Big carb (Quick Fuel 850) on a small engine?

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glockr

glockr

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Just a question(s) - don't know much about carbs, that's why I'm asking. One thing I see is that a lot of people running fast cars (even street cars, not just drag cars) are running carbs a lot bigger than what is the "standard recommended" cfm. I've also read in a few places is the real reason big carbs don't work on smaller/lower rpm applications is b/c of weak booster signal, not the size of the carb. Couple that with "with a dual plane intake, the engine only sees half of the carb's cfm".

Assuming both are true, could a carb that's way to big by "conventional standards" be tuned to work OK on a 318 if it had annular boosters for a stronger booster signal? Probably with an RPM Air Gap, Promaxx 171 heads (not the Shockers), and 236 @ .050 cam? Or would it be waste of time and money trying to get it to work?
 
You could run it and tune it to work fine, but I don't think it would be optimal size. I think it would be easier to tune a 750 for the project.
 
318willrun said:
You could run it and tune it to work fine, but I don't think it would be optimal size. I think it would be easier to tune a 750 for the project.
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Would you run annular boosters on the 750?
 
kdeboy said:
Would you run annular boosters on the 750?
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maybe if it was seeing a lot of street time. If it's going to live on the track in the upper rpm's, down leg would be just fine.
 
Say your building a 400 hp engine and say 400 hp is a about 550 cfm, i'm not talking carb size but the actual amount of air pumped through the engine each minute "550 cubic feet". Now no matter what carb you put on a 350 450 550 750 1050 etc.. About 550 cfm with be displaced each minute making all these carbs a 550 cfm carb at full throttle just at different vacuum levels.
Why I said "about 550 cfm" cause carb size will slightly change how much air the engine will displace cause of restriction, it's better to think of carb as different levels of restriction like header diameter size. Smaller carb more restrictive making less hp so needing slightly less cfm over optimal size carb.

So since the engine above uses 550 cfm you'd think 550 cfm carb would be perfect but a carb is only 550 @ 1.5" hg which is fairly restrictive still so say you put on a 750 cfm carb @ 1.5" hg
on this engine it now will be about 550 cfm @ 1" hg (1" hg is a guess).
 
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I ran a 1050 on my small block with down legs. Went mid 10's @130 through the exhaust 3650 lb. street car. A straight leg carb didn't work for me at all.
 
Continued..

Go larger is gonna stop given hp gains at some point say on the 400 hp engine eg. Above 850 stop showing real gains so 950 made 400 hp and 850 did the same a 750 was down 5 hp, 650 down 15 hp, 550 down 25 hp etc... Choose your compromise :)

Plus people just know what works there's probably near zero mopar V8 application where a 600/650 carb is too big even 750 proven themselves on a wide variety of combos from mild to wild.
 
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As with everything else, it's all in the tune. I'm getting ready to swap a 650 double pumper on my slant 6. It had a 450 vacuum secondary carburetor on it now. I'm not the least bit concerned that I cannot get it tuned with all the modern tuning it has built into it. Might take a little time to get it dialed in, but it'll get there.
 
For street use, bigger isn't always better. Tuning it will take some experimenting, but it can be done. I've always run the Holley 4779 4 point idle circuits on my street cars & race cars. But, then again, I've been fooling with them for about 50 years & learn something new everytime I build one. No two carburetors will run the same out of the box so that is where the tuning curve comes in. I'm not saying don't run the 850, just be prepared to tune it using plug readings, performance (butt dyno) & reading the plugs. In the end, you can brag to your friends & maybe help them with their carb tuning.
 
Big carbs will have lower vacuum in the venturi meaning weaker pull on the fuel. This results in poor responsiveness and difficulty in tuning.
 
If your not building an all out race 318 motor run a 650 or 750, lie at the car show that its an 850 or 950.
 
1MeanA said:
Big carbs will have lower vacuum in the venturi meaning weaker pull on the fuel. This results in poor responsiveness and difficulty in tuning.
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So wouldn't the question be what's the ideal vacuum compromise for different applications eg.. Race only, street strip, street machine, daily driver etc.. Seems like almost no one run's 1.5" hg.
 
mderoy340 said:
If your not building an all out race 318 motor run a 650 or 750, lie at the car show that its an 850 or 950.
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I'm not going to lie about it, I just find my self strangely obsessed with the Holley 830 annular booster model. Heck, I prefer sleepers so if I was gonna try to fool people I'd try to make it look like a 2 bbl :) But... talked to the machine shop that's going to be doing the engine for me and they don't have a 318 core so I might get stuck building a 360. It'll either get the 830 or a 4779.
 
1MeanA said:
Big carbs will have lower vacuum in the venturi meaning weaker pull on the fuel. This results in poor responsiveness and difficulty in tuning.
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That's why I was thinking about trying annular boosters.
 
1MeanA said:
Big carbs will have lower vacuum in the venturi meaning weaker pull on the fuel. This results in poor responsiveness and difficulty in tuning.
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You can address that, make more power everywhere and not lose any drivability.
 
Rat Bastid said:
There are far better options out there than any Holley annular booster.
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What would you recommend? Engine will have around 236 @ .050 on the intake, 9.5 - 10.5 compression, probably Speedmaster or Promaxx as cast aluminum head. Early A body with manual transmission. Needs to run on 91 pump gas.
 
If I was building the engine you have and I already had the 850 CFM carb I would definitely use it, and tune it to work. If I was looking to buy a new carb for that build I would go with a 750 CFM carb. Another question is, are you talking about an 850 double pumper or an 850 vacuum carb? An 850 DP is very difficult to tune with a smaller engine, unless you have a manual transmission, or a high stall converter (3,500 RPM+), and low gears, like 3.91 or 4.10. A vacuum carb is much easier to get working. If you build that engine with 3.23 gears and a 2,500 stall and put a double pumper 850 on it, you may never be able to get it working optimally.
 
HSDemon said:
If I was building the engine you have and I already had the 850 CFM carb I would definitely use it, and tune it to work. If I was looking to buy a new carb for that build I would go with a 750 CFM carb. Another question is, are you talking about an 850 double pumper or an 850 vacuum carb? An 850 DP is very difficult to tune with a smaller engine, unless you have a manual transmission, or a high stall converter (3,500 RPM+), and low gears, like 3.91 or 4.10. A vacuum carb is much easier to get working. If you build that engine with 3.23 gears and a 2,500 stall and put a double pumper 850 on it, you may never be able to get it working optimally.
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Car is manual, will be putting in 4.30 gears this summer.
 
kdeboy said:
What would you recommend? Engine will have around 236 @ .050 on the intake, 9.5 - 10.5 compression, probably Speedmaster or Promaxx as cast aluminum head. Early A body with manual transmission. Needs to run on 91 pump gas.
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I’m using 20 hole boosters from Lightning Carbs. I have the tool to install them.

IIRC the Holley boosters are only 8 or 10 holes. And they have a different shape.
 
Rat Bastid said:
I’m using 20 hole boosters from Lightning Carbs. I have the tool to install them.

IIRC the Holley boosters are only 8 or 10 holes. And they have a different shape.
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So Holley style carb but with different (non-Holley) annular boosters?
 
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