dog-leg performance boosters WTF

I was looking on eBay at carbs and parts...and.....I read this statement...

This carb is the more desirable performance 780 with dog-leg performance boosters from the muscle car era, :shock:

WTF..... does that mean....o No: o No: o No:

:idea2: educate me guys ray:


:bglasses: :TU:

 

Read this article

http://www.carcraft.com/techarticles/116_0508_carburetor_showdown/index.html

 

thanks.....:bglasses: :bglasses: :bglasses:

 

I have two completely/correctly calibrated Holley carbs for my engine using both types of boosters.

The Car Craft article touched on it, but didn't do it, which was using the airbleeds to clean up or calibrate the fuel curve in lieu of just changing jets. Common error or problem with Holley and Holley clone tuning.

I performed street and track testing with both of these carbs, and had them on the dyno a few times as well. On the street, I find the annular boosters more "sensitive" to very light throttle openings compared to the downleg boosters. The engine is slightly smoother right off idle, and slightly improved throttle response for "normal" driving.

Beyond that, very little difference in felt power, etc.

At the track, the downleg booster carb runs more mph, as the annualar boosters give up some cfm which my 600hp engine requires. Even so, both carburetors run very close in ET. I've never observed over .08 seconds between them, back to back testing, and most of the time the difference is closer to .02-.03 seconds in favor of the downleg carb.

For tuning, the downleg booster carburetor requires much larger jets to get the fuel curve where I want it, and it pulls a tad harder on top end, as the annular boosters give up some cfm.

The best part of the testing I saved for last. Either one mounted on my dual plane RPM intake ran slower in both ET and MPH to my 1977 Q-jet. The larger 850cfm 4781-2 made 2 LESS HP on the dyno, which was reflected at the track when it ran .02 seconds and .30 mph slower than the q-jet!......Cliff

 

Cliff,
Someday when you have nothing to do :laugh: could you elaborate more on tuning with air bleeds?

 

Will do. The theory/principle of the airbleeds as they relate to fuel curves is much more complicated than the tuning process involved with them. The folks who have figured out the airbleed deal, lead the industry in building those types of carburetors.

A Holley or Holley clone is basically the same unrefined design used back into the 1960's (far back as I can remember). Two float bowls, two metering blocks, two accl pumps (DB models), 4 airbleeds per side in the main casting, four boosters, idle curcuit, and various feed/enrichment curcuits in the metering blocks, not to mention at least a dozen places to leak fuel below the fuel level.

They are very simple designs, and it doesn't take much effort to get the most out of any particular one. The large bore areas comprimise fine metering required for part throttle smoothness/throttle respose/fuel economy. This is why we often see larger and/or more sensitive boosters being used in modern units. The booster designs create a greater signal to the main system, and improve fuel delivery and atomization at light throttle openings.

When it comes to street driven cars, these things are important. For full race stuff, the "toilette" principle applies. It only has to work good when you flush it, and mix the correct proportions of fuel with the incoming air across the engine's speed/load range.

Holley designs are, and always have been less than desirable for daily driven vehicles. Modern materials to reduce/eliminate fuel leaks has brought them a long ways, as have improved booster designs. We've seen folks come up with pretty good results tuning them for fuel economy and driveability, armed with a 02 sensor and plenty of airbleeds/jets/power valves.

I've dabbled with them myself, but keep coming back to the Q-jet, it's simply a much better overall design for street driven cars, and with correct tuning will run right with the big Holley carbs at the track. We still have to feed them with a "garden hose" on really high HP stuff, so for many folks it makes more sense to use a carb with two huge fuel bowls fed by two needle/seat assemblies.

These days I enjoy the "shock" factor, so run a stock intake and q-jet on my car running nearly into the 10's in full street trim. That look you get when you put the hood up in staging lanes in final rounds after making a hard run.....PRICELESS!:TU: ......Cliff

 

thanks Cliff, that was highly educational......:TU: :TU: :TU:

PS.....what Q jet do you recommend :idea2:

 

I use and prefer the 1976 and later larger cfm units, front or side inlet, doesn't matter.

The best early divorced choke carbs are the 455 Buick models from 71-74.

The best early "hot air" choke models are the 73-74 Pontiac Super Duty carburetors.

All of the carburetors mentioned above are equal from a performance standpoint.

Next in line would be the 1971 Pontiac HO single ring units.

Yes, later emission years units that most folks shy away from, are as good as any of the early stuff as far as performance goes, and superior in many other areas, such as the APT (Adjustable Part Throttle) system for fine tuning the part throttle A/F ratio.

The get side lined and avoided simply because they have very lean idle and off idle calibrations.

The worst units are the 1968 and older models with the large floats and center float fulcrum (hinge pin) location. Oldsmobile and Cadillac carbs continued with this design till 1974. All Marine units used this design as well. (Yes, the Olds crowd hates me for these comments, they woln't even let me sign up on one of the Forums)

These designs can be made to work, but require a LOT of fuel flow at relatively low pressure for really high HP applications, as they are very sensitive to fuel pressure.

Q-jets as a whole get a bad reputation from these early designs, and we still see the same crap posted about them on nearly all the websites about not being able to handle any fuel pressure, etc.

Later designs will easily handle 7-8 psi (many racers run 9-10 psi) with a relatively large fuel inlet seat and small float.

I have a customer running Super Stock with an early Camaro, 355cid, into the 9's with an early 70's Chevy divorced choke carb. He runs two 250gph pumps and pretty high fuel pressure thru a .125" inlet seat. Considering the power level, even the smaller 750 cfm units are fine for most applications......Cliff

 

thanks Cliff...:bglasses:

 

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