Coming from you Jon - I should have known it wasn't gonna be that easy 8). You
are correct as far as I'm concerned. Careful machining and flowbench testing
will reveal what you describe. Even some very minor adjustments to a stock TB
can show improvements. Similiar to the gains obtained when port matching
intakes/heads as well as smoothing out casting flaws and the like.
I was mostly referring to the general case where TB's are just bored oversize
w/out attention to these subtle details. Also the results of dragstrip runs
before and after can be greatly over (and under) exaggerated. This is I'm sure
due to the subtle smoothing effects coming into play more at just off-idle
conditions compared to WOT runs at the dragstrip where the low-rpm torque loss
is much more apparent due to the low-speed air travelling through a larger
orifice. At the dragstrip can exhibit a slower ET whereas the same vehicle can
feel more perky light to light.
Good analysis,
Shane
Jon Smith wrote:
> No, I meant what I said. What you say about the intake manifold is
> correct, but the TB is different. This is due primarily to the hour-glass
> shape of the bore, resulting in a shrouding effect over 1/2 of each bore.
> This design helps reduce shock off-idle (to both occupants, driveline parts,
> as well as payload... horse trailer). OTHO, if you install a much larger
> bore TB, you can loose low-end... howerver with a port job by a
> knowledgeable person, a larger TB will not hurt ANY performance at ANY rpm.
>
> > I think Jon meant to say that DECREASED low-end is unavoidable w/a ported
> TB.
> > This is due to the fact that air moves slower through a larger orifice.
> The
> > benefit is higher volume potential as realized in the upper-end of the rpm
> scale
> > at a cost of low-end torque. This is also the basis of the factory
> "beer-keg"
> > intakes as installed on all 5.2 & 5.9L magnum engines. The smaller and
> longer
> > port runners give high speed airflow at low rpm's resulting in lots of
> low-end
> > torque but at the cost of limited upper-rpm flow potential.
> >
> > Latr,
> >
> > Shane
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