Use the rev limiter!

Discussion in 'Track Days & HPDE's' started by Steve, Jul 25, 2010.

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  1. Steve

    Steve Administrator
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    It's been two years since I thought of asking someone this question but forgot about it and just remembered again today.

    Two years ago I was on VIR with an instructor in my car. Still learning the track and trying to avoid a late up-shift shortly before needing to hit the brakes, I hit the rev limiter. The instructor yelled "use the rev limiter, it's a technique!"

    I didn't understand but it was time to hit the brakes anyway so it didn't matter at that moment. Then I got caught up in the fun again and forgot to ask him what he meant.

    Can anyone explain?
     
  2. Minidave

    Minidave Well-Known Member
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    Don't watch the tach, concentrate on your line and braking points, let the rev limiter tell you when to shift.....................would be my guess.
     
  3. cct1

    cct1 Well-Known Member
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    There are two spots on blackhawk where I would have to shift from third to fourth then almost immediately hit the brakes. I know where these spots are, and I'll glance at the tack and keep the revs just below the rev limiter (as close as I can get to it). I'm giving something up--a little more acceleration--but gaining more in return--no upshift followed by an immediate downshift then threshold braking.

    So if you know where you're hitting the rev limit, and it's safely on a straight, and you're confident of your braking points, checking the tack so you come in just underneath the rev limit might be beneficial. I don't like hitting the rev limit--the car's revs drop after that, it's smoother to come in just underneath and keep it there, if it doesn't make sense to upshift.
     
  4. Dr Obnxs

    Dr Obnxs New Member

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    It is a technique...

    I don't know if it's a good one. Reason I say this is that the rev limiter has to be good enough to keep you from accelerating. I'm guessing in most situations it is, but there may be times when it's not (this would be a downhill). Anyway, the idea is that you may go past an ideal shift point, but holding the revs trades more torque at the wheels for less shifting, as stated earlier.

    Holding revs is usefull when the next "event" (crossing the finish line, next braking zone), come soon enough that having more torque at the wheels is for such a short time that it doesn't make up for the time that you're not delivering power to the wheels. Think of the acceleration curves you see in car mags for manual transmission cars. At every gear change there's a bit of a hiccup. It takes some time and distance to get this back.

    Matt
     
  5. Minidave

    Minidave Well-Known Member
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    At every gear change there's a bit of a hiccup. It takes some time and distance to get this back.

    Not in my car!








    (It's an automatic) :D
     
  6. Dr Obnxs

    Dr Obnxs New Member

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    yeah...

    but with an Auto, can you even get to the rev limiter?

    Matt
     
  7. Minidave

    Minidave Well-Known Member
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    Yes, but your assumption is right, it shifts at redline....

    If I'm in manual mode it will touch it briefly before shifting up to preserve itself, if I don't....

    It actually works pretty good, as sometimes I find myself concentrating so hard on my line, braking point or the car I'm passing that I don't pay enough attention to it. That's the difference between a real race driver and an occasional track guy like me, those skills fall away if you don't use them often enough.
     
  8. istara

    istara New Member

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    I hit the rev limiter on a regular basis in autocross when there is a slow feature that happens just after I should be shifting to second. Turns out bouncing off of it it 10+ times a weekend has put some serious premature wear on my cam, so.. you might want to watch it.
     
  9. Dr Obnxs

    Dr Obnxs New Member

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    One of the media events I get to do...

    has me jumping in and out of really cool cars for 3 laps of Laguna each. Just enough to get a taste. And there are so many different ways the dual clutch and autos deal with redline, and a lot of them are different depending on the mode they are in. My favorite is the Jag, for causual dating. But because we're all so unfamiliar with all the cars, you hear a lot of beating off the rev-limiter on the front straight in 2nd before the shift to 3rd.

    Matt
     
  10. BlimeyCabrio

    BlimeyCabrio Oscar Goldman of MINIs
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    Not on mine you can't. Unless you're in 6th. And, well, that would have to be one h3ll of a downhill.
     
  11. Steve

    Steve Administrator
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    This is sort of what I expected. This guy was supposedly an experienced track rat and the immediate impression I got based on how and when he said it was that he was talking about a recognized technique. I felt like an idiot when I couldn't work out on my own what he might have meant or guess why it might be a good thing...feeling better now.

    After eight years with this car (six at that track day) I'm pretty familiar with the sound and feel (vibration, pull, etc) of it all the way up and down each gear, and I use those to decide when to shift and when to leave it alone...don't we all do that once we're really familiar with a car? It's sort of second nature now like not needing a gauge while walking to tell me that I'm tipping to one side and about to fall over. I rarely glance at the tach and when I do it's usually because I don't understand something that's going on and my brain needs to see a number to make sense of it. If I hit the limiter it's rare and by accident because I'm concentrating on something else and very distracted, and trying to figure out WHY it would be a recognized "technique" to intentionally ride the limiter always leaves me shaking my head and confused.
     
  12. minimark

    minimark Well-Known Member

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    See no reason to be using the Rev limiter as a shift point, the power and torque would be falling off there anyway so all your accomplishing is upsetting the car and not maximizing available power on either side of the shift and extending the off power time during the shift. 2 cents
     
  13. btwdriver

    btwdriver New Member

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    I do 30+ track days a year, normally 4 sessions per day. Out of those 120 sessions and countless laps and shifts, I might hit the rev limiter a dozen times a year. Always by mistake, usually I am at the exit of a corner and just don't get a new gear quite fast enough.
     
  14. goaljnky

    goaljnky New Member

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    I am with you guys on this. I just don't get it. My rev limiter is set at 7200. I know that if I get that high in the range, then I am loosing time as my ideal shift point is around 6750. I had a related conversation over the weekend regarding my shift light. Some implied that I was somehow less capable because I used it. The statement was along the lines of "I know my car and I can hear and feel it". Well, I know my car and I know that when I am out on the track with the windows open and the wind whipping around and other cars around me with the super charger screaming, I have a difficult time distinguishing 6700 rpm from 7100 rpm.

    The few times I have bounced of the rev limiter I found that it upset the car balance enough where I did not care to do it on purpose.

    And back to shift lights: if the guys making a few million dollars a year driving for a living use them, I think I should heed their example.
     
  15. minimark

    minimark Well-Known Member

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    Shift lights are great and I use one, doesn't work as advertised but I can set a certain RPM range that it starts coming on at. Don't rely on it to determine every shift though but use it as sort of a backup warning for when I might be too much in the heat of the moment. Warns that redline is approaching and is a good backup to the mod between my ears...

    If someone says your a wiener for having one please direct them to any F1 in car video.....
     
  16. goaljnky

    goaljnky New Member

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    My point exactly. I have the FES shift light. I actually have it programmed so the yellow light kicks in about 1k early to get my attention. Green lights up when I should shift and then it switches to Red just before the rev limiter. The scheme works for me.
     
  17. ScottinBend

    ScottinBend Space Cowboy
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    :Thumbsup:

    Exactly........
     
  18. Dr Obnxs

    Dr Obnxs New Member

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    That's not the point...

    the point is if you'll have to brake or downshift very shortly after where you'd hit the limiter, you just ride it and stay at the top speed for the gear and not have all the time not under power of the shift to the next gear, and the immidiate downshift shortly after.

    It's just like running past the optimal shift point, having less torque at the wheels, but only momentarily. The way cars run rev-limits now, it's doable. Yes, it sounds funny, but as long as one isn't going down a very steep place, shouldn't result in over-revving the engine. I'd want to know how my car responds to it to make sure that the torque change isn't so severe that it would upset the balance though.

    Matt
     
  19. btwdriver

    btwdriver New Member

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    I would encourage people to see what the RPM difference between gears is. I have an R56 so keep these numbers in mind. In most cases you will be talking about hitting the limiter in third, the RPM difference between the same speed in third and fourth gear is only around 600 RPM. On most cars that don't have race cams you start to lose power towards the top end.

    I am a proponent of grabbing the next higher gear rather than run the limiter. You really only pay for the up shift. The downshift will be done under braking and doesn't generally cost you time. (There is some debate about not being able to left foot brake and that costing some time, but I have been unable to see this in data afterwards.)

    All that being said, there are times when running the car to the limiter makes sense, but I find that in most cases you can drop lap times by shorting into the next gear.
     
  20. Minidave

    Minidave Well-Known Member
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    I agree, especially with the turbo motor.

    With the supercharged motor, I think you can go either way as there is not such a pronounced drop off in power as you get near red line, like there is on the Turbo motor....
     

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