[Parkerfe]

Has any one else noticed when you accelerate hard in 1st gear that the rev limiter cuts in at 6500rpm ? It doesn't cut in until 7000rpm in the other 5 gears. My dealer checked it out and claims that the engine was outrunning the tach with the engine turning 7000rpm while the tach is reading 6500rpm. If you slowly run it up to 7000rpm in first the rev limiter doesn't cut in until the tach reads 7000rpm. What do you guys think ?

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[greg]

Your dealer is right. You're outrunning the tach.

 

[RaMsEs]

so much for shifting with precision...

 

[JEM]

If you're watching your tach that closely while under full-throttle acceleration you've got other problems.

BTW, some engines these days have 'predictive' rev limiters. They're set up to measure the rate at which engine speed is increasing, and will cut power early to prevent (for instance) flywheel inertia from overrevving the engine with the clutch in.

 

[MEnthusiast]

Cool stuff- although with full throttle there is no way Im watching the tach in the first few gears- the power is too much- gotta keep the eyes on the road in front!

ME

 

[RaMsEs]

No wonder the M5's are getting spanked by the E55's at the streetlight races...the drivers aren't looking at their gauges!

Seriously though...how can you extract maximum performance without shifting at the right point? I've driven a friend's car at the dragstrip which had a shift light equipped because the revs mounted so damn quick, and even then I could afford a couple super quick glances at the tach to see where I was at...I would hope a streetable car like the M5 isn't worse than a 500+ HP Mustang drag racer in this respect.

Granted a dragstrip is a controlled environment as opposed to stoplight races, I wouldn't race on public streets unless I had a straight and empty road ahead.

So you guys are saying for a fact an M5's tach is useless in fist gear during full throttle acceleration? This poses a problem for me at least, hmm.....

The predictive rev limiter makes a lot of sense. I hope this is why the fuel cut-off hits a little early as opposed to outrunning the tach. It's the inprecision that bothers me. I guess I'll find out for myself in October.

 

[Mr. Grinch]

I can understand needing to shift by the tach when the car is new to you, but after a few months, don't you feel that you know the sound well enough to shift by sound?

 

[Parkerfe]

Originally posted by greg:
Your dealer is right. You're outrunning the tach.

So you're noticing the same problem ? I don't normally watch my tach when I'm driving but I did start when I noticed the rev limiter kept cutting in when I was accelerating hard in first gear. The revs build so quick is hard to fine the right point to shift without bumping the limiter.

 

[RaMsEs]

Mr. Grinch,
I know exactly what you mean, I'm at that point with my current car...but at the dragstrip or when I need to perform at my absolute best, a few quick glances at the tach are necessary for a couple reasons.

First, upon an aggressive launch, I monitor the revs as I "feather" the clutch. It is important to keep revs at a safe point before the clutch is let all the way up to avoid wheelspin. If I know that my car will bust the tires loose at above a certain RPM in 1st gear, then I must acheive that point while feathering...and then go full throttle once I know I'm truley hooked-up to the tarmac.

Second, while in the first couple gears, the rpms pile on real fast. To bang the rev limiter or to shift 300 rpm too early could easily result in 5 thousandths of a second or more lost.

I'm not doubting any of your abilities to drive by "feel"...I do the same pretty well too. Maybe I've run one too many bracket races where consistancy = trophy. Or maybe I've lost one too many races by .001 secs. Or maybe I'm just too anal!

Here's a little story so you guys know where I'm coming from:
There was this one time when a friend of mine decided to run his car down the 1320. He made many runs driving the car according to him "the best I know how to"...the results varied by as much as .7 seconds. I asked him to let me give it a try. He says "man I've been driving this for 3 years, you think you can do better than me?" My first run, I smashed his best time by .3 seconds. Why? Before I ran it, I called up another friend to check the best shift points off of this Toyota enthusiast website. I adhered to them stricly, and got a beautiful launch...as a result beat the best magazine times by .2 secs.

It is this level of precision that gives cars with good automatic transmissions (E55) an edge over others in day-to-day situations. Such cars will give 99% of their performance 99% of the time. A stick?...hmm...well that depends on the driver and how masterfully he can use it.

-Shawn

 

[MEnthusiast]

you will discover the right point and it will become something you do by feel: the sound of the engine, the feeling through the various parts in the car.

ME

 

[rickw]

It's pretty much a given that fast-rev'ing high-performance cars will over-run analog tachs in 1st gear.

Digital tachs don't have this problem, but are less desirable in almost every other way (in my opinion)

Besides, I agree with Grinch and MEnthusiast about the importance of watching where you're going first and the tach 2nd. It doesn't take me long to get used to the pitch of the engine and the feel of the torque curve and be able to shift effectively out of 1st while keeping my eyes on the road.

Perhaps there's another 0.1s or two to be found by hitting the mark perfectly, but even then you can recalibrate your tach mentally. It may be lagging, but it should lag consistently. Note what the tach is reading when the limiter hits and shift earlier.

Rick W

 

[RaMsEs]

rickw,

That sounds like a great idea to me, but won't be an easy task.

At first it seems logical to take the rpm the rev limiter is known to cut in at and formulate a percent error using the percieved rpm at which the cut-off occurs. But this probably won't be correct since the tach's needle surges faster at peak performance points (or so this problem would suggest). A dyno can't help either...you shouldn't go full throttle in 1st gear on a dynometer.

I'll play around with it after my car is broken in. It will be some time though since I don't even have it yet!

I must say that it is discussions like this (require me to intensely imagine piloting the beast) that I come away from feeling like I've actually driven the. How sweet it will be

 

[rickw]

Whoa.. don't overcomplicated it there.

The only time you care about shifting at the absolute perfect point is when you are doing a full-throttle, wind-it-out, acceleration run right?

If so, then just learn at what displayed RPM your car hits the rev limiter. That should be consistent under those same circumstances. Then back off a few hundred RPMs for your shift point (experiment, as necessary, for best results)

It's also worth noting that, not even counting over-run, most analog tachometers are not perfectly accurate anyway. At a steady-state RPM, they are typically around +/- 2% which is a possible 300rpm variation between different tachs at redline.

Basically, my point is to learn the sound and feel of your car and enjoy it.

Rick W

 

[intrepid one]

I am a new M5 owner, and also have a 325i with an automatic. As I am getting comfortable with the M, I totally belive that you could "outrun the tach" in 1st gear. ....no problem..

 

[E55AMG2]

So you're noticing the same problem ? I don't normally watch my tach when I'm driving but I did start when I noticed the rev limiter kept cutting in when I was accelerating hard in first gear. The revs build so quick is hard to fine the right point to shift without bumping the limiter.

Every M5 does it. Just learn where the rev limiter cuts in on 1st and 2nd gears and shift there.

 

[DRM]

Well......

So you're noticing the same problem ? I don't normally watch my tach when I'm driving but I did start when I noticed the rev limiter kept cutting in when I was accelerating hard in first gear. The revs build so quick is hard to fine the right point to shift without bumping the limiter.

Try running an S3 @ WOT from 1st gear ; especially when you're old and have slower reflexes, like me. 'Had to go back to 3:15 diff just to buy an extra couple of ticks to shift when nailing it out of low speed, low gear turns at the track!!oohhh:

 

[Blockdoc]

Try running an S3 @ WOT from 1st gear ; especially when you're old and have slower reflexes, like me. 'Had to go back to 3:15 diff just to buy an extra couple of ticks to shift when nailing it out of low speed, low gear turns at the track!!oohhh:

Gee Dan,
That's a tough problem to have! I'd be happy to trade you for my stock Beast if it bothers you

Brian:cheers:

 

[intrepid one]

Just a question, wouldn't changing to a 3:15 or 3:45 diff just make it "happen " quicker..I'm thinking that redline in both 1st and 2nd gear would be gotten to just a little quicker, ergo increasing the phenomena of outrunning the tach...Oh by the way this is a terrific board. Everyone seems so supportive and helpful..

 

[CSBM5]

Just a question, wouldn't changing to a 3:15 or 3:45 diff just make it "happen " quicker..I'm thinking that redline in both 1st and 2nd gear would be gotten to just a little quicker, ergo increasing the phenomena of outrunning the tach...Oh by the way this is a terrific board. Everyone seems so supportive and helpful..

He went from the S3's 3:45 diff back to the stock 3:15.

Chuck

P.S. This is one of the oldest threads drawn out of retirement I've seen in a while.

 

[intrepid one]

I have a rev limiter question. As a new M owner, I was ahhhhh..."checking the car out"...I bounced it off the rev limiter in the first three gears. Some very nice lady was taiking to me thru the dashboard, and for the next while, I could only get to 3000 rpm..I was a little too busy to pay much attention to her. After I got home, and the car had set in the driveway for a half hour or so, It seemed to reset...Everything seems ok now..Is this normal?