Smoother without the APC!

[trackside]

I have disconnected the APC on my 91T16 to run on base boost as I'm sure I'm starting to hear some pinion noise - Ah well not bad for 175k miles. However I now find that the car is much nicer to drive - especialy the power pickup between shifts. With the APC working there was a definate hesitation in the power which would make shifts a bit lumpy. Now it's silky smooth and it's possible to get a nice smooth continuous power delivery while changing up through the gears. What gives? - something to do with some boost being bled off while the solenoid is switched on? BTW whats the going rate for a recon gearbox (fitted) these days in the UK - pref London area?

[Jezzadee]

Most fitted recon gearboxes seem to be about £600 round this way, although London may be more expensive. I did hear of someone getting one for £500 but it was a total bodge.

[saab90089]

What kind of boost were you running that you are getting pinion noise? My dad never had that till he learned how to mess with the APC. He started messing with the APC (holding 17psi) and about 15K later he got the pinion noise.

Ian

[Alex]

I've been running without the APC for several years now, though not quite down at stock base boost You also get a much smoother torque curve when the influence of the APC is removed. The boost always cycles slightly due to the closed loop boost control (this is much more visible on an external boost gauge as the in-dash one seems to have a fair amount of damping.

[trackside]

I was running about just under 1 bar - about 1/3 into the red and had gradualy reduced the p so I was getting a pretty smooth boost without any sudden surge. I understand the boost fluctuation with solenoid cycling but I'm getting smoother shifts well before boost and at lower revs than the APC starts to take over - weird. Anyway I'll keep it this way till I need a new box.

[deniss]

So if you run without APC, you just have to crank up base boost up high?

What's wrong with APC control then? Does boost rise rate need to be adjusted or something? There are two things I don't really like about the way the power is delivered in my SPG:

1) shift-up from 1st to 2nd is *always* sluggish, and I have no idea how to make it better except by keeping the engine really wound up high in between the shifts, otherwise when you let the clutch out, it takes a while to get going... But I don't really want to do that for the sake of my transmission...

2) there isn't a whole lot of pick-up until it gets to like 2700-3000 RPM, at which point the boost comes on so rapidly that it's a little challenging to control the car and keep the tires from breaking loose. I can't imagine this being healthy for the transmission either. I am beginning to develop an intermittent whine in the 5th (178,000 miles on original tranny), but I'd still like for my tranny to last at least until the summer.

[Jezzadee]

Both the things you describe sound typical. The 1st / 2nd gear sluggishness is designed to save your tranny. The way round it is to fit an MBC - just press the pedal and hang on. You do have to develop good right foot control with an MBC. The boost coming in with a bang at 3000 rpm sounds like a typical Garrett turbo - the Mitsubishi, being smaller, spools up earlier and less suddenly. Mine (quite a few mods) comes in at 2500 rpm which is about right for me - used to be about 1750 rpm, but that got a bit mad in traffic

[trackside]

You can crank up the wastegate but you may get surge and creep at high rpm boost as it won't be able to regulate the boost anymore. Also try moving the actuator pressure hose to after the IC but before the throttle as this takes away the pressure loss across the IC from the pressure signal the actuator gets.
"there isn't a whole lot of pick-up until it gets to like 2700-3000 RPM" - quite normal for a Garrett turbo - later Mitsubishi turbos spool up much earlier in the revs and are much more civilised as everyday drivers. To get more low end power you may want to set your ignition more advanced, get a smooth flowing downpipe, check your wastegate is not weak. To stop the surge - turn down the P on the APC.