godblessmud said:

A related question, right before i yanked the 302 from my truck I was having deiseling issues as well, timing was good, injectors were good, spark plugs gapped correct. It was bad enough that i had to pop the hood, jump out and yank the battery cables sometimes to kill the spark. I never thought anything of it until i read this because i pulled that engine shortly afterwards but could my old 9mm plug wires have had something to do with it?

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Stoner Dan said:

Thanks for the ideas, guys.

Yeah, the builder feels bad about it and wants me to bring it in, but that's easier said than done (I live in San Francisco) and I need to do a leak-down on it to get some better info first.

That's an interesting idea on the linkage--I wouldn't be surprised at all if that was part of the problem and I'm gonna check that out first, since it's so dang easy.

And I've set the timing on this pig several times and it seems to help for a while and then goes right back to the same old tricks, dangit.

I appreciate the thoughts and looking forward to any more you might send my way!

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Mil1ion said:

Tune the carburetor properly.

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One of the most asked questions we receive at PRO-SYSTEMS is, how big will my carburetor be, how much will it flow? Most customers are expecting to hear big numbers. They want to hear that their 4150 will flow 1150 cfm or that their 2 inch throttle blade Dominator will flow 1400. Unfortunately, that's not going to happen. In all the designs I've flowed, designed, calibrated and tested, it's just plain physically impossible. Builders will give out dry numbers (meaning cfm ratings without the disturbance of the fuel cone) but these numbers are not what the engine actually sees and are for reference only. Some builders still use the inflated cfm creating 28 inch rating used by an aftermarket manufacturer. Enough has been said on that subject.

But really cfm is not the main level of importance. Fuel shear, atomization properties and fuel curve are your main areas of concern.

To size a carb for the application, you're looking to achieve minimal restriction at the finish-line yet have enough signal at launch as to be sure that the booster is atomizing the fuel and supplying the proper air to fuel ratio.

Horsepower equals air flow (of course). Launch rpm/trap rpm equals a reference of the range of the air flow.

If the carb is too big or signal/curve is too poor at the launch rpms created airflow, the fuel does not properly atomize and plates out (turns back to raw fuel) on the intake. Losses of 10-12 percent of available torque at launch can easily be recognized without a lean cutout or backfire. Then as rpms increase, the plated fuel is picked up and alters the air to fuel ratio down-track as it is cleaned out of the intake. More loss of power. So you jet it down to compensate for the plated fuel being picked up and the launch gets even worse. See the dilemma.

The wider the range of rpm you're going to subject the design to, the more you need to look at the range of airflow and available options.

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

CID x RPM x V.E. / 2820 = CFM
350 x 6600 x .9 / 2820 = 737 CFM

Now you'll be a little closer.

A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out assault on the engine blocks stress handling capabilities.

Remember, if we have a heavy vehicle and a two speed we will require a slightly smaller carburetor, than a light vehicle and a stick. Also, if we have a booster/emulsion/air bleed configuration designed to operate and shear fuel at lower rpms we can increase the cfm. An increase in cfm is usually a guaranteed increase in power, but it takes a design that'll still pull and shear fuel at launch to pull that off. That's when the builder starts altering the entry and exit angles of the booster, the emulsion layout, air bleed configuration/well diameter, etc. All in an effort to fan the fuel cone to increase impact, supply the proper air to fuel ratio throughout the rpm band and emulsify the mixture prior to decrease plating for the air speed being encountered. All those mods cost money and they're not easy to do.

But return on investment is the deal when purchasing a carburetor. Oftentimes a customer is thinking of purchasing a this or a that, when the same money spent customizing his current model will yield more performance.

Remember, as we talked about earlier, the loss of torque we record at launch and the subsequent rate of acceleration you lose at the start of the race will be carried throughout the rest of the shifts. So a good leave (excellent fuel shear and proper air to fuel ratio at launch) is getting the reciprocating mass to carry this rate of acceleration to reduce E.T.'s. But if you have too much restriction at the finish-line, the mass will be slowed as a result and E.T.s will increase and none of us want that.

Thanks for the E's.

Have a great season everyone...see you at the races.

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UTfball68 said:

Well with an 850 cfm carb, you're drowning your engine. Keep in mind, 460's came stock with a 600cfm carb...if Ford thought that was good enough, their engineers know more than I do. And if you use the first age old formula Aaron posted, a 460 that travels at no more than 5000 rpm with 90% efficiency divided by 3456...gives you 599 cfm. Goo 600cfm if you want a touch more low end torque or go 650cfm if you want a touch more top end hp. To be quite honest, I'd try to stay in the 600-700 range.

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Stoner Dan said:

So, if I'm belching raw fuel back up out the top of my carb and the thing is running like **** (even though I keep tuning it), it sounds like a new carb is in order, right?

Maybe I should start there and see what, if any, improvements I'll find? Now, I hear you guys saying to get a 600 or 650, but what exactly makes you say that? I do alot of pulling with this truck, etc. ––*is a 650 what I really need to be looking at or is there another recommendation as far as cfm?

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Troy Patterson said:

Hello all, Nate told me about this thread so I thought I'd come over and share my two cents worth.

I am impressed that there seems to be more or better understanding here generally of the whole carb cfm issue than I often read.

I refer to my carbs based upon venturi diameter. The reason I do this is that it is the diameter of the venturi relative to engine displacement that determines the air speed or velocity through the carburetor - which I consider a critical factor to application.

As was alluded to, the limiting factor to carb sizing (at this point of development) is the ability to meter fuel accurately. So, we can keep increasing the size to the point the carb is no longer able to be sensitive and responsive to subtle changes or fluctuation in the metering signal. In the case of Nate's 289, it is running a 1.50" diameter venturi over a 1.75" butterfly base plate. On a flow bench, this works out to around 900 cfm. In my opinion, Nate's 289 / car combo could happily handle a 1.60" + diameter venturi / 1.75" butterflies (or 1000 + cfm) carb properly prepared.

When I started playing with carburetors, I to thought that smaller cfms equaled better throttle response. The logic seemed to make sense - high velocity = quick response, and to some degree it's true. However, as it turns out, small carb / high velocity is more forgiving of a poor tune (or calibration, I use these terms interchangeably) and therefore to the novice there is greater likelihood it will produce the "expected" results when tested.

As my understanding progressed and I was better able to tune the fuel curve to the engine's needs, I found that I could run progressively larger and larger carburetors without loss of tip-in throttle response or low end torque, and always making more torque and horsepower.

What's interesting to consider is that as the carburetor is increased in size, manifold vacuum @ WOT is less - less vacuum means (relatively speaking) more pressure - in the intake track to fill the cylinder. If you picture what's happening at the intake valve: flow into the cylinder is not initiated into the combustion chamber until the pressure is greater on the port side of the intake valve, than on the chamber side of the valve. A large venturi carburetor by reducing the vacuum in the intake @ WOT, effectively decreases the vacuum - effectively increasing pressure - at the port side of the intake valve. A little bit like adding boost compared to running a small carburetor isn't it!?

All this means flow will be initiated sooner therefore more air / fuel mixture will be feed into the cylinder - and more power will be made.

Where running large to really large venturi diameter carburetors get people into trouble is if the carburetor isn't tuned well to the application and / or if it is a conventionally built / modified carb intended for a more conventional / typical cfm to displacement application.

Another benefit (power producer) to consider when running larger cfm carburetors is that because air speed through the carburetor is lower (by virtue of the larger dia. venturi / butterflies), the air / fuel mixture enters the plenum chamber at with less velocity - which enables it to make the transition or turn (with fuel still in suspension) into the intake runners. The little carburetors literally shot the fuel droplets straight at the floor of the plenum chamber.

By keeping a greater percentage of fuel in suspension vs. impinged on the floor of the intake manifold's plenum chamber and walls and running into the cylinder in fluid form, you maintain better fuel distribution of the a/f mixture reaching the combustion chamber therefore better combustion efficiency = better throttle response, more torque and horsepower.

As Nate mentioned, the Weber Power Plates although a bit pricey, improves the carburetors ability to accurately meter fuel, providing greater flexibility and broadening the usable rpm range of the engine and carburetor @ WOT.

All these issues together, allow me to run 1.50" - 1.60" + dia. venturi over 1.75" dia. butterflies on 289 / 302 engines with great success, and without compromising low end in any way. The results are as Nate experienced. It's not uncommon to see a half second reduction in quarter miles times and a proportionate increase in speed, or faster lap times, or whatever - and still be perfectly streetable.

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dragogt said:

No Less than a 750...



460 x 5500 x .9 / 2820 = 807.4 CFM

Also IF a 600 is good enough for a 460 why would GM put a 750QJ on a 350???????

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dragogt said:

I still think your timing is out of wack.. Check that first before you go buy a new carb...

http://sbftech.com/index.php/topic,8087.15.html




http://www.tmpcarbs.net/

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