View Full Version : Specified Northstar Max RPM Vs. Actual on Tach

05-05-04, 12:33 PM
This may have been discussed in the past, however, I am concerned about the safety of WOT at highway speeds.

Today, I was going about 60 MPH in the left lane, waiting for the guy cruising in front of me to realize he was in the passing lane. When I finally had an opening, I floored my STS to pass and, saw the tach go well past 7000 RPM briefly, before I backed off the acceleration. It is probably fine, however, I never fully understood how accurate the tach is at the upper end of the RPM reading.

Can anyone explain?

Best Regards.


05-05-04, 01:56 PM
I think you are just seeing the tach needle overshoot a little as it swung from a low reading to a high one. The STS should upshift about 6500 RPM so if it was nearing the upshift point just after a down shift (it will downshift and accelerate pretty quickly to the redline...) you just caught the tach needle at it's overswing point.

There is an electronic rev limiter at 6800 so you didn't go that high in RPM or you would have heard the engine stumble against the rev limiter.

The RPM information from the crank sensors is going to the PCM which is making all the RPM related decisions for the engine/trans, governing the shift points, etc.....and.....sending the tach info to the dash. On a rapid transient there might be a slight lag and then overshoot on the tach as you describe.

Quit looking at the tach and floor it.....it will take care of itself. The trans shift points and the rev limiter are all continually active and make "decisions" MUCH MUCH faster than the tach is updated or can respond. Trust the PCM to have the correct RPM data realtime.

By "safe" did you really mean something related to "safety" or were you concerned about the engine/trans operating correctly. "Safe" is not a "good" word to use unless you are really concerned about safety. The only thing non-"safe" about your description is taking your eyes off the road while doing a WOT to look at the tach....LOL. Quite looking at the tach.

05-06-04, 06:22 AM
Yeah, I guess I was more concerned about engine safety than anything else. That split second of looking at the tach is where I saw it. Once I got past the guy on 131 South, I had 4 lanes of no traffic.

Thanks for the excellent explanation. I was hoping to read your response. This is what I was wondering about there being a little lag in regards to the ACTUAL RPM already being processed long before the tach has the chance to catch up.

I'm not sure if you guys live in the millisecond or microsecond world, when discussing response times of the processors, such as the PCM etc.

It's funny, the reactions that you see from people when you are in an agressive passing manuever in the STS. The sound of the stock exaust alone, at high RPM, makes their head quickly turn and look our their side window while you pass.

You've got to look quick though to catch that surprised look though, or else they'll be nothing more than a small dot in your rear view mirror. LOL

05-06-04, 11:21 AM
Most of the updates to the various dash and other modules are discussed in the millisecond range....but...depending on the priority of the signal it could still add up to 300 or 600 milliseconds for an update...i.e...torque limit requests, traction control interaction with the brake controller, etc...will have priority over the serial data link compared to a tach update info......LOL.

Keep in mind also that the tach is taking a digital update over the serial data link to the dash microprocessor and converting it to an analog signal and voltage to drive the tach motor that moves the tach needle....so, there is some lag and some chance of descrepancy there in terms of the actual readout on the tach face.

Just floor it (often) and don't worry. There are multiple levels of "engine protection" in the system. Even if you TRY to put it in lower gears manally and floor it the engine really can't be over-revved due to forced shifts and the normal rev limiter built into the system.