76 Porsche 912 Subaru 2.5 swap

[Andy Somogyi]

This is a 76 Porsche 912 that originally came with a factory swapped VW bus engine instead of the Porsche flat-6.

For me, that’s simply doesn’t feel like a sports car, so I’m swapping it with something lighter and about 3 times the power, a normally aspirated Subaru EJ251.

I’ve had a build blog on Facebook for a while, and thought I’d shadow it here also. Original blog is at

https://facebook.com/ArrowBlau

Up until now, I’ve mostly been working on setting up my shop equipment, milling machine, welder, and doing engineering calculations. Now I’m happy enough with the design that I’m starting on fabrication.

Radiator just arrived, gave it a test fit.

Looks all right, it sits about as low as the 911 SC air conditioner condenser. I still need to fabricate a nice cage / guard for it out of cro-moly tube, and of course cut the duct and fab the brackets. I’ll also re-locate the side hose outlets.

Got this radiator off eBay, it’s for a Renault Alpine A310, which originally had a 275 hp V6 turbo. So cooling wise, should be ok for the 200 HP naturally aspirated Subaru motor I’m going with.

But finally starting to look like a project.

















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[LukasM]

Have been following you on Facebook Andy, will be cool to see what you come up with on this build. I just bought previously V8 converted '76 911 without an engine on Pelican (1977 911S Roller - Pelican Parts Forums), so might do something similar with it eventually.

Awesome job on setting up your workshop by the way! I've got a nice space and most tools, still missing a lathe, mill, TIG welder, and more importantly, the skills to use them however!

[Andy Somogyi]

Originally Posted by LukasM

Have been following you on Facebook Andy, will be cool to see what you come up with on this build. I just bought previously V8 converted '76 911 without an engine on Pelican (1977 911S Roller - Pelican Parts Forums), so might do something similar with it eventually.

Awesome job on setting up your workshop by the way! I've got a nice space and most tools, still missing a lathe, mill, TIG welder, and more importantly, the skills to use them however!

Thanks man, it's getting there. I'm going to start facing the tube bender probably tomorrow, because I've finally got all the towing to make the dies.

Have you thought about what kind of engine you want to put in your 911? I'd say think about all the cars've you've driven, and what kinds of things you like about them. What kind of 'feel' are you looking for?

[Andy Somogyi]

Calculations of coolant flow through a variety of commonly available tubing materials.

The main engineering goal here is to maximize cooling based on the vehicle shape constraints.

I've been able to cobble together a variety of different data here, and some estimates.

Looking at how much heat each kind of tube dissipates along it's length and how much of a friction head they incur.

It is vitally important that the cooling system (pipes + radiator) does not incur too much of a friction head because water pump flow is inversely proportional to friction head. Looking at the attached chart (measured from a GM 3.8 V6), we can see that flow rate drastically drops when the pressure hits about 1 bar. We can estimate the friction head of the radiator at about 0.5 bar, so to ensure enough flow, we need to make sure the pipe system is no more than about 0.5 bar at most. This happens around 26mm ID.

Therefore, the absolute minimum sized pipe is 26mm. 1" EMT or aluminum rigid conduit meets this value.

But better yet is reducing the friction head even further, which increases water pump flow.

We can see from the calculated values that the STATIC heat dissipation of the pipes, the aluminum tube is roughly twice as effective as the steel tubing. However, the total amount of heat dissipated through the tubing is small compared to the total amount required.

Therefore, I conclude that it is better to choose a pipe based on flow rates rather than heat dissipation capacity. Increased flow rate allows the water pump to work more effetely and transfer more heat to the radiator.

In my 912, the maximum size tube OD is possibly 38mm, but I need to fit the rocker covers back up and do some more test fitting.

So, I'm going to look at 1.25" EMT as this is commonly available, and easy to bend, and has sufficient flow capabilities.

Also note, because the aluminum tube is thicker wall wise, it reduces the heat transfer capacity, so it's not as effective as you'd think for heat transfer.

[Andy Somogyi]

Did some test fit ups with the rocker covers on.

34mm, that’s about the largest diameter OD tube that will cleanly clear. This corresponds to 1” EMT.

I’m thinking that at least in the rocker sections, I want to go with steel tube — it’s much more durable. Then in the wheel wells, I can go with a slightly larger diameter aluminum tube.

Remember, pressure drop is directly proportional to the length of the tube. So, if a thinner diameter tube is only used in the rockers and a larger tube is used elsewhere, that’s not that bad.










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[Andy Somogyi]

Water pipe flow calculations. Figuring out Reynolds number, friction head, flow rate, flow velocity and all that fun stuff.

I wrote a program here in Mathematica to calculate the optimal pipe size because there's so little room in the rocker panels to run pipes.

Working on optimizing the pipe size based on vehicle constraints. To optimize here means minimizing the friction head insured by the pipes, which in turn maximized the water pump flow volume. Also looking at how much heat dissipation the pipes themselves will perform. So, lots and lots of variables, it sure makes coming up with a design pretty challenging.

Remember, from the previous pics, there's only about 33mm under the rocker panels to play with.

[Andy Somogyi]

Coolant line mock-up try 2.

The 911 ran two lines per side, a pressure and return line. Originally I was thinking about following the upper factory line route. But it’s a pretty torturous route around the Jack point, and gets rather nasty in the left rear wheel arch.

So, frustrated with that mount, I figure, let’s try the bottom route.

This I think is better. It’s WAY easier to fabricate and it’s a clean straight shot for the coolant.

The lower line only becomes visible when looking at the car with your eyes about fender height. And then even I can just paint the lines black with some high temp paint, so it looks like part of the undercarriage.

I’m using 1” PVC here, this is the same exact size as 1” aluminum rigid conduit, or 1.3” OD and 1.11” ID.

The other advantage with this approach is the whole line is exposed, so better cooling.















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[Andy Somogyi]

Poor man’s laser scanner.

Measuring a series of 3D coordinates off my IROC bumper to build an Autocad Fusion360 model of it.

Will use the 3D model to design a bumper reinforcement / impact bar.

I’ve got a nice honeycomb mesh design using 6061 aluminum that I’m going to try.

Made a crude model of the factory bumper and it’s not that good of a design structurally.










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[Andy Somogyi]

Working on making the models of the IROC bumper. This is a massive pain in the ass because this thing has SOO MANY compound curves. Hopefully tomorrow, I'll be done with this part, and I can start actually layout out the radiator and start designing the bumper impact bar.

[Yan2947]

Andy, are you going to run your coolant lines under each rocker like in your post #7? One line per side? How are you going to sweep into each wheel well area?

My 89 has the rear radiator but I just got an 80 sc with a stroker and the guy ran the lines through the main cabin on the passenger floor side. It has a renegade setup and the car is a wide body slant nose. I plan on using the large rockers to hide it but I will need some couplings in case I need to get to the torsion bars. The lines will be in the way. Any new mods in your coolant lines?

[Andy Somogyi]

I’m routing them on the inside of the rocker panels, basically following the factory oil line route, but mirroring it on both sides.

This is the approach Mike used in his swap, which is the cleanest swap out there.

I’m using 1.25” aluminum rigid conduit for most sections, and 1.25” tube for the lower parts because it’s a tighter clearance.

Wheel well clearance is fine on the right side, Porsche made a clearance pocket in the frame. But the left side is tight, Mike just ran his pipe straight through, above the torsion bars, but I think I’ll fabricate a flattened aluminum manifold in this location to ensure plenty of wheel clearance.

My pipes will be all aluminum with TIG welded elbow, and silicon connectors between the long sections.

My radiator will also be front mounted, outside the tub. One rigidly hard and fast design goal for me was absolutely no radiator sticking up in the trunk, the trunk had to be 100% usable and had to look factory.

Take a look at Mike’s thread, it’s been a big source of inspiration for me. His is without a doubt the cleanest and best swap I’ve ever seen.

DSC_00861410389629 by andy.somogyi

Help me style this beat-up old 912

Originally Posted by Yan2947

Andy, are you going to run your coolant lines under each rocker like in your post #7? One line per side? How are you going to sweep into each wheel well area?

My 89 has the rear radiator but I just got an 80 sc with a stroker and the guy ran the lines through the main cabin on the passenger floor side. It has a renegade setup and the car is a wide body slant nose. I plan on using the large rockers to hide it but I will need some couplings in case I need to get to the torsion bars. The lines will be in the way. Any new mods in your coolant lines?

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[Yan2947]

Andy, feel like making a second set of those pipes?...

Which is Mike's build you are speaking about?

Amazed at your skills!

[Andy Somogyi]

This is Mike and Matt's 912, this is a quality build, here's a link to his thread. The pipes above are Mike's pipes.

Sure, I can make another set. I'm currently measuring the insides of the fender wells so I can make an exact mechanical drawing of the pipes.

Now, what size engine do you plan on going with? I'll be using about a 275 hp EJ205, that flows about 150 liters per minute, so based on my calculations, a 1.1" ID pipe is OK. If you're going with a larger engine, with a higher flow requirements, you'll need a bigger pipe.

The one tricky area will be in the left rear wheel well, depending on tire and offset size. I still need to do some close measurements in that area to check on the exact clearance between the tire and the wheel well. My car came with 6x16, +36 offset Fuchs, all the way around, and the seem pretty tight on the inside clearance.

Help me style this beat-up old 912

Originally Posted by Yan2947

Andy, feel like making a second set of those pipes?...

Which is Mike's build you are speaking about?

Amazed at your skills!

[Yan2947]

Thanks for the link! I have not seen or read that build. Looks amazing. I just picked up an 80 sc with a 383 Stroker. Already has the Renegade radiator setup but the guy ran the rubber hose through the passenger area and it needs to be re-routed. I believe its 1 1/4 hose. It has 460HP. A beast. Just trying to make it more streetable and cleaner like my 89 ls1. I have a wide body kit with large rockers but following the route you discuss seems to be the best option.

[Andy Somogyi]

For an engine that big, I think it's going to require about a minimum of a 1.25" ID pipe. Flow rate is going to be up around 200 liters / minute. Note, cast iron needs a higher flow rate than aluminum blocks because the heat transfer is much lower of iron to water vs. aluminum to water.

With an iron block Chevy, I'd recommend even going with a 1.378" ID pipe on the low pressure side. The internal water flow of the SBC isn't as well thought out as the LS, and typically needs a bigger pipe on the low pressure side. Though, if you're going with an electric water pump, you can probably bet by with same size pipe both low and high pressure side.

For that setup, I'd recommend a triple radiator setup, with a pair of radiators in the front fenders, and a smaller center one. So, here, you could runner a smaller center radiator (like the one I'm using) in parallel with a pair of side radiators. You can get custom radiators like this made pretty affordably. That way, you can still have a usable and factory looking trunk.

You're also going to need to run an electric water pump.


PVX build thread: one view from the slope

Originally Posted by Yan2947

Thanks for the link! I have not seen or read that build. Looks amazing. I just picked up an 80 sc with a 383 Stroker. Already has the Renegade radiator setup but the guy ran the rubber hose through the passenger area and it needs to be re-routed. I believe its 1 1/4 hose. It has 460HP. A beast. Just trying to make it more streetable and cleaner like my 89 ls1. I have a wide body kit with large rockers but following the route you discuss seems to be the best option.

[Yan2947]

Hey Andy, the car was setup as a race car. The coolant system is working and as far as I know the previous owner had no problems on the track. He ran an 1 1/4 and and 1" rubber hose through the cabin. Quite messy. No electric pump just mechanical water pump. Here is a photo of the hose through the cabin...

[Tremelune]

I love Subie swaps. Are you going to try and get a Subaru transmission in?

Have you considered using the stock oil lines for running coolant? You could run two sets on each side of the car in one big loop. I suspect the lines together would have enough flow.

The main down side is cost vs PVC and complexity, I guess...but you wouldn't have to hide them behind rockers, and there would be less custom everything, as you could use the various oil coolers available for these cars (one in each fender and a center cooler if you really need it).

[Andy Somogyi]

Originally Posted by Tremelune

I love Subie swaps. Are you going to try and get a Subaru transmission in?

Have you considered using the stock oil lines for running coolant? You could run two sets on each side of the car in one big loop. I suspect the lines together would have enough flow.

The main down side is cost vs PVC and complexity, I guess...but you wouldn't have to hide them behind rockers, and there would be less custom everything, as you could use the various oil coolers available for these cars (one in each fender and a center cooler if you really need it).

I'm going be making welded aluminum hard-lines that will follow the same path as the factory oil lines. OMG, no, I'm not going to use PVC, LOL
, I just put the PVC up there to try the fit, as a mock-up to see if the diameter was too big.

Even using both of the factory oil lines are too small for sufficient water flow, remember, pressure drop for a given flow rate is proportional to 1/D^4. So, as the pipe gets smaller, pressure drop tends to infinity very very fast. I worked out that an internal diameter of just over 1 inch is sufficient, so I'll be using 1.125" ID tube. I'll be welding them together here like Mike did. I think I'll just run a single line down each side that follows the lower factory oil line path, so that it's a straight shot, and don't have to curve around the jack point.

For the trans, I'm keeping the Porsche 923 (915 with VW input shaft) trans. It works, and I like the feel, seems appropriate for the car. I'm also going to great lengths to get the engine to feel like it's period correct, in that all the radiators will be completely hidden. Engine wise, I've got a pair of cams custom ground with the Porsche 356 Super 90 profile ($300), and it has the same firing order as the porsche/VW flat-4s, so shooting for a similar exhaust note, and engine feel.

[Andy Somogyi]

My overarching goal with the swap is to have something as "factory" looking as possible, to be as period-correct looking as possible. That means absolutely no radiator in the trunk, and cooling system needs to be stealth.

I've found a pair of side, fender / wheel well mounted radiators that'll work. These are shifter-kart radiators on ebay, and looks like they'll require a little bit of modification (weld in larger water neck, etc...), but not much. They've got a core size of 7.9x11.8x2.25 inches, or 93 square inches of surface area. This will make a pretty easy fit, along with custom fiberglass fan mounts and ducting, so they should get a good pressure drop when mounted under the headlights.

Planning on pairing these with a trimmed Renault radiator in the center that'll have a core area of about 7x25" or 175 square inches. I'm going to shave the bottom couple inches off the radiator, re-weld the tanks to make sure it has plenty of ground clearance, and give enough room for a lower bash bar.

This three-radiator setup is pretty similar to how the new Porsche 911 turbo does it. I've read a number of papers of the design of the new 911 cooling system. They actually restrict the exhaust flow into the wheel well, in order to reduce air flow, and hence reduce aerodynamic drag. But they could enlarge the port if they needed more cooling.

With the three radiator approach, that should give a total of about 350 square inches of radiator area. Figure that should be plenty for a built EJ251 crank out about about 200 hp.

I think I'll run the two shifter kart radiators in series, and plumb those parallel with the center Renault radiator.

Also going to have a front trunk mounted burp tank, where I'll run a small bleed line from each of the three front radiators, so that the whole system will be self-bleeding.

What I'm not sure about, is is the air pressure inside the fender well lower at the top, or the bottom of the fender??? Is it best to have the fender well hot air exit ducted to the bottom of the top of the wheel well.

This is the Renault radiator mocked up, I still need to cut about 2" off the bottom for sufficient ground clearance.

[Yan2947]

Andy, for comparison, my 89 911 with the Toyjet rear radiator setup has exposed fin area or 22.5 by 18 for 405 sq inches while the 80 SC Slantnose has the renegade Hybrid front mounted setup which is 25 by 16.5 for a total of 412 sq inches. I've never overheated with the toyjet setup.