Hi everybody. I'm in the process of converting my '85 3/4 ton 4X4 suburban to TPI. I have a complete manifold, but the runners have a big ding in them. I will eventually be changing the engine to a 406 ci engine that I'm rebuilding. I'm not too worried about top end or HP, I'm after the low-end torque that the TPI gives compared to a TBI or LT-1. www.thirdgenresource.com has a good deal on SLP semi-siamesed runners right now. From my understanding, siamesed and semi-siamesed runners are better for top-end, with a noticeable loss of bottom end torque? Given the planned increase in engine size, would these be a good choice, or should I hold out for a set of conventional "large tube" runners? Thanks,

Jake Kooser

 

Stock runners will give you better low end torque than large tube, everything else being the same.

 

Here are a few facts to consider.

The stock MAF flows 530cfm, removing the screens adds 180cfm
for a total of 710cfm, removing the fins adds 40cfm for 750 total.

The stock throttle body flows 650cfm

The stock intake flows 575cfm, fully porting it will add 125cfm
for a total of 700cfm. The base and upper can be opened up
to flow way more than that but the runners can not.

A 406 motor with a stock profile cam and heads, at 4500 rpm will
require 450cfm, at 5500 it will require 515cfm, at 6500 - 625cfm

Formula = inches X rpm / 3456 X .80 for a "standard" motor.
Formula = inches X rpm / 3456 X .90 for a "race" motor.
The last factor being Volumetric Efficiency, how much air actually
gets into the cylinder. A mid 70's smog motor fills the cylinder
at about .70 or 70% efficiency where a well prepared race motor
will run up to 95% efficiency or possibly more.