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81 931 ready for new turbo and wastegate

 
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streetracer931  
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PostPosted: Tue May 21, 2002 11:13 am    Post subject: Reply with quote

Hi everyone

I've own my 931 for about 11 yrs now and I think it's ready for a fresh turbocharger and wastegate.

I found the wastegate not working about over a year ago. But I still drove it nicely till now, and no I haven't blown my motor yet. The fuel pump cuts it off when it should (@24psi). Turbo Data will rebuild my wastegate for $275.

Has anyone tried to use an adjustable boost controller from TurboCity? If I were to use this item, I should have my wastegate rebuilt to stock 0.75 bar boost right?

My turbocharger is boosting fine, but it leaks oil, so Turbo Data is rebuilding it with 944T intake turbine and "cut 10-deg off the exhaust blades" (don't really know what that means but I'm quoting his words) and machine the housing for the bigger wheel. All this suppose to get faster spin-up, which I think I'd like to have. Mod/Rebuild price $600.

Sound like a ok deal right? Let me know what you all think.

Thanks

PHIL
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Rick MacLaren  
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PostPosted: Thu May 23, 2002 2:07 am    Post subject: Reply with quote

Hi Phil,

Just a few points:

1. You said "Turbo Data will rebuild my wastegate for $275" That's probably not too bad an estimate, but it's quite easy to do it yourself. I did mine a year ago and it's still working just fine. Now, if they REMOVE and REINSTALL it for $295, I'd say that's a damned good deal, but if they're just doing the rebuild, and you have to pay/DIY the REMOVE and REINSTALL, it's only a fair deal at $275.

2. You said "If I were to use this item, I should have my wastegate rebuilt to stock 0.75 bar boost right?" I do NOT know that specific boost controller, but if it's a banjo bolt style controller, that might be a good course of action. I opted for a 1 bar boost spring, and counted on some depletion of boost from the existing vaccuum system (i.e., old hoses tend to be loose fitting and a tad leaky) and I pull only 12 PSI which is about 0.80 bar boost.

3. You said "My turbocharger is boosting fine, but it leaks oil, so Turbo Data is rebuilding it with 944T intake turbine and "cut 10-deg off the exhaust blades" (don't really know what that means but I'm quoting his words) and machine the housing for the bigger wheel. All this suppose to get faster spin-up, which I think I'd like to have. Mod/Rebuild price $600" That's a great price on a rebuild. But I'd wonder why they wouldn't build up the turbo to the performance specs of the 88 944 Turbo (S) equivalent. It might be worth your while to look into the feasibility of that...apparently the later 944 Turbos have a longer running boost, so it doesn't taper off as you accellerate. Also, you could build up the turbo using I think K 26 specs from an earlier 80 924 Turbo.

How severe is the turbo oil leak? And where's it leaking?

Mine blows out a wee black puff of smoke as it kicks in. I don't worry about it. A bit of oil there doesn't hurt anything. 944 Turbos do it all the time. And I'd wonder at a high performance car that doesn't blow off some oil occasionally. Maybe it's nothing to worry about.

Doesn't sound like they're ripping you off. But have they done it before? Anyone else who's had that job done you can talk to to see if the work and performance panned out?

[ This Message was edited by: Rick MacLaren on 2002-05-23 01:09 ]
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TroyDest  
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PostPosted: Thu May 23, 2002 5:26 am    Post subject: Reply with quote

As far as adding a boost controller and changing the wastegate spring: why not use the stock .5 bar spring? The lower spring force you have, the more adjustable range you have. The .5 bar spring will let you turn the boost down to that level if you need to, the stronger ones won't. I added a pressure line into the top of my wastegate and am using the stock spring. I love it. I can turn the boost up as far as I dare and can also turn it down to 7 psi on rainy days. -Troy
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larso  
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PostPosted: Thu May 23, 2002 4:19 pm    Post subject: Reply with quote

The only real advantage to the boost spring is that the wastegate will not seep as much during lower rpms, whereas the controller will still allow it to seep. In my opinion for reliabilty, the .5 bar boost spring with a controller is a better idea though. There's no point in having too much boost and sacrificing daily reliability (AHEM, RICK. BOOM)
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Rick MacLaren  
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PostPosted: Fri May 24, 2002 2:34 am    Post subject: Reply with quote

Good point Lars.

But I'm a bit confused. I thought the stock boost on the '80 924 Turbo was around 0.70 bar (circa 11 PSI) would be right, but the tech section states:

"...Porsche performance and racing circles, sells a 1.0Bar wastegate spring (stock 931 boost is 0.80Bar)."

This might be for the later models.

SO where did 0.50 bar come from?


[ This Message was edited by: Rick MacLaren on 2002-05-24 01:39 ]
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TroyDest  
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PostPosted: Fri May 24, 2002 9:37 am    Post subject: Reply with quote

Rick, check the itty bitty spec book. Page 42 shows USA cars should have .43 to .47 bar (6-7 psi).

As far as wastegate "seep" at lower rpm's, you can actually prevent that by installing a pressure relief valve in the line going to the bottom of the wastegate. It's commonly referred to as "creep" because the wastegate slowly opens in a range of about 3psi. Originally, mine would start to open at 4 psi and be fully open at 7psi. I ran the pressure line into my center console and teed off of it. One line goes to the top of the wastegate with a pressure regulator in it and one line goes to the bottom of the wategate with a relief valve in it. Now, I set the regulator for my desired max boost then set the relief valve to open just before that. The wastegate remains completely closed until I reach my desired boost level which comes on suddenly. Oh yeah, it's nice! The "boost enhancer" they advertise for the 951 is just a relief valve that works the same as mine. -Troy
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numbers  
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PostPosted: Fri May 24, 2002 9:50 am    Post subject: Reply with quote

Yep, stok boost on my '80 was 7 PSI read right off of the aftermarket boost guage I installed.
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larso  
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PostPosted: Fri May 24, 2002 2:40 pm    Post subject: Reply with quote

I'm really starting to like these terms; seep and creep haha. Good point about the controller. The only other thing I can think about is the stiffer spring causing the wastegate valve to pop closed faster for better boost hold during shifting, or when when jumping on and off the throttle. Reading with the discussion on the BOV, it probably woudln't make enough of a difference to notice anyway. With the boost controller, do you ever have spikes of boost come in..where it goes over the boost amount you wanted, then pops back down to your desired boost? Say when you are at WOT and accellerating hard, does the boost jump up to 1.1 bar for a split second and then pop back down to 1.0 right away? I would assume that there is some time delay when the wastegate has to open up.

Maybe the softer spring would help keep the spikes away (are there really any spikes or is the response fast enough anyway), since it's easier for the wastegate valve to open and close with a SOFTER spring, where as the stiffer spring might cause greater spikes.
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TroyDest  
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PostPosted: Fri May 24, 2002 8:47 pm    Post subject: Reply with quote

No. No spikes whatsoever get to the engine. I watch my VDO boost gauge very closely. The only spike I've seen is upstream of the throttle body when you slam the throttle shut between shifts. It doesn't really matter to the engine since it doesn't ever get past the TB. I'm sure it puts quite a load on the compressor wheel though. I've since relocated my boost gauge and overboost switch downstream of the TB. I still have the engine protection of the switch but it doesn't see the spikes between shifts. -Troy
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Rick MacLaren  
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PostPosted: Sat May 25, 2002 1:24 am    Post subject: Reply with quote

Oh jeeze.

I hope to God I haven't busted anything with the larger boost spring! Well I checked my plugs and there are no signs of detonation. And the motor still is strong. Some hesitation though. Still have the old boost spring too...installing that tomorrow, dyno'ing it with a wide range sensor setup to check air/fuel mix across the range.



[ This Message was edited by: Rick MacLaren on 2002-05-25 00:47 ]
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numbers  
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PostPosted: Sat May 25, 2002 8:53 am    Post subject: Reply with quote

As far as the seep/creep goes, I can't see where it will have much effect. Up to three grand, I have little or no boost. After the boost kicks in, I go from three to six almost instantly. So, unless the seep/creep problem is going to give me significant boost under three thousand, I don't think I would see much gain.
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Rick MacLaren  
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PostPosted: Mon May 27, 2002 2:42 am    Post subject: Reply with quote

Back down to 9 PSI now...had it out for a run this morning and it was great. Whew. Feels much smoother and no spikes.

[ This Message was edited by: Rick MacLaren on 2002-05-27 01:42 ]
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TroyDest  
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PostPosted: Mon May 27, 2002 7:14 am    Post subject: Reply with quote

Numbers, you will definitely feel a difference if you install a wastegate creep control. You maximum boost will be at a slightly lower rpm and it will get there quicker. It's not a huge difference, but it's a kick in the pants. I know how proud you are of your hardware store boost control, but...you might consider a valve from TurboXS. It uses the same boost signal restricting that you are already using and has the anti-creep built in. You just turn the adjustment to where you want your boost level and it doesn't open until it gets to that point which is also regulated. That way you don't have to add a separate pressure line to the top of the wastegate like I did. Of course you could probably just add a relief valve downstream of your boost valve.

Rick, let's hear more about your motor now that it's finished. You put a lot of work into that thing! Does it seem to rev quicker now that it's lightened? -Troy
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Rick MacLaren  
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PostPosted: Mon May 27, 2002 11:48 pm    Post subject: Reply with quote

I haven't really noticed quicker revving, but I have noticed that before the rebuild, I'd pump the clutch in 1st at 5500 RPM and I'd wait for the lag after shifting into second. Now, when I do the same, there's no lag.

I think I'm gonna go to a crank fire to speed things up all round. Yeah, I'll post pics soon...just tidying things up first...



[ This Message was edited by: Rick MacLaren on 2002-05-27 22:48 ]
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