I asked both Holley and Summit this question and neither could answer it. We've all read that cars with manual transmissions should be used with mechanical secondary carbs (like DP's) and automatics should go with vac secondary carbs. But why exactly is this recommended?

Does that mean you can't run a q-jet with a manual trans?

So far i've asked everyone about this, from GM mechanics to performance shops. I got a lot of contradicting opinions from- "You'll lose 20 hp with a vacuum secondary carb." to "The cam in that motor isn't big enough to cause power loss." to "I traded my vacuum secondary on my rustang with a T5 for a double-pumper because it bogged between shifts." So who's right?

I searched all over the net and found many sites that just repeated the same guidelines from Holley, BG, and here. These are good general guides, but what happens if you put a vac secondary carb on a motor with a manual trans? And vice versa. Why specifically is it not recommended?

 

Many cars fall on their face between shifts with a VS carb & manual trans, and especially so when you let off the gas pedal between shifts.
You CAN use a VS carb with a manual but you WILL lose some performance. The mechanical secondary carb will perform much smoother when running a manual trans.
The reason the VS carb loses performance is because the the secondaries can try to close again when you shift... then it takes some time for them to re-open. During this transition you will lose some power,
AND power at shift recovery is VERY important in drag racing.

So it's not that you can't do it, it's just that nobody wants to.

Personally I do not like to run a VS carb on an automatic either. The only place I've ever seen a VS carb perform as good as a mechanical is on an engine dyno where things are more steady-state.

 

What about a q-jet, auto or stick

 

Obviously you CAN run a Q jet with a stick. hundreds of thousands of gm cars did. Q jets can actually cope with it better than aftermarket offerings for a couple reasons. A. heavy air doors and B. a very sensitive booster design. It's still not optimal and suffers the same theoratical drawback as any VS carb, they're basiclly just tuned to function with it.


Long story short, VS carbs, counting carters/edelcocks/holleys/whatever, can only open sloowwwwly, and can't open all the way. With sticks the problem is put into sharp relief with every shift, but it will still cost you at least some power on all but the most ***** of auto cars. On a dyno, or on a 5000lb car with 2.43 gears, VS's can keep up just fine.

 

How much power are we talking about? 10 hp? 20 hp or more? On paper, I'm sure Dp's are a lot better than VS's, but will performance drop noticeably in the real world?

Since edelbrocks are mechanical secondary carbs, would those give better performance than VS Holleys or Q-jets on manual cars?

Bottom line, is there a carburetor that gets good performance (doesn't lose 15+ horses), decent mileage (12-15mpg), and won't foul spark plugs every few weeks?

Thanks for all the replies, let's keep 'em coming.

 

No Fbird88 you've got it backwards. The real world is where the VS carbs lose the performance. On a dyno they are equal to or better than the DP's.
When you're actually driving is where the power loss occurs.
Edelbrocks/Carters generally do not have the problem
and mine have never had it because I remove the air valve making it purely mechanical.

 

I've said this before, I'll say it again. EDELCROCKS, CARTERS, AND Q JETS ARE NOT MECHANICAL SECONDARIES. They are VS carbs....just via a little different method than a Holley VS. They are subject to the same limitations on manuals too...altho they tend to handle it better because they come with tighter secondaries. Unless you remove the air valve anyway, then it's a little different. I don't personally recommend that either...altho some knowledgible people do.

A double pumper is the best canidate for all those criteria. They make the most power, and are the most fine tunable across all operating ranges. Ideally the most tunable system on any motor can give you the best economy. It's up to the ability of the human factor to take advantage of it or not.

To answer your other question, I have seen proven gains of close to 30 hp between an Edelbrock and Holley DP on a mild small block.

 

Str8, that clears things up a lot more. I guess I'm gonna tune up my DP and see if I can get it to run right. Hopefully, I won't have to do anything drastic, like drilling out PV channel restrictors or anything. Thanks again for all the replies, guys.

 

I don't know much about carbs, but what is the advantage of VS?

 

They are usually better on gas mileage.

 

Thats it?

If you wanted gas milage, you wouldn't use secondaries right?

 

Yep, I get lousy gas mileage (8 mpg) with my Holley 600 DP and never use the secondaries.

 

I get about the same with a Qjet... and its on top of a VERY mild 350 (about 200hp kind of mild)

 

get it tuned. i know nothing about carbs, but i had a carb shop put a rebuilt quadrajunk in my car, and i get about 15 mpg around town.

 

you carb is either in bad need of a rebuild, or your car is terriblely out of tune. your mild 350 should get about 2.5 times better milage....well on the highway at least.

 

If you talked to Edelbrock, Holley, and Carter and they couldn't answer your question, you must have reached the idiot line!(thats the guy that doesn't get paid much to answere the phone so the real tech's don't have to be bothered). To answer your question about VS vs DP here it is.... If you put a DP on an auto trans car every time you step on the gas your stuffing to much fuel down its throat that it can't use and usually causes a bog. that is why they put VS on auto's so the secondary's open only by engine demand. So, if you put a VS on a manual trns, you lose performance because the secondarys will open to slowly. With a manual secondary, you are forcing the secondarys open and you need to cover the sudden air opening with a shot of gas from the secondary squirters. You have to remember that cars were made to drive on the street so driveability is a big factor in what carb goes on what trans with what gears for what purpose.

 

Yesterday I took out my Carter 625cfm carburetor and replaced it with a vacuum sec holley.

One thing which promped me to swap out carbs with my friends is that holleys always felt sharper for some reason. The carter would do well on the low end but on the high end it would litterally push the car back and rev like a raped monkey. If i downshifted w/ the carter and i was at fairly high rpm 3500 and up it would violently accelerate. With the holley it just feels smooth the whole way up. Not flat mind you but stedy pull. But i miss the violence my carter gave.

I WANT MY CARTER BACK. I think im going to get my carter back remove the stupid airflap and get the pump shot to maximum and see what happens. My concern with this is that i dont know if the accelerator pump will keep up with the barrels opening up that quickly. (reason for double pumps on DPs) What do you guys think.

I cant believe it im putting a blower on it soon and Im still playing with NA carbs. I love carbs

Wow I missied the point of the thread didnt I :-) SORRY

 

If you take the flap out of the carter you'll get a bog big time. The primary squirter won't supply enough fuel to cover the secondary opening up. the holley might need more jet size in the secondary to get back to the level of performance you had with the carter.

 

Thats what I figured on the high rpm pull of the carter (richer up top). I also think that the spring is set up to open way to late on the holley.

BOG!! One of you guys said that some people that know their stuff recomend this (air flap removal) if so then how is this acomplished correctly (on carter edelbrocks.) Looking for documentation but have not found any.

Weird the air flap is not supposed to open up in neutra but if i wack the throttle twice really hard and fast the secondary flap will smack open. (My engine hungry!!)

 

I forgot, you didn't say what cfm the holley was, or what engine you had. if you put a 600 holley on you loose perf. assuming you have a 350. I don't think holley makes a 625. If you put a 650 on you can work with it !

 

Its on a little 305. It runs mid 14s w/ my old 600cfm holley.(secondary metering plate :-(

The Carter is the 625cfm one.

 

What did the carter run?

 

I never ran it with the carter on. I know I could have tuned the low end deficit on it but it was giving me descent fuel milage so I never touched it.

does anyone want to buy a 3310 holley carburetor rebuilt about 2 months ago its got the quick secondary mettering spring thingy changer.

Im going to two carters anyways for the blown setup. Im afraid of the blowers demand smaking the secondary flaps wide open. I would be like running without the flaps NA. (This is from what im told)

I could compensate for it by richening up the primaries enough and using the largest shot on the linkage.

 

The two carters is a good choice. They fit good in front of each other. the holleys would have to go on sideways because the bowls would hit each other ! For now, I would put the carter back on. I think the carter has slightly smaller primary venturis than the holley that's why you have better top end with the carters large secondarys. The holley will have better primary performance than the carter thats why you don't feel the big "kick" you felt with the carter BUT, the performance is still there with the holley, you just don't feel the big transition when the secondarys open up. If it aint to much trouble, put the carter back on then go to the track and compare.

 

If you remove the flapper you'll need to move up a few steps in squirter size and maybe increase the pump travel as well.

Carter MODs

 

All those mod's will work good at the drag strip! But how about the street? Cold weather? you'll have to sit in the car for 5 or 10 min. untill it warms up enough to idle. If your driving at part throttle and decide to floor it you won't have enough pump arm travel left to cover the secondary's opening up! I've lost sleep at night trying to figure out how to get .010 of a second faster, and when I did, I had to give up some drivability on the street. It's your choice, I had to drive my car to work every day, so I oppted for consistency at the track and went with other parts and tuning.