LZ9 3.9L V6 in 1985 944 - GM 60 degree V6 swapped

This has been an ongoing project since August 2020 but about 3 weeks ago I was able to drive the car for the first time. It is faster than I anticipated and also louder! Still a lot of work to get the rest of the car buttoned up but it is drivable.

Summary of build:
- 1985 Porsche 944 (early) with stock torque tube and 951 transmission
- Microsquirt ECU and custom fuse/relay box under the hood (rewired entire car)
- 2006 Chevy Impala 3.9L V6 (60 degree family, OHV), custom intake and exhaust manifolds
- GM F23 FWD manual transaxle cut up to make a bellhousing
- GM FWD flywheel/pressure plate (fits Cavalier, Fiero, and everything in between)
- 9" Ford Ranger clutch disc (1"x23 spline fits perfectly)
- Buick Regal/Saab 9-3 slave cylinder
- Honda Civic 5/8" clutch master cylinder (made adapter to attach to firewall)
- 1988 BMW 325 brake master cylinder without boost (22mm/17mm bores vs 23.8/19 for 944 MC)
- Mustang-II type manual steering rack mounted on custom crossmember
- Jaguar XJ6 engine mounts
- Relocated alternator on custom bracket, repositioned idlers to make unique accessory drive to fit

The engine is the first pushrod VVT engine that GM made. However the VVT is only used for cruising MPG - it starts off fully advanced and retards depending on load and RPM condition. I didn't want to bother so I locked the cam phaser fully advanced (+9*) and set the cam gear 1 tooth retarded (-7.2*) so the cam is fixed at 1.8* advanced from "straight up". My initial drives prove this to be successful, it hauls!

Stock the engine is rated 240hp/240tq with that tiny VVT cam. The per-cylinder displacement and head flow are roughly equal to a 5.3L LSV8, so there is a lot of head room for power gains. Aftermarket cams/springs/etc are available that should make ~300hp attainable.

Why did I pick this engine?
- Priorities were 968/turbo S type power, but lower running cost, greater parts availability, and more working space.
--- I played with 944 engines for 16 years but the parts are too expensive now, and there are only about 3-4 places in the US that know what Alusil is and how to work with it. Just became too much of a PITA for me.
--- This whole swap has cost about the same as buying just a tired 968 engine, but parts to run it going forward will be a fraction of Porsche prices, and usually in stock at the local auto store if needed.
- This engine is 6 inches narrower than an LS, 5 inches shorter front to back, sits almost entirely behind the front axle, and weighs only 10 lbs more fully dressed than my 944 engine did, with over 50% more displacement.
- Any kind of maintenance I may need to do I will have tons of working room, something you can't always say with a 944 engine and definitely not a 951.

Below are some photos - top is taken after the first drive, the others are earlier "mid-swap" just to show the small package of the engine overall.

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You still have to modify the oil pan to clear the crossmember with this swap? It looked that way in the picture. If so, how much did you have to modify it to fit?

Killjoy wrote:You still have to modify the oil pan to clear the crossmember with this swap? It looked that way in the picture. If so, how much did you have to modify it to fit?

No, I did not modify the oil pan at all. I had considered it originally but ended up not needing to.

I would love to see and hear in more detail about your custom crossmember and Mustang II rack. Havent heard of anyone else doing that. I'm setting up an early 924 with early 944 brakes and transaxle, and since it has the steel crossmember I'm curious to see some custom crossmember solutions. I was set on an ls swap but I find your setup very interesting.

I'd also like to see what you had to do to the intake and exhaust manifolds.

Killjoy wrote:I'd also like to see what you had to do to the intake and exhaust manifolds.

lower intake is stock. upper intake you can see in the first photo. a guy made a cad file of the flanges, i ordered them laser cut, then welded up the rest of the manifold on top of the flanges.

the exhaust manifolds are just made out of Weld-Els, very simple design to get around everything else in the engine bay.

here's a shot from a while back showing the crossmember and steering rack and motor mount arms (which have since been modified/trimmed). it is mostly 2.5" square tubing with a notch cut out to clear the oil pan. the 2.5" dimension was chosen since it's very close to the outside width of the control arm bushing.


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Why the change to the mustang rack? And you used parts of the factory exhaust manifold?

Seems like my 924 having the steel crossmember actually should help for this swap, easier to modify and weld.

Killjoy wrote:Why the change to the mustang rack? And you used parts of the factory exhaust manifold?

the stock 944/924 rack mounts stick up above the rack tube about an inch. i needed the engine to sit as low as i could. the mustang rack mounts are both below the rack entirely so i could put the oil pan closer to the rack.

i did not use the factory manifolds at all in the end - i had hoped i could but it wouldn't have cleared the steering shaft on the driver side.

Good info. Thanks!

Do you think I could have a copy of your intake flange CAD file?

I can't tell exactly from from your mock up photos, but approximately how far back would the motor have to go to clear the factory crossmember altogether? How close at the valve covers to the firewall as it sits in your car. I'm thinking I would maybe rather cut the firewall back and shorten the torque tube and drive shaft some than have to move the rack and crossmember around.

Killjoy wrote:I can't tell exactly from from your mock up photos, but approximately how far back would the motor have to go to clear the factory crossmember altogether? How close at the valve covers to the firewall as it sits in your car. I'm thinking I would maybe rather cut the firewall back and shorten the torque tube and drive shaft some than have to move the rack and crossmember around.

Sorry, didn't realize you had posted again.

I don't remember exactly but I think it would be around 4 inches to clear the x-member.

I had to shift the whole drivetrain forward 1" (by adjusting the trans mount) and still had to bash the firewall a bit to clear the driver side valve cover. Currently I have maybe 1/2" clearance back there. If you are up to cutting out that bump in the firewall you could make this fit a lot better.

Very cool build. It won't sound like a Camaro and you have lots of room to add a turbo or supercharger if desired later.

I was thinking about using the 3.8 out of a Pontiac GTP. Is this essentially the same engine that you used?

Cockney wrote:I was thinking about using the 3.8 out of a Pontiac GTP. Is this essentially the same engine that you used?

the 3.8l and the 3.9l are completely different engine families. the 3800 has its routes from the buick v6, its a 90 degree engine. the 3.9l is from the gm 60 degree v6 family that includes the 2.8, 3.1, 3.4, 3.5 and 3.9l.

Cockney wrote:I was thinking about using the 3.8 out of a Pontiac GTP. Is this essentially the same engine that you used?

No; the 3.9 in my car and the Buick 3800 you are looking at have nothing in common, aside from the bell housing bolt pattern.

But I did spend time over the years looking at the Buick engines and I think they would fit well and be a good match to the 944 too.

My 3.9 makes the same power N/A as a 3800 does supercharged, and it's 6" narrower than a Buick, which were big factors to me. But a 3800 Buick would probably fit under the hood better.