850 or 950 Carb

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69Chrgr

Charger/Cuda
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Jan 29, 2007
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Location
Midlothian, Texas
Would I see an improvement from my QF 750 DP to an 850 or 950 DP on my 408 Eddy headed Magnum? Heads OOTB Eddies, M1, Hughes Roller, 242 and 246 @.050, .555 and .565 lift, Crane 1.6 rollers, Windage tray, forged pistons, not sure on compression, but about 170 to 180 psi on compression test, MSD distributor, Dynamic 3800 converter, 998, and 4.11 gears. What do ya'll think?

Thanks,
Darren
 
Well 340's came with a 800 cfm carb from the factory so a 950 should work well on a 408. But then I'm a big fan of lots and lots of air on my engines.
 
A QF 750 flows more than 750... I think with the setup you have, the 950 would be an improvment. I dont see the 850 doing much for you going from the QF. You cannot overcarb a 4" engine with a 3800 stall... I run 770 Avengers (wet flowed) as the low end/small carbs on anything with a 4" crank.
 
Maybe, maybe not. Using a 7,000RPM redline and 90% volumetric efficiency (which is rarely attained) your engine should not ever pull in more than just under 750cfm of air. Going bigger may actually result in reduced performance because the air velocity through the carb, and therefore the booster signal, may never reach the level required for the carb to go full rich. You can use jets the size of your finger and you will never acheive a proper A/F ratio. You will always be a little too lean because, due to the low air velocity, your carb will "think" you are at only part throttle. My car has a 383, .030 over TRW 11.5 to 1s, worked 906 heads with big valves, and a solid cam with 528 lift and 241 duration @ 50. When I bought it there was an 850 on it, on a single plane. The best MOPAR guy in town suggested that I pull the 850 and put on a 750. It was one of the best things I ever did. Once I sorted out the correct jetting it made better power and was more responsive than with the 850. When it comes to carbs (and cams) bigger is usually not better.
 
If a performance engine cannot eclipse 90% efficient, you're correct. Except most can. The longer strokes tend to easilly reach 112% at peak torque, and will carry that well past HP peak. So you are correct, but the 383 is a short stroke. You could also use the percentage of stroke increase and multiply the carb choice the general "rules" say. 750x 120%=900.
 
We dynoed 4 different QF carbs on our 414 stroker , 750 , 850 , 1050, 1050 dominator. There was 30 hp. difference in the 750 - 850. we lost a coupel hp. with the 1050 , and the 1050 dominator, but she sure sounds mean with the dominator.
We have raced with the 1050 dominator ,and the 850 switching them each round to see if there was any difference, O difference sooo we race with the 850 now . Because some of the classes only allow 850. Dane



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moper said:
If a performance engine cannot eclipse 90% efficient, you're correct. Except most can. The longer strokes tend to easilly reach 112% at peak torque, and will carry that well past HP peak. So you are correct, but the 383 is a short stroke. You could also use the percentage of stroke increase and multiply the carb choice the general "rules" say. 750x 120%=900.
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I just used 90% as a good number for most engines and a 7,000RPM redline since none was listed. At or slightly above 100% an 850 would be perfect. Have you seen one over 100% without forced induction? I know a supercharged engine can top out at 112% to 115% but N/A can as well?
 
I have no way to measure VE in the garage but I do have a vacuum gauge. If anyone feels that they need a bigger carb, just hook up a vacuum gauge to manifold vacuum on a long hose and tape it somewhere where you can see it on a full throttle run. Shoot for 1.5" of vacuum. If you have more, you need a bigger carb. :)
 
I've seen as high as 118% in a N/A 4" stroke engine. It makes 549hp from 408CI on pump fuel. When the induction is right, inertia and pressure wave physics take over. Think of the sonic tuned ram manifolds. The technology has gotten much better, and most race engines reach that high at peak torque. I know of a couple iron head Modified Tour 360s that get 112% with the Magnum heads and small cams, and make over 450hp that way. Forced induction goes way beyond 115%. at 7lbs of boost the numbers should go beyond 150%.
 
moper said:
I've seen as high as 118% in a N/A 4" stroke engine. It makes 549hp from 408CI on pump fuel. When the induction is right, inertia and pressure wave physics take over. Think of the sonic tuned ram manifolds. The technology has gotten much better, and most race engines reach that high at peak torque. I know of a couple iron head Modified Tour 360s that get 112% with the Magnum heads and small cams, and make over 450hp that way. Forced induction goes way beyond 115%. at 7lbs of boost the numbers should go beyond 150%.
Click to expand...
The science is amazing. Just the physics of getting the perfect headers is fascinating.
 
As you might expect, I'm a fan of the Dominator. We made big gains from a 950 HP to the Dom. But I suspect every engine is different. More is better........... too much is just right. Kind of.........
 
ramcharger said:
I have no way to measure VE in the garage but I do have a vacuum gauge. If anyone feels that they need a bigger carb, just hook up a vacuum gauge to manifold vacuum on a long hose and tape it somewhere where you can see it on a full throttle run. Shoot for 1.5" of vacuum. If you have more, you need a bigger carb. :)
Click to expand...


Manifold vacum. Is that the one with Vac at idle Or just off idle? Just cant remember and would like to try that. Thanks.
 
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