4 Barrel on a stock slant

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flatiron

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I have a header and A Offy intake for my 71 Dart Slant six. Is a small 4 Barrel to much for a stock Slant ?
 
IMO, the Holley is a mistake on small engines like the slant and even a 318.

The Holley is a square bore carburetor, meaning all the barrels are the same size. The slant will respond better with a carburetor that has smaller primaries, such as the smaller AFB style carburetors that Edelbrock makes. The 500 would be a good choice.

Some may say it's too big, but since it has small primaries, it will work well. They are more forgiving on smaller engines.
 
RustyRatRod said:
IMO, the Holley is a mistake on small engines like the slant and even a 318.

The Holley is a square bore carburetor, meaning all the barrels are the same size. The slant will respond better with a carburetor that has smaller primaries, such as the smaller AFB style carburetors that Edelbrock makes. The 500 would be a good choice.

Some may say it's too big, but since it has small primaries, it will work well. They are more forgiving on smaller engines.
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If that's the case wouldn't a vacuum secondary carb be even a better choice?
 
toolmanmike said:
If that's the case wouldn't a vacuum secondary carb be even a better choice?
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The AFB is a hybrid design in that area. While the base plate is mechanical, they have an air door that regulates secondary operation.

Chrysler chose the AFB for that very reason on the Hyper Pack.
 
Throwing a 4bbl and headers on a stock Slant-6 is an efficient way to waste effort, money, and time. The money waste is ongoing, too: poor fuel economy with not much to show for it in terms of performance.

Much better for waking up a stock-ish Slant-6: a 2bbl and Dutra Duals.
 
AJ/FormS said:
The point was what Dan said. The SuperSix 2bbl is almost more than what a stock slanty knows what to do with.
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I get that, but the OP asked about a 4 barrel.
 
I happen to be working through this right now.
225 inch engine, 9.5 compression, RV cam, ported head with stock size valves. Hooker long tube headers, Offy intake.
Carb is a 600 AFB. From what I am determining, the carb itself is an ideal size for this engine, but the stock jetting was drowning it.
I keep taking fuel away from it, and she keeps running harder. .98 jets on the primary side, with original drop rods (stock was .103) and I'm down to .081 in the secondaries (started at .098)
Very happy on the front side of the carb, but it looks like something in the mid .070s will be perfect on the back side.
Running the car this weekend, so should know soon.
BTW, not set up for the street at all, but running this car around the industrial park by my shop, she is an absolute kitten. Drivability is excellent, and with the small jets, full throttle roll-ons are smooth and clean.
 
By that formula every car is over carbed.

A 650 holley front barrels is the same as a 390 two barrel and a 750 front barrels is the same as 500 two barrel.

I don't think anyone would argue about putting a 390 2bbl on a /6, since the vacuum secondaries only open as needed can't seeing it cause too much problems.
A 4bbl with smaller primaries would be better. The gains would probably be small over a stock 2 bbl. The only way is check the restrictiveness by the vacuum level at full throttle run and go from there.

Don't forget a just because you throw on a 1000 cfm carb on a engine that only needs 500 cfm that is not gonna give twice the fuel and air to the engine over a 500 cfm carb.

Carb size is more about restriction then it's actual flow numbers since the rating is based on vacuum levels 1.5 for 4bbl and 3 for 2 bbl. So if your engine produces a different vacuum level at full throttle then you carb will flow more or less then rated.

So if your engine see a vacuum levels of 1.5 with the 500 cfm carb and then you throw on a 1000 cfm carb the vacuum will drop so the actual cfm of the 1000 cfm carb will be about 500 cfm too. And on the dyno would probably gain hp only because of less restriction but wouldn't work cause of drive ability, idle and part throttle.
 
Those formulas are meaningless, unless you use them as a guide for minimum carb size.
Giving the engine the right amout of fuel at the right time is what counts.
 
Ok try this
take your 225 and figure every revolution pulls in three cylinders worth of air right? So that would be 225/2 =112.5 cubic inches at 100% volumetric efficiency.But that poor stock slanty would be lucky to pull in 70%;so 112.5 x .7= about 80 cubic inches per revolution.
So lets rev it till it floats the valves, say 4200rpm. So now it is gonna pull in 80x4200= 336,000 cubic inches per minute. Ok so converting that to cubic feet is; 336,000/(12x12x12)=194 cfm

But let's say you put a big cam into it and ported the snot out of those miserable heads,And bumped the compression up, and headers etc, and you finally got the volumetric efficiency up to 85%.
Now you are up to 236 cfm@4200, or 281@5000,or 337@6000

Then bear in mind that the efficiency peak is usually at or near the torque peak,not at max rpm. So a slanty-cam that shifts at 6000, might power peak at 5200, and so torque-peak at 4000.
Now you are down to 225 cfm;imagine that!
I am very impressed that you can make a 500 perform.
Yeah, I know;lot's of guys have made it work. And I'm impressed by every single one of those/you guys.
 
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All that math does not mean a thing unless you want mediocrity. I have run a 750 cfm AVS on engines as small as a 289 and 302 High Performance Ford engines. Neither would give me back the Carb, it made such a huge difference from the 600 Holley they were Dorking with. I've run a 71 750 cfm TQ on a 273 and it would have all of it. None were stock engines but the math still would not recommend anything that large for that size engine. That being said, I love the Super Six 2 barrel setup for any slant six. What most people don't realize is how much time and work it takes to correctly set up an aftermarket intake and carb. Read between the lines of whitepunkonnitro's post. He's almost there with a 600cfm on a 225 slant six.
 
AJ/FormS said:
Ok try this
take your 225 and figure every revolution pulls in three cylinders worth of air right? So that would be 225/2 =112.5 cubic inches at 100% volumetric efficiency.But that poor stock slanty would be lucky to pull in 70%;so 112.5 x .7= about 80 cubic inches per revolution.
So lets rev it till it floats the valves, say 4200rpm. So now it is gonna pull in 80x4200= 336,000 cubic inches per minute. Ok so converting that to cubic feet is; 336,000/(12x12x12)=194 cfm

But let's say you put a big cam into it and ported the snot out of those miserable heads,And bumped the compression up, and headers etc, and you finally got the volumetric efficiency up to 85%.
Now you are up to 236 cfm@4200, or 281@5000,or 337@6000

Then bear in mind that the efficiency peak is usually at or near the torque peak,not at max rpm. So a slanty-cam that shifts at 6000, might power peak at 5200, and so torque-peak at 4000.
Now you are down to 225 cfm;imagine that!
I am very impressed that you can make a 500 perform.
Yeah, I know;lot's of guys have made it work. And I'm impressed by every single one of those/you guys.
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According to science, the bumble bee can't fly. Its wing size and speed are not enough to get it off the ground.
The Bee flys, because it doesn't understand science....just like a Slant with too big a carb
 
The problem is the cfm the formula gives which are pretty close to what the engine actually uses but a carbs cfm is a rating doesn't actually correspond to the formula numbers. Just like head cfm it's a different rating also.

Take a built 700 hp block that needs a 1000 cfm carb but instead you bolt on a 500 cfm 2bbl (which would even be rated less if rated like a 4 bbl) but that engine will still pull close to 1000 cfms through that 2 bbl but obviously cause of restrictiveness will make less power and use a little less cfms.

Why not just measure it, like I said earlier run a vacuum gauge on the engine do a full throttle run and see what vacuum the engine is pulling if 3 and under a carb switch is unlikely gonna help much.
 
Drivability is all about the primaries. Restriction only comes into play at wide open throttle. The stock 1bbl on the six provides decent economy and drivability, but is incredibly restrictive at wide open throttle. Based on this, a progressive 2bbl carb would be perfect, but who makes one? The BBD 2bbl can be tuned for decent drivability and similar economy, and at least has some headroom for WOT. IME it vastly improves passing and hillclimbing power on an otherwise stock engine (I do have the larger exhaust manifold exit and a bigger pipe). But the only way to get a progressive carburetor is with a 4bbl. At least in theory, this should work so long as the primaries are not too large, and you can jet them lean enough. If the secondaries are too big, so what? The engine will pull what it can flow, and there won't be any restriction; again the only issue is to keep from flooding it. I would think the AFB from a 273 Commando might work -- it has small primaries, and is tuned for a small port engine. The one thing about a 4bbl is, you would ideally want it mounted sideways, so the primaries are facing the head (to ensure even mixture across the cylinders.) That makes for wacky linkages. Or you could use a big plenum. I don't know what the Offy intake looks like inside.
 
whitepunkonnitro said:
According to science, the bumble bee can't fly.
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Bulk wrap. That's one of those hee-haw stories people who don't know what they're talking about love to trot out. It's not true and never has been. And "Hurr, derp, it can fly anyway because it's a bee and it can't understand science, haw haw haw, just like a slant-6 doesn't know its carburetor is too big, haw haw haw" might seem clever to any 3rd-graders reading this thread, but for the grownups…not so much.

"Science" means knowledge of how stuff works (and doesn't). That's it. It's not an esoteric religion or anything. If what you're looking at seems to defy science, you don't understand what you're looking at and/or you don't understand the apposite science. That's all.
 
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