[Diy_efi] Throttle before turbo on EFI

[Derek]

How's this for a idea? I'm not talking about your normal carbed "draw-through"
type of setup. Rather a intercooled large turbo (aprox T66) with a huge (105mm)
throttle body in front of it.

Let's not let the things associated with older draw-through applications be the
basis of critisim of the above idea. I know very well those problems.
Specifically they are vacum at the compressor inlet (due to insuffciently sized
carbs causing a pressure drop), and having to flow fuel through a compressor.
Those concerns won't exist in my case. With a 105mm throttle body, it's just as
big as a 4" inlet to the turbo, which is what I have. So at WOT, there should be
full atmospheric pressure at the compressor inlet. I'm still EFI, and use port
fuel injection, so no fuel will pass through the turbo or intercooler.

Advantages I can think of.
1. No blow off valve needed
2. Quicker return of boost between shifts (compressor doesn't slow down nearly
as much or decelerate suddenly)
3. Longer turbo life? I'm guessing here but the removal of harsh transients to
the compressor would put less stress on the shaft, bearings, etc.
4. Less pumping losses at cruise/idle? The turbine will no longer be pushing on
a compressor that is trying to push air that has no where to go (against a
closed or barely open throttle).
5. Quicker spool up? Won't the compressor be at a higher RPM in general
(cruise), making for quicker spool once the throttle is cracked?

Disadvantages. All of these are a big MAYBE. They may not be a concern at all.
1. Greater tendency for oil seal failure in turbo. Off throttle vacumn will be
sucking on the bearing. However if you think about it, the strain on the seal is
just the delta pressure between the oil pressure and the compressor pressure.
Having 15" of vacumn in the compressor is the same as extra 7.5psi or so of oil
pressure. So it shouldn't make a difference.
2. The generally higher compressor RPM will cause more wear. I'm not sure if
this is even an issue, or if the savings in transient wear will more than make
up for it.
3. Time to decel. The extra volume in the system may cause the engine to still
make power for a brief instant after the throttle is lifted, as the engine
gobbles up the air that in a regular system would shoot out the blow off valve.
In my case, I don't need to worry about this. My engine computer shuts off
fueling when the throttle is lifted, and above 1500RPM.

Feedback anyone?


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