OK. I have a 71 LT1 350 4 bolt main, Edelbrock heads, cam, intake, MSD, headers, etc. I have a 750cfm vaccuum secondary carb on it. The car has 3.70 gears and a 4speed. I knowI want a mechanical secondary carb. Does anyone have the right combo as far as jetting and cfm with this trans and gear setup? The car is lazy and was before I rebuilt it. There is no snap to it. I have had mechanical secondary carbs before on 4 speeds and have found them to be great. With an automatic vaccuum secondaries work much better. So, carb size, (probably 750 cfm) jetting and accelerator pump size I need help with. Any help?

 

This what I learned: all depends on the cam size. Any duration @ .050 liftstock-to 220 degrees, a vacuum secondary is fine, 220-230ish degrees you want to run a 750 mechanical secondary, any cam 240+ degrees you need an 800 cfm mech. secondary because those cams are for high RPM engines. Whats the specs on the cam? Car shouldn't be lazy with 3.70 gears. Could be a bog because the secondaries are partially open because of the low vacuum?

 

The cam is pretty big. I just tried to get the specs and the catalogue doesn't list them. I put an Edelbrock heads, cam, intake package on it. Says here it's good for 435 horse. Feels like it too. Nice lopey idle etc.I thought that the secondaries were opened by the lack of vaccuum, as in full throttle rather than presence of vaccuum as in idle. Am I wrong here?

 

Hi z16375ss, so your saying that you think the engine is lazy due to the carb? Do you have any other carbs you could try before you re-jet/purchase new, etc.? Are you using the stock LT-1 pistons, what size chambers are the Edelbrock heads? Is the ignition timing and valve timing correct? Just thinkin' out-loud here. I guess I would swap carbs first and go from there.

 

Welcome to the Corvette forum. You are correct re: vacuum secondary opening as the vacuum bleeds off. Q-jets also have an air valve that lifts the secondary metering rods as the air flow increases and this valve has a wrapup spring that controls the rate of opening. So if you have a Q-jet, check to see that all of the adjustments are correct.

I'm guessing that you probably have a Holley tho. But it's secondary also has a spring involved which controls how fast it opens and there are several springs that you can use. Good luck.

 

Carb size (cfm) is just a math excercise. I have posted the math before, but basically, you have 350 cubic inches. The "perfect" gas/air mixture at sea level is 14:1 (slightly more at elevation). If you are filling/emptying the cylinders (350 cubic inches worth) 5000 times per minute, you are moving just over 500 cfm. If you up your red line to 7500, you will be running a perfect mixture at around 750 cfm.

CFM = Cubic Inches x Max RPM's / 3456 x VE%

VE is volumetric efficiency, or how well your engine uses the fuel that it is being fed. Most stock carb'd engines sit around 75%-85%. VERY well tuned engines can get to 100%, especially with hot cams.

sooo...

CFM = 350ci * 5000rpm / 3456 * 85% = 430cfm
350ci * 5000rpm / 3456 * 100% = 506cfm
350ci * 7000rpm / 3456 * 100% = 708cfm

So a great 350ci engine with a red line of about 6000 rpm, and a great tune can take advantage of 600+ cfm carbs. Anything bigger is just throwing gas out of your exhaust or not living up to full throttle (squirting more gas in than your engine can suck air/fuel mixture to burn, making full throttle too rich).


bill.

 

Big cams reduce vacuum and the secondaries open pre-maturely.

True that most 350's don't need anything more than a 600-650.

Mostbigger carbs wastegas out of the pipe because customersdon't know how a carb fits into the whole mix of engine theory, and typicallybuy a carb based on information available to them and from the people around them.

See... the problem I've ran into is that there is only one carb in the world that will do a 350 +big cam the right way: a 650 double pumper from Holley.

MOST 600ish cfm carbs are tuned for economy and aren't meant for any cam duration over 220 degrees because most customers demand so much "drivability" they just want the best of everything... so... most carb companies recommend their 700ish cfm line of carbs for "modified" engines, even though the engines will never need that much, because those bigger carbs come with the internal guts to keep up with the increased RPM range ofbigger cams so the customers don't have to do as much tuning, its for out of the box performance. Besides, most carbs will adjust their fuel flow rates to the loaddemand of the engine vacuumanyway, the CFM rating is just correlation tothe size of the holesin the bottom.


Here's the theory: if you take a 600 cfm Edelbrock, and an 800 cfm Edelbrock WITH THE EXACT SAME INTERNALSSPRINGS/RODS/JETSETC.AS THE 600 carb, they should run NO different for most 350's that never see over 6000 RPM. Just because the bigger carb has more potential for air flow doesn't mean the engine will ever demand it, because the piston acts like a big syringe, pulling in thefuel/air mixas needed. If anything at all, an 800 with 600 internals will run a littleleaner, asCFM ratings are typically based on ambient air blown through the carb, and not the mixed stuff that gets sucked into the engine, and it could also reduce the mixture velocity at low RPM, the same way an intake port that is WAY to big will.

Most people are just plain scared of cracking open a carb and all their voodoo, so they just buy what they are told by the company, or their buddies, or the forums.



If something is not right here, feel free to tell me, I am always learning

 

Damn. You guys are good. This has been a great lesson but no one yet has told me what the hell to run!. Right now it has a Holley 750 vac sec on it. Runs like **** if you ask me. Just blah.Timing is all in at the start 36 degrees locked into the MSD distributor. I'm gonna go with a Holley 750 dp and jet it until the plugs look right. I know this may be too much but they don't make a 700. Thanks for the help. You guys really know your ****. I do too but thought I might get the magic combination off this board from someone running the same setup as me and was happy with it. I'll get it sorted out if it's the last thing I do! Thanks again!

 

I read that vac secondary carbs are for cars with an auto trans and mechanical (double pumpers) are for use with stick.
My 74 is set up pretty close to yours and I run a 650 Holley double pumper and the Vet flys.

 

I'm going with a 750 dp sqare bore andI got jet kit with it. I think it is a little overkill. But we will see. Thank you for the help. Schoon, I like the paint scheme, very slick.