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Britain Smith's Avatar
 
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Location: Cornelius, OR
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Porsche 924 Racecar Build Thread (AAN Conversion)

We have been racing a 1980'ish Porsche 924 Turbo (931) in Lemons, Chumpcar, and now Lucky Dog going on about 6 years now. The car has gone thru several iterations of changes, from the stock 931 configuration, to a built 931 engine, to now an Audi AAN (which is awesome). I have build custom suspension, added unique cooling solutions, and overall made the car damn fast. Here is my attempt to document some of the things we have done, please let me know if you have any questions.

We started with a pile of parts that was a 1980 924 with 931 drivetrain and suspension. This car was a pile of $hit but it came with 2 engines, 2 transmissions, and a bunch of other crap.


We determined really quick that the car was too far gone to be a good candidate and a friend just happened to hear about the project and had a solid 1978 chassis sitting around that he donated to the cause.


We quickly stripped the chassis down and added a Hanksville Hotrod 944 cage.



The first engine we built for this car used stock internals, however we ran with a Subaru TD04 turbo to get a water-cooled turbo in there, a 944 intercooler, and complete EFI set-up on a Microsquirt ECU. We also used an S2 intake manifold which had the round instead of oval throttle body and a Subaru throttle body to get the TPS sensor.




Here is the car at the first track day...




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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 12:35 PM
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Not to bore you with the misc issues and problems we encountered over the years (cracks in the frame, blowing up engines, melting transmissions), I will show some of the good stuff.

At a race in our first year we tagged the wall with the front right corner. This later propagated into cracks in the front framerail and suspension pick-up points. To address this, I cut open the front frame rails in the wheel wells and plated this area to reinforce the mounting points.







At another race, we had a front control arm failure...in response I plated a set of control arms top and bottom to reinforce them. Never failed again.





There is a junkyard across the street from The Ridge Motorsports park here in the PacNw, they just happened to have a god-awful looking 924 in there with a hideous fiberglass bodykit. I made a deal for the front bumper set-up and went thru making a valance for our car. I wanted to keep the aluminum front bumper for crash protection. I sectioned it, removed the headlight areas, made a plug and laid one up to fit it to our car.





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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 12:48 PM
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At another race, we had a spin on the front straight at PIR in the wet which took out the back half of the car...you could actually see the profile of the concrete barrier. That same day we drove up to that same junkyard and cut off the back half of a rust 924 and loaded it into my truck. Over the winter we cut all of the spotwelds and fitted a new rear end to the chassis. Fortunately there was no damage ahead of the rear cage bars so it was fairly straightforward.









While we were there, I picked up a used 911 cup car wing from Rothsport Racing in Tigard and proceeded to fab some uprights to mount it to the back of the chassis. This set-up works great because you can still take off the rear hatch and it is solid.






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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 12:54 PM
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Ok...so now we get into the good stuff. After several years of blowing up 931 engines, we finally decided to do an engine swap to an Audi AAN 5-Cylinder. These engines were used in the '93-ish Audi UrS4's and actually bolt to the 931 bell-housing...sort'of.

This was the first test-fit of the Audi engine in the 924 chassis. I fabricated some beefy engine mounts and the engine is solid mounted to the chassis.




We had to relocate the throttle body to clear the master cylinder and we made it point nicely to the 944 intercooler. We are still using a Subaru turbo, however we went to a slightly large VF-39 used on '05 2.5L STI's.


One fitting issue was the oil pan around the cross-member. Lots of cutting and fitting later, new oil pan fabricated.




Here is the final iteration of turbo fitting, plumbing, etc. Everything not needed to run the engine has been removed to simply things. We run a cast manifold from 034 Motorsports and then an up-pipe from that to the turbo with an external Turbosmart Wastegate. You can also see the smaller alternator we attached to the waterpump mounts.



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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:06 PM
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Britain, always wanted to hear about the build of your car, especially the AAN swap.

Interesting that you started with a 1978 924; that was our first car we started in Chumpcar with back in 2011.

We are now on the 1987 924S, since that one can also run 944 SPEC.
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Old 06-21-2019, 01:13 PM
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Staying on the topic of the engine, the bellhousing set-up is what makes this conversion a reality. I run the Porsche 931 bellhousing with an Audi 01E spacer which is about 3/8" thick. There is some material removed here and there to provide clearance and give provisions for the Audi starter and the flywheel sensors.



The next challenge was converting from the 931 pull-center pressure plate to the Audi style push-center pressure plate. For these I had Clutchmasters make me a custom annular throw-out bearing and mounted it on a plate in bolted into the bellhousing.



To make everything fit, I run the early 7A single mass flywheel with a AAN pressure plate which is thinner. You can see that the clearance is tight, but it all fits. We also run a custom solid-center friction disc from Kennedy Engineering.





__________________
'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:15 PM
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While the engine swap was underway, we decided to tie-in the front shock towers to strengthen the chassis. This wasn't too bad as I ran the tubing thru the frame-rail areas and plated the shock towers.





Here is the chassis going back together. I had the wiring harness paired down to only what is needed by Marc at EFI Express and ran it out the firewall in a similar place that it was routed on the Audi to make sure everything reached.



The engine almost all back together. We went with a 7A valve cover which is a lower profile than the AAN valve cover, however we had to machine the holes to fit the Audi coil-packs.





With the new engine comes a new fuse panel to support the needed electronics.



Here is everything back together and the car ready to run on the track.



__________________
'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:24 PM
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With the extra power comes upgrades to things like transmissions and brakes. We run the G31 transmission which has been great. It is plenty strong and keeps the weight forward of the rear axles. One way we keep it alive is keeping it cool. We previously ran an electric pump but it eventually stopped working and we always forgot to turn on the switch...therefore I built a mechanical pump that runs off the rear axles output flange. This goes to a cooler in the rear of the car and feed by a NACA duct in the quarter window.





Another thing to deal with is brakes. We previously ran 944 turbo calipers which worked fine...however we wanted more. I got a set of Boxster calipers and proceeded with make my own brake hats with a set of Coleman rotors from Clint at Rebel Racing. I first mocked them up using some parts I printed on my 3D printer.







Once that was confirmed, I had a friend of mine do some quick FEA and cut them on his CNC.







__________________
'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:33 PM
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We also upgraded to a bad-ass water-cooler wastegate from Turbosmart. This thing is awesome and works consistently over a 8hr race where we previously were getting failed diaphragms due to the heat. These are fed from some 3D printed ducts that I mounted in the turn-signal locations.



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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:38 PM
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After years of abuse and another hit in the front right corner, the frame rail had enough and actually split down the seams.





To stop the madness, I cut-off the front framerails back to just before the shock towers. I then plated the framerails and proceeded to built an entirely removable front-end that holds the radiator, intercooler, bumper, splitter, etc.









Also built a entire set of ducting to direct air to the radiator and intercooler.

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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:46 PM
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This past year we were leading a race at Thunderhill when we got tagged in the rear corner and actually separated the torsion tube from the chassis. It torn the mounts off the sheetmetal and the entire thing was being held on by the top bolts on the spring plates. To fix it, I decided we would go to a coil-over set-up and I built some 935 style spring plates with 944 aluminum control arms.

I started by cutting out the rear seat pan.



I sources some heim-joint hardward from Clint at Rebel Racing and cut the stock spring plates to make it all match the stock parts. I machined a puck to weld into the rear torsion carrier and raised both the inner and outer pick-up points by 1".











To cope with the address stress of coil-overs, I reinforced the rear frame rails by plating the suspension pick-up areas and tying them into the cage. I also welded the torsion carrier into the chassis.







__________________
'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 01:57 PM
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To go along with the rear suspension changes, I built a set of tubular control arms for the front.





I also welded on some ball joint extension to the hubs to match the 1" raised pick-up point in the rear. This also required extending the tie-rod to cure the bump-steer and I built some heim-joint tie-rods.









I also built some billet caster block mounts and a brace to go in-between the two to tie everything together.



With the raised hubs, I started running short on shock travel and wanted to address it before hitting the track. I cut a set of raised mounts on the waterjet and welded them in place. These were set-up to accommodate a set of camber/caster plates.



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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 02:29 PM
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To further help with air extraction, I fabricated hood vents and side vents which have helped out immensely. I have sense made a lot of these in various sizes for all sorts of cars.







Here is how the car sits today.

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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-21-2019, 02:32 PM
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Wow. Everything very nicely laid out - and you even left plenty of room for the Cruise Control Motor !!!

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Old 06-21-2019, 03:50 PM
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Ha...cruise control would be nice on the track...set it and pass everyone
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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-26-2019, 08:37 AM
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Nice work.

You should have those AL hubs analyzed regularly. Those old cast AL hubs aren't great.

If those are 986 non-S Boxster brakes, they are the same piston sizes as 944 Turbo/ 968 brakes. The only change is to a radial mount and monoblock design.

The removable carrier is cool. That's what Audi has used for a while (I think the AAN cars may predate it).

You've clearly put a lot into the car, very cool. I like the development with the raised rear pickup points and 935-style adjustable spring plates.
Old 06-27-2019, 08:00 PM
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Quote:
Originally Posted by FrenchToast View Post
Nice work.

You should have those AL hubs analyzed regularly. Those old cast AL hubs aren't great.

If those are 986 non-S Boxster brakes, they are the same piston sizes as 944 Turbo/ 968 brakes. The only change is to a radial mount and monoblock design.

The removable carrier is cool. That's what Audi has used for a while (I think the AAN cars may predate it).

You've clearly put a lot into the car, very cool. I like the development with the raised rear pickup points and 935-style adjustable spring plates.

Yeah, we check all the parts regularly and have thrown away a set of hubs after a few season of use.

The calipers are 996/Boxster-S. The monoblock design, larger pistons, and increased pad surface area is very noticeable on the track.

Which removable carrier are you talking about? The cross-member under the engine or the whole front sub-frame?

Thanks on the suspension...the 935 style rear spring plates greatly improved the track characteristics.

-Britain
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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 06-28-2019, 10:25 AM
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Quote:
Originally Posted by Britain Smith View Post
Yeah, we check all the parts regularly and have thrown away a set of hubs after a few season of use.

The calipers are 996/Boxster-S. The monoblock design, larger pistons, and increased pad surface area is very noticeable on the track.

Which removable carrier are you talking about? The cross-member under the engine or the whole front sub-frame?

Thanks on the suspension...the 935 style rear spring plates greatly improved the track characteristics.

-Britain
I was referring to the removable front clip that you now bolt to the frame rails. Some modern cars have that, although the attachment point is not as far back. Interesting setup. Makes servicing easier.

I think Kokeln used to make similar spring plates. They also made a torsion bar carrier (for torsion bar delete), but don't recall if it incorporated raised pickup points. (Some of there stuff was good, some of it built like garbage).

Do you get those spindles annealed or x-rayed after welding them?

Last edited by FrenchToast; 06-29-2019 at 04:14 PM..
Old 06-29-2019, 03:51 PM
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Well...if you do this long enough eventually you are going to wreck your car.

We had an incident at the race this past weekend that resulted in contact with the wall down the front straight of PIR at abut 60mph. The data showed that the car hit the wall at about a 120deg angle with -1G of LongAcc and -2.5G of LatAcc. Not the hardest hit in the world, but enough the tear up the front end of the car.





Started tearing it apart last night and I think that their is minimal frame damage and it can be repaired. My plan is to get the suspension damage repaired to get the car rolling and then take it to the alignment shop to measure the chassis to make sure it is ok before repairing everything up front.



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'69 912 w/ 2.3L Type-4 Turbo Engine
'74 914 1825lbs, JDM Subaru STi Spec-C Engine, Lotus Suspension, 930 Trans.
'80 924 AAN 5-Cyl Swap - Team UBoot Rennwerks
Old 07-08-2019, 10:21 AM
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Nicely done. Get her fixed up and get out there and race some more.

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