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Old 07-25-2013, 04:28 PM   #1
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Default Opinions on a carb and torque converter

Any opinions on the carb size and stall speed for my '65 Mustang?

289 Block with 331 stroker kit
Roller Cam with .544 lift, 272 duration and 112 spread
1.6 Roller rockers
Stock rear end
Old Edlebrock Performer intake

Goal is to make it look as stock as possible. I was sold a 750 cfm Slayer 4BBL and I think it is too big. Also suggested stall speed for converter, maybe 2600 rpm? I used to be a gear head way back in the '70s but many things have changed.
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Old 07-25-2013, 06:25 PM   #2
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750 should be fine if it's a decent carb. Stall depends on the rpm range on the cam and what you want to do with the car, transmission and gear ratio etc.
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Old 07-25-2013, 08:06 PM   #3
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On a normally aspirated engine, CFM is determined by the total engine displacement & VE. Why in the HECK would someone think a 750 would have any drivability is beyond me? If you are going to use it as a track car, and it will spend all of it's time at 7000 RPM's then maybe a 700cfm (Maybe).

But, for a street motor that will make live day-to-day at 5k, I would still stick with a 500 or less. If it is a track style motor, you still need a tick under 650cfm.

Most people wash down cylinder walls.

Take a look at this calculator.

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Old 07-25-2013, 09:23 PM   #4
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This isn't 1972, carburetors have changed.

A cammed 331 will be fine with a 750 if it's a decent carb (good metering blocks, downleg or annular boosters), provided the engine has good heads. As long as the carb can meter fuel at low throttle and rpm, it'll work, whether it's 650cfm or 1,050cfm.

Most EFI throttles are around 1,000cfm. Carbs tend to be smaller to create a velocity increase to meter fuel, since newer carbs are much more sensitive they can be larger than in the past and still meter fuel.

I had a QFT annular 650 on my 302 and it was too small....the metering system was so damned sensitive I could barely settle the thing down. Had to give it a ton of main air and emulsion bleed to reduce signal strength.

Washing down cylinder walls is another load of BS related to "too big of a carb." Washing cylinders is a result of running too rich, which can occur with carb or EFI regardless of throttle size.

It totally depends on the carb and the engine setup.

That said, a carb that's a bit too small will tend to run better than one that's so big it can't meter fuel properly. And the power loss isn't that much when running a tad too small.
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Old 07-26-2013, 03:45 PM   #5
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Lots of BS out here about carbs and their sizing. Merely changing metering on a carb built to flow 750 CFM won't work for long IMHO. As stated, engines perform better with carbs on the too small size better than the opposite. Look for something in the 550-600 cfm range. Better yet, find someone locally who knows carbs to help you. Out here, you don't know who to trust. Also, sealevel experience may not apply to high altitude driving. I am in the 800-900 ft range and would set my carb up differently than, say, Denver, Colorado.
As for a stall converter, that would depend upon the operating range of the camshaft you are using. If, for example, the spec sheet calls for off idle-6,000 rpm, stay with a stock stall. If the spec sheet calls for 2,400-6,000 rpm, get a stall set up for lock up around 2,400 rpm. For overall streetability and economy, lower stall is preferable.

Last edited by Al Newman; 07-26-2013 at 03:52 PM.
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Old 07-26-2013, 04:06 PM   #6
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Also keep in mind, if you go with more stall due to cam range or usage (strip), make sure you run a cooler. The slippage will get the fluid really hot really fast.

The carb you have is a QFT (Quick Fuel Technologies), which makes pretty good carbs. It's a vac. 2nd carb with a good primary metering block. If your 331 has decent heads then it will be fine. If the 331 has stock or modified stock heads, then it may be a tad on the large side. But being a vac. 2nd carb, it will be more forgiving.
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Old 07-26-2013, 04:08 PM   #7
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Id run a 750 carb on that engine all day .
If it were stock 289/302 I wouldn't go bigger than a 600. But with the lift on that cam and the stroker kit go for it I think if you go smaller than a 750 youll be cheating yourself.
Just remember larger carbs make hp smaller carbs make torque, decide what you want to do with the car .
You want some nice bottom end torque go with a 650 , If you want a lot of top end power go for the 750. You can use a 750 carb and put a 1inch spacer keeping the air separated until its in the intake and it will increase velocity which will give you a little more torque at the lower end while still keeping the power there in the high end.
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