everyone has a fav.brand of carb.a fav.size,and style for a given a application.some people like double pumpers on street cars,some vac.sec.some people put 850cfm on 350ci.i know a mechanic that swears a carter 500cfm is the hottest thing smoken on a 350.for me the carb formula has held true.i have a holly 750 vac.sec.a Q-jet 750 vac.sec. a holly 650dp an a summit 600.all of these carbs have been on my car.i have also had a edelbrcock 750 on it.my 600cfm out performs the others.it gives me the best seat of the pants feel.its the quickest and has best throttle response.i put a 50cc accelerator pump in the 600cfm.i like holly.and the summit carb takes all the holly parts. im sure holly makes them.600cfm is great on my motor.355 sbc,rebuilt bottom,sealed power flat top short skirt pistons,39 quinch,10to1 compression.lt4 hot cam,218-228 dur. @.50. 525in 525ex lift,procomp 190cc heads,lunati lifters,lunati 7.29in pushrods,1.6 stainless steel full roller rockers,headmen shorty headers,dyno max dual ex.air gap rpm intake.what do u like?vac sec,holly,eddy.big carb. small carb?what works for u?why?

 

For what it's worth, you're correct in that the Summit carburetors are a Holley design.

I'm not the carb expert, but I can tell you what Mr Five7Kid has to say about carbs:

-Edelbrocks are never the best carburetor for any application!

-If you have a manual transmission, you must use a mechanical secondary carb!

-For the rest of you, the ccc Q-Jet that came with it is the best carburetor you can have, why would you change it out for anything else dumb%&$??

You're welcome


To answer your question somewhat though, I personally think it's best to err on the side of undersizing a carb, as opposed to oversizing. We all overestimate what our motor needs anyway, we just want to boost our ego by thinking it needs 750 or 850 cfm

 

by putting diff.carbs on my car i found my smaller carb is instant.an it pulls great all rpms.

 

The Rochester and Carter Qjets are designed so that even though they are "larger" carbs, they only pull what the engine requires as opposed to Holly's and Edelbrocks that are constantly running at 750 or 650 or whatever size they are.

 

Not exactly true

 

Carter = Edelbrock Performer.

Holley vacuum secondary will also limit flow. It does it in a different manner, however.

There's no such thing as a "vac sec Q-jet".

If I'm reading you correctly, the Summit 600 CFM is the old Holley 4010 model that they quit making several years ago. Not sure what Summit did, but I'd guess they talked them into bringing the line back up with the Summit name on them. The selling point of the series was supposed to be no gaskets below the fuel level, but they were a little more complicated to tune. They are vacuum secondary.

I'm not sure what objective standard you're using to determine which one ran best, but assuming equal tuning attention and adequate stall, I'd take the 650 DP at the drag strip on your engine, and the Q-jet for economy on the street (assuming a spreadbore intake manifold that doesn't require an adapter).

 

why do u need a mechanical secondary carb for a manual trans car?

 

You don't need it, but it tends to work a little better.

 

why does it make a difference and how do i kno if the carb is a mechanical secondary one?

 

The throttle closes and reopens between upshifts, so with a VS carb there can be a delay on each shift while the secondaries open, instead of opening immediately. When everything is appropriately sized and tuned, it's not a major problem, anyway.

 

Mechanical secondaries are mechanically linked to the primaries.

As stated, you don't Need them with a manual, but they work better. Why?

With vacuum secondaries, they will stay open monentarily even after you've lifted your foot off the throttle. At high RPM's, you can essentially be "powershifting" your transmission without meaning to, putting extra abuse on it.

Air valve secondaries, like an Edelbrock, have the opposite effect. The secondaries will not open immediately when you open the throttle, leaving a slight "lag" after you let off the gas for a shift.

Mechanical secondaries also often offer better "response", which also makes them popular with a manual.

 

ohh ok thanks. any visual way i can tell the two apart?

 

The secondary linkage closes the secondaries immediately when the primaries close.

 

Vacuum carb's have a diaphram/bellows attached to the carb main body, mechanical carb's have only linkage, no diaphram, to control the secondaries.

 

well, my car is an 86iroc.700r4 with shift kit.the rest of the drive train is stock for now i think.i dont know the gears,but some of the guys on here know gear formulas, so my car reads 2000rpm @ 60mph.fourth gear is.70 rear tires are 255-50-16. what i use to judge what carb i run is thottle response.my 600cfm is so instant off idle. much better then the bigger ones.i dont feel any less pull @ 3000-4000-5600rpm.shifts @ 5600rpm.i can brake the rear loose at 50-55mph with the 600cfm.i play a lot at the lights.smoke mustangs,i dont know the top end.it only reads 145mph.but its still pulling hard.there is no
time that the larger carb feels better.the 650dp feels a hair stronger 4000rpm up,but the low end sacrifice is not worth it,and the GAS!

 

i have found that using the formula of displacement times rpm devided by 3456 gets you at the optimol carb for your app.for the best gas mileage and performance,choosing between a vac.or mechanical sec.should be on how you intend to drive,all out performance,mechanical,if you going to drive it around vac,if you just going to race it or ride in it is the question you have to ask yourself,im willing to give up a tenth for better fuel economy,if you want the best of both worlds go for the fuel economy and put a small nitrous system on it,just my way of thinking.jplp

 

I'd say, "If you are going to tow, vacuum secondary. Any other use, mechanical secondary."

Assuming we're talking Holley's that is. Q-jets are fine for towing and driving around. Not too shabby for racing, either.

 

I like Qjets but dont know how to tune them, I think they are a "smart" carb and work real reliably.

For most auto cars a V.S. works real well unless its cammed to the moon, big stall gears etc then would move to a D.P.
I always got the smoothest operation and mpg from a VS

Almost all stick cars Id use a Holley DP. Current car requires it, deep gears large cam stick etc.


To the OP

You dont need that 50cc pump on that 600 carb, you only need that thing when you use a squirter around a size 40 which requires a hollow screw which yours never will.

Sounds like there was a tip in stumble that someone stuck that on there to compensate for, take it off and tune it right youll like the difference.