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Boost vs CFM- Turbo rebuild planning stage
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Dave951M  
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PostPosted: Wed Jul 17, 2002 11:29 am    Post subject: Reply with quote

I found a guy here fairly local that specializes in rebuilding turbos and the price is very reasonable, like under $400usd worst case. From what he's telling me, the stock K26 is more than adequate for a 2.0 931. The only possible changes of any benefit on this turbo is increasing the cfm by changing the turbine. That being the case, there will be a point at which there are no real gains to be had. Does anyone have any experience with turbo upgrades and are the upgrades worth the time and effort over stock?
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Cbass  
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PostPosted: Wed Jul 17, 2002 2:08 pm    Post subject: Reply with quote

No experience, but I would think upgrading to a watercooled turbo with a turbo timer would be a big advantage.

I was looking at prices for Garrett turbos, and they are quite reasonable...
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John H  
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PostPosted: Wed Jul 17, 2002 3:47 pm    Post subject: Reply with quote

To fit a garret you require some mods to the adapt to the exhaust manifold. 931 is a triangular shaped flange and 3 bolts garret are square and 4 bolts. Also you need to massage the frame to fit the garrett.

The 924 Carrera GT used basically the same turbo as the 931 but had changes to the compressor wheel to alter boost characteristics. The Carrera GTR turbo used some of the parts out of the early K26 turbo (1979) and diffeernct compressor and compressor housing. You can still get the parts from KKK to change the early 931 turbo to the same unit as used by teh Le Mans cars. Cost a few years ago was around the US$1000 for the parts then rebuilding on top.
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Dave951M  
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PostPosted: Sat Jul 20, 2002 9:19 pm    Post subject: Reply with quote

The particluar mod to the turbo I was refering to is changing the compressor wheel. The shop owner tells me that it will up the cfm rating of the turbo significantly, but to go much further, more will have to be done to the hot side. What I would like to know is, has anyone done any flow bench work on the 931 head to utilize any extra cfm that would be available. I'm thinking at least port matching of the intake.
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TroyDest  
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PostPosted: Sun Jul 21, 2002 8:14 am    Post subject: Reply with quote

What you and I need are kkk compressor maps. I am about to have a turbine housing jet-hot coated and the turbo rebuilt. I have two extra turbos that I could use. The 931 k26's used several different turbine housings, compressor housings, and impellers. I'm not sure which combo would be best for me. Without compressor maps, I'm just guessing. Unfortunately, kkk does not advertise their maps. I have seen a few photocopies on Audi websites but they are not exactly the same as the 931.

Another item that would be nice is to know the A/R ratios for the kkk housings and I can't tell from all the numbers on them if they are even there. I can tell you for sure that there are larger turbine and compressor housings that cam stock on the 931. The turbine housings use the same turbine wheel but have different flange spacers to set the turbine depth into the housing. The larger turbine housing that I have is considerably larger and I think it will flow much better to give more top end, possibly with more lag at the bottom end. Same with the larger compressor housing and impeller.

If anybody knows where to find the kkk maps or finds them, please let us know. -Troy
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Rick MacLaren  
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PostPosted: Mon Jul 22, 2002 9:24 pm    Post subject: Reply with quote

CBass: You said "No experience, but I would think upgrading to a watercooled turbo with a turbo timer would be a big advantage."

Well, yeah and no. You COULD just get a turbo timer for $50 and let the car run a minute, or three minutes, or five minutes, depending on the setting you like best, and save your turbo that way too. I've been doing it with my early '80 car and it's far less expensive and just as useful for cooling (and for preventing 'coking' on the turbo) as a water cooled system.

Isn't a turbo upgrade kind of 'guilding the lily'? By that I mean, we don't actually use all the boost available to us with the stock 931 turbo (at least I don't on my '80 931). Not meaning to be a stick in the mud here, but wouldn't the same cash be more effectively spent on a digital ignition to USE the boost you've already got more effectively and efficiently? I know right now, if I had a $400 to a couple of thousand to spend on replacements and enhancements, I'd be putting it into ignition timing. Maybe a LINKS system or HALTECH.

Is your turbo baffed? Or is it working?


[ This Message was edited by: Rick MacLaren on 2002-07-22 21:27 ]
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kyhm  
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PostPosted: Tue Jul 23, 2002 6:32 am    Post subject: Reply with quote

Quote:
On 2002-07-21 08:14, TroyDest wrote:
What you and I need are kkk compressor maps
... Unfortunately, kkk does not advertise their maps ...


I've seen them posted somewhere, someone like Vaughn or Bob Dodd had 'em... I didn't snag a copy when they were, my bad, or I'd repost 'em...

Quote:
On 2002-07-22 21:24, Rick MacLaren wrote:
it's far less expensive and just as useful for cooling (and for preventing 'coking' on the turbo) as a water cooled system.


IIRC, the real advantage to a watercooled turbo is you can fit a timer-run pump to continue circulating coolant for a minute or so after switch-off... You don't have to leave the engine idling, but can lock up and go.
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Dave951M  
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PostPosted: Tue Jul 23, 2002 7:27 am    Post subject: Reply with quote

My turbo is toast, spews oil and the shaft on the hot side is a trifle bent. No mechanical damage, the shop tells me that this looks mostly like a heat problem. Knowing the propensity of most American drivers and turbo cars, somebody was driving it hard and just shutting it off. The reasons I was contemplating the upgrade were:

future expansion
"while I'm in there"
cost of upgrade only slightly exceeds the amount I budgeted for the rebuild.

Even if I don't do much else initially, I have the basis for a stompin' good engine. A turbo timer is also in the budget and is a must item. That should help with the coking problem.

Dave
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John H  
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PostPosted: Tue Jul 23, 2002 4:10 pm    Post subject: Reply with quote

Turbo timer is a must - I used to work in the earth moving industry and all our turbo charged equipment had to left idling for up to 15 minutes at the end of the day to cool down. Normally the guys would carry out some maintenance while they were running down.

Also my GT has had a timer on it since 1987 and it has yet to destroy a turbo - in fact it's stil the same turbo and it's was repalced at about 70,000 miles (before I brought the car) and the car's now done 260,000 plus miles. So I'd definitely recommend a timer and you have to use it.

As for a water cooled turbo - biggest problem is finding the room to fit it. The standard turbo is a tight fir and anything bigger and you'd be struggling to fit it in the space available
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Rick MacLaren  
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PostPosted: Wed Jul 24, 2002 4:30 am    Post subject: Reply with quote

Yeah I use a turbo timer and just set it to 30 seconds for a gentle trip to the store, 1 minute for a slightly hard run to work, and 3 to 5 minutes for temporary moments of insanity where I have to check my rearview for clouds of smoke and flashing lights...

So Dave, what difference do you expect the bigger turbo will make on the car? Is it the duration of boost you wanna change? (i.e., have the car run longer at full boost) Or is it a reduction in the time needed to achieve full boost that you desire?



[ This Message was edited by: rick maclaren on 2002-07-24 05:09 ]
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Wed Jul 24, 2002 4:40 am    Post subject: Reply with quote

Would you guys care to remind us where and for how much the turbo timers are?

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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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Dave951M  
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PostPosted: Wed Jul 24, 2002 8:20 am    Post subject: Reply with quote

Rick- I'm shooting for both. Bad news from my turbo guy- only the hot side housing is salvagable. I have a three bolt turbo that I was saving for a core. Is the three bolt the same as the four except for the number of bolts, ie is the compressor side the same? Actual turbine isn't an issue since it will most likely be replaced. Temporary setback.

My goal with this car is to build a very solid base for a future track car, keeping it streetable somewhat until that time. From my hotrodding days over 20+ years ago, I learned early on, get what you might need later, rather than rebuild everything. Some parts will be way overboard for street use, but they will be there for the day when the track gets first priority.

Turbo Timers- Greddy, Apexi and others, do a google search using the terms turbo timer
You'll get about 56k hits.

Dave

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Dave Miller
82 931 Guards Red
86 951 Guards Red

[ This Message was edited by: Dave951M on 2002-07-24 11:38 ]
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Rick MacLaren  
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PostPosted: Thu Jul 25, 2002 1:08 am    Post subject: Reply with quote

I've got an HKS turbo timer...

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Cbass  
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PostPosted: Thu Jul 25, 2002 6:15 am    Post subject: Reply with quote

I have no idea what the state of the turbo is, I haven't even looked at the engine yet. I'm planning a worst case scenario, since the whole thing will have to be rebuilt anyways. I'm not looking for crazy boost levels, I think 10-12 lbs@8.5:1 static compression is more than enough for a daily driver.

My concerns are reliability and cost. For a little more than the cost of a rebuilt K26 oil cooled, I could upgrade to a T3($600USD or so), and an electronic wastegate tied into a electronic boost controller(cheap, as in Saab APC).

Rick, the ignition would also be a priority. Tell me more about this Links system. Can it control the wastegate too?
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John H  
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PostPosted: Thu Jul 25, 2002 10:06 am    Post subject: Reply with quote

Quote:

On 2002-07-25 06:15, Cbass wrote:
My concerns are reliability and cost. For a little more than the cost of a rebuilt K26 oil cooled, I could upgrade to a T3($600USD or so), and an electronic wastegate tied into a electronic boost controller(cheap, as in Saab APC).

Cbass, I'm not trying to be rude but read the posts carefully - the big problem with a T3 or any other turbo other that the K26 designed for the Porsche 924 Turbo Is fitting it. There is limited room and if you go for a bigger turbo you have to massage the chassis rail with a big hammer to get ot to fit and you have to modify both the exhaust housing and the exhaust as it exits from the turbo.
A K26 that is well maintained will last a long time.
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