A 350" engine spinning at 6000 rpm with 100% volumetric efficiency can only swallow 608 cfm. For a L-48 a 500 cfm carburetor is more than enough. It is absolute pointless to slap a 750 cfm vacuum secondary carburetor on any L-48 engine because at wide open throttle the secondaries would only be open maybe 1/4th of the way. In street use running thru mufflers the volumetric efficiency can seldom exceed 85%.

And double pumper carburetors aren't suited for street use at all. They're made for engines that don't drop below 4000 rpm such as in track racing or in drag racing where the car is launched at over 4000 rpm and it remains above 4000 rpm. People buy these things on the advice of their ignorant friends who learned from other ignorant friends. That is why every factory engine comes with an air valve secondary carburetor to prevent bogging.

 

Q junk is an on-demand carb. Almost never tuned to run at its potential. It was designed as a frugal smog carb. Never intended for true high performance.
Most stock vette engines will run great with 600-650 cfm Holley.

 

Lars posted this awhile back....................................

I have done quite a bit of testing, including dyno testing, of mechanical sec and vacuum sec carbs. What I have found is that vacuum secondary carbs (there are only 2 brands of vac sec carbs on the market - Holley and BG) will never get the secondaries wide open, even with the lightest springs installed. This impedes performance and results in the carb not delivering its full cfm potential.

I have tested mechanical sec carbs on a wide range of engines, and have found them to perform very well. I recenty ran a 750 double pumper BG Speed Demon on a little 302 Ford on the engine dyno at Westech Performance. It ran with no hestitation, no off-idle stumble, and brutal performance from off-idle to wide open throttle. The 750 mech sec carb outperformed a 650 across the entire rpm range on the 302, and had better throttle response.

I used to be of the opinion that vac sec carbs were good for street cars and that mech sec carbs belonged on racecars. After testing on the dyno and from all results I have achieved on the street cars I've built, I am now of the opinion that the mechanical secondary carb is the way to go for overal best performance and tuneability. Use the vacuum secondary if you're running a motor home, 4WD, truck for trailer pulling, or other applications that see wide open throttle at low rpm for extended periods of time. Use the mech sec DP type carb on any performance application such as a Vette.
https://www.corvetteforum.com/forums...ly-driver.html

 

Thank you again! Off the topic for a bit: I recently said goodbye to a co-work, who everytime he shared his experience it was like getting a lecture from a well rounded professor (wealth of knowledge). Hence I nicknamed the time he shared (Stananland University), it is because everytime he talked, it was an education and if you were not listening you missed something.

Hence, when I meet someone who just bought a corvette and never owned one before. I always recomend to them to join a club, there you will get to know everything you need to know about your car. I was one of the new "Vette" owner 6 years ago, to get to know this 72 LT1, I joined a local corvette club and joined this forum.

This post I started sure has educated me about fuel delivery. Thank you everyone.

My 72 LT1 sure runs well with this 600 CFM with mechanical secondary, I am testing it this week end to make sure the WOT performance is as it should. Then to the Dragon.

 

Carbs are a passionate discussion for most car buffs. And one in which everyone has an opinion.

You really do have to decide on the application.

I do not debate the benefits of a double pumper, or the cfm size of your choice.

Only the application? Is your car for the street or the track? Do you really want WOT all the time?

Yes, you can tune and mount a monster double pumper on your stock or mild small block chevy.



But does it make sense?