Short shift to drive faster on track...
#61
Rennlist Member
GRAPHS for and against short shifting
I know there has been a lot of discussion already, but i thought I would provide two examples of were to short shift and where it makes no sense to short shift.
Again, for this discussion, its ONLY in a straight line.
Case for short shifting: (First graph)
To ross Bentley's point, (and he has 2 major points in his video) the first graph is a case for short shifting.
you can see that the HP falls off very rapidly and if you take a standard gear box and the common RPM drop of most gears, (say 75% of the redline RPM post shift) you can see that it pays to short shift this engine. Good new for the owner, there is no need to run the car to redline ever.... his new redline is 6200rpm, based on a 75% post shift RPM.
Next is a case FOR never short shifting: (Second graph)
It is a 911 and you can see the redline is 7,000rpm and post shift , using the same common .75% RPM multiplyer post shift, he ends up at 5200rpm and maximizes hp in doing so. However, if he short shifts, say at 6200rpm (which is peak HP) what happens?? his post shift RPM is 4500rpm and at a spot where his average HP drops significantly as well as post shift HP. (Almost a 45hp difference there.) the real net effect is 208 average HP vs 195 average HP. a net loss of about 12 by short shifting. that's tossing out 12 hp for no reason, other than to save the engine RPM level, which for making your engine last longer is not a huge sacrifice if you are not racing toe to toe. (slightly more with a wider gear spacing and slightly for close ratio gears).
the first graph is a rarity. there are very few cars with engine HP curves that look like this. those are the ones that Ross is referring too in his video.
one other point i should make, is that the feeling of acceleration is tough to gauge because as you accelerate in your race car, acceleration goes down naturally with speed. so, to "feel " the rate change based on one gear to another at redlne vs post shift, is near impossible. its analogous to airplane vertigo as a pilot on instruments you feel like one thing is happening, and it isnt.
Again, for this discussion, its ONLY in a straight line.
Case for short shifting: (First graph)
To ross Bentley's point, (and he has 2 major points in his video) the first graph is a case for short shifting.
you can see that the HP falls off very rapidly and if you take a standard gear box and the common RPM drop of most gears, (say 75% of the redline RPM post shift) you can see that it pays to short shift this engine. Good new for the owner, there is no need to run the car to redline ever.... his new redline is 6200rpm, based on a 75% post shift RPM.
Next is a case FOR never short shifting: (Second graph)
It is a 911 and you can see the redline is 7,000rpm and post shift , using the same common .75% RPM multiplyer post shift, he ends up at 5200rpm and maximizes hp in doing so. However, if he short shifts, say at 6200rpm (which is peak HP) what happens?? his post shift RPM is 4500rpm and at a spot where his average HP drops significantly as well as post shift HP. (Almost a 45hp difference there.) the real net effect is 208 average HP vs 195 average HP. a net loss of about 12 by short shifting. that's tossing out 12 hp for no reason, other than to save the engine RPM level, which for making your engine last longer is not a huge sacrifice if you are not racing toe to toe. (slightly more with a wider gear spacing and slightly for close ratio gears).
the first graph is a rarity. there are very few cars with engine HP curves that look like this. those are the ones that Ross is referring too in his video.
one other point i should make, is that the feeling of acceleration is tough to gauge because as you accelerate in your race car, acceleration goes down naturally with speed. so, to "feel " the rate change based on one gear to another at redlne vs post shift, is near impossible. its analogous to airplane vertigo as a pilot on instruments you feel like one thing is happening, and it isnt.
#63
Jackie Stewart - what could he possibly know about driving? Besides physics has completely changed since the early 70's. The great thing about physics is you never really know what to expect.
-Mike
-Mike
#64
Allow me to explain - short shifting and driving the car delicately to manage weight xfer DOES NOT WORK on old Hoosiers. Case closed.
#65
Rennlist Member
Did you even look at the graphs? I have literally hundreds of E36 engines that show the same characteristics too.
The main point here is maximizing HP is more important than post shift engine torque. does that make sense?
Do you not agree with the messsage the thrust curves i posted say?
2Bwise.. thats a great video talking specifically about gear choice, in and out of turns. even at the 2 min mark, he is talking about being in a gear higher as the car "can be " more docile. and he is talking turns, not the straights. He talks about not banging the curbing, on the brakes smooth and easy . off the brakes smooth and easy. All great info for those cars running Monte Carlo and other tracks.
#67
Rennlist Member
#69
I'm in....
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#70
Rennlist Member
I hope all that were interested got a little benefit of some work i put in to the graphs and posts. they were meant only to inform, give perspective, and not to start a battle.
Race on!
Mark
Race on!
Mark
#71
Rennlist Member
I see another "This message is hidden because mark kibort is on your ignore list." I guess he just doesn't understand no one gives a crap about anything he has to say? Bob, BANNING works, why can't you just go ahead and get rid of this knat?
#72
I am certain that many, many people understand Physics better than Sir Jackie, but he was very good at the practical application of it during his career.
Roger
#73
Shifting at redline for most all cars, (race and street) will give you the most acceleration forces over a speed range for both the gear you redline in and the gear you end up shifting into.
Ross miss spoke on his video, that shifting at redline is just "noise" in most cases. this is not true. as shown by the thrust curves below, you can see this is not true. its a common mistake to think that just because power is going down after redline, or engine torque is falling that shifting earlier helps. it doesnt BECAUSE, the HP in the next gear at anywhere besides RPM will 99% of the time, be much less. this is a fact and is easily proved.
to your last point about shifting at reline being "useful" only if power peaks happen at redline, is not really true as well. Anytime there is a greater thrust force at redline vs the post shift RPM, you need to take it to redline to optimize forces.
Actually, Bentley's video is the only thing im providing clarity for. I know you are not intending to spread "disinformation" because it is a topic that can be a little confusing.
You are absolutely right about not redlining, or even shortshifting, during turns, corners, and traction limited conditions such as rain. ive found this very useful in my racing experiences.
my apologies for making the corrections using the wrong light. Ive made a few here that just stay on topic and provide information that is correct and can help anyone trying to take their car to the limit at the track on the straights.
Below are the thrust curves that have all the answers for most all race cars you will see on the track. Notice at all redline levels, the thrust force is greater that if there was a short shift.
Ross miss spoke on his video, that shifting at redline is just "noise" in most cases. this is not true. as shown by the thrust curves below, you can see this is not true. its a common mistake to think that just because power is going down after redline, or engine torque is falling that shifting earlier helps. it doesnt BECAUSE, the HP in the next gear at anywhere besides RPM will 99% of the time, be much less. this is a fact and is easily proved.
to your last point about shifting at reline being "useful" only if power peaks happen at redline, is not really true as well. Anytime there is a greater thrust force at redline vs the post shift RPM, you need to take it to redline to optimize forces.
Actually, Bentley's video is the only thing im providing clarity for. I know you are not intending to spread "disinformation" because it is a topic that can be a little confusing.
You are absolutely right about not redlining, or even shortshifting, during turns, corners, and traction limited conditions such as rain. ive found this very useful in my racing experiences.
my apologies for making the corrections using the wrong light. Ive made a few here that just stay on topic and provide information that is correct and can help anyone trying to take their car to the limit at the track on the straights.
Below are the thrust curves that have all the answers for most all race cars you will see on the track. Notice at all redline levels, the thrust force is greater that if there was a short shift.
#74
Thread Starter
Rennlist Member
Because most cars do not have the very close ratio transmission that the 997.2 GT3 RS had...and most cars have a longer drop off after HP peak well before red line. Shifting approaching or at peak power (in a straight line), putting the motor into the meat of the torque curve in the next higher gear, in most cars, is faster than wringing the car out to redline well past peak power. His example is very self-serving to his point of view, and he is making a universal extrapolation based on it, whereas Ross and I am others know that this particuular powertrain combination is a rarity. Ross and I an others here have driven a LOT of cars on track. Mark has not, yet claims that Ross and I and others "mis spoke" based on the GT3 RS example above? Pffttt. This is why so many have Mark on ignore.
Last edited by Veloce Raptor; 06-25-2016 at 12:02 PM.
#75
In my experience (which I don't claim is good evidence), performance engines are designed to give increasing power as high up the rev range as possible. There are special cases, e.g. where power is deliberately restricted, where this may not apply. Formula 1 engines rev very highly, producing higher power as revs increase, but with very low torque compared to most performance cars. You would not short-shift if you are accelerating in a straight line and have enough traction.
You do see Formula 1 cars short-shift (during straight-line acceleration) when they have engine issues or need to conserve fuel.
Thanks,
Roger