Rusty Bowtie
General Category => GM Tech Questions and Answers => Topic started by: Cool53 on January 02, 2014, 01:28:26 PM
-
http://www.holley.com/applications/CarburetorSelector/CarbSelection.asp. This is a link to get a general idea of what size carb you should start with. But you and I are dealing with a lot of factors. No two performance cars, hotrods or customs that come in this shop have the same parts.
You need a bigger carb if;
You have a lot of horsepower and torque,
You have a low rear end ratio
You have a high stall speed converter
Stall speed is a big one. Going from a stock converter to let's say a 3500 stall, you can go from a 650 to a 750. That much.
If you need help sizing a carb forget the formula, contact someone who can help you make a one time decision.
-
Jef
Back in the day when we couldn't go to the internet to plug in the info about engine size and intended use I used try to size my carburetor by figuring the amount of air the engine could pump per revolution and then do the math to see what might work. For example if you had a 350 cu Chevy engine it can pump approximately .2 cubic feet of air per minute at wide open throttle. If my maximum rpm was going to be something like 7500 then I would multiply 7500 times .2 which equals 1500. Because the engine is a 4 cycle you need to divide this by 2 which would be 750. That was the ball park of what I would want for a carb. Admittedly there all sorts of variables that affect how much air an engine can truly pump but at least I wasn't installing two 850's on a 283 thinking that it was going to make a ton of horse power. Even with all of the tools and information that we have to work with today I still see a lot of cars that have way too much carburetor for the engine size and intended use.
Moose
-
Moose I like that formula. Sounds like a good way to get yourself in the ball park. Like you said there are a lot of variables but you can take those into consideration. Thanks to you and Jef posting all this great info.
Tom
-
This is the formula extracted from what you are saying, Moose.
CFM = Displacement X RPM X VE / 3456
Example: CFM = 350 CI X 6000 RPM X 90% Volumetric Efficiency / 3456
CFM = 350 X 6000 X .90 / 3456 = 546.87
CFM = 546.87
Volumetric efficiency for street engines is usually based on 85%, race engines at 110%. But carb sizing is like everything else, it's all about penis envy. Mine's bigger than yours. Which of course is counterproductive.
I have guts with brand new Corvettes come in that want long tube headers, a bigger cam, and call that. I get tired doing work just for the money so I tell these people that if they have wrung the car out completely and it's not enough for them, then I will do the work. I have plenty of work. But if it's just for bragging rights then I tell them if they buy me lunch then I'll give a receipt to say we did the work and they can drive 35 mph to a gathering and brag about things that might be.
Guys want big carbs, big cams, big intake manifolds, and overdrive transmissions but won't put gear in the car, which is what most cars need. But let's face it,mew live in an era where the phrase "crate engine" has become a good thing.
Most cars have way too much cam and end up going slower and running worse. If you don't have a really good exhaust, a decent amount of compression, good ignition, carb, intake, heads and gear then stay small and enjoy what you have. With what we have today there's no way I will run my stuff at less than 11:1 compression. But that means staying on top of things, too. I see guys racing that change the if oil every year whether the car needed it or not. Sheesh.
-
I've always found for a street engine you need to figure out which size to use (there are a number of methods) and then go one size down. It has a lot to do with volumetric efficiency, I'm not going to get into that, but only a race engine at wide open throttle will have even close to 100%.
An example is 1968 impala with a very mild 454 and T400 fitted with a 3310 Holley 750. Ran ok with terrible economy. I lent him a 1850 Holley 600 while I checked his carb. The 750 was in great condition and when he came to pick it up he wanted to keep the 600, better throttle response, more power and way better economy.
It's not always about size!
Mike.
-
Amen, Mike.
-
But one carb might be better not so much because of size but by what parts are used. Straight leg boosters won't pull main fuel as quickly as down leg boosters, and annular discharge boosters will bring a 750 down to about a 680, just because they are much bigger and restrict air flow. Hat makes a much crisper throttle response, gives better fuel atomization. But companies like AED make such terrific changes to the area under the centuries that carb sizing on a conventional Holley gets thrown out the window. But stay smaller.
-
So using the formula Moose posted for my 283 I should be looking for a Holley 390 CFM?
CFM = Displacement X RPM X VE / 3456
Example: CFM = 350 CI X 6000 RPM X 90% Volumetric Efficiency / 3456
CFM = 350 X 6000 X .90 / 3456 = 546.87
CFM = 546.87
CFM = 283 X 5000 X .85 / 3456 = 348.02
-
It still depends on compression ratio, manual or auto, if auto what stall speed, etc. but if it's stock it's better to go small.
-
Helpful site Cool53. I checked it out and I may be a little under-carbureted, but I'm happy.
-
It still depends on compression ratio, manual or auto, if auto what stall speed, etc. but if it's stock it's better to go small.
All stock except for headers, Edelbrock Intake and a GM HEI distributor.
Helpful site Cool53. I checked it out and I may be a little under-carbureted, but I'm happy.
Using that link to Holley it suggested the 390 CFM model 0-8007 or the 570 CFM Model 0-80570 for mine.