[Mr Twister]

How do you determine what size or type of throttle body to use on an EFI conversion? I am going to get one and do not know which one to get. I think that sizing is different than a carb because there is no requirement for a differential pressure thru a venturi, just airflow. Maybe bigger is not bad?

[Tom Vaught]

Normally the throttle body size is the same as the ID plumbing on the rest of the AIS (Air Induction System) of a production engine.

Say you have a short AIS system, the Hydraulic Diameter of the pipe can be smaller vs a long torturous system where there will be more pipe losses. Hydraulic diameter has to increase in those cases so the Throttle Body diameter also gets larger.

In the old days carbs were calculated for airflow by the formula:

(Displacement x RPM x Volumetric Efficiency) divided by 3456

or say (455 x 5500 x .85) divided by 3456 or 615 cfm

You could figure out the "effective flow co-efficient" of say a 850 cfm carb by calculating the added throttle blade diameters vs the airflow. A 850 carb had a "effective flow discharge co-efficient" of about .70 (flow area of 1.75" diameter throttle blade x 4 blades x .70 Cd)

A chart is provided below of a 4 bbl 850 Holley vs a single bore throttle body. A 4 hole efi throttle body would act just like a carb throttle body.

TB (mm)******(in)******CFM****************CFM (4 bbl Holley)

44.5mm*****1.750*****212******times 4 blades******848 cfm
50.8mm*****2.000*****276
57.5mm*****2.250*****350
63.5mm*****2.500*****432
69.8mm*****2.750*****523
76.2mm*****3.000*****622
82.5mm*****3.250*****730
88.9mm*****3.500*****847
95.2mm*****3.750*****973
101mm*****4.000*****1107

So you can see that a 90mm Throttle Body (single blade) has about the same cfm (when flowed the same as a 850 cfm Holley).

Tom Vaught

[stobin]

Are you wanting to use a universal 4 barrel type, or a single blade. I use an Accufab 1215 cfm unit on my car. Instant throttle response. As you stated, EFI is different than a carb in reference to sizing. It is acting as an air door. There is no fuel in the plenum to worry about.

[Mr Twister]

I am still confused about the sizing. Wouldn't head flow, and VE be factors in the selection. What I mean is, if the heads flow 300 (times 8) and VE is .8 wouldn't you need a Throttle body that could support 1920 CFM??I have not figured out which one to use I think I want to use the least expensive one I can find that will work well whichever kind it ends up being. So far I have a super Victor, a MEFI 4 harness incl the heated 02 sensor and ECM intended for a 502. I need injectors and the associated small parts. A friend of mine owns the MEFI programming software to tweak the program to work on my 400.

[Tom Vaught]

Originally Posted by Mr Twister

I am still confused about the sizing. Wouldn't head flow, and VE be factors in the selection. What I mean is, if the heads flow 300 (times 8) and VE is .8 wouldn't you need a Throttle body that could support 1920 CFM??I have not figured out which one to use I think I want to use the least expensive one I can find that will work well whichever kind it ends up being. So far I have a super Victor, a MEFI 4 harness incl the heated 02 sensor and ECM intended for a 502. I need injectors and the associated small parts. A friend of mine owns the MEFI programming software to tweak the program to work on my 400.

The heads do not flow 300 cfm times 8. All of the intake valves are not open at the same time.

1) The crankshaft goes around twice for one revolution of the camshaft.

2) All of the Intake valves open in sequence based on the firing order.
Cylinder #1 will open on the driver's bank then cylinder #8 will open on the passenger bank. Then Cylinder #4 will open on the passenger side, and finally Cylinder #3 will open for that revolution of the crankshaft.

3) Typically you can use the formula to size an Carb or Throttle Body that I posted earlier in this topic. The fuel in a carb will take up about 8% of the mass flowing into the engine. With a Throttle Body you will have 8% more air flow through a throttle body but the same air/fuel mass for the same power after the injector and going into the head.

4) The ONLY reason why you need massive amounts of air flow like you described 300 x 8 x .8 would be if you had a individual runner/ throttle for EACH cylinder. I ran 4800 cfm of airflow on a Pro Stock 355 CID Chevy engine in the 70s when I worked for Holley Carburetor. It had a IR manifold. and TWO 2400 cfm Experimental Dominators.

Tom Vaught

[Mr Twister]

Tom, Thanks for clearing that up. I understand now.

[Tom Vaught]

(;>)

Tom V.