Well Frank, first off thank you for this info. Now, seeing that the stock
manifold flows around 650 CFM, and a worked over
stock plate TB will flow about the same, how much work are we looking at? Is
this as easy as maintaining the lower bore
diameter up to the top, or is that 670 CFM reached by a thinner shaft,
countersunk bolts, and tapered bores...etc.
The reason I ask is that I have access to a shop that bores motorcycle carbs.
What do you guys do to the $90 TB's
BTW, where are you located?
Marc Federici
99 SLT+ CC 5.2 5sp 4x2 3.55sg
<
<After a tremendous amount of work both on regular chassis dynos and DynoJets
I figured that I would give my two cents <worth. In stock form, the V-8 TB's
flow approx. 570 CFM @ 24 inches. With stock plates and work, the TB can be
pushed up <to approx. 670 CFM @ 24 inches. Larger plates can be installed,
but the size for a streetable TB is limited by the web <thickness between the
two bores (actually a support for the throttle shaft) and the throttle shaft
bearings themselves. We have <used larger plates (out to 2.025 inches)
without compromising the center web and without actually hitting the
bearings. Flow <@ 24 inches nets you approx. 710 CFM with the larger plates.
All work has been done with either the stock shaft or one of <our SS half
shafts. Whether you use larger plates or the stock plates depends on your
needs and your goals. The stock <intake actually becomes the limiting factor
at around 650 CFM. If you are contemplating a different intake (MPI) and
using a <healthier cam, !
<then you will reach a point where you need to go to more CFM through the TB
whether you choose to use a 4 BBL or <increase the plate size in the stock TB
is up to you. One thing you need to remember is that our engines use a MAP
sensor, <the setup is actually a speed density setup whereby the amount of
air reaching the engine is calculated based on MAP signal <at the base of the
plates. The problem with larger plates is that the velocity through the TB
actually decreases, the computer <recognizes this and supplies less fuel. The
results are a reduction in peak HP through the mid/upper RPM ranges. You can
<see this with scan tools and also see it on the dyno. This doesn't
necessarily mean that big plates are bad, you just need to <take the
appropriate steps within the computer to adjust accordingly. If you don't
plan on going the custom route with <software, then smaller plates are better
(you need to keep the velocity up through the TB). If done correctly, the
factory plate <size will net you more HP and Torque throughout the powerband
with all after-market bolt-on equipment then larger plates will. <This
includes cam changes. We have been down both roads and it really doesn't
matter to me if I use larger or factory size <plates. The goal is to match
the work to the intended modifications.............
<
< Frank
<
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