GeoffSchultz

It seems that what I have read indicates that a mechanical secondary, if dialed in right, can give slightly better response than a vacuum secondary. However, a vacuum secondary generally gets slightly better gas mileage.

What is you guys' opinions on which one is better for a pure street carb? I would think vacuum secondary, since I don't care about track times, and would like the additional mileage. But I'm not sure.

BTW, what comes stock on C3 Vettes? I have a 1980 L-82.

Thanks.

Eddie 70

Most of the C3 corvettes came with vacuum secodaries, for just the reason you listed.

toddalin

Main reason vacuum secondaries get better mileage is because everytime you mash the peddle you've only got a squirt on the primaries instead of the double-shot for the primaries and secondaries. A vacuum seconday carb is also good if the carb is too big for the engine.

On the other hand, with a mechanical you can physically see if the secondaries are opening all the way. This is not always apparent on a vacuum secondary carb.

noskillz

I have a mechanical "double pumper" on my L-36 and get 16 MPG with a 3:70 rearend. Granted, the carb is a 650, but with the small venturi and the mechanical secondaries the car really jumps when you open it up. Bout the only thing vacuum 2nds do is open on a non-linear curve.

Given that you don't even use the 2nds while cruising, between 0 to 70 mph I doubt there is a big difference - BUT you will have less trouble with the mechanical.

mbeeman350

:iagree:
I have had both and I prefer the mechanical secondaries......you know for sure by the seat of the pants ....that they have kicked in!!!
Mark :cheers:

GeoffSchultz

Thanks guys.

Seems so far there is no consensus. I guess if stock vettes came with vacuum secondaries thats what I would go with.

Any other comments?

Tominator

Yes the great thing about this forum is a lot of opionion here. I have used both types of carbs and like the vac. sec. best. I love the feeling of the sec. kicking in.

Probably due to have a vac. sec. as a kid and when they kick in I thought I was driving a bad boy.

All in all I think that the mec. sec. would probably be faster in the 1/4

lars

Geoff -
Stock Vettes did not come with vacuum secondary carbs. The Q-Jet is a mechanical secondary carb with an airvalve. This is not a vac sec carb.

I have done quite a bit of testing, including dyno testing, of mechanical sec and vacuum sec carbs. What I have found is that vacuum secondary carbs (there are only 2 brands of vac sec carbs on the market - Holley and BG) will never get the secondaries wide open, even with the lightest springs installed. This impedes performance and results in the carb not delivering its full cfm potential.

I have tested mechanical sec carbs on a wide range of engines, and have found them to perform very well. I recenty ran a 750 double pumper BG Speed Demon on a little 302 Ford on the engine dyno at Westech Performance. It ran with no hestitation, no off-idle stumble, and brutal performance from off-idle to wide open throttle. The 750 mech sec carb outperformed a 650 across the entire rpm range on the 302, and had better throttle response.

I used to be of the opinion that vac sec carbs were good for street cars and that mech sec carbs belonged on racecars. After testing on the dyno and from all results I have achieved on the street cars I've built, I am now of the opinion that the mechanical secondary carb is the way to go for overal best performance and tuneability. Use the vacuum secondary if you're running a motor home, 4WD, truck for trailer pulling, or other applications that see wide open throttle at low rpm for extended periods of time. Use the mech sec DP type carb on any performance application such as a Vette.

Brettmc

I'm running a Holley 850 DP (mech secondary) carb on my mild 468 which is "too big" according to the gospel. It ran like feces when I pulled it outta the box. After emailing Lars to death and following his tuning paper, it now runs wonderfully. I had to go inside it and make some minor changes, but it was well worth it. Would I buy another Holley? Sure would, but I would look at BG too. Some of their carbs have adjustable air bleeds, which makes tuning them a lot easier, I would think.

Brett :thumbs:

Blkblt

Mech secondary. Period.

I have run both Hollys and BGs in both arrays on my 427, and the best, by far, is the present mech/sec 750 BG set-up. I never could get full opening on the vac carbs, even with Lars pulling out all his tricks. :D I don't believe much of the "street gospel", preferring instead, to believe hard data such as Lars brought back from his CA dyno experiences. I probably will go to an 850 BG mech/sec in the future. :eek: :yesnod:

A Corvette is a performance machine. Treat it and yourself to a performance carb that will let you know when its "all in"....even if you only go all in once in a while. :thumbs: :steering:

:cheers:
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Jeff; Charter CCC
1966. Modified L72
The Silver Beast
Keep on
******************** :cheers: :seeya :flag :auto:

noskillz

WOW Lars! I am truly AMAZED the 302 was able to drink from a 750! Must have been a pretty nice engine to turn that kind of airflow!

lars

Actually, you'd be amazed how big of a carb you can install on an engine before it becomes "overcarbureted." We just ran an 850 DP on a 350 out here in the thin Denver air. No problem - ran great. The 302 with the 750 was a stock short block engine with a stock '92 Mustang roller cam and a set of AFR heads with headers. Other than that, it was all stock, and the 750 ran great. Engine put out 370 horse on the dyno, so although it was respectable for a 302, it wasn't a monster engine. If the carb is properly set up and the engine is correctly tuned, a big carb runs great and makes lots of power with no problems.

noskillz

"Engine put out 370 horse on the dyno, so although it was respectable for a 302, it wasn't a monster engine"

:lolg:

Getting that form a 302 is MONSTER to me! I assume the HP figure is at the crank. Do you have an idea what the enging was producing gross before the mods?

SaltyNuts

Lars--

Thanks so much for the input.

Is tuning a mech. secondary carb. easy enough so a standard (performance) mechanic can do it correctly? I am going to have the engine rebuilt by the same guy who put a ROD 6-speed in place of my 3-speed auto in my 80 coupe. In your opinion, is correctly tuning a mech. secondary carb. within someone like that's capability? I am totally ignorant about carbs so have no clue.

Thanks!

AlwaysWave

Something is eluding me here. At any given engine speed, the engine vacuum will draw only what it needs into itself. So how can a larger than needed cfm carb add power?

Maybe mechanical secondaries do work fine when correctly adjusted, but a vacuum secondary carb will open the rears to suit the vacuum draw. That’s why they may never open fully.

Matt Gruber

carbs are rated at 1.5"
and flow a lot less at lower vacuum.

gerry72

Single 4bbl carbs are air flow rated at that pressure drop. 2bbl carbs are rated a 3in hg pressure drop.

And about the only problem area in going really big on cfm for a street-driven car is that throttle response can be a bit soggy. It's really all part of the overall combination. In other words, you wouldn't be doing yourself any favors by plopping an 850 mechanical secondary on a completly stock 350ci, big, heavy Impala with 2.56 gears.

lars

noskillz -
Yes, an engine dyno measures power at the crank.

The engine was built in the configuration that it was run - it was not run on the dyno as a "stock" Mustang engine. The short block was stock, including the stock 1992 Ford hydraulic roller cam. Heads were AFR. Intake was Performer RPM. MSD ignition with total timing set to 36. BG Speed Demon 750. Tri-Y headers. Nothing trick or unusual - just the normal street-type hop-ups. We were actually hoping to see 390 out of it, but couldn't get there.

As far as "soggy" response (one of the comments above) out of bigger carbs, I 've never seen it. The 750 on the little 302 had crisp and instant throttle response. The 850 on the 350 was simply brutal right off idle and was no degredation over the 750 that came off.

I've heard all the arguments about "overcarburetion", and I used to spout the same gospel. But after having been involved in a bunch of actual testing with real documentation of the various results, I've had to change my philosophy. Same with the vac secondaries: all my actual test results show a measurable performance improvement with a mechanical secondary carb on a performance vehicle. Dyno numbers don't lie...

GeoffSchultz

bump

gkull

I just wish that i had Lar's tuning ability!:) I've run Edelbrock vacuum sec. and now Demon mechanical secondary carbs in my Vette. I couldn't really get my 825 CFM Race Demon to work just how I wanted on my little 383. Off idle to WOT always gave an instant hesitation then the tires would go up in smoke in first gear.

At the drag strip I had to learn to ease into it even when power braking.

So I had to go back down to my fully fiddled with 750 Speed Demon.
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I do know one thing in defence of Vac. Sec. carbs. Many of the track champion bracket racers use them because of consistancy beats a few more hp every time.