84CraZ28

iTrader: (1)

Hey guys i'm swappin the 305 l69 out of my 84 z28 for a gmpp 350/290hp crate engine. i'm thinkin about using the sugested holley 670 4barrel but b4 I decide I want to look into the 650 double pumper.
-How does the double pumper work compared to an average 4 barrel?
-Is it to much carb for my new motor?
-Any other info you may think would help?
All info/ answers greatly appreciated Thanks Guys!

andy74

a double pumper is a mechanical secondary carb,in my opinion and experience they are better suited for racing,a vaccum secondary carb is a ton better on the street,ive used both over the years,and a vaccum secondary carb has always been a better choice for street driven cars

Apeiron

A vacuum secondary is only better on the street when you've bought too big of a carb. Size it right and the DP is just fine or even preferrable.

cuisinartvette

Secondary is activated strictly by pedal position you can feel when youre on the primaries. 600 DP should be enough for what youre doing.

five7kid

iTrader: (14)

A vacuum secondary carb being better for the street or automatic transmission is a complete myth.

Not sure exactly what you're asking, though. Are you asking for the technical details of their operation? Then check this site (linked from the Holley site linked in the "welcome to the carburetor forum" sticky): http://www.holley.com/data/TechServi...ech%20Info.pdf

If you're asking how well they function, then I agree with Apeiron. Having run both, a double pumper is just fine on the street.

As to the size of carb for that 290 horse 350, you'll be fine. Check out this site: http://www.holley.com/data/TechServi...Carburetor.pdf
There are two charts there, the first being the minimum size you need, and the 2nd the largest you can use. On the first chart, I believe the 290 horse engine makes max power at 5000 RPMs, but even if you assume 5500 RPMs, you don't need more than 600 CFM. On the 2nd chart, you didn't say what transmission you're going to use, but assuming a stock TH700R-4, it stalls at something around 1500 RPMs, I believe. Draw a line starting at the "1500 RPMs" point on the left, going through "350 CID" in the middle, and a 750 CFM double pumper would work. So, a 650 is in the middle, and therefore would be fine.

Having said all that, why don't you just put the stock carb and distributor on the 350? It'll flow more than 700 CFM, but won't be "too big", because the secondary air valve won't open more than what the engine can use. It's a great street performance carb, better than either type of Holley.

PeteZ28

I haven't been convinced that the original cc-qjet is a great performance carb. Whether or not manufacturing quality is better than a Holley, I'm not sure, but my experience is that these carbs are borderline in adapting to a modified 350. Some people have had success, others haven't. This can be due to variability in the manufacturing of inidividual carbs. Mine seems to be lean at low engine speeds until about 2000rpm. There doesn't seem to be the capacity to tune the idle/transition circuits to flow the fuel at off-idle and light cruise conditions. It can be made to perform well with modifications to idle restrictions and idle bleeds, but I'm hard pressed to call it a great performance carb as is. A great performance carb is one that runs decent out of the box and can be tuned for best performance without drills.

As far as the flow, seems like a qjet with the air valve only partially open at say 3-4000rpm would be flowing less air than a wide open double-pumper. In this regard, the qjet is similar to vacuum secondaries.

As far as buying a double-pumper, I've looked at prices and noticed the Holleys are pretty close in price to the Quick-Fuel HR series carbs ($400) on Summit's web-site. For a few dollars more the QF HR carb comes with 4 corner idle, change-able idle restrictions and air bleeds (don't need drills!) on double pumpers and vacuum secondary carbs. The QF double pumpers also have an adjustable secondary throttle linkage.

Good luck

five7kid

iTrader: (14)

By that definition, no factory carb qualifies. Very few aftermarket carbs do, either. For one thing, the box had better have been chosen based on the engine, because there's no such thing as a universal, out-of-the-box carb.

PeteZ28

Quote:
Originally Posted by PeteZ28
A great performance carb is one that runs decent out of the box and can be tuned for best performance without drills.

Quote:
Originally Posted by five7kid
By that definition, no factory carb qualifies. Very few aftermarket carbs do, either. For one thing, the box had better have been chosen based on the engine, because there's no such thing as a universal, out-of-the-box carb.


Are you saying the mentioned Quickfuel carb would require drilling fuel circuits to be tuned for best performance for the average modded 350 most people put into 3rd Gen's? I don't think that is the case. For the engine '84CraZ28' asked about above many aftermarket carbs would run really well with moderate tuning and no drilling. The Quickfuel carb could certainly be tuned for 'best performance' without extensive mods. A holley probably could too. That's not something that one could say for certainty of the cc qjet. There not bad carbs, but one may or may not need considerable tuning/mods to run really well on a modified engine. I've used Holley's before and got good smooth responsive performance with the usual re-jetting, vacuum sec, accel pump tuning. The adequate range of adjustment is just not there in my cc qjet to run smoothly under 2000 rpm (without changing idle/air bleed internal restrictions, etc.)

five7kid

iTrader: (14)

The 290 horse crate engine doesn't require anything special. The stock cc q-jet would be fine. A lot less hassle than the QFT carb, might run a little better WOT with secondary rods/hanger change.

cuisinartvette

Go to youtube and enter in 4150 double pumper carb

Holley has a vid on there that illustrates and explains the basic differences.