Throttle Body Size, Blown Application

JAM · Post by **JAM** » Sat Feb 04, 2006 7:39 pm

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 · Post by **Uncle Bob** » Sun Feb 05, 2006 2:00 am

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 · Post by **bobnova** » Mon Feb 06, 2006 12:52 pm

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 · Post by **JAM** » Wed Feb 08, 2006 12:58 pm

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 · Post by **JAM** » Sat Feb 11, 2006 4:57 pm

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

Any Feedback?

Big Creek · Post by **Big Creek** » Sat Feb 11, 2006 9:13 pm

That should be more than enough.

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

Todd

JAM · Post by **JAM** » Sun Feb 12, 2006 7:10 pm

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