Carburetor Formula

[petes74ttop]

Thanks for the help guys, I want to stay with the stock carb & manifold and try to get better milage. I can take a carb apart and change the float or the accelerator pump assembly, but I'm not sure about the rest. Should I order differant primary metering rods and power piston rods and what size should thay be? PG

 

[Vettehead Mikey]

Pete, the message is a stock set up will get you somewhere around 20 MPG. If someone has been under your hood and you now get less than 20 MPG, try to undo the damage, not throw more good money after bad.

 

[Kamie]

Holley Carburetor

Hi all . .

I'm about to replace my stock carburetor of my 1980 Corvette 350 ci L48.
My question: is Holley 0-80457S - 600 CFM fit with my stock intake? or shall I go for Edelbrock 1406 - 600 CFM

Thanks.

 

[TMPJoe]

Hi all . .

I'm about to replace my stock carburetor of my 1980 Corvette 350 ci L48.
My question: is Holley 0-80457S - 600 CFM fit with my stock intake? or shall I go for Edelbrock 1406 - 600 CFM

Thanks.

Holy old post!

Get the replacment quadrajet that fits. If you are not doing mods and want to keep everything in order. Or just have yours refurbished

 

[Crimson Thunder]

Those carbs won't fit your stock spread bore intake. They make adaptors to fit but you would run into hood clearance issues. Your best bet would be to buy an Edelbrock performer(not the RPM) intake to go with your new carb. Welcome to the CAC.

 

[TimAT]

And some real worlds application experience-and observations

3300 pound car, 350, 4 speed, GM #3927140 cam, 2.02 heads, Edelbrock Torquer intake, 3.73 gears. The cam likes 3000 to 7000+RPM.

All 3 carbs were Holley double pumpers mechanical secondary- first carb:

650 CFM. That thing rocked off the stop light. Crazy power on the low end, but flat died at 5000 RPM.

850 CFM. Wouldn't pull a string out of a cat's butt off the stop light- Dead, no torque at all until 3000 RPM. Choices were slip the clutch or light the tires. Above 3000, it was time to hang on. I've turned it to better than 7500 and it never did feel like it quit pulling.

750 CFM. Kind of the best of both worlds- still a little soggy off the stop light but liveable. Pulled good to 7000 and then felt like it nosed over a bit.

Ended up pretty much running the 750 all the time. Back in the Mid 70's kicking the snot out of a 455 Trans-Am was no deal.

And on to today- Ol' Red's LS-7 (454) with more cam and better heads- I'm running an 850 Demon- I've also got an Edelbrock 2x4 setup that works out to 1200 CFM (2x600CFM carbs). It REALLY, I mean REALLY, likes that 1200 CFM. But then too, if you look down an intake port with the manifold off and the intake valve OPEN, it looks like you could roll a dime on edge right into the cylinder and never touch a thing..

 

[Hib Halverson]

Hi all . .

I'm about to replace my stock carburetor of my 1980 Corvette 350 ci L48.
My question: is Holley 0-80457S - 600 CFM fit with my stock intake? or shall I go for Edelbrock 1406 - 600 CFM

Thanks.

Neither of those carbs will fit your stock intake manifold.

Nothing was said about any existing mods, but if the engine in your 80 is stock or mildly-modified, retain the existing Quadrajet carb.

Holley's are good carburetors, but offer no advantage to a stock or near-stock L48.

 

[Peer81]

I don't agree with Mr. Minifridge1138,

The text sort off says the carb is no restriction in the airpump system the engine is. But the carb IS a restriction in the same way the intake runners in the heads are a restriction! A small intake runner will not flow as much air as a big intake runner.

Like Tim say's a small carb (like small intake runners) has high flow at low numbers but doesn't reach the high rpm's. Otherway around for the big carbs. No flow at low rpm but at high rpm like a rocket.

I think the only way to test this is to put a 750cfm q-jet with engine on a dyno and tune it till max output is reached. Change the carb for a 800 or 850 q-jet and also tune that till max output is gained. I would almost say the 750 will win with the torque number and the 850 with the hp output.

Of course the formula is a great way to see how much cfm carb you will need minimum, not maximum.

Greetings Peter

 

[herminator]

Back to the basic theme of the thread, you are saying that mod's only increase volumetric efficiency and only a change from NA to turbo will increase the carb size requirement? What about ram air?

 

[JohnZ]

What about ram air?

There's no such thing as any benefit from ram air until well over 100 mph; the only benefit from "ram air" intakes on the street is reduction in intake charge temperature, and it's minimal (about 1% power increase for every 10*F reduction vs. underhood air is the rule of thumb).

 

[minifridge1138]

I don't agree with Mr. Minifridge1138,

The text sort off says the carb is no restriction in the airpump system the engine is. But the carb IS a restriction in the same way the intake runners in the heads are a restriction! A small intake runner will not flow as much air as a big intake runner.

Like Tim say's a small carb (like small intake runners) has high flow at low numbers but doesn't reach the high rpm's. Otherway around for the big carbs. No flow at low rpm but at high rpm like a rocket.

I think the only way to test this is to put a 750cfm q-jet with engine on a dyno and tune it till max output is reached. Change the carb for a 800 or 850 q-jet and also tune that till max output is gained. I would almost say the 750 will win with the torque number and the 850 with the hp output.

Of course the formula is a great way to see how much cfm carb you will need minimum, not maximum.

Greetings Peter

You're correct that the carburetor is a restriction to the air-pump that is the engine (that's why it's called a throttle).

You're also correct that the formula calculates a lower-bound for the amount of air/fuel that you'll need.

I've spent some more time playing (i've had plenty since I started this thread) and I've found that playing with the secondary control spring, accelerator pump cam, and jets can have a dramatic impact on the performance with only a mild impact on gas mileage.

I would still argue that bigger is not better and that this formula and that was the reason I posted in the first place. It will put in you the ballpark for your engine (my SB 350 does not need a 1200 cfm duel 4-barrel setup). I'd also say that it is better to err on the big side than get one that is too small.

Enjoy the drive, everyone!!!

 

[Peer81]

I agree!

Greetings Peter

 

[84Turbo]

Your formula is comparing apples to oranges. Carburetors are rated at a pressure drop (1.5" Hg, IIRC) (not a mathematical formula with assumptions as in the engine example). Pressure drops in the intake reduce power. There's a reason Grumpy Jenkins used to run a pair of 660's on his 331" SBC (for less pressure drop). A simple formula would indicate that he was way off in his carb selection. His winning percentage indicated otherwise.

For non-drag racing applications throttle response is a desirable thing. However, decent power is also desirable, hence any carb choice is a compromise between power and throttle response/drivability.

 

[lars]

As 84Turbo indicates, there is actually more to carb selection than the formulas provided by some of the carb manufacturers.

The carb manufacturers provide those formulas as a guideline in order to limit their warranty claims. A small carb is generally much more forgiving to tuning errors, and will run reasonably well even if it's not set up right. A larger carb is more sensitive to tuning errors. For people expecting a carb to be bolted on "out-of-the-box", a smaller carb will provide the least amount of problems.

For those of us who have spent a considerable amount of time on engine dynos, doing tuning and parts comparisons back-to-back with different components, it is clear that an engine does not follow the formula if you're wanting to produce the best possible torque and power curves - there are a lot more variables involved. As correctly stated earlier in this thread, once the carb size has been "maxed out" for a given engine, the use of a larger carb will not produce more power. But an engine will produce better power with a much larger carb than that indicated by the standard formula provided by the carb manufacturers. These larger carbs, to run well, do need to be tuned correctly, though. Once correctly tuned, a large carb will produce outstanding throttle response and drivability, even in the lower rpm ranges.

As an example, my last dyno session was on a 407 street engine I had just completed. I ran a 650, 750 and an 850 on this engine: back-to-back testing. Each carb was optimized for air/fuel ratio so the A/F was identical for each carb. The 650 ran OK - probably accceptable for most people. The 750 raised the torque and power curves through the entire rpm range, with average power between 2500 and 6000 rpm up 24 hp over the 650. The 850 duplicated the 750's curves within 2 hp and ft/lbs at any point on the curve, indicating that the engine was no longer cfm limited. But the 850 did not destroy power or detract from any point in the performance. The smaller carb produced a significant power limitation, even though the "formula" will show that it should have been more than adequate. For most people, it probably is. This engine is now running the 750, and the car is severely traction limited at 2500 rpm...

I have also run back-to-back testing of Q-Jets, Holleys and BGs of similar cfm ratings (Q-Jets are in the 750 cfm range). Once the carbs are properly tuned to produce the same A/F ratio, there is little or no power difference between a Q-Jet, 750 Holley, and a 750 BG ("Demon"), except the Q-Jet will produce quite a bit more torque below 3000 rpm than a similar sized square-bore carb. We've run bracket cars where we've swapped between the Q-Jet and a Holley with no detectable difference in ET or MPH. Don't expect to make more power by simply removing your Q-Jet and installing a 650-750 Holley in its place. If doing so produces a power increase, your Q-Jet was not set up right to begin with, and you could have saved yourself $450 by just fixing the Q-Jet.

Bottom line: Use those "formulas" as a guide. Run the carb brand that you are most comfortable with tuning. For a stock engine in a daily driver where mileage is a concern, run the Q-Jet and set it up right (use my paper as a guide). If mileage is not a concern and you don't mind changing intake manifolds, run a Holley, BG, QuickFuel or whatever. If you're not good at tuning and setup, run a smaller carb per the formula. If you can tune and set up a carb, and you're after the best performance possible, I wouldn't run anything smaller than a 750 on a 350-400, and I run 820-850 carbs and anything larger than 420 ci on the street.

Also, just as a little sidenote, it is interesting to note that a properly built street engine with good heads and a well-matched cam will actually produce over 100% volumetric efficiency in certain rpm ranges, even if normally aspirated. An engine's capability to run at over 100% VE should be considered in carb sizing if you're trying to make peak power and torque numbers.

Lars

 

[minifridge1138]

I apologize if anyone thought I was saying that this formula and ONLY this formula should be used to determine what sized carburetor to run. There are too many variables.

I was just trying to point out that a lot of people (not necessarily on this forum) go out and spend a lot of money on 850 or 1000 cfm carburetors for their 327 or 350 because it's a high performance carburetor and should be better.

If they are capable and have access to the proper tools, then they should be able to make any carburetor work correctly for their engine. I wish I had a dyno in my garage.

As Lars said, the 650 cfm produced power in a 407 that was acceptable for most people.

Bottom line: Use those "formulas" as a guide.

That's all I was going for.

Thanks for the extra information. I know i've learned a lot as this thread has grown.