Carburetor CFM and Carburetor Selection

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Scotty54
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Carburetor CFM and Carburetor Selection

Post by Scotty54 »

Using the standard carb formula:

cfm = (c.i.d. x rpm x VE) / 3,456

My stock 401 engine requires only 493 cfm. That's assuming a top RPM of 5000 and VE (volumetric efficiency) of 85%, sort of the standard figure for a stock engine. 5000 rpm is not likely but I would estimate that for a stock 401 redline. Mostly road time with some light wheeling.

I am carb shopping and this is my first step. I have reviewed dozens of prior posts on carbs but none that directly addressed CFM. I would like to hear from you if you are running an aftermarket carb and what CFM you selected.
1977 Cherokee Chief 401 QT
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tgreese
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Re: Carburetor CFM and Carburetor Selection

Post by tgreese »

The lower the CFM rating, the more vacuum per volume of air and the better control of fuel mixture at a given speed. Raise the CFM rating and you'll put more air/fuel in the cylinders at a given RPM, with less control. Which do you want?

A carburetor uses vacuum as both a control signal and as a source of power. The more restrictive a carburetor is, the more vacuum there is and the better fuel control. Restrict the air and reduce the performance.

EFI avoids this issue by using the vacuum as a signal, not as a source of power. A throttle body will be much less restrictive than a carburetor for the same displacement. All that's needed is enough vacuum to measure.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
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Re: Carburetor CFM and Carburetor Selection

Post by Scotty54 »

I want throttle response and drivability, and I don't mind periodic adjustments. The 493 cfm figure looked incorrect at first, but when I played around with other engine/carb combinations I see that it makes sense.

From your explanation, I would shoot for a higher cfm but not that much higher. I ran a 750 double pumper on a 401 powered Spirit which was a perfect combo, but for the Cherokee I am thinking 600 cfm might give the best of both worlds, as you describe. At 600 cfm that also gives me many brand options.
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Re: Carburetor CFM and Carburetor Selection

Post by tgreese »

JMO - the CFM "ratings" are quite arbitrary. Not much to retrieve from those numbers.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
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Re: Carburetor CFM and Carburetor Selection

Post by Scotty54 »

Please explain further. If those figures are arbitrary, what other rating is available to the consumer when selecting a carb? They can't be completely arbitrary - maybe more like a "ball park" figure?
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Re: Carburetor CFM and Carburetor Selection

Post by fulsizjeep »

I have been using Holley Truck Avengers for a while, 670 CFM. I think that's over kill and believe that if you are running a fairly stock 360 or 401 engine, the 470 CFM should be adequate. For 2 barrel, I recommend the Holley 500 CFM. The Holleys are fairly easy to tune and change the jets. I use the blue reusable gaskets. Before the Holley, I ran a 750 CFM AFB on my 401 for a few years. It's bored .030 over, RV cam, Edelbrock SP2P intake, headers and dual exhaust. To me, AFB and Edelbrocks are tedious to tune and always run too rich even with smaller jets for higher elevation. I consider them wheel chocks.

I am not a techy guy when considering a carburetor. I have been driving AMC V8s all my life and find 500-600 CFM is plenty.
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Re: Carburetor CFM and Carburetor Selection

Post by sierrablue »

Personally I'm not a huge fan of the Holleys (I respect them; I'm just not a fan). Because they don't have metering rods, and most of them don't have vacuum secondaries, they don't control the fuel level real well, which leads to poor mileage and less power. If all you're doing is WOT and idle they're just as good as any other carb, but the middle is where they suck (no pun intended).

I loved my Quadrajet when I had it on mine; the problem I have with it is that it's an old carb that's hard to find parts for, and much like a used vehicle, I was always fighting something on it. The smaller primaries increase the velocity, which as Tim was saying improves power and fuel economy, since the air can be better metered. It also has some massive vacuum secondaries for when you want it to go. I know the Avengers are different; I like the Avengers but they're $$$$.

I've had pretty good luck with my Edelbrock 1406. The vacuum secondaries make it so that you're not going to overdo it on the air/fuel mixture (just like the Qjet).

If you're ever planning/considering taking it out into the mountains sometime, I would recommend going a little over what you need for cfm. I realize the engine can only suck so much air and fuel in, but with less O2 in the air you need to give it more air to maintain the same power output (aka more throttle), so you want to be able to use every last bit of flow you have.

If you're ok with a little bit expensive, and fiddling with it some, the stock '70s 4-barrel intake for the AMCs takes the Quadrajet pattern, I believe. Maybe the bolts don't line up, but it is spread bore. Lots of people used them for a long time, lots of threads on using them on IFSJA.
'71 Wagoneer (DD)
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-'74 D44 front (nonpower discs)
-custom headliner
-Front shoulder belts (rears eventually)

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There are 2 major differences between new Wranglers and FSJs. FSJs are meant to be both utilitarian and capable, not just capable. FSJs are also rarely initially recognized as Jeeps by the average American.
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Re: Carburetor CFM and Carburetor Selection

Post by tgreese »

Scotty54 wrote: Fri May 12, 2023 5:15 pm... They can't be completely arbitrary - maybe more like a "ball park" figure?
Well, I guess it's most meaningful if you are comparing carburetors within a single line. You could expect that a 500 CFM Holley 2100 probably makes 30% less vacuum at WOT on a specific engine at a specific speed than a 350 CFM 2100. You can get a CFM number from your displacement, RPM, etc. Likely such calculation will get you in the ballpark, but I think the rating is mostly provided for marketing. Enthusiasts realize what "sizes" of carburetors will generally work on which engines; they need some kind of number to make sales.

Realize that a new 670 CFM TA is nearly $700. https://www.summitracing.com/parts/hly-0-90670 Are you buying a $500 manifold too? That will get you within a few hunderd more for a complete Sniper system that bolts to your existing 2V manifold. Likely it will have as good a performance as the carbureted system, with the well-known advantages of an EFI system, especially on a Jeep.

Ok, so it's a 401, with the oddball 4V manifold. You either need a new manifold or perhaps the adapter from Holley to 4350 is still available. Realize that you don't need secondaries with EFI - even the Chevy 454 used a 2V throttle body. You could use a 2V adapter (if you could make one or it exists) or you could use a factory 2V intake. You should be able to get a 2V iron manifold for scrap prices. Holley makes a 2V system that's limited to 650 HP. That should be enough for a warmed-up 401. https://www.summitracing.com/parts/sne-550-852

Also, what's wrong with the current 4V carburetor? I think they are ok if the top is not warped.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
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Re: Carburetor CFM and Carburetor Selection

Post by Yeller »

I always find carburetor discussions interesting. I’ve never ran a carb on anything very long accept a lawnmower. I started doing TBI conversions in 89 as a bet I couldn’t make it work in high school, been running fuel injection since.

For some reason though I’ve never felt the urge to do the programming side. Even though I find it helps and sometimes is very necessary. 1’s and 0’s have never been my thing, even though I like what they do lol
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Re: Carburetor CFM and Carburetor Selection

Post by Scotty54 »

I considered fuel injection but have heard more bad experiences than favorable . That being said, I don't want to derail this thread into a carb vs injection. I know that FI is superior. But, I like carbs and enjoy tuning them. I'm not really intimidated by the 0's and 1's either - my early years were in computer programming and repair but I still understand mechanicals better then electronics.

I should have mentioned that I already have a Edelbrock carb on a Performer manifold, so I am looking at square bore 4-barrels. I have rebuilt the Edelbrock with only so-so improvement.

I have used them all, even a Holley 500 2V which I really liked. I don't care for the AFB but had one that was tuned perfectly and stayed that way! The 4V Motorcraft was always a good one, and I have had good luck with Holley 4V's. My previous experience with the Motorcraft has me looking at the Summit 600, and I am also looking at the Quick-Fuel line-up.

It's not 1970, when you only had two options other than stock. There are many to choose from.
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Re: Carburetor CFM and Carburetor Selection

Post by tgreese »

In my experience, Holleys are really easy to work on and easy to tune. I have run the 2300 and the 4150. Supposedly (Ristow at IFSJA) the TAs have a mid-range stumble that you can never fully tune away. This is said to be why Holley markets the TAs as for off-road only. Dunno - you can search IFSJA for more about this.

There are also new Rochester Quadrajet repops that might be worth looking into - check Summit. The Quadrajet is reputed to be an excellent carburetor for the street or trail. I've heard it's not so simple to tune, but flexible and maybe you'll like that. The smaller primaries seems like an obvious advantage. It's a spread bore design, but I expect Summit has an adapter plate.

I've been following these forums for nearly two decades, and twenty years ago the Quadrajet was the hot setup for a Wagoneer. I think they've fallen out of popularity perhaps because a) good junkyard cores are all gone, and b) there are so many TBI options. Funny, my impression is the opposite of what you found; everyone that has EFI loves it. Maybe ask Tony (babywag) at IFSJA. Seems likely the Holley product will put the custom installers out of business, given the price and self-tuning feature.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
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Re: Carburetor CFM and Carburetor Selection

Post by Yeller »

If… I was to run a carb it would be a q-jet. Imo they are the most stable, get it tuned the way you want and they stay that way until the throttle shafts get sloppy. I personally don’t like tinkering with any of them, just want it to run and if life gets in the way, and it sits for 6 months, it needs to run without a rebuild. IMO q-jet is as close as it gets filling that bill.

I deal with a lot of the new integrated throttle body injection units. I know several installers that will no longer install them, failures are too high. But there seems to be 2 camps that use them. The street car crowd that doesn’t get a lot of deep heat soak love them, and seem to have good reliability. The other is the old school SUV guys that have small poorly ventilated engine bays that spend a lot of time a slow speeds creating a lot of deep heat soak that seems to kill the electronics in them. They seem to have horrible luck with them. So it’s either old school Howell type kit an MPI kit like Edelbrock’s seem to be only second to a factory injected engine swap for reliability.
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Re: Carburetor CFM and Carburetor Selection

Post by Stuka »

Q-Jets really aren't an option on an AMC engine without a funky adapter.

I like Edelbrocks for street or light trail use because they are easier to tune without taking the carb apart. Put the offroad kit in them and they are fine for light trails.

But for anything more than light trails, I would go with a Truck Avenger. Its hands down the best offroad 4v carb (Yes, even better than a Q-Jet).

A 470 would be fine for a stock engine IMO. In my Experience, even 600cfm is too much for a stock-ish AMC 360 (I have not owned a 401). The 670 is the better option if you have a cam and some head work, or higher compression. Anything that will raise that VE up so that it can utilize that extra CFM.
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Re: Carburetor CFM and Carburetor Selection

Post by will e »

I have had good luck with my 670 cfm Truck Avenger on my 401. My main focus is off road and it works well for this purpose. Yes, it's a carb. Sometimes heat soak is a problem. Sometimes when it gets really bumpy off road it will stumble. It does great on hills both going up and down.

It worked right out of the box but I fiddled with it and changed the jets and power valve. Holley's are one of the most adjustable carbs, which is a plus and minus. If you are trying to shave a half second off the 60 foot time there are a lot of options. For us normal folks a jet change, a power valve change and a bit of fooling with the choke and idle will get you where close enough to where you need to be.

My opinion of carb vs EFI goes back to wheeling. Yes, EFI is great at any angle. But the Holley Truck avenger is pretty darn good as well. EFI has electronics that can 'fail'. Carbs almost never fail. The worse that can happen is a needle/seat gets something in it or the gasket gives way. Maybe the power valve blows. Or the transfer tubes leak. You can limp off with many of these problems but a few spare parts in the tool box and you are on your way again. Carbs tend to 'wear out' and not fail. Yes, you can carry spare control modules, injectors and such for the EFI too.

FYI. The two barrel holley carbs are rated different than the 4 barrel carbs. A 500 CFM 2 barrel carb flows about the same as a 350 CFM 4 barrel carb.
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Re: Carburetor CFM and Carburetor Selection

Post by sierrablue »

I would also note that a lot of the time on these engines, the 2-barrel intake tends to have longer (but less efficient) runners. The effect is giving it very similar torque to the 4-barrel, lower in the RPM band. However, the intake doesn't flow as well, creating a massive drop in horsepower, as well as worse mileage, usually. Idk the numbers on the AMCs, but on mine, going to the stock 4-barrel gives me about 100 more horsepower, and about 25-50 more lb feet of torque, about 500 rpms higher (3200 vs 26-2700). Now, that is a combination of carb and intake, but it's mostly intake. I also didn't used to be able to get out of the 12s-13s for mileage, and now I've been pulling 14 regularly in the summer, and I've gotten as good as 16.9.

Qjet should just need a bolt pattern adapter right? Also the Edelbrock 1900 (or is it 9000s? Something like that) that are basically a new Qjet, with a full custom-tune kit available, I'm pretty sure come with both bolt patterns of spread-bore, so it'll bolt right on.

If you're planning to keep it long-term, you'd be ahead to get Sniper EFI or something similar. Yes it's expensive and frustrating upfront, but your starter will last longer, you'll save gas money, and if it sits a lot, you'll end up spending at least as much rebuilding the carb, replacing the mechanical fuel pump, etc. as you did on the EFI. And because it'll be running right all the time, the whole engine will last longer.

Electronics don't mind sitting, whereas mechanical stuff hates to sit. Same is true of being used ALL THE TIME. Yes, when they die it's an all-out failure, but they last a lot longer before they die.

We had the 401 (.060" over, Edelbrock intake, headers going into a 2.5" y pipe and 3" single) dialed in with TBI on the '88, and it was kind of amazing. It sat for like 8 years, and we went out, dropped a battery in it, and tried starting it. After letting the gas break loose all the junk on the ends of the injectors, we hit the key again, and it fired right up. Sat again for 6-8 months and we sold it, and it fired right up again. It also was making enough torque (low altitude here in the midwest) that it'd spin the limited slip/3.54/31x10.5" tires, all the way through an intersection. It also got 15 mpg, 727, so no O/D. Once they're dialed in, those 401s are awesome with injection.
'71 Wagoneer (DD)
-B350 (HEI, iron 4-barrel, Edelbrock 1406), TH400, D20
-'74 D44 front (nonpower discs)
-custom headliner
-Front shoulder belts (rears eventually)

viewtopic.php?t=23070

There are 2 major differences between new Wranglers and FSJs. FSJs are meant to be both utilitarian and capable, not just capable. FSJs are also rarely initially recognized as Jeeps by the average American.
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Re: Carburetor CFM and Carburetor Selection

Post by tgreese »

Regarding the Sniper, this is the first I've heard of them having problems with heat. It's understandable that they would have problems with reliability/longevity, given their design. Putting all the electronics on the TB could be trouble; depends on the quality of the components and how it's built. Presumably you could build the computer with the finest high-temp-rated mil-spec components, but it would be more expensive. Even automotive-grade components sell for a premium price, compared to what's in your radio or smart phone.

The systems that use the GM ECU (like Howell) locate the ECU in the passenger cabin. Makes a lot of extra wires, and not as attractive on your open-engine hot rod, but much easier environment for the transistors. Plus it was GM that invested in the component quality at build time, and you'd probably pay a lot more for a new ECU built to their standards.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
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Re: Carburetor CFM and Carburetor Selection

Post by sierrablue »

My issue with the Howell stuff is that unless you have the computer and the programming skills, you're never gonna get it dialed in right. The Sniper is a quick easy install, and pretty much anybody can get it programmed right. Also because they use the LS injectors, it's easy to find the parts that die/have problems. It's just a pain IMO.
'71 Wagoneer (DD)
-B350 (HEI, iron 4-barrel, Edelbrock 1406), TH400, D20
-'74 D44 front (nonpower discs)
-custom headliner
-Front shoulder belts (rears eventually)

viewtopic.php?t=23070

There are 2 major differences between new Wranglers and FSJs. FSJs are meant to be both utilitarian and capable, not just capable. FSJs are also rarely initially recognized as Jeeps by the average American.
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Re: Carburetor CFM and Carburetor Selection

Post by Yeller »

I’ve been watching sniper, fitech and Atomic with interest for many years. I’ve seen enough issues first hand to easily say no. If I’m not doing OEM injection there are 3 options, in no particular order…. Edelbrock proflow, Holley a terminator X max or mega squirt. All depends on how deep you want to go. Holley is the most versatile, home tunable without a lot of experience. mega squirt is all custom, lot of programming and individual parts selection. Edlebrock is simple plug and play that is self tuning and does well on mild engines. Of course just retrofitting an old engine, the old GM TBI is my go to.

We’ve strayed way away from the OP’s question which I think we answered, no silver bullet but up to what you want to try with some good testimonials. Hopefully the fuel injection part of the discussion helps someone with a decision.
The bus I ride is so short it is a yellow Smart Car full of squirrels, monkeys and clowns.

1970 J2500 Resto Mod
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Re: Carburetor CFM and Carburetor Selection

Post by Scotty54 »

Yes, original question answered and all the additional info has been very helpful.
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Re: Carburetor CFM and Carburetor Selection

Post by weeegoneeer »

I have a street avenger 670 and it is way too big, can confirm what was said earlier - I run a AFR gauge and it's all over the map (though runs OK). Also ran a 470 TA for a bit and it was no bueno for street use. Holley was actually awesome here, they swapped out my TA for the SA for free.

Eventually I'll probably move to a 450 CFM or so 4 barrel with small primary venturis, see what that does. Really want to steer clear of TBI but may get there if a small street carb doesn't work.
1984 GW
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