You know how there are many places you can go on the net and find a nice chart to choose the correct carb size? I have even seen several math formulas that produce those charts.

Does anyone know or have a link to a web site that has those same formulas and charts to choose the correct size throttle body? (I'm looking for the actual math formulas... or engineering type charts please.)

Thanks in advance.

 

Here is what I've found so far, with no surprises

The engine cfm requirement is RPM x Displacement, divided by 3456 (a constant), Which I already had. This of course is the same for a carb or EFI. There are other formulas that depending on the engine Volumetric Efficiency.

I found my way to the Accufab website http://www.accufabracing.com/index.html and found some flow numbers for different throttle bodies.

It appears that there is no formula for flow on a throttle body based on plate diameter. The bad thing is that we are left with manufactures advertised flow numbers.

However, Accufab did tell me that you can still get reasonably good foot response in mid throttle with a TB that is up to 150% of the required cfm. I'm sure most of you know that too big will behave more like a on/off switch and will kill mid throttle drivability. As for me... I think I will stick with about 125% of required because I need very good part throttle behavior.

That's it for now

 

Off the top of my head, heres how I see it...

RPM x Displacement x Volumetric Efficiency / 2 = Required CFM

I'm dividing by two because only half of the cylinders will be drawing in an air charge. Keep in mind, that displacement is usually in cubic inches, so you have to convert to cubic feet. There are 12 x 12 x 12 inches in a cubic foot, or 1728. 100% volumetric efficiency would mean multiply by 1.0. If you wanted 85 % VE, multiply by .85.

Example: Assume a 350 engine with 100% volumetric efficiency at 5000 RPM

5000 RPM x 350 cubic inches x 1 ft / 1728 cubic inches x 1.0 (VE) / 2 = 506 CFM
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Just punched in the same numbers in your formula from accufab, and it yields 506 CFM as well.

FYI, stock TB flows about 700 CFM, and the figures in the above example are stretch even for a best case scenario TPI motor.

 

Here are my thoughts. In theory you cannot over throttle body a fuel injected car. However with a very large throttle body of lets say 1300cfm you will let in a lot of cfm with very little movement of the throttle blades. This will give you jack rabbit starts with little pedal movement.

So if you are planning some mods down the road get a 58mm throttle body and don't worry about it. You will have that base covered.

 

That helps. 700 cfm out of a stock TB

The engine's cfm requirement is what I already know. I was hoping to find a formula that related TB plate size to cfm through the TB. But it appears to be much more complicated than that due to intake/TB design...

Thanks

 

There isn't such a formula. There is more involved, as you already acknowledge, than just a simple relation between throttle blade size and flow.

Generally speaking,
Stock 48mm TB is about 700 CFM
Stock 48mm TB w/ airfoil is about 750 CFM
Aftermarket 52mm TB is about 850 CFM
Aftermarket 58mm TB is about 950-1000 CFM

Mathematically speaking, all are overkill for any TPI motor. If you are going to go with a throttle body that has 125% of the required flow, you are going to have to fabricate a smaller throttle body than stock.