The Stovebolt Site Forums The Shop Area Engine Shop 292 and Holley.. how many CFM?

Hi All,
Do any of you recommend which Holley carburetor might be best for a stock 292 (with split header exhaust)? Important to me is LOW end TQ and good gas mileage. Holley site takes me to a 600 model.. :o I think that is waay too much.. After doing some reading, 350cfm seems plenty for street. I m learning but dont know a whole lot.. I do have a aussie intake on the way.. it can handle a 4bbl or 2bbl. Thanks again guys for helping me and I hope to see some of you soon when all of this nasty virus thing is over with.

A 350cfm carb would put you right where you need to be. On PAPER a 292 at 4000rpm would pump 338cfm. With that intake have you considered using a self tuning TBI? We don't use carburetors anymore and everyone has been very satisfied with the result. An O2 bung in the exhaust, high pressure fuel pump, and very simple wiring is required. Zero hot-cold starting problem and MUCH better economy.

Apples and oranges, however:

My Ford van 300 CID 4 speed 400 CFM Carter AFB on Offenhauser intake consistantly delivered about 40 percent better fuel economy plus more torque than Dad's factory fuel injected Ford van 300 CID automatic. Probably some of that could be attributed to the 4-speed versus automatic.

Jon

The Holley 7448 350 CFM 2 barrel we were required to run at the Nashville Fairgrounds 5/8 mile oval would let a Chevy 350 run slightly above 6K RPM, so the carb will flow a lot more than its rated CFM- - - -it just won't do it at the specified manifold pressure where the flow is rated. Jon's suggestion of the low-CFM Carter 4 barrel is probably a much better idea. Holley carbs in general are a "one size fits none" situation without a bunch of custom tuning.
Jerry

Thanks for the idea. Those setups are way to much money for me right now. :/ I was reading one of the sponsor sites that suggested 500cfm so I thought I would be safe with a holley , and 390+ cfm.. until I started to look into it up myself.. and for driveability the lower cfm was looking better..

The Aussiespeed intake I bought actually says use holley 390+ to fit.. So I guess I will have to get some sort of adapter for a 350 or the carter 400 you guys are mentioning.

Aussiespeed Chevy 250, 292 Chevrolet in line 6 cylinder 4 barrel intake manifold, this performance inlet manifold offers high velocity ports, divided center plenum port allowing for even flow to all runners. the Aussiespeed Chevy 6 inline manifold is available in to suit square bore Holley 4 barrel carbs the manifold is supplied to accept either forward facing or sideways mounting carburetors.
230 250 292 Chevrolet Premium intake manifold only.
No Adapter is required to mount a single 4 barrel.
The intake manifold comes drilled for both positions forwards and sideways mounting of a 4150 style 4 barrel square bore
Holley Carburetor from 390 CFM to 850 CFM

The Holley 390 CFM 8007 carb was popular on the Buick V6 engines. I think some were using this carb on Jeeps with inline 6 engines. More research required...

For reference only.

Here is a picture of the Carter AFB 9400s (400 CFM) on a GMC 300 (not mine).
Also the same model carburetor (mine) flipped so that you can see the small primaries and secondaries as well as the bolt pattern.
Also a clipping from 1978-79 Carter catalog showing STREET (modified++ from stock) engines.

The Holley 390 in theory seems to be just about right for a six motor in terms of 4 bbl carbs but the reality is that they require a bit of tinkering to get them right because they were not really designed for the use that we put them. Its not terrible work, but it takes a little time to get the accelerator pump and pump shot right to get rid of any stumble, the power valve usually has to be changed out, some jetting changes. But once done, they work pretty well.

Be careful comparing cfm values between the 2 bbls and 4 bbls. They are measured at 1.5 inches of depression for the 4 bbls and 3 inches for the 2 bbls which in direct comparison means that the equivalent advertised flow rating for a 2 bbl is about 35% or so smaller in direct comparison to the advertised flow rating of a 4 bbl. So a 350 2bbl is roughly in the 240 cfm range using the 4 bbl conversion.

283 chevy motors in the late 50s used a carter WCFB. Those carbs were rated anywhere between 400 and 500 cfm. The most streetable 2X4 set ups (at least in my opinion) all used dual carters. The best 2X4 intake set up I ever had running on the street was way back in the late 70s on a 327. It was an offy medium rise intake and two carter 500 cfm carbs. The Carter's were easier to get running right for a street driven car then were the hollys. Little more fuss to get the top off but the rod and jet system for the Carters is pretty good.

I have never run a carter single 4 on any of my motors only because I love multiple carbs. But if I was, I would definitely consider one of the small carter's, like above, for a mild street driven 250-292 motor. Its in the right flow range, both carter and edlebrock sell the carbs and the parts to adjust them. I think they are a better call then the Holley 390.

Other than "eye candy", is there any real advantage to running an expensive 4 barrel carburetor on an even more expensive aluminum intake manifold over the tried and true single barrel carburetor? Hotrodders are a little like Marines- - - - -they tend to believe their own Bovine Scatology!
LOL!
Jerry

I have read a lot of people using the 390cfm 4-barrel carburetors on the 292.

I went a different route ended up using a modified 750cfm on a mildly built 292.
- modified from a 4-barrel to a 3-barrel essentially. Fuel flow blocked from one main primary (with butterfly of primary blocked off from air flow), this brings the carb down to ~560cfm, and allows the one primary to open a little more at lower RPM for supposedly better air velocity.

My 292 has larger valves, lump ports, head decked, .30 bored, new cam.

Hasn't been road tested, only run for break in, but runs very well.


I'm not sure on that aussie intake (they look real nice, and I thought about getting one, but wanted water heat/cooling to the intake) so I went with Clifford. Supposedly Offenhauser intake is better for low end torque and clifford for higher end HP.

12bolt has a lot of good info on this, as well as Leo Santucci (two published books/power manuals I highly recommend)

Should someone wish to consider the small (400 CFM) Carter AFB, there were two of the 9000 series aftermarket version.

(1) 9400s was released about 1972.
(2) 9410s superseded 9400s about 1975.

The difference in the two carbs is that 9400s came with no EGR port. The casting was machined for an EGR port (it may be plugged), and the new number 9410s was supplied.

Both have GM linkage, and 12-volt electric chokes.

For those that wish to modify these carburetors, Carter produced a "strip-kit". The strip-kits contained a selection of different orifice fuel valves, different orifice jets, and different diameter metering rods. The original strip-kit was 10-101. There 16 different strip-kits in the original "100" series. Due to dealers griping about the number of different kits, Carter consolidated parts into only three kits, the "200" series , one for AFB's, one for AVS's, and one for TQ's. The new number for the consolidated AFB kit is 10-201. While all parts in the 10-201 will fit either the 9400s or the 9410s, the calibration of the parts in the original 10-101 kit are more useful for the 9400s and 9410s.

In addition to the strip-kits, Carter also produced "blister-pack" sets of vacuum piston springs. There were two different (and please forgive me, I do not remember the numbers); one contained 2 each of 3 different value springs, the other 2 each of 5 different value springs. The numbers started with 61P something. 61 was the Carter code for spring, and the P meant performance. There was also a blister-pack of pump squirters, part number starting with 48P something.

I HIGHLY recommend that if one is going to use modify one of these Carter carburetors, that genuine Carter parts SHOULD be used; not the modern imitations. When the imitations first came out, I bought several hundred step-up rods, only to find the quality control was less than horrible! They ended up in scrap brass! Rods that were marked "64" when checked with a micrometer could be anywhere from 60~68 thousandths.

EDIT: just read Jerry's last post. Jerry - on my personal 300 CID Ford engine, the Offy intake and the Carter 9400 added a significant amount of torque, and almost DOUBLED the fuel economy of the original Holley single barrel that was used by Ford. Again, apples and oranges.

Jon.

Good points, Jon, and I can see the advantage of having small venturis with increased velocity over a single big one with lazy airflow through it. That's the concept the triple-stack venturi and the tiny primary barrels the Quadrajet uses. The big sixes (Ford, Chevy and International) were never really intended to get good fuel mileage, as they were primarily used in medium-duty trucks hauling max cargo loads on a daily basis. I wonder if anyone has ever tried using a Q-jet on a 292? One for a 305 Chevy or a 307 Olds engine should be in the ballpark for initial calibration, and the adjustable air valve spring makes it easy to adjust the tip-in point of the secondary barrels.
Jerry

Jerry - I agree completely on the Q-Jet; and in fact, Pontiac used a Q-Jet on their 230 CID and 250 CID OHC 6 cylinders.

However, it is sometimes difficult to explain primary CFM and secondary CFM to someone that asks "if 600 is way too big, why isn't 750 even more too big?".

CFM ratings

Carburetor selection

Probably the biggest issue in many trying to use a Q-Jet is that Rochester NEVER sold them as aftermarket, thus not a lot of information (exception: books by Doug Roe, and later Cliff Ruggles), and not a lot of aftermarket tuning parts availability.

Jon.

Ran a 390 Holley from Summet on my 54 235 , 261 cam, corvette exhaust, clifford intake, just what that truck liked, never ran better. Most of this speed equipment I picked up at swap meets or trades so didn't have to shell out big money.

Ditto on the swap meet stuff- - - - -I had an Offenhauser 4 BBL intake for a 250 that was given to me by a bolter in Nashville who was cleaning out his garage, along with a bunch of other stuff, including a mildly hotrodded 230 engine. I passed the manifold on to another stovebolt member for the cost of shipping. If I ever put the 230 back into operation, I'm planning to do some major modifications including a home-brewed intake system using three Mikuni side draft carbs for a Harley and a roller cam, plus some pretty radical cylinder head mods. That one's also going into a super light weight chassis, not a truck.
Jerry

You know I pretty much always agree with Hotrod Lincoln but, If you have bigger valves and a larger cam you will find that you will have more torque above your cubic inches. An incredible amount of h.p. to torque. I raced one with larger valves and an Isky cam. It does affect the patience in you to get it to open up quick (accelerator pump wise), but, the end result is magnificent. A 390 Holly is bigger than I ran. I would take on a stock 350 any day. It is best that I don't brag, one cannot do anything like this with a 235. Doc.

Jerry - you would know better than I, but wouldn't sidedraft Carters be preferable to the Mikinis? We put four brand new Mikinis on a 2 liter Ford race engine, and after one summer of weekend racing, the slides and housing were worn completely out! Toward the end of the season, synchronization was difficult due to the wear.

Jon.

I'm a Navy guy but I may fall into that category. This is my setup for the 292, Possibly slightly arrogant. Chrome won't get you home but it might get you picked up on the side of the road.

Jon, I read with interest the side draft carter inquiry. Three carter YH side drafts might very well work. I always wanted to try that set up but the problem with using carter side drafts is just finding them in the first place. I know they were used on the early corvettes, Nash used them on their metropolitans, I guess there are others. But in any event, they are not all that easy to come by. And then you run head first into getting access to some rods for tuning. Main jets are no problem as they use the same main jets as the WCFB carbs (or AFB I forget now. I have a load in the basement but have not used them for some years after I switched to the webers), but the rods are a different story. I have an old print out from Carter with the call out numbers for rods. From my review, the part numbers do not correspond to any kind of progressive metering. It looks to me that as a need arose for a specific diameter or design rod was called out, Carter engineers designed such rod and then the next available part number was assigned. At least that is my take on it. So to really run the Carters and dial them in, you would need three rare carbs, and a real working knowledge of the rods you would need. There is a real learning curve to that.

Don't get me wrong, I loved my Carter YFs when I ran them. If I ever go back to single bbl carbs, or restore a 235 back to stock, you can be sure I will search out a carter variant, but in terms of what is easily available today in a new or easily obtainable carb, the pickings are slim. At least I think so.

Mike - there are four generations of the Carter YH. None of them are cheap, but all of them are quality.

(1) Corvette and Nash
(2) Mid '50's marine applications, Latham supercharger
(3) 1961~ mid-1964 Corvair, also marine applications
(4) Mid-1964 ~ 1966 Corvair, Clark Cortez, marine applications.

Probably the easiest to match would be the Dearborn marine applications circa 1957~59.

You are right about the rods.

You are also right about the learning curve, but if it were easy, everyone would do it.

I have an advantage, as I can make any rod I need, to any configuration.

But if I did not have that capability, I could still get close by a lot of mathematics, and using main jets (AFB, or WCFB).

If you ever decide to go that route, call, and I can give you additional information.

Incidentally, we have sold a ton of these for multiple carb use on various sixes, plus a bunch more for a 4 carb set-up on V-8's

Jon.

I post this out of curiosity and for discussion (or my education).
We didn't ask which year 292 so the original stock carburetor would have been the one barrel Rochester B, Carter YF, Monojet or 1-MV.
Why did the manufacturer's choose a one-barrel? Cheaper to make? Better low end torque?
General consensus in this forum is that the YF is a great carburetor so why doesn't the forum automatically recommend the Carter YF 4371S (for California) 1965-67?
Jon said the Carter YF was tested and designed for A/F ratio delivery (I would assume to meet Chevy's/GMC's engine requirement) and not CFM.

CFM was not a value of discussion until the mid to late 1960's, and then only on aftermarket carbs.

Most carburetor manufacturers did not post CFM on O.E. carbs until the late 1970's.

As Buoymaker suggested, the carburetor manufacturers were interested in pleasing the O.E. manufacturers (in this case G.M.)

While both Stromberg and Zenith published CFM charts for engine requirements and for some of their aftermarket carburetors as early as 1930, enthusiasts did not care. Enthusiasts were interested in venturii area.

And while if the folks publishing CFM were honest, CFM would be a more accurate value for comparison than venturi area, as it would take into account differences in air intake (eddy currents), as well as the efficiency of the air flowing through the carburetor; whoever claimed that these folks were honest? (Does anyone REALLY believe that sales wanted a 600 CFM carburetor, and the engineers experimented with bore diameters and venturii shapes until exactly 600 CFM was flowed)? The numbers were either rounded, are just plain fudged. I know of one company that did not have a carburetor rated at a specific size to compete with another manufacturers carb of the same size, so the sales folks simply TOOK THE NEXT LARGER SIZE AND RENUMBERED AND READVERTISED AT THE LOWER SIZE. Guess which of the manufacturers got the higher HP rates when the carbs were tested by an "independent" laboratory?

Jon.

As I mentioned earlier, a well-tuned 355 cubic inch small block Chevy will turn 6200 RPM with a "350 CFM" Holley 2 barrel. It just won't do it at the manifold vacuum where the flow rate of the carb is measured.
Jerry

buoymaker - Maybe it is bec in some cases we have made modifications, like in my case, split headers and a higher volume intake.. perhaps also because there are some more MPG efficient set ups in the example of the 400CFM carter that has been mentioned? My engine is from 1984.

I'm 3 steps behind you guys. Sorry.

A stovebolt engine breathes like a neck being strangled. Making the mouth wider (bigger carburetor) doesn't make breathing better.
I get that point. The optimum is to somehow match (+/-) the carburetor to the restriction or reduce the restriction or use force (turbocharge)?

Back to the YF. Jon said that Carter tested the YF's A/F at various vacuum steps.
Turn back time.
GM engine designers asks Carter carburetor designers to build a carburetor for them.
GM would have to provide Carter a graph of actual measured engine vacuum steps and the desired A/F ratio for each step?
Carter would design idle ports, low speed jet and more importantly a custom fuel metering to meet GM's requirement.

How did GM measure engine vacuum without a carburetor attached?
A big variable speed DC motor connected to the driveshaft and a Pitot Tube inserted into the intake throat opening?

I posted a link on the split header thread that showed a larger carb making a good improvement in power..


Removing that strangle wherever it is probably makes even more power.

Edit - this is a 292 tho. And i realize “good power” is subjective. 😊 as one of the guys stated.. smaller older engines do not benefit much without other customization! I agree.

38 1.5 Ton, Do you have this setup running yet? I am curious to see how it turned out.

If you follow this and the following video, the bolt-ons (header, cliff intake and big carburetor) added a 20 HP gain. The video creator was expecting 175 HP. He stated that there needs to be improvement in the head and cam to get more flow. This sounds like what previous posters have experienced. Then comes the porting, swirling, shaving, special cam and other tricks. It does give improvements but not as much bang for the buck.

The test doesn't show street-ability. Idle up to cruise. Stoplight to stoplight. Fuel mileage. Starting up on a frigid morning.

What's really funny in the following (3rd) video is that the engine started misfiring and they ran out of dyno time.
Later the misfire was found to be a worn-out set of points. They/he started the whole test without changing to a new set of points.
The test has to repeated to make sure the data is correct.

"Speed is expensive- - - - -How fast can you afford to go?"

"There's no sunstitute for cubic dollars!"

My kids had a pretty nice home to grow up in, and ate a lot of good meals, bercause people wanted to go around in circles in a hurry. They thought they were "big wheels!"
Jerry

I would love to meet you guys in person, I bet you have a ton of zingers to throw around at everyone, along with some great stories of years gone by. It would be a lot of fun to sit down and share a brew and chew the fat with you guys. Unfortunately I am a working stiff that doesn't have enough time on the books to take a two week vacation to travel across the country to meet you all at the Reunion's.

Now, back to the scheduled programming. 38 1.5 Ton, Do you have this setup up and running yet?

Double dingo I went to one Kansas City reunion and met a few Stovebolters well worth the trip then a homecoming in Mary land I got hooked and been back to both events can’t get out of my country now but put it on your bucket list to meet some of the guys had the pleasure of lunch with HRL as well in Tennessee with Don King

After a failed EFI attempt on my 235, I settled in a 350cfm Holley 2 barrel on a Clifford heated manifold. I have Fenton exhaust with dual pipes. Threw in a Langdon mini HEI and could not be happier. After some accelerator pump tuning, she runs like a dream.