woodenichols

Trying to decide which carb to buy. Some discussion going on other thread about Demon vs Holley and others but I need to know difference between mechanical and vacuum secondaries. Any opinions?

lars

If you're going to drive it on the street, vac secondaries are the way to go. This will prevent any problems with bogging going into the secondaries, and will provide an overall "smoother" level of performance. Also, a vac secondary carb will produce better fuel economy because the mechanical carbs are jetted quite a bit richer to avoid the secondary bog problem.

We just pulled the mechanical secondary Holley off of a Forum member's '64 350 Vette and installed a vac secondary - just to try it out. The 350 in this car is a pretty radical setup, and it's a 4-speed. The car was brutally fast with the mechanical carb on it, so we were anxious to see what the vac setup would do. The car actually got faster and had a much stronger transition into the secondaries with no lag, bog or stumble. The mechanical carb is now in a box in my garage....

woodenichols

Thanks Lars. Any thoughts about the Holley "Street Avenger" compared to a Model 4160 Kieth Dorton Signature Series?

ODennis

I don't really have any way to compare but I will share my experience with the Vac. Sec. Carb. I've got a dedicated drag race car set up for bracket racing. I'm running a Holly 750 Vac. Sec. with the lightest spring. It gives me what I need when I need it. I wouldn't consider a Mech. Sec. carb unless I had a 4000+ stall and a tranny brake. Vac. sec. carbs (IMHO) are underrated and Mech. sec. carbs are overrated. I have come to believe that a vacuum sec. carb is the most efficient in power, economy and reliability for ALL street or street/strip cars.

My drag racing friends tell me the car will be more consistent (that's what drag racing is all about) with a mechanical but my average dial for 110 competitive passes in 2001 was 12.06 and my average ET was 12.069. I'm not going to argue with them, I'm just going to continue to beat them! :smash:

1979toy

I had heard that engines with mechanical secondaries revved faster than vacuum secondaries. Comments?

ODennis - After reading your post I clicked on your web page to see how you got a C3 in to the 11's. That is a sweet ride! :cool:
Full leather interior, roll cage, and weighing in at less than 3000 pounds is a major achievement.

73Ken73

I have had the same Holley 700 with mech secondaries on my car for 12 years. I have never had a problem with bogging or hesitation. I can't say anything about the vacumm secondaries but I would be surprised if my car could run better.

Vetterodder

Over the years I've used both mechanical and vacuum carbs and have good results with both. Generally, for most applications and, more importantly, for most drivers, I'd recommend vacuum secondaries. Mechanical secondary carbs are less forgiving if you over carb but, if properly matched, can give much better throttle response. I replaced a 600 vacuum Holley on my 327 with a 700 dp last year, no other changes. The perfomance difference was well worth the cost and there was no loss of drivability, just better throttle response at all speeds. If I were running an auto tranny though, I probably wouldn't be so pleased. If everthing else were equal, neither type would have an advantage once the secondaries are fully open and the secondary accellerator pumps were through pumping. Another consideration might be how the carbs are set up. Typically, mfgs assume a higher performance use of mechanical secondary carbs (as they often do for those factory equipped with manual chokes vs electric too) and set them up with richer jetting out of the box.

The vacuum secondary carb limits the opening rate of the secondaries in order to prevent a lean bog. A mechanical secondary carb (except those pos things sold on ebay that someone had tried to convert from vacuum to mechanical) uses accellerator pumps on the secondary side (hence the term "double pumper") to provide additional fuel to go with the additional air. The end result is the potential to better match the air flow rate and the fuel need for a particular situation. This can provide a more immediate power delivery during the transition as long as you don't try to give it more throttle than conditions (rpm vs load) dictate. Think of it like auto vs manual shifting.

Steve439

Boy, you'll get some different ideas here. My take is that the Holley double
pumpers are race carbs. In other words they're not designed to idle well
on a street car and the off idle/part throttle response isn't great. They
are designed to whale on the track.

I tried one 850 DP and my car just wasn't right. The idle screws showed no
response and it didn't run well. Now, a lot of guys will say I should have
spent some time with it to get it tuned. That's probably true, but instead
I dropped the same size vac sec on it and it ran pretty much perfectly
right out of the box. 16 inches of vacuum with about 1 1/2 turns out,
outstanding response, and a solid pull through the range.

That said, Barry Grant (Demon carbs) says if your car can take it a
DP is the way to go. I have no idea - his carbs might be different or
I might have had a bad experience. I just bought a 750 vac Demon
to try it out over a 750 vac Holley.

I was interested to see Lars say what he did. After hanging out here
for a while I'd pay attention to his thoughts.

GDaina

This is a debate like religion and politics. ask 100 guys and you will get a 50-50 mix.

Me? i'm running the factory tripower and second to lightest spring. Would not change to mechanical if they were given to me free.

Les

I agree w/ your respect for Lars- I guarantee that he knows his stuff & can set up almost any carb to work well, whatever his preference.

That being said, I'll throw in my own experiences. Shortly before Barry Grant carbs became Demon carbs I was running a well setup/carefully jetted Holley 780 vacuum(3310)- strong throttle response w/ no bog whatsoever. I was hoping to gain more HP, so I contacted Barry Grant for recommendations. The tech there said "Your car will love the Stage 3(been a few years but pretty sure it was Stage 3) modded Holley double pumper 750.". This carb had choke tower removed, 4 corner idle, had been hand ported, etc. and could flow up to 1040 cfm.

I took their advice and spent the big bucks for a new one. I worried about having too much carb & had always heard that a double pumper on an auto trans car wasn't a good idea. After bolting it on w/ no adjustments at all, I found throttle response to be remarkably better, as was the idle. It idles cold after about a 30 second warmup, which the other carb couldn't begin to do.

I then took it to the track, which I did alot of back then. Again, with NO other mods or adjustments, I promptly improved my best ET by .35 seconds w/ an increase in trap speed of over 4 MPH. I hadn't told my neighbor, who I race with(he owns a 440 Super Bee), about my mod and you should have seen him come running w/ his buddies to confront me about what in the heck I'd done to the car.

I'll be the first to say that the mods on the carb have much to do w/ the improved idle & performance but I tell my story to illustrate the point that, if the carb is properly set up or modded, a double pumper can be very well mannered & deliver a nasty punch. Conventional wisdom about double pumpers & auto trans cars isn't necessarily true. Just my 2 cents worth...

je68

I saw a big change with my demon 850 mec but it did take some time to get it setup up right but once I did like I said big difference. I did have to learn about the air bleads and spend some time with it which is why I bought the car.