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.

ORIGINAL: z16375ss
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?


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. :D 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. :eek:



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.

ORIGINAL: SCHOON

I read that vac secondary carbs are for cars with an auto trans and mechanical (double pumpers) are for use with stick.


Not sure about the L-88 but I believe the rest of the Holley carbs on Corvettes were vacuum secondaries.

Thisis just something I read a couple of years ago.
They say the response is faster with double pumpers.
I think I read it on Holley's website.

I suspect that would be true with mechanical secondaries since you would have a bigger hole to cover when the throttles are slammed open.

Absolutely Clayton!! :D It isn't just the CFM that counts, it is the jets, the guts, strength of the springs, etc.. I'd rather run with a smaller CFM carb with great internals (high end Demon or 4150 or better Holley) than a bigger and "more capable" carb with cheezy internals. If your carb is mixing right and holds up to vibration and heavy use... you will be happy with the results.

I've got a polished and coated Holley street avenger on mine (with mild cam). Overkill on CFM, but responsive and no drama or maintenance.

bill.

All right.I put the Holley 750 dp square bore on tonight. The car already idles better. I have the feeling the carb I have been running was just plain worn out. I crackedthe throttleafter it warmed up and it had that snap I'm looking for. It sounds really good.I think that I could have rebuilt the old carb, put a looser vac spring in it, rejetted it, put a new accelerator pump on it etc and it would have run well. I got a jet kit so I can get it just right but it seems to run great out of the box. I'll check the plugs tomorrow. I tried to set the idle mixture screws but found they were exactly where they needed to be. The box says each carb is completely wet flow tested so maybe these guys set these up right the first time. We'll see. So far I'm happy. What more can I ask for?

More power Scotty.

Took the car out for a shakedown and found the throttle response about 100% better. I checked the plugs and they look a little rich. I am going to leave it alone though. The drivability of the car is wonderful. No more hesitation or bucking. A real pleasure to have something you try work.I think the carb I had on there to begin with was just junk. More power Mr. Scott!

While running rich will give snappy acceleration, a minor pitfall is the extra gas tends to "wash down" the cylinder walls. At least that's what I've read and heard. :eek: Might want to lean the primaries a tad. Just a thought. :)

Good point. I forgot about that. It could be leaner anyway so I'll do it just in case.

That's the good news and bad news about Holleys. There a lot of things you can tweek. Unless you have a dyno, sometimes you just say uncle, that's good enuf.

Amen, Brother. The engine in my car is a fresh 350/350HP. I putabrand new 650 Holley Street Avenger on the car and could not get the mixture or response right. I changed the vacuum secondary spring several times, changed jets several times, adjusted the accelerator pump several times and nothing seemed to work. The car had a good idle and a good off idle. Now under acceleration and at highway speeds it was a different story. It would almost lay down under hard pedal and at highway speed it would surge every two seconds. I threw the Holley in the corner and put a 625 Demon on it. I put the carb on, adjusted the four corner idle, set the idle at 750 rpm and drove it. The only problem Ifound was that the vacuum secondary spring was too weak and the secondaries were opening a little too soon. I put in a stiffer spring and it fixed that. After only 300 miles of driving with the Demon, the plugs were burning clean instead of black and my mileage went from 14 to19 mpg. I have always run Holleys on all of myCamaros and never had a problem. I have heard from other people that the quality of theStreet Avenger Series is nowhere near thequality that they used to be. I have a 750 on the wifes69Camaro that I put on right out of the box in2000 and it still runs perfect. The reports that I have received indicate that there is some internal leakage in the newer Street Avenger Series which will definately effect tuning and performance. Enough ranting. Have a great day everyone!!!

We can be seen on the Hot Rod Power Tour this weekend!!!!!

I wasn't going to reply to this as it's pretty straight foreward...with the exception of the newer "Demon" carbs which are supposed to be superior in every way as far as performance goes, forever (my foreverbeing a 35 plus year period) any small block, from 327 to 400 c.i., (and even a worked 283) with alittle guys to it, runs the best w/ a 750 cfm carb, performance-wise. That's been the concensus forever.


1969 L46: Interesting and good info stated. Sounds like you're up on things...

The info put out in the string was superb and theonly reason that I responded to it at all was so that the readers had the piece of info concerning the Street Avenger series.

I have been working on cars since I was 12 and I am 53 now so, 41 years later and I am still not through learning nor will I ever be. In our car club we all pitch in and help others with their projects.

The OutlawCar Club is an eclectic group. We have mostly Corvettes (59, 65, 69, 72and a 78 Pace Car)andCamaros (68,three 69's, and two 70's)but we also have a70 Hemi Cuda, a 71Challenger R/T, a68 Javelin, a70 Monte Carlo,a 67 Nova, a 70 Malibu Wagon, two71 Chargers,abone stock 65 Mustang, a bone stock 66 Shelby GT 350H,a 23 T-Bucket, a 32 Ford Coupe, a 35 Ford Sedan Rat Rod, a 57 Ford Pickup, a 58 Chevy Pickup, and achopped big block powered 39 Chevy Sedan. I may have missed a few but you get the picture.

Although I am a Chevy man at heart I have a broad range of experience with other name plates and love to pitch inand share with others. I love to see the younger people that are interested incars and I support their interest even if they burn rice.

Well, I've put some miles on the new setup and the drivability is excellent. Just what i was looking for. The car is not really faster but it gets up NOWwhen you hit it. The Holley 750 dp square bore has been the carb I have used the most in my Hot Rodding life. Starting at 13 and now 46. It is a combo that is hard to beat.I still don't know why someone would put a vac secondary on a 4 speed with steep gears. It just doesn't make sense.

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