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More on overboost (longish).
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Posted by Brooks G (more from Brooks G) on Sat, 26 Mar 2005 15:38:42 Share Post by Email
In Reply to: Re: 20 second 272 lb ft overboost mode, Montreux, Sat, 26 Mar 2005 14:51:35
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Montreux,

There is another nanny in a modern car. It is called a rev limiter. When you get to the redline, the engine management takes over to prevent you from over revving. This came with first with electronic fuel injection where they would just shut off the fuel at the redline. This is a hard rev limiter which can lead to some wild bucking as the engine goes over and than back under the redline.

Many more modern cars have soft rev limiters where they start by reducing power progressively for a certain rev band before shutting it off at the redline. I think the latter is what you get in a 9-5. As the ECU (Engine Control Unit - I think) controls the throttle (you don't, you just tell the ECU what you want to do), the fuel injection, the ignition and turbo boost, cutting back on power is easy for the ECU to accomplish.

Also note that max torque occurs between 1900 and 4000 RPM for a 9-5, and has dropped way off by the redline. There is little need to rev up to the redline and not much torque left anyway, you might as well shift and get back in the torque curve. This is why I'm not positive if the 9-5 has a hard of soft redline, don't recall ever hitting it. Now my two previous cars had engines that needed lots of RPMs to perform at their best so I know full well what kind of rev limiter they had.

Brooks

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