Slingshot Seven

One of the world's best luxury limousines that's had over $32,000 invested in a big turbocharged power-up. Sounds like a leather armchair ride to ecstasy to me...

Words by Michael Knowling, Pix by Tony Rabbitte

Click on pics to view larger images

This article was first published in April 2000.

Undoubtedly, one of the most delectable executive limousines ever is the BMW 7 Series. Its long tail and wide stance gives it the most gracious - yet distinctively aggressive - appearance amongst all cars in its class. The Seven series certainly has a presence like few other vehicles on earth. And aboard this luxury liner, there's more pampers than you'd find in a child-care centre. But what of its straight-line performance? Sadly, the average 735i gets dusted by just about anything with a sporty engine. The V8 model and - to a greater extent - the silky-smooth V12 are the only powerplant offerings from BMW capable of getting this sizable beast really rolling. Shame about their cost though.

IT industry consultant Maarten Briede is a BMW 7 owner who's recently grown that all-too-common desire for sizzling performance. Initially, though, he simply desired the supreme comfort and overall rigidity that the big BM had to offer - grunt wasn't that big an issue. Its role was to give comfortable long distance cruising (Maarten gets around the place a lot with his work) and to tow his MG club racecar.

After taking over the reign from the previous lease owner, Maarten's already owned this immaculate 1988 735i for a whole seven years. With 50,000 kilometers on its odometer, the fully BMW serviced purebred vehicle was purchased for a sum of $70,000 - but as Maarten found out "it's a bit heavy to be a real performance car". Of course, the only way to spice up a scale-tipping vehicle is to add a herd more horses under the bonnet. The more the better in this case, coz the wonderful BM chassis is more than capable of taking it.

The search for a reputable BM tuner wound up at the workshop doors belonging to one Herbert Gattermeier. His business, Bavariacars. After discussions, it was obvious there was only one effective way to satisfy the requirement for power - forced induction. Herbert drew from his experience with his turbo 5 Series (already featured in AutoSpeed) and pieced together nearly a carbon copy of that particular engine. And according to Herbert, there are no weaknesses in the large-ish BMW M30 six, so the build-up was therefore fairly straightforward. The only real change was to reduce the static compression ratio to accommodate the push of a turbocharger. Although Herbert wasn't too keen on giving out the type of pistons he used or the CR, we do know they're forged and we'd guess the compression ratio to be hovering around the 8-ish:1 mark.

Other than the pistons, the 2-bolt main engine remains as BMW intended - complete with an un-touched 2 valve per cylinder SOHC head. It's what's slung off the exhaust side of that cross-flow alloy head that really matters. An in-house cast exhaust manifold puts the six's waste gases through a water cooled Mitsubishi TD-series turbocharger, which operates between 7-15 psi depending on where Maarten wants to have the in-cabin adjustment set.

A K&N air filter is positioned up-stream of the compressor, with the forced airflow going to a custom made front-mounted air-to-air intercooler and into the standard throttle body (which Herbert says is around 75mm diameter). The intake manifold is left stock and un-match ported because "it's all very good stuff". An HKS throttle-closed blow-off valve is plumbed in to vent intake air to the atmosphere when a preset manifold vacuum is reached. Meanwhile, the whisper quiet rustle from the standard BMW exhaust system is lost in preference to a high flowing 3 inch system with equal diameter cat converter and rear muffler. It's not loud enough to spoil the luxury feel onboard though.

The late 80s Bosch injection system would have had a terrible time trying to take to its turbocharged mate - so the only option was to go down that increasingly familiar programmable injection route. A MicroTech MT8 system was hooked up to all the sensors and gave the opportunity to remove the standard BMW airflow meter (that usually sits atop the valve cover) and replace it with a MAP load sensor. Again, the type of injectors used is a bit of a secret - but we can confirm that they are Bosch high flow units and there were no modifications required to the fuel rail. Interestingly enough, the car retains the services of the standard 735i fuel pump (and its associated lift pump) which Herbert says very good for making large power. No surge tank is used given that the car is not intended for track work. The other active half of the MicroTech MT8 brain sees to ignition matters, using a single Bosch coil, Magnecore leads and NGK spark plugs.

As we said earlier, Herbert claims that the M30 engine has no particular design weaknesses. In a wild one-off build, he estimates that it would probably be possible to pull 450-500 horsepower from the standard bottom-end (except for forged low CR pistons). That's running it "on the borderline" though. But to maintain this car's everyday reliability and mild street manners, the Seven's motor is tamed down a quite a bit from that dizzy level. On a chassis dyno (with boost set at 15 psi) this classy six can p-u-r-r out 260-270 rear wheel horses. That's around 370 at the flywheel according to Herbert - a pile more than the standard engine's 212hp.

With the 3.5 litre six primarily set up for a blend of reliability and performance, Herbert advised that the driveline should follow suit. First to go was the 4-speed auto trans and in its place was bolted a 5-speed Getrag manual with the accompanying flywheel, Sachs clutch and modified pressure plate. A BMW hydraulic clutch system went in as well. Heading further south, the new distance separating the gearbox and the diff required that a custom 70mm tailshaft be constructed - which is also guaranteed not to break. And the differential itself was cause for some concern. Herbert says the standard car's 3.45:1 ratio wasn't originally fitted to Maarten's car when he first saw it. His actually had shorter gears. So the guys pulled this from its mounts and put in a LSD centre spinning at a 3.15 ratio - and, yes, there's still more than enough torque to get off the line smartly.

On the road - where it really matters - is where this intelligent build-up really shines through. The car now hikes up its executive woollen socks at only around 1500 rpm and runs to over 6000 without sweating through its Armani suit. Torque is they key word. Maarten often finds himself changing up a cog at a lazy 2000 revs... That says it all. A bonus is that fuel economy even remains near standard, with (under similar driving conditions) it consuming an average of around 12 litres of PULP per 100 kilometres.

But there are limitations of having a body of such stature. Whenever Maarten wants to tackle the narrow roads of Sydney, he has to hop into his stock Impreza WRX - what he calls "a spritely shopping car". And Maarten's automotive collection also extends to a pristine stock VL turbo, so you can hardly say he favours any one sort of car. Except performance ones, that is!

You'll be pleased to hear that the BM is no longer relegated to towing or any other hard-slogging work. Maarten has changed his tune and now regards it as "a good project car - a unique car". He just wants to "keep it clean - and keep it straight" he adds with a laugh.

And let's be honest, you'd park it in your living room at night if you could wouldn't you?

Contact:

Bavariacars
+61 2 9879 7557

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