Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

[Screaming Detroit]

I was at the track yesterday for a test and tune and had been having carb running concerns with the 650 dp mech secondaries. So my friend gave me a 750 vac that was set up to run on his Cleavland. I put it on and the difference was so much better. I think I definitely am liking the vac secondary carb.

[Monk]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

I've had both.......my '69 Z 28 had mechanicals and liked 'em.

My 427 Nova had vac and performed well.

Current 409 with dual quads has mechanicals and they're
kick ass.

[Barry Donovan]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

I could see a 302 taking it all. :o

I like vacuum secondaries, especially symmetrical. (non bogro jet)

the engine determines it... following math for displacement, correct cfm...

a low vacuum in inches better is with vacuum secondary...something to work for. especially a heavy hauler..
big bores little strokes tend to be quite the suckers.

I ponder weber 38 on a 1781cc boxer (22 inches of vacuum as oem.. plenty to play with)
the 302 chavy... that is a rare case of one taking it all from the get go...and really liking it. ;D

[min301]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

Set up correctly, either one will work.

I'd prefer a vac. sec. carb for the street.

[Thumpin455]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

My 455s dont care either way, my Clevelands like the vacuum carbs unless there is a bunch of gear behind them. I can tune and build either, but I like the Qjet over the Holley for a street carb. Holleys are simple and easy to tune but I get better drivability and mileage with the Qjet. I like the Demon 850 I ran on my GTO for years, its mechanical and it was the only carb that didnt make that engine idle rich as hell.

I dont have a preference as to the secondaries, both work for me its all in how you set them up.

[squirrel]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

Vacuum secondaries act like the carb is half as big (assuming square bore carb), or 1/4 as big (spread bore)...so it's a lot easier to set them up in a street car, compared to a double pumper.

My blower motor has two double pumpers on it, and instant throttle response. A mild small block with stock stall and mild gearing will work better with a vacuum secondary carb, unless the carb is just the right size. If it's a bit too big, you'll get a lean bog when the pump shots run out, but the air is not flowing fast enough thru the venturies so the main circuit has not started flowing fuel yet.

[SuperBuickGuy]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

The only time a DP worked well for me was when the motor was blown.

The Buick loves the vacuum secondary because it's a heavy car, all the DP would do on it is make it use more gas for the same hp. As an aside, you can make a vacuum secondary act like a DP simply by tying the vacuum pot open... but then it'll run pig rich.

[Gary 351C]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

I prefer vacuum secondaries for a street car.

[Scott Liggett]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

I have two street cars. Both have Holley's. One has a DP, the other has a Vac Sec. They both work great when set up properly. I also think Edelbrock's vac sec, AFB type work just fine, but I'd prefer Holleys. I'm still learning about Q Jets, but my experience with them is that their smaller primaries lack part throttle response I like.

[IRONHEAD]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

vac 2nd q jet..
street..

[SpiderGearsMan]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

double pumpers and solid rollers

[yellomalibu]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

For performance applications, DP without question.

For the daily driver, Vac Sec.

[fast Ed]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

Vacuum secondaries for the street, especially on an auto. trans. car. Mechanical for something lighter with a manual trans. and some gear that is quick to rev.


cheers
Ed N.

[Falcon67]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

I prefer a properly sized DP. Your 650 may have an issue - those are the most robust carbs around. I ran one for about 5 years - it won me a lot of $$$$$ bracket racing and cost all of $40. His carb was just tuned better than yours.

If you are just messing around, either is fine and you can run a bigger VS carb because of the design. Note that there are WAY more DP sizes than VS sizes, which means you have to think more to match the carb to the application. On my tests, a 750 VS in good tune would run the same ET and MPH as the 650DP. However, for bracket racing you will want the DP as it is more consistent. You can make the VS somewhat more consistent by putting in a purple or stiffer secondary spring. That's a "secret" - everybody reads half the Holley book, then puts in the lightest yellow to make the secondaries open quick. Then they stand around and wonder why their ET is all over the place. ;D

I run a 600 DP on the 302, and matching "750" ProForm parted up DPs on the 351Cs. Those flow 800+CFM. The 302 drives and races real nice with the 600DP.

Note that the Edelbrocks are "half and half" - they are a mechanical secondary carb with a weighted air vane. I believe that design is also more consistent on the track than the Holley VS setup. We ran a 600 Ed for a long while on the Mustang and it also won a lot of rounds. The 600 DP was quicker by .02 and provided a better 60' in our specific application. But if I had to choose a "vacuum" street carb from between a Holley VS or an Ed, I'd pick the Ed every time.

[jcharliem]

Re: Vaccum Secondaries or Mechanical Secondarys whats everybodys preferance

I always thought the general rule of thumb for a street/strip ride was... vac sec for an auto trans, mech sec for a manual box. I would think a street ride would be just fine with a vac sec regardless of trans.

<------ 800DP on an itsy-bitsy small-block.