle opening at relatively low RPM. Vacuum secondary carbs provide slightly better fuel economy since there is only one accelerator pump enrichening the mixture and ONLY open the secondaries when engine flow demands it. With mechanical secondaries YOU are in control and occationally most people open up the throttles too quickly resulting in decreased flow through each barrel of the carb, which reduces the efficiency of the fuel atomization in each active venturi.

I run a 750 DP, but may go to vacuum secondaries the next time around.

C3 Stroker

Good tip from StickShiftCorv.......I now use a mech sec. Demon on my 383, and it is definitely faster ( quicker ET) than before with an Edelbrock or Holley vac. sec. carb.

73, Dark Blue 454

I prefer a DP vs. Vac secondary. The key is to not over-carb which is easy to do with the DP,..and hard to do with a VS. Yes, mileage will suffer,..figure about a 15% difference (no joke!). But I think they're much easiere to tune. Set your float, idle, idle mix, and the only things that I've done as far as tuning is change the cams on the primary accelerator pump (easy to do) and jets. I can never get the VS dialed in,..and never get the secondaries to do what I want (many love VS Holley,..but I'll take a Q-Jet for the street).

750 DP shoud be ok for healthy 383,..or a 700.

Good luck!

SmokedTires

From what I understand, vac sec are good for automatc transmissions and DP are good for manual shifts

Gordonm

Having second thoughts on this?

I prefer the mech. secondaries on all occasions. Unless you have a mild motor the DP should work very well if setup correctly. I ran a vacuum and a DP on my 350 and was much happier with the DP. Yes fuel mileage will suffer slightly but if you want a fuel mizer get a ricer!

painterman52

I have second thoughts on everything. I get it on tuesday. It will be on Tuesday night and I`m sure I`ll be very happy. All this stuff is new to me. So ya I`m a 54 year old nervous kid.

henderjl

Corvettes are for having fun and a DP is way more fun than VS. Hit it and it is all there. If it bogs, you need more cam.

mandm1200

I never knew that a mechanical secondary carb would provide a noticable difference in gas milage versus a vacuum secondary carb? I thought that most driving was done on the mechanical primary's (both carbs use a mechanical primary). Now if you're the type of driver that needs to use all 4 barrels driving block to block, red ilight to red light, then I can see a little more fuel being used with a mechanical secondary carb.

For racing with an experienced driver, the mechanical secondary is preferred. For a daily driver or weekend warrior a properly setup vacauum secondary is fine. Mash the throttle and the secondarys will open up as the engine demands the additional CFM.

427Hotrod

I personally prefer a mechanical sec..but I do build stuff a little hairier than some folks.

If you are looking for a vac sec 750 Holley, my buddy has an excellent one that he isn't using anymore. It was on his '69 Shelby Mustang show car. He had a 351 Lightning roller shortblock and ported TFS heads. It made 410 HP in mild pump[ gas mode.

It has been converted to use metering blocks on the back side instead of the plates. This way you can change jets as needed. It also has the quik-change vacuum can where you can easily change springs. He later had it tricked out and flowed by a local guy in Houston with a great rep. It has smooth throttle plate screws and all the little tricks..different boosters etc. It's a real nice carb that he just doesn't use. He's going back resto on me! I think $175 will buy it. Definitely a bargain for someone!

He's a Forum member (has a C-5 too!) and I can put you in touch with him if interested.

JIM

chewy

Almost all street cars and many non serious (bracket type) race cars do better with a vacuum carb. Don't let the throttle response in first gear fool you any other time a vacuum carb is faster.

CGGorman

My DP has two set of shooters, yes, but only the primary side squirts unless I get into the secondaries. I'm getting over 20 MPG in a 68 vert with a solid-cammed 427 and an 850 DP (with a 5 speed).

It never bogs, either.


It's all just a matter of tuning.