A 500 CFM carb is adequate for a 360 up to about 5000 RPM which would cover the OPs original stated objective of a non radical build with acceptable mileage.

 

Well, that was simply an example. It is dependent on the cam, heads, usage, etc, etc.

If I was going to build an engine with a carb, I would use one of the CFM calculators either from Holley, Edelbrock, or this one: http://www.4secondsflat.com/Carb_CFM_Calculator.html to determine what would work best.

Personally, I prefer fuel injection...

 

Even with fuel injection the CFM of the throttle body must still be calculated to determine its size.

I used 100% volumetric efficiency as a rough measurement in my calculation knowing that the only way you can reach 100% VE or higher is with a blower or a turbo. Using 100% removes the need to take into account the cam, heads, or manifold and assures that you are dealing with the max CFM the engine can require under any naturally aspirated scenario.

I use my desktop calculator. The calculation is simple. 360 cu in times max RPM divided by 2 divided by 12 3 times = max CFM the engine can use. Dividing by 12 3 times converts cu in to cu ft. Dividing by 2 is because each cylinder only has an intake cycle every other revolution.

 

not really...there are only a couple CFM sizes available for throttle bodies. FAST only offers a couple sizes 1000 cfm, 1350 cfm and I think they have a 2k. Edlebrock has 2 or 3 as well. They are not like carbs since the computer is calculating the amount of fuel to input into the engine where as with a carb it is the "draw" of air into the engine that determines how much fuel goes in and why the CFM calculators are handy.

For fuel injection the critical concern is the size\flow of your fuel injectors
http://witchhunter.com/injectorcalc1.php4

 

A carb has two functions. It regulates fuel and allows air to enter the engine. With fuel injection those two functions are done by the TB and the injectors. If either one of those parts is selected wrong the engine will not be able to reach its potential. There is a choice that must be made since there is more than one size. That means there has to be an informed decision on which CFM TB would be the best for each application and that means you need to do the appropriate calculations just like you do for carb applications.

Every part effects the ability of the engine to function and must be chosen carefully. Why build in a limiting factor?

 

I agree completely with your last statement...but my point is where as carburetors come in many different CFM sizes T-bodies only come in a couple basic sizes due to a different delivery method

Recall that the carburetor has several functions: 1) it combines gasoline and air creating a highly combustible mixture, 2) it regulates the ratio of air and fuel, and 3) it controls the engine's speed. Remember gas is not flammable in its liquid state it needs to be atomized and mixed with air (vapor).

Fuel injection is less dependent on air intake\t-body CFM since the fuel is injected at high pressure nearly fully atomized directly above the intake valve.

Speaking in most basic terms: what I said\meant was that the CFM or flow of a (4150\4500 style) throttle body is less important than the injector sizing with FI due to the fact that computer is controlling the flow of fuel which in turn allows the engine to utilize additional air. This is why there are so few different CFM T-bodies since the fuel flow is controlled by the computer via injectors. Large CFM T-bodies are designed with long duration\wide open throttle performance in mind but with only minor performance loss if to large of a t-body is utilized.

Great conversation Seal!

 

I know how all of this stuff works as well as I know you do. We are headed for the weeds though with this conversation. I would like to focus on my statement you disagreed with to understand your thinking.

"Even with fuel injection the CFM of the throttle body must still be calculated to determine its size."

This is the statement you disagreed with. What is "not really" about that statement? I'm curious. Do you not believe the engine would be starved for air if the TB were too small? If the TB were too large, idle and lower end control would suffer. Yes,no? No matter what use the engine is built for, street or track, performance would suffer if the wrong TB were used. The fact that there is more than one size TB even if there were only 2 means you must decide which one to use. The only way to know what is the correct CFM TB to use is to calculate the needed air for that application. My only point with that statement is that the calculation must be done for every part and you can't dismiss it as less important than any other calculation in the engine design.

Surely you must admit that there would be a real problem if one of the following TBs fit an application but the other was used. One is a 1000CFM and the other is a 2000CFM.

http://shop.advanceautoparts.com/web...ody#fragment-2

http://shop.advanceautoparts.com/web...ody#fragment-2

Having correctly sized parts (TB, injectors, rings, bearings, or pistons) is an important thing for engine performance as we all know. I just don't think any one of these parts is more important than the other. Some are more expensive but not more important. JMHO

I find it intellectually stimulating also.

 

What I'm getting at is that the CFM of a T-body isn't as critical as it is with selecting a carb. If it was there would be many different size TB's to choose from.


Do I think an engine would be starved with a TB to small - Yes; but with a 1,000 CFM T-body being the smallest you wouldn't need to worry about that unless you had a very large engine and\or one running very high RPM's,

If it were to large would idle suffer - No not at all as the computer is only going to allow a small amount\just enough, air pass through the IAC passage since it doesn't idle through the "bores" of the throttle body. Would it loose some power at low end - Yes it would move torque higher up the RPM band.

 

You say it isn't critical and yet you agree that the wrong one would effect the running of the engine. I just believe anything that effects the ability of the engine to perform as designed is critical. I guess we will just agree to disagree. JMHO

 

I'm not trying to say your wrong but simply that in the case I'm referring to it isn't critical.

Remember I'm talking about a common plenum\runner type intake style FI (standard 4bbl intake type) which in this case the 1,000 CFM (smallest available) will work for nearly all but large displacement, forced induction, or sustained hi-rpm engines.

Here the difference between a 1,000 CFM and 2,000 CFM throttle body would have no effect on start or idle (IAC controlled); it would move the torque curve higher in the RPM band and chances are the driver would never know at normal driving and would only see the differences at the track or on a dyno.

Recall that dodge used the same t-body on both 318's and 360's with different injectors. GM used the same t-body (different injectors) on 4.3, 5.0, 5.7, and 7.4's before switching to port fuel type.

I agree whole heartily that the correct throttle body size is essential for the tuned port, long runner, type FI for which there are formula's for calculating the sizes.