Re: Is this a good deal?

[Matt S Bower <m.s.bower@cummins.com>]

Squash wrote:
> 
> I don't get this argument.  If your engine takes 750cfm to run at WOT/5000 rpm, and 500cfm at
> WOT/3500 rpm, how can a 750cfm TB be better than a 900cfm at anything but WOT/5000?  Anything
> under 5000 rpm and you are over-sized with a 750cfm unit, right?  Is a 900cfm unit at WOT/3500
> worse than a 750cfm unit at WOT/3500?  Or WOT/2000?
> 
> Squash

As some of the others said, it's a drivabillity issue more than many
others.  With the larger throttle for a engine you will use a lot less
throttle to get the same air flow into the engine, with this you lose
air flow control resolution.  It's like putting a 60# injector in
instead of a 30#. You can still put in the fuel but controlling the low
fuel ammounts gets to be more difficult the larger the injector gets. 
The other issue is on the air flow with an n/a engine is that
maintaining the negative pressure at the correct levels can keep the air
flowing through the ports after the piston reaches tdc were without the
restriction the manafold pressure is high enough that air flow can stop
when the piston reaches tdc and limit air flow.  As Bruce said back to
back testing is the only way to 100% sure on any aplication but going to
large can definately screw up driveabilty of a car imo.

Matt

corrections welcome.
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