Carb and dual snorkel issues

[svtrichie]

My 82 GT has a performer 302 intake and a edelbrock 750 carb which I believe is too big. What carb would you recommend going to?? Also, I want to put the dual snorkel back on but not sure how or if I need to connect all the vacuum fittings. Or can I just leave then unconnected.

[dilligaf2]

My 82 GT has a performer 302 intake and a edelbrock 750 carb which I believe is too big. What carb would you recommend going to?? Also, I want to put the dual snorkel back on but not sure how or if I need to connect all the vacuum fittings. Or can I just leave then unconnected.

I don't really know about the carb question, but as far as the air cleaner goes I think the green and grey plastic vacuum switches just control the 2 hot air diverter valves on the snorkels. I don't remember what the metal one in the bottom did. If the vacuum schematic in still on the hood then you can figure it all out with a little time and you can always post it if you're having problems reading it.

[billkandi]

I think the factory size was a 600 Holley IIRC.

[richpet]

If it were me I would go with a 500-600 carb. A 570 street avenger worked great on my modified 306 years ago... 750 is WAY too big for a 302.

[homer302]

As long as the air filter is inside of the holes AND (AND) there are no vacuum leaks elsewhere it should be fine. The holes in the air cleaner housing by themselves are not an issue if you have a proper filter inside the housing. If the stuff that is supposed to plug in to those holes are vacuum leaks then you have an issue of course. So it's hard to answer. That carb is too big but if tuned properly will run fine on your car. You are correct that it is too large but your car can be made to run fine with it. A Carb pulls fuel out of the bowl by vacuum. It will only pull what it needs.

[Lowetlx]

I ran an 1850 Holley and the car ran great with a stock engine and cam. Then i outgrew it when I replaced the heads and cam. I ran a avenger 670 and had issues with a lean spot in the transition that could not be tuned out without drilling idle feed restrictions. I now have a hr650 quickfuel. I should have gone with quick fuel from the beginning, it’s a great carb. I would pick something from quick fuel lineup. The 650 double pumper would probably be too big for a stock engine combo.

[Walking-Tall]

Absolutely nothing wrong with running a 750cfm on a 302. Dial it in as per the Edelbrock tuning literature. Less cfm only limits upper end HP, and more cfm with idle and transition dialed in has no bottom end, cruise, or overall average torque penalty. I wouldn't put anything less than a 750cfm on even a stock 302... and don't fall for the marketing from Holley or others today... a "670" and spooky sounding 'Street Avenger' is still, by it's bore and venturi sizes, a 600cfm carburetor, the same as an 1850 or 80457 etc. They have not in 2018 figured out how to (naturally aspirated) shove more air through the same size venturi, lol... On the subject, QFT offerings often have transfers slots in the base plates that are too wide and/or long, creating a marvelous masking of any/all tuning issues with abundant transition fueling, or vice versa... ultimately requiring transfer slot restrictions if any kind of sane MPG is wanted, or idle/transition fueling increased just for correct function... as well as sometimes all of the other "new and improved" nonsense of over-"emulsion" and main air bleeds too big etc wreaking havoc on proper function... ask me how I know...

[AJ1398]

A Holley 570 Street Avenger or 600 CFM cab works well on a 302 up to about 6500 rpm. At that rpm a 302 moves 568 cfm with 100% volumetric efficiency, and a 302 with a carburetor is probably about 85 to 90% volumetric efficient over it's operating range. Sure, you can tune a 750 cfm to work, but it's more carb than the engine needs.

[Walking-Tall]

Respectfully... decades of magazine articles, rarely composed by someone with the engineering education or experience to be somehow since and now still dictating consumer and hot-rodding procedure, and/or also not proof-read by anyone with any such credentials, is at work when this subject of "too much carb" comes up... Unless too small, the size of the carburetor hasn't much affect on how much air the engine will pump. The velocity through the intake tract isn't affected by the size of the carburetor bolted on top of it, unless of course again, the carburetor is too small enough to be affecting it. A larger carburetor installed will always increase manifold pressure (decrease vacuum, airflow restriction) up top and also allow increased air density to go in, producing more power all over, especially up top of the rpm range, as well as more peak and average torque amounts from off-idle to the red line... instead of use up substantial amounts of power having to breathe through a small carburetor.

Adds: IMHO... yup, I'm crazy like that ^^^...

[gr79]

Since the 50's, large single and dual 4v carb on small cube engines were offered by auto co's.
Not done for street cruising drive ability or fuel mileage.
Notable factory production small block engines with large Holley:
1969-1970 Ford Boss 302, Holley 780 cfm
1967-1969 Chevrolet Z/28 302, Holley 780 or 800? cfm
1969 Chevrolet Z/28, DZ 302 over the parts counter option, Cross Ram intake, two 4bbl Holley carbs.

Funny how cars 'feel' faster with larger carbs. And show well.

[Walking-Tall]

Yup, and perfect drive-ability and maximum fuel mileage are only a few tweaks away with calibration. If some time is taken, the results are pleasantly surprising. Ditto with multiple carburetors for very efficient function, with also usually very much so enhanced air:fuel distribution. Win, win, win.

<--- that red car (daily driver, 365 days/year) you see at the left sports a 500cfm Holley 2-barrel on it's 232 cu-in V6 engine, for over two years now. Whacko, right? Not with crisp throttle response/drive-ability and any amount of acceleration, tired engine to boot with more power than it ever had, and averages 30 MPG out the highway on a bad day.

[homer302]

I wonder if a lot of the misconceptions came about because people want to just buy a carb and put it on without tuning anything and Holley or whoever doesn't like a lot of tech support calls about how to tune. So they sell a carb advertised at a certain CFM but really it's the jets and carb tuning that are designed for a smaller CID engine maybe. So even though the housing is the same, I'm sure a 600 CFM Edelbrock comes with smaller jetting than a 780 CFM does. Just a theory. We all said the same thing. The carb too big thing is a myth.

[emerygt350]

But isn't a 500 cfm 2 barrel different than a 500 cfm 4? I remember reading some very in depth stuff on that. Something about how it is measured.

[Walking-Tall]

But isn't a 500 cfm 2 barrel different than a 500 cfm 4? I remember reading some very in depth stuff on that. Something about how it is measured.

Yes, theoretically, 2-barrels are rated at 3.0"Hg as opposed to 4-barrels rated at 1.5"Hg ("Hg = WOT vacuum), and with the LIST-4412, it's all about a flow rating and vacuum level on a race engine, which is where 4412 500's were intended to be used. In a manual I have, a 2-barrel rating of 500cfm is to be multiplied by 0.71 to theorize an equivalent 4-barrel rating of 355cfm. Potato, potawto... apples and oranges based on more flow division with a 4-barrel. Regardless, on my engine or anything smaller, less air-pumpin' engine than a bigger healthy V8 racing engine, it generates nowhere near even the 4-barrel 1.5"Hg amount of vacuum at WOT... a LIST-4412 Holley 500 2-barrel (with down-leg boosters like my -2 has) has identical bore and venturi sizes (therefore airflow) as the front barrels of a down-leg booster LIST-3310 780cfm 4-barrel. So, at the very least, it ought to flow about 48% of what a wide open 780 4-barrel would, 373cfm... call it a 500, a 355, a 373, a whichever... according to the uneducated/inexperienced and snowballed mythology, it's too big for my measly 232 cubic inches... to which I say, BS, based on results.

[erratic50]

CFM - cubic feet (per) minute.

A perfectly cammed 100% volumetricly efficient 302 pumps 302 cubic inches of air with every revolution. How many cubic feet per minute is that? It depends upon if you are talking about 500 revolutions per minute or 6500.

How much fuel is needed? That depends upon the density of the oxygen in the air space which is impacted by both temperature and elevation......

Its all just a guessing game anyway to land at something close enough.

A lot of times too big of a carb on too small of an engine makes it run lean from failing to draw enough fuel — at least that’s my experience at part throttle. When we tweaked the “quadrajunk” off of a 455 rocket olds to work on a 350 Buick, we had to increase the fuel level to get rid of a horrible dead spot that the 455 never had and we had to do a few other similar things.

when done, it got 20 mpg on the highway on the front barrels. If you dared drop the hammer, when the 4 barrel was wide open it could throw fire several feet out each side exit exhaust pipe like a bad-ass Hotrod that doesn’t have cats or much for mufflers to speak of should. Over fueled? That’s a little bit of a myth as it takes very little remaining fuel to throw flames. Some combustion chambers really aren’t all that efficient at burning completely. We tuned it so it ran the best ETs in the 1/4 and it just so happens it threw fire like that. Any less fuel and our exhaust temps got insane and it was prone to head gasket failures.... figured it out after a few of those. Grrr

Jetting and all sorts of metering problems can come up when going off the ranch a ways on engine size vs carb size.

Any carb needs to be properly setup for the engine it’s on by someone that knows what the hell they are doing is the bottom line.

Fir those who have read some of the deep dive articles here on carbs — there are guys here who definitely learned from the master. Some more like me with the school of hard knocks too.

My only lesson is never put a 360 degree rise intake on a motor you need to start when it’s below zero. It gets so damn cold in the intake tract before the motor come up on temp that fuel will pour out the exhaust and yet your lean when it comes to atomized fuel vs air..... tuning jail for you until you buy a block heater.... and have an approach to prehearing the fuel

[854vragtop]

The best carb I've used on my '85 Mustang is the Summit 600 CFM carb. Tunes EXACTLY like a Holley, uses a lot of Holley parts, no vertical gasket surfaces, can fine tune it with Holley accelerator pump cam kit, can install a Holley quick change secondary spring kit, (4) annular discharge boosters, crisp throttle response. The body is a much lighter aluminum casting than the potted metal that's found on a genuine Holley. Because of that, it does seem to absorb heat a little quicker and cause the fuel in the bowls to boil if you don't run a good carb spacer to insulate it. The only symptom is hot hard starting on 95+ degree days after the car was shut down and started after sitting for 20 minutes. You could see the fuel boiling through the sight windows before I installed a good phenolic carb spacer. I liked it so much, I bought another one for my '68 Cougar. https://www.summitracing.com/parts/s...xoCC-EQAvD_BwE