Chasing Response: Air-cooled Flat-Six with VGT Turbo Tech

[TopTuning]

Figured it was time to start a fresh thread for my latest “science project”

Background

About 15 years ago, I was one of the first to run a Holset VGT turbo on my MKIV Supra. I’ve always been fascinated by turbos and the gains they can bring. That project turned out decent—I used a simple pneumatic control setup and managed close to 450hp with very little lag compared to the single turbo I had before. My skills weren’t exactly up to the task back then, but sheer youthful determination made up for it.

Fast forward to today, and we now have VGT turbos designed specifically for gasoline engines. Even better, used units are becoming more common and surprisingly affordable.

I’ve wanted to experiment with these for a while. I even tried convincing Bruce Canepa to use them on the 959 SC builds, but he was happy with the performance from the twin EFR 6258s we were already running—and understandably didn’t want to restart development. Fair enough, but it left me itching to try it myself.

So, my development car is about to get a whole lot more “test mule” treatment.

The Turbo

This unit comes from the 718 Cayman/Boxster S. It’s compact, yet has relatively large housings. The factory CNC-machined compressor wheel is a nice bonus, though I’ll probably upgrade it.

And yes, there are upgrades! Several companies offer larger compressor wheels, with the biggest options capable of flowing around 64 lb/min—supporting up to 600hp.

Even the stock unit performs impressively. Here’s a comparison of a 718 2.5L 4-cylinder with just a reflash versus completely stock:




The most interesting part to me is the massive torque down low. While many chase peak numbers up top, enthusiasts who truly enjoy driving turbo cars value responsiveness and midrange punch. That’s where this shines. It transforms the overall driving experience, even if it’s not about outright straight-line speed—which, honestly, isn’t what a Porsche is really about anyway.

Making It Work

I picked mine up for about $1600, but there’s a catch: absolutely zero aftermarket support. No fittings, no flanges, no adapters—nothing. So I got to work and designed my own.

Here are a few of the custom pieces so far:

Oil feed adapter

Intake adapter

Coolant manifold fitting

Exhaust flange (titanium, of course)








Now we’ve got a universal oil inlet, a 3-inch intake adapter, and the start of an exhaust solution.

Challenges & Solutions

Clocking the turbo
The VGT mechanism is pinned, so the turbo isn’t easily clockable. The compressor housing must stay closely aligned to the CHRA for the actuator to function, unless the linkage is reworked. The solution: open the turbo and redrill the alignment dowel holes. This gives six possible turbine housing positions. Luckily, one of them was nearly ideal for a flat-six layout.

Wastegate actuator
From the factory, the actuator is a vacuum-operated pull-type. Not very convenient. I converted it to a more common push-type actuator. A universal bracket (with some mods and reinforcement) allowed me to clock it to match the new turbine position. The flapper arm needed reversing, so I grafted one in from a rebuild kit.




Mounting & Exhaust

Next, I designed a simple mounting bracket that ties into the original support bracket. SendCutSend makes this kind of fabrication painless and affordable.

[TopTuning]

(continued. Why do we still have a 10 image limit here?)

For the exhaust, I didn’t just want to slap together a laser cut inlet flange and massaged to fit pipes. Instead, I designed a tight-fitting Y-pipe inlet with a bank divider to improve flow velocity and reduce turbulence. It took about 10 design iterations, plenty of fitment checks, and CFD simulations, but the final piece should work beautifully.

I'm using the the plastic 3D-printed prototype for fitment—final version will be 3D metal printed in GH3625 Inconel, a seriously impressive “super alloy.” Also the stock 718 turbine inlet as a comparison.





Stock 718 exhaust:




I chopped up my old setup and tacked in some new pipes from the headers (321 stainless). ER-347 rod welds it beautifully to GH3625. I’ll eventually build new headers, but my bargain-bin set has somehow held up to years of abuse, so they’ll do for now.

VGT Control

This is where things get really fun. As a MoTeC dealer, I have access to all the right info. MoTeC USA shared the actuator control details, pinouts, and connector part numbers. I also have full MoTec mapping for the engine and VGT system, so integrating it into my M150 ECU should be straightforward.

I’m also considering developing a standalone universal controller for these VGT actuators. Not every ECU can handle VGT control, but a simple programmable microcontroller (like Arduino) with some trim pots and supporting hardware could make it possible for nearly anyone.

Current Status & Goals

The turbo is mounted, and most of the puzzle pieces are in place. I’ll finish the exhaust once the engine goes back in—which should be in a couple of weeks.








Performance target:

Similar peak power to the EFR 7670 it’s replacing (around 414whp)

But with boost coming in as much as 1000rpm sooner

That change alone will completely transform the driving dynamics for the better.

More updates to come! Once everything is thoroughly tested, I’ll also make the adapters and components I’ve designed available to others who want to try this tech.

[Ryan_Cunningham]

This is awesome. Once life settles down we're definitely going to be talking!

[eastbay]

Super cool, that is some great work.

[75 911s]

Nice work. Following with interest.

[TopTuning]

Got bored last night and got my new back purging setup so I went ahead and welded the pipes up. 321 is hard (especially used, despite how much you clean it) Titanium isn't any easier (only my second time welding it!)

Results are decent cosmetically (maybe a bit too much heat), but full depth welds and back purged so they should be full strength. Back purging gives an equally smooth interior weld, which I smoothed out a little more with my bore sanding air tool attachments.

Just waiting on parts from my other big project (custom flywheels for the 996 gearbox swap) to put the engine back in now.