924T turbo.....turbo rebuildable?
08-02-2009, 11:38 PM
#16
Three Wheelin'
I dont think hes made one for a while, I've had mine for ages. He used to tell people to send him the core + cash. He doesnt post as frequently on 924board unless he's chiming in on questions like these. I talk to him mainly on email. He had a genuine turbo from a CGT not long back.
08-03-2009, 08:56 AM
#17
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The most logical way to proceed is not to replicate 30-yr-old technology, but use the latest available to do better. So that means using a 951 IC, instead of the crappy CGT IC, and re-doing all the engine management to Megasquirt or the like. Same deal with suspension; why waste time and money finding the original bits when you can just use the commonly-available aftermarket parts?
08-03-2009, 11:53 AM
#18
Instructor
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Certainly the best route to an intercooled 931 is to put a 951 IC in the nose... however its not exactly bolt-on.
http://www.blankemeijer.com/
Go to "Foto's" and click on the 924 Turbo to see a photo history of a 951 IC install. And even as proper as this install was, ideally he ought have created a new intake that was similiar to the 924GTS intake and EFIed the engine or installed a digital WUR.
At any rate adapting a 951 IC for use on a 931 isn't trivial and will quickly cost a few thousand dollars.
http://www.blankemeijer.com/
Go to "Foto's" and click on the 924 Turbo to see a photo history of a 951 IC install. And even as proper as this install was, ideally he ought have created a new intake that was similiar to the 924GTS intake and EFIed the engine or installed a digital WUR.
At any rate adapting a 951 IC for use on a 931 isn't trivial and will quickly cost a few thousand dollars.
08-03-2009, 09:55 PM
#19
Three Wheelin'
For the rules for the racing the cgt racecar was in, i dont believe EFI was a permissible mod otherwise i think it would have been done.
this was my stepdads setup. I couldnt persuade him to coldbox the airfilter while he owned it.
08-04-2009, 11:15 AM
#20
Former Vendor
Back to the original question, rebuilding the turbo is definitely doable. To follow up on a couple of comments above, I highly recommend working with John Walzel at Majestic Turbo. I've worked with him on two of the aforementioned hybrid 931+951+964 turbos (931 hotside, 951 center section, 964 cold side). Here are some related threads:
Bolt-in off-the-shelf water-cooled turbo for 931
Hybrid Turbo for Ideola's Ultra Wide Body 931
K26/6 to K27/6 Turbo Mod WITH pics
I'm going to have to disagree with an earlier comment about the S2 being more desirable than the S1. Regarding the turbo itself, the S2 is smaller, so boost comes on sooner, at the expense of running out of breath higher up in the power band. So a blanket statement that the S2 is better isn't accurate. It depends on what your goals are. On a related note, the S2 has an ignition system called DITC: Digital Ignition Timing & Control. It takes a boost signal and an intake air temp signal and a crank position sensor to digitally control timing. On the surface, this is a Good Thing. Unfortunately, DITC is notoriously finnicky, and replacement crank sensors are NLA. Period. As a consequence of its age-related unreliability, many S2 owners have retrograded the ignition system to the earlier S1 setup, while retaining the S2 intake, throttle body, exhaust manifold and turbo. My "941" is a good example of this arrangement.
But again, back to the original topic. There are two key issues to be aware of, both related to the oddball 3-bolt flange.
First, the exhaust manifold. There are two US versions of the exhaust manifold. The S1 version (part number 931 111 594) is prone to cracking due to a variety of engineering realities that conspire against it. A combination of poor bracket design, bracket hardware that has a tendency to back out on its own if it's not safety-wired, and the pulse collisions common to log style manifolds result in the S1 manifold almost always cracking on underside of the #1 or #2 runners. I've never seen an uncracked S1 manifold. Ever. So the first thing to check is the last three numbers of the exhaust manifold, easily visible from the top of the car just beneath the valve cover gasket. If the last three numbers are 594, you will need to start searching for a S2 manifold. The S2 manifold (part number 931 111 596) has reinforcements in the casting on the underside of all runners. I have never seen a cracked S2 manifold, but it does happen occasionally. You're only option for finding one if you need one is to source one from a donor S2 (1981-82) 931. Here's some pix and a related thread:
Crack on underside of #2 runner, S1 manifold:
Detail of reinforced S2 manifold:
Now, secondly, the biggest problem with the 931 turbo unit is the hot side (i.e. turbine) housing. As mentioned, it has a completely unique three bolt flange. Quite simply, unless you have $2-$3K to spend on a custom fabricated manifold, you really have no option for getting away from the 3-bolt flange. Some guys have attempted making adapters, but when you do this, it changes the required geometry for mounting the turbo. The bracket that attaches the turbo to the block has an integrated oil feed line. Plus, the turbo is already very low in the car, and experiences oil drainage problems (which I'll discuss more in a moment). Using such a flange adapter will only exacerbate that issue. So bottom line, you want a S2 manifold, and you need a good 931 hot side housing. The problem is, every single hotside housing I've ever seen has been cracked, without exception. Here's photos of where the typical cracks develop:
And a related thread.
Bottom line, you probably won't find an uncracked 931 turbine housing. The only way you can tell if it's usable is to remove the housing, clean it thoroughly in the blast cabinet, and inspect the interior scroll area very carefully. The damage is typically NOT visible from the outside, you MUST disassemble the unit. That said, IF (and it's a big if) the turbine wheel is undamaged, that is an indication that the turbine is probably OK to reuse. If the turbine wheel IS damaged, then the housing is probably not usable because the damage almost certainly came from the turbine fins interfering with the raised surface of the cracks, which are susceptible to deforming when heated up.
So, if you buy this car, your first step is to find out what manifold you have, and then to promptly remove and disassmble the turbo so you know whether or not you can reuse the turbine housing, or if you'll have to find a donor. They are unobtanium from Borg-Warner (KKK). John Walzel told me that they could probably be welded and remachined, but that given the design and material, it's really a crap shoot as to whether or not a repaired unit would hold for any length of time. His advice to me was to find units that weren't too badly cracked to be reused. I have two such rebuilt units on the shelf, with a third donor on the bench should the need arise.
Now, finally, the turbo drain issue. As mentioned, the turbo sits low in the car, and oil drainage was a problem. There were a series of iterations on the drain line and the breather system. On the final iteration, the drain line has a funky square box that performs an air-oil separation function, along with a tee back into the so-called 931 head "breather update." If you're looking at an S1 car, it is highly likely that you have the breather update, but a total crapshoot as to which version of the oil drain is there. Suffice it to say, you want the latest S2 version of both the drain line and the breather update, OR you can investigate removing the stock setup and plumbing in a dedicated oil scavenge pump.
Bottom line in all of this, you have to REALLY want a 931. Personally, I love these cars, and I'm undaunted by all of this, as I have been fortunate to acquire many spares of all the unique and oddball things that could possibly need replacing. But for a 931 neophyte, it can be a rude awakening, and frankly has led to the rapid demise of more than one 931. I'd strongly suggest that you peruse the monster Performance Upgrades thread on 924board.org. There is a ton more to be said on the intercooling topic, but I'll save that for another post. In the meantime, here are a couple of additional very pertinent threads with more info on the nuances of the 931:
All the KKK / K26 info you ever wanted to know
Impact of 931 Breather Update
If you have follow up questions, feel free to post here or to send me a PM. Good luck!
Bolt-in off-the-shelf water-cooled turbo for 931
Hybrid Turbo for Ideola's Ultra Wide Body 931
K26/6 to K27/6 Turbo Mod WITH pics
I'm going to have to disagree with an earlier comment about the S2 being more desirable than the S1. Regarding the turbo itself, the S2 is smaller, so boost comes on sooner, at the expense of running out of breath higher up in the power band. So a blanket statement that the S2 is better isn't accurate. It depends on what your goals are. On a related note, the S2 has an ignition system called DITC: Digital Ignition Timing & Control. It takes a boost signal and an intake air temp signal and a crank position sensor to digitally control timing. On the surface, this is a Good Thing. Unfortunately, DITC is notoriously finnicky, and replacement crank sensors are NLA. Period. As a consequence of its age-related unreliability, many S2 owners have retrograded the ignition system to the earlier S1 setup, while retaining the S2 intake, throttle body, exhaust manifold and turbo. My "941" is a good example of this arrangement.
But again, back to the original topic. There are two key issues to be aware of, both related to the oddball 3-bolt flange.
First, the exhaust manifold. There are two US versions of the exhaust manifold. The S1 version (part number 931 111 594) is prone to cracking due to a variety of engineering realities that conspire against it. A combination of poor bracket design, bracket hardware that has a tendency to back out on its own if it's not safety-wired, and the pulse collisions common to log style manifolds result in the S1 manifold almost always cracking on underside of the #1 or #2 runners. I've never seen an uncracked S1 manifold. Ever. So the first thing to check is the last three numbers of the exhaust manifold, easily visible from the top of the car just beneath the valve cover gasket. If the last three numbers are 594, you will need to start searching for a S2 manifold. The S2 manifold (part number 931 111 596) has reinforcements in the casting on the underside of all runners. I have never seen a cracked S2 manifold, but it does happen occasionally. You're only option for finding one if you need one is to source one from a donor S2 (1981-82) 931. Here's some pix and a related thread:
Crack on underside of #2 runner, S1 manifold:
Detail of reinforced S2 manifold:
Now, secondly, the biggest problem with the 931 turbo unit is the hot side (i.e. turbine) housing. As mentioned, it has a completely unique three bolt flange. Quite simply, unless you have $2-$3K to spend on a custom fabricated manifold, you really have no option for getting away from the 3-bolt flange. Some guys have attempted making adapters, but when you do this, it changes the required geometry for mounting the turbo. The bracket that attaches the turbo to the block has an integrated oil feed line. Plus, the turbo is already very low in the car, and experiences oil drainage problems (which I'll discuss more in a moment). Using such a flange adapter will only exacerbate that issue. So bottom line, you want a S2 manifold, and you need a good 931 hot side housing. The problem is, every single hotside housing I've ever seen has been cracked, without exception. Here's photos of where the typical cracks develop:
And a related thread.
Bottom line, you probably won't find an uncracked 931 turbine housing. The only way you can tell if it's usable is to remove the housing, clean it thoroughly in the blast cabinet, and inspect the interior scroll area very carefully. The damage is typically NOT visible from the outside, you MUST disassemble the unit. That said, IF (and it's a big if) the turbine wheel is undamaged, that is an indication that the turbine is probably OK to reuse. If the turbine wheel IS damaged, then the housing is probably not usable because the damage almost certainly came from the turbine fins interfering with the raised surface of the cracks, which are susceptible to deforming when heated up.
So, if you buy this car, your first step is to find out what manifold you have, and then to promptly remove and disassmble the turbo so you know whether or not you can reuse the turbine housing, or if you'll have to find a donor. They are unobtanium from Borg-Warner (KKK). John Walzel told me that they could probably be welded and remachined, but that given the design and material, it's really a crap shoot as to whether or not a repaired unit would hold for any length of time. His advice to me was to find units that weren't too badly cracked to be reused. I have two such rebuilt units on the shelf, with a third donor on the bench should the need arise.
Now, finally, the turbo drain issue. As mentioned, the turbo sits low in the car, and oil drainage was a problem. There were a series of iterations on the drain line and the breather system. On the final iteration, the drain line has a funky square box that performs an air-oil separation function, along with a tee back into the so-called 931 head "breather update." If you're looking at an S1 car, it is highly likely that you have the breather update, but a total crapshoot as to which version of the oil drain is there. Suffice it to say, you want the latest S2 version of both the drain line and the breather update, OR you can investigate removing the stock setup and plumbing in a dedicated oil scavenge pump.
Bottom line in all of this, you have to REALLY want a 931. Personally, I love these cars, and I'm undaunted by all of this, as I have been fortunate to acquire many spares of all the unique and oddball things that could possibly need replacing. But for a 931 neophyte, it can be a rude awakening, and frankly has led to the rapid demise of more than one 931. I'd strongly suggest that you peruse the monster Performance Upgrades thread on 924board.org. There is a ton more to be said on the intercooling topic, but I'll save that for another post. In the meantime, here are a couple of additional very pertinent threads with more info on the nuances of the 931:
All the KKK / K26 info you ever wanted to know
Impact of 931 Breather Update
If you have follow up questions, feel free to post here or to send me a PM. Good luck!
08-04-2009, 02:23 PM
#21
Race Car
Thread Starter
Thank you so much for that in depth analysis of the turbo's, very helpful. Sounds like a bit more than I want to deal with right now. I know not much about the 931's (even less of turbo ins/outs) and this is the type of stuff I need to learn first. I learned as I went with my 993 and its been the greatest car, and I have a little hobby business because of it so...... 
You've been a big help, feel free to keep it up, very interesting stuff.
You've been a big help, feel free to keep it up, very interesting stuff.
08-04-2009, 06:42 PM
#22
Formula One Spin Doctor
Rennlist Member
Rennlist Member
I found a 924T FS, guy says the Turbo was going bad (not sure what that means exactly) but not knowing much about them, are they rebuildable or is there a better alternative to the original? Thinking about creating a 924 CGT or something close to get my 993 off the track
Whats silly money? I drive a 993 C4S now and was thinking of getting on track in something cool and for much less. Thought a GT clone would be interesting and much cheaper. Would 10k be silly? And once done wouldn't it be one hell of a cool car (if done right)? Some of the junk out there is being sold as if it were gold. If I found a $1,500 beater and resto modded it, I figured I'd have a cool little beast.
08-04-2009, 07:51 PM
#23
Race Car
Thread Starter
Yeah sounds that way! My next choice is a little 914.....love those things 
I really just want to get intimate with my welding machine and do a little body work, not re-engineer a propulsion system, but this is great stuff to know. I'm itching for a little project car and the 914 fits the bill.
I really just want to get intimate with my welding machine and do a little body work, not re-engineer a propulsion system, but this is great stuff to know. I'm itching for a little project car and the 914 fits the bill.
08-05-2009, 08:45 AM
#24
Addict
Rennlist Member
Rennlist Member
Yeah, 931's won't give you much welding work, just a lot of wiring etc... great fun, it's my DD all summer long, but it's not for everybody...
