Throttle Body Size, Blown Application

[JAM]

We seem to have some concern deteriming a good throttle body diameter
for a blown Ford Flathead that we are building. Here are the details:

271 C.I V8 Flathead

Front-draft (crank driven) Detroit 3-71

~8 of boost (max)

2 Throttle bodies

*Previous research we have found on similar Flatheads shows we need around 750CFM to flow through the throttle blades without causing resrictions*

I have found info on NA engines, but I realize that boost is a different situation. We also dont want to over do the size as to not cause the related probs.

If you have any info on the topic, or examples of throttle plates on
engines with boost let me know.
Thanks

[Uncle Bob]

as a general rule, you use the same throttle body size you would use for a non-blown engine of simular calibar.

For example, a "reasonable" turbo'd V8 (stock 350 SBC for example) engine will usually be happiest with a 600CFM carb. An extreme turbo'd V8, will be 800+ CFM.

I don't know what your HP goal is, but I would base it off what the engine is capable of NA. Its easy to get confused on the difference between CFM on a boosted engine, because CFM doesn't represent the pressure and velocity of the air charge.

[bobnova]

It makes a fairly large difference whether the throttle bodies are before the blower, or after the blower.

If they are before it, treat them exactly the same as on a n/a engine making the same amount of power (BIG!), if they are after it, it isn't nearly as important, as you can push a lot more air through a given size TB then you can pull it.

[JAM]

The throttle bodies are before the blower, so i assume that making them slightly larger would be better than making them smaller. Is there a conversion from estimated CFM to TB bore diameter?

[JAM]

P.S.
We are looking at up to 300 H.P., and running 2 x 2.5 inch Throttle Blades.

Any Feedback?

[Big Creek]

That should be more than enough.

http://www.megasquirt.info/v22manual/minj.htm#throttle

Todd

[JAM]

Does anybody think that this will be overkill and cause drivability probs?

[Jack]

Does anybody think that this will be overkill and cause drivability probs?

You can stage the two TB's with a progressive linkage, so you start on one and the other comes on later, but faster, with both gettng to full opening together.

Jack

[JAM1]

I think that is the way we are going to do it, we were forced to do that on an intake we made for drivabilty issues. A large number of large blades makes for dificult drivability any other way than progressive linkage, which can get a little tricky. Thanks for the input.