Thread: Which Carb

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Need help with what carb to use....I have a Quick Fuel 750-Q and a 850 Speed Demon


The engine is a 402 it's 30 over....Should be right at 10.1 compression

TRW Forged pistons...L2328F30.....319 dome

Comp Cam....Hydraulic Roller
235/240 @ 50 Duration
570/578....Lift
110 Lobe separation

820 Oval port heads 112cc

Unless you'll be doing some racing, the 750cfm will be plenty. Although I don't know what a 750-Q is... Is it like a Holley or a Quadrajet?

Formula for carb size is:
CFM = CID x maxrpm/3456 x VE
Where VE is volumetric efficiency and is about 85% for street cars and 110% for race cars. Rule of thumb I use is CIDx2.

-Chris

Hi Denny,

Don't know the exact specs for this cam but judging by the duration and lsa I would be very surprised if there were anything left much over 6000. With a very generous 100% VE that puts you at 705cfm for a 406. Say you pull to 6500 you're at 760 with the same unlikely 100%. Do you disagree AFR has nothing to do with CFM? Why go lean at the top?

Anyway depends what you're doing with the car... If you're racing put the 850 on there and have fun... 750 is plenty big enough for virtually anything you want to do and you'll still have some throttle response. IMO 750 will be plenty even on the strip.

-Chris

^^^ I'm not convinced... Choking the motor with not enough air/fuel cfm is not the same as leaning the motor for not enough fuel in the air/fuel mix, no? In fact, you might expect the mix to be richer at the limit of cfm capacity as the booster signal will be strongest. I'd like to hear anyone's opinion on this... Maybe we should take a vote

-Chris

Okay... We're obviously on different pages here and that's fine I'm talking about fluid dynamics (air and fuel both being fluids)... I've already stated and restated the point I'm trying to make so I will not repeat it here.

Enjoy your day

Chris

http://en.wikipedia.org/wiki/Fluid_Dynamics

...198plus asked which carb to use and I suggested the 750 unless he's racing it... That was the first reply to this thread. I hope we haven't thoroughly confused him since then

I'm alright........Continue on......Carb Tech 101

Originally Posted by skids72

http://en.wikipedia.org/wiki/Fluid_Dynamics

...198plus asked which carb to use and I suggested the 750 unless he's racing it... That was the first reply to this thread. I hope we haven't thoroughly confused him since then

The 750 cfm carb will do fine.... save the 850 for track day... The Quick Fuel carbs seem to work quite well considering their cost... But the Speed Demon would be the better carb (when tuned and jetted correctly) at the drag strip...