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Join Date: Jul 2003
Location: Glorious Pac NW
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Originally Posted by JakobM View Post
Ok, so what I read is you cannot get more boost than spring rate, right? What is your spring rate btw?
Exactly; Tial F46 with .5 bar spring ~= 9 PSI manifold.

Originally Posted by JakobM View Post
And you let the WG open at 130kpa, I assume this is 1.3 bar and not 0.3 bar, correct?
Motec say to begin controlling boost early, rather than switch on/step in as it reaches desired boost; just their stock advice. Which seems to match what you're saying elsewhere about early crack of the WG.

kPa is absolute, rather than relative to manifold; so both my baro and MAP sensors read 100 kpa before I start the car - 130 kpa is 1.3 bar absolute, or .3 bar over manifold.

So boost control starts at 130 kpa, target boost set for 200 kPa (1 bar boost), and overboost cut for 240 kPa.

I see boost pressure spike briefly past 10 PSI-ish on the mechanical VDO gauge - maybe 11 PSI - and then settle back a little. Motec logs 160, 162 kPA on the MAP sensor.

162 kPa = 23.49 PSI, - 14.7 == 8.79 PSI over atmo.

0.5 * 14.7 = 7.35.

Those two result numbers are so close it doesn't look like coincidence to me.

Just behaves pretty much exactly like there's a .5 bar spring installed (which there is) and no boost control at all (which there should be/it worked at one time)...

I might just fit a .8 bar spring...

Originally Posted by JakobM View Post
Such issue is typically due to one of two reason from my experience - or both: 1) too high back pressure and not enough force to hold WG valve closed, or 2) leak(s) some where else that vent the air to the birds. Other factors can play in, but these two are the most common

You will properly get earlier boost when you fix the issue
I'm going to have to read your detailed suggestions carefully/several more times yet, I think. Many thanks!

I'm leaning strongly raising the spring rate; just to check that turbo (K27/HF)/exhaust (Euro muffler) behave as expected/as they did when car was CIS; eliminating things that aren't the problem should make it easier to understand the problem. And if necessary, I can run a stiffer spring and not bother with control.

When you say "cover plate" for the WG - you mean a bolt-up flange with no hole to completely disable the WG right? Wheee..

Connecting the top port is an easy test - and there's guys selling ready-made test caps on Flea Bay for $40, so that's no biggie. Lack of compressed air in my garage would be more of an challenge, really...

So; you bolt the test cap up to the turbo inlet and pressurize with all hoses/gadget connected. I'm guessing this allows you to pressurize charge side against the throttle plate, look for leaks/see how fast that bleeds off, right?

Could be very useful, that...
'77 S with '78 930 power and a few other things.
Old 07-20-2019, 08:56 PM
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