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PICKING THE RIGHT CARBURETOR SIZE FOR YOUR MOTOR

I listened to about 5 minutes in the middle of that video. Truly some mis-information in there.

As I suspected, their "chart" is using the 45 year old RPM x cu.in. / 3456 = CFM formula. Then multiply by VE and make your number even smaller. This formula has been proven wrong just about every time it is tested.

If all you want to do is putt around town, use the formula. It will drive real nice once tuned. If you really care about performance, as measured by track et and mph, take your car to the track and try several carbs. If they are tuned right, you'll find a carb lager than recommended in that video will perform the best. Don't take my word for it. If anyone is serious about doing this, let me know. Keystone Raceway and Quaker City Raceway are both local track to me. I'll bring a couple of my dozen or so carbs.
 
750 cfm on a 340, no problem at all. We've run the 4779 on several 340s. They run great.
Normally I can blindly agree. There is not enough information on this particular problem to judge and make blanket statements.
Remember, just because you can doesn’t mean everybody else can. Make up any excuse you want, that’s just the facts.
 
..............just because you can doesn’t mean everybody else can. Make up any excuse you want, that’s just the facts.

Hopefully you didn't offend anyone. Personally, I see it more like: ....if I can do it, anyone can do it...

And, can you direct me to one or more of the excuses that I made up? If I have done that I'd like to know so I can correct my behavior. Or are you implying that I will make up excuses? Just trying to understand.
 
Now now boys, no fighting ! I don't race, so I guess that puts me in the "put around town" crowd with the occasional stop light to stop light brawl ! .... I have no doubts I'll lose some top end performance going down to a 650 from a 750, but 90% of my driving is put putting around town...BSB67, I wish you were closer to me. I'd absolutely give you a shot at dialing in this carb to my motor !
 
Now now boys, no fighting ! I don't race, so I guess that puts me in the "put around town" crowd with the occasional stop light to stop light brawl ! .... I have no doubts I'll lose some top end performance going down to a 650 from a 750, but 90% of my driving is put putting around town...BSB67, I wish you were closer to me. I'd absolutely give you a shot at dialing in this carb to my motor !

I don't race either, but I do love the track and testing and tuning. And honestly, I think a lot of people would like it if they tried it.

I'll be in the Hampton area in early October, but don't think I'll be able to break away, otherwise I'd love too. If something changes, I'll let you know.

I'm way okay with with the putting. That is why I qualify my statements as being for the more serious performance orientated, understanding that this is not the priority for most.

And I hope you get what you're looking for with the smaller carb. But also understand that I would hate for you to actually believe that the AED carb size is the issue, as that is doubtful, IMO, and simply changing the carb is not really proof.

Good luck
 
I think the hot temps are getting to everyone,lol!
 
This is my story. My 72 Charger had a 750 vacuum secondary carb on it since I bought it in 2010. It is a 360 engine, mild cam, RPM intake and headers. The 750 ran well, or so I thought, but mileage was horrible. After I installed an air fuel ratio gauge I discovered it was cruising pig rich, and even richer at WOT. After a very long learning curve on tuning a Holley I finally got it dialed in.

Then I picked up a used SA670. Rebuilt that and put it on the car. It was a lot easier to get dialed in. Cruise ratio was much better, mileage was better, but my seat of the pants feel the 750 felt stronger.

Then I had the opportunity to put the car on a portable chassis dyno at a local show. I brought both carbs.

The 750 made average HP/TQ of 247/355 at the wheels
The 670 made average HP/TQ of 256/378 at the wheels

IMHO, a bigger carb is not better, the right size carb is best. On Holleys site it tells me a 600 CFM carb is what it needs. Going by that, a little bit bigger carb, a 670 is better.
 
This is my story. My 72 Charger had a 750 vacuum secondary carb on it since I bought it in 2010. It is a 360 engine, mild cam, RPM intake and headers. The 750 ran well, or so I thought, but mileage was horrible. After I installed an air fuel ratio gauge I discovered it was cruising pig rich, and even richer at WOT. After a very long learning curve on tuning a Holley I finally got it dialed in.

Then I picked up a used SA670. Rebuilt that and put it on the car. It was a lot easier to get dialed in. Cruise ratio was much better, mileage was better, but my seat of the pants feel the 750 felt stronger.

Then I had the opportunity to put the car on a portable chassis dyno at a local show. I brought both carbs.

The 750 made average HP/TQ of 247/355 at the wheels
The 670 made average HP/TQ of 256/378 at the wheels

IMHO, a bigger carb is not better, the right size carb is best. On Holleys site it tells me a 600 CFM carb is what it needs. Going by that, a little bit bigger carb, a 670 is better.

Paul – That is great information. Thanks for posting. You have done a really good job at extracting the most out of your combo, and I have always been impressed with it.

Here are a few thoughts regarding carb size in your application:

1) If you use the “right size formula” to size a carb for your engine, it comes out to like 540 cfm, assuming 100% VE (490 cfm @ 90% VE). So based on the formula, your 670 is way oversized.

2) Dyno results don’t necessarily follow and track results. The key difference for this discussion is what happens after peak power. Usually the best track results with street cars with relatively tight converters are with shift points several hundred rpm above peak, where the larger carb does not fall off like the smaller carb. The real benefit is not above the peak power, it is the large power gain on the shift recovery rpm.

3) My guess, at the end of the day, for your 250 hp automatic tranny, small block B-Body, a well tuned 750 would at best be a wash with the 670 at the track. And of course, the 670 will be better around town.

So how is this different than the other poster? He has a 340, with Eddy heads and a larger cam in a 4 spd A Body car. These alone are enough for me to develop my stated opinion on improved et and mph with a 750. The improvement might be small, or larger if his compression is higher.

However, on the flip side, if the exhaust is restrictive, say with factory exhaust manifolds, 2” compression bent pipes, there would likely be no improvement.
 
Very good info here.. as far as my exhaust goes, TTI headers and full TTI 2.5" exhaust. Dynomax ultra flo mufflers .. 3.55 gear .... if it wasn't for that part throttle stumble I'd leave the 750 alone, but I honestly don't know where else to go with it ... not too mention I took a 400 mile trip with that carb on there and fuel mileage dropped big time ! But that might be the difference between vacuum and mechanical secondaries on the Hwy!
 
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