Originally posted by bret:
i just doubt roush would go thorugh the trouble of using a larger tb to obtain more hp, and not max it out...
No real trouble at all using the 60mm on the SVT. The SVT throttle body is just the 3L Duratec TB casting with a different bracket for use in the Contour. Design costs were minimal, production costs were minimal. Saying that "Roush gave us a 60mm TB, it must be the correct one for max power on our cars" is like saying "We don't need an LSD or baffled oil pan, because Roush didn't give us one". The determining factor in designing ANY car is never what will give it the most performance/reliability, it is the bottom line ($$).
what purpose would they have not to use the most efficient tb?
Like I said, "Hey, if we use this TB from the 3L, we won't have to make up a new casting for this project".
don't you think they would have loved to pull another 5-10 hp out of the engine with something as easy as a larger tb? Don't you think they didn't use plots, and anayalsis for choosing the proper size of the tb?
"Hey look, according to this, a higher output 2.5L just happens to require the same size TB that we're using on the Taurus, that's great!"
Boy, that'd be a hell of a coincidence.
even sho-shop only went to a 63mm throttle body and they upped displacement by a half litre. I would love to see the dyno plots for this, because i bet this thing really bogs from 0-3250 rpm in the torque range, and then looses hp from there on out... just get the graph over the original tb on the same car at the same time...
Roush started with a 53mm TB on the standard 2.5L and upped it 7mm to 60 for the "HO". The SHO-Shop stared with a 60mm and upped it 3mm for the "HO" version of their 3L. Are you implying that the same principle applied to the 2.5L won't work for the 3L?
Moreover, I too would like to see a dyno plot for the 3L using both size TB's. You refer to expecting to see "bogging" in the lower rpms from a dropping in torque. The overall size of the TB has nothing to do with low-end air velocity on a Duratec. The primary intake runners determine air velocity, because they are the point of restriction on the induction side of the engine. It doesn't matter what size the TB is, be it 53mm or 100mm, the engine will still be pulling air through the primary runners, which will only be able to pull a fixed volume/velocity of air through them at a given time. Those are the McDonalds straws that you refer to. Increase the diameter of the runners and air velocity during low-speed operation will fall, which in turn will drop torque. You comment about it loosing HP from there on is ludicrous. High-end power is determined by one thing only, air volume. You?ve got 2 intake runners at your disposal from ~3500rpms and up. One is significantly shorter than the other. The engine at this point is limited by how much air it has at its disposal. The TB is the determining factor, as both runners have the potential of pulling more air than the TB can supply. If someone were to dyno a flow optimized car (headers, catback, air filter, etc.) with a chip, this would be shown by a HP falloff around 7100rpms. To imply that an engine of more displacement would be better suited with a smaller TB is crazy.
