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spuggy spuggy is offline
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Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 3,409
Originally Posted by parasailsam View Post
If indeed it is to high as Spuggy says, Should I S-can the turb idea until I have proper time/funds to make it right?
Seems a shame after it's all bolted on there. You do need to make it run properly and safely.

I say see what you can salvage for as little as possible, if you're strapped for cash.

You can't get rebuild kits for the factory wastegate anymore (at least, not readily, some are rumored to still exist), and the only aftermarket springs are even higher than stock.

However, there's a Tial 44mm wastegate that is a bolt on and is available with a range of springs - including 0.2 bar (2.9 PSI), 0.3 bar (4.35 PSI) & 0.4 bar (5.8 PSI). Ben (mb911) sells Tials.

(There's also very, very cheap Tial knock-offs on Evil Bay. I'm not sure I'd trust those to work at all, not to put too fine a point on it.)

0.3 bar with 9.3:1 (if that's what you have) would put you right @ 11:1 dynamic C/R according to that graph. Still kind of on the high side for a non-intercooled application (compare where the factory were at with 6.5:1), but at least it's on the chart...

One other thing; the way factory 930 gauges read, folks talk about the positive manifold pressure as the boost level.

What's not always clear from that figure is that you're gaining all that positive pressure over what would be manifold vacuum (e.g. on an N/A), before boost levels go over atmospheric.

This is somewhat more intuitive to 951 (944 turbo) pilots, whose factory boost gauges read 2.0 bar at full boost, when the car actually makes 1 bar manifold pressure over atmospheric...

To explain, see this gauge:

At idle, my car makes 20 In/Hg of vacuum. On the overrun in N/A mode (e.g. no boost. dump throttle shut), it makes closer to 30, IIRC.

On an N/A car, when you floor the throttle, the motor makes as much vacuum as it can (and that's what's sucking in the intake charge).

On a forced induction car, the turbo will spool and manifold pressure starts to rise.

So you can see that before you're making any positive pressure at all over atmospheric, PSI #1, you're already packing the cylinder head with as much extra gas as is represented by the manifold vacuum you would have had on an N/A car - which the turbo has already overcome.

~4-5 PSI would still be pretty useful - and noticable - extra grunt for climbing hills or passing until you can plan what you want to do next.
'77 S with '78 930 power and a few other things.
Old 01-26-2009, 10:18 PM
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