Dieselgate, Bosch, and the Series 1 Porsche 944

This article was written and published by Chris Duffy back in 2016 when the Volkswagen “Dieselgate” scandal was happening.  So how is the infamous “944 Shudder” related to VW’s problems?  Read on…

944 TachNearly every early 944 has “The 944 Shudder.” When lifting off the throttle from any position, the revs drop below idle, and then the engine recovers. If you’ve never experienced it, it looks like this:

So what’s that got to do with the surprising, unsettling, completely unprecedented mass deception event that is dieselgate? More than you might think.

The 944 was developed during the late 70’s and early 80’s. Economy of all types, and particularly fuel mileage, were a bit of a concern around then. Porsche, like all manufacturers at the time, was very interested in finding ways to make more power and further distances on less fuel.

Porsche had a design goal, so they chased it. In 1981 at the 24 hours of Le Mans, they reaped their reward. Two 944 prototypes placed very well in that race, even though they were based on production cars and therefore much slower than the purpose built race cars that made up the majority of the competition.

Porsche did well in that race because they had developed a very advanced, very reliable, and very efficient fuel injection system, which meant that their cars spent less time on pit road refueling than any other car in the field.

Fuel economy was again the secret weapon. Thanks to the engine management system, the 924 GTP stopped only 21 times in 24 hours for fuel and only spent an amazing 56 minutes on pit lane.


This fuel injection system was moved to the production 944, which showed up in Europe in 1982 and in the US in 1983. The 944 did deliver on it’s fuel economy promises, and it still makes respectable fuel mileage numbers today.

There were a couple problems, though.

First, the fuel mix between Europe and the US was significantly different. Therefore Porsche delivered two different engines, the US getting a slightly lower compression engine to match the lower octane fuel.

Second, the emissions regulations.

One of the ways that Porsche had figured out how to save fuel was by cutting off fuel entirely when the throttle was closed. This made the long braking areas at Le Mans use no fuel whatsoever, because the engine was effectively off whenever the throttle was closed. Great idea, and a great improvement over carburetor and earlier EFI approaches.

The issue is that once you’ve turned the fuel off, you have to turn it back on at some point to keep the engine from actually shutting off when the RPM drops past a certain point. The engine was computer controlled, so this was a matter of simply monitoring RPM and turning the fuel back on when a threshold was reached, so that the engine could easily settle into an idle.

For the European market 944s that threshold was 1,600 RPM. By turning the fuel back on then, the engine would be “running again” by the time the RPM fell to a 900-1,000 RPM idle.

For US 944s, that threshold was set to 1,350 RPM, which is simply too low. This is why almost all US series 1 944s have the shudder, the fuel comes back on too late, so the RPMs drop too far and the engine has to recover.

Porsche intentionally spent development time on this market-specific change – which made the overall product inferior. Why?

Burning less fuel means producing less emissions. Keeping the fuel off longer meant it was easier to pass smog checks.

In 1982, right as the 944 production was starting, California passed SB 33. This made California the 20th state in the US to do vehicle smog inspections, showing that emissions testing wasn’t just a fad.

Worse, the effective date for implementation of inspections in California was March 1984. This is after the first cars would be delivered but close enough to the new model launch that if any 944 failed inspection, it would look like a terrible failure on Porsche’s part. They had to be extra cautious about emissions.

They also had to think of something quick, which didn’t require a major retooling, since they already had cars in production.

Some solution that would help them score as well as possible on a “steady-state 30 mph and drop to idle” emissions test. Not real world emissions, just the test.

A brilliant engineer noticed that the fuel “turn on” threshold could be lowered. “What’s the harm in that? Nobody will notice, and it will improve emissions on that particular test!”

That brilliant engineer, and the entire equally brilliant fuel injection system on the 944, was from Bosch.

So when VW’s lawyers screamed that Bosch has been an “active participant” in the dieselgate efforts, all I could think was “well, of course.”

VW is liable for what they sold; but engineers build their systems to the tests their systems will experience.

Over 30 years later, we’re still modifying our 944s to remedy the infamous Shudder, the Shudder that didn’t need to be there if it wasn’t for a change to try and do better on an emissions test.

And now you know.

Author: Kevin Duffy, 924S944.com LLC, DeLand, FL

After retiring from a career in Law Enforcement, Kevin Duffy turned his attention to one of his passions, Porsche 944's and 924S's. He owns 924S944.com LLC in DeLand, FL, rescuing and restoring forgotten Porsches, bringing them back to a useful life. He is especially interested in the rare-but-beautiful 924S Special Edition. He can be found at Porsche Club events, including track days, tours and shows, as well as other car-focused events around the southeastern United States.

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