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If it were possible to have the perfect cvt, the RPM needs to be at peak HP, NOT peak tq.. Acceleration would be the highest if the engine was kept right at peak HP rpm.mbrooks wrote:
if it was possible to have an infinite number of gears or an infinitely variable ratio, then the rpm could remain at peak tq. and thrust available curve would be a smooth curve.
1st how is Bills post a double? he says that with a CVT it would run at peak HP, not peak Tq, and then goes on to give a real world example of shifting wayy past peak HP to get the highest average HP.mbrooks wrote:
if it was possible to have an infinite number of gears or an infinitely variable ratio, then the rpm could remain at peak tq. and thrust available curve would be a smooth curve. it should also be apparent that it is important to have a relatively flat engine tq. curve over as broad an rpm range as possible. if tq. falls off sharply or has noticeable dips due to intake ram tuning and exhaust tuning effects, it will be more difficult to fill the holes in the thrust curve by changing gear ratios.
it makes the common practive of rating race engines by peak hp. look relatively meaningless.
all the above from racecar engineering & mechanics by paul van valkenburgh
i think it just reiterates what some of you are showing with the graphs and previous posts is all.
oops, just saw bills post, almost a double.
Sounds like Chevrolet and Lotus missed the boat on that one.MadBill wrote:FWIW, many years ago I was doing tests on a ZR1 Corvette (with the Lotus 4-cam engine). I was moving the shift point higher and higher, looking for the magic point where the Gs after the shift matched the 'before'. The nominal power peak was as I recall, in the low six thousands, but I got right up to the rev limiter at 7,200 and still saw less accel after the shift. When I looked at the dyno sheet I knew why: Although the peak was indeed in the low sixes, the drop off was almost nil right out to the redline. For this engine and gear ratios, the optimum shift RPM would been ~ 7,600 and would have resulted in delivering an 'average power' output only a few horses less than the peak!
bill doesn't say hp, he says power, then later refers to torque in a different sentence.F1Fever wrote:1st how is Bills post a double? he says that with a CVT it would run at peak HP, not peak Tq, and then goes on to give a real world example of shifting wayy past peak HP to get the highest average HP.mbrooks wrote:
if it was possible to have an infinite number of gears or an infinitely variable ratio, then the rpm could remain at peak tq. and thrust available curve would be a smooth curve. it should also be apparent that it is important to have a relatively flat engine tq. curve over as broad an rpm range as possible. if tq. falls off sharply or has noticeable dips due to intake ram tuning and exhaust tuning effects, it will be more difficult to fill the holes in the thrust curve by changing gear ratios.
it makes the common practive of rating race engines by peak hp. look relatively meaningless.
all the above from racecar engineering & mechanics by paul van valkenburgh
i think it just reiterates what some of you are showing with the graphs and previous posts is all.
oops, just saw bills post, almost a double.
now you went on to say:
if it was possible to have an infinite number of gears or an infinitely variable ratio, then the rpm could remain at peak tq.
while it would most certainly be possible it will not yield the greatest performance, unless the pk TQ rpm was exactly equal to the peak HP rpm..
b/c at a higher RPM with higher HP the gear can be lower in effect multiplying the TQ.
BTW, remind me not to read books by paul van valkenburgh ...
Absolutely. But in reference to a previous post you'll find that when you only have 1 gear to play with (i.e. dirt oval etc.) you want to reach the "shift point" before its time to lift rather than just at the time to lift, in order to generate the quickest time down the straight. That being said, as you mentioned, there are other important factors to consider, available traction off the corner, what you want to do with the car at the end of the straight (depends on track, track conditions, traffic, etc. etc.), how much overrev the engine can tolerate. So in the end even the quickest straight line roll-on may not generate the quickest laptimes.A Atwood wrote:The ideal shift occurs EXACTLY when the horsepower has fallen to the point that the shift recovery rpm matches the hp before the shift.
ARN
A Atwood wrote:My whole point to this topic is that acceleration is based off of average horsepower output.
Can we agree that an engines potential to accelerate a vehicle is highest at its power peak and NOT its torque peak? I'm not talking about a "given gear", just its raw potential.
ARN
I assume when you say "the high end of the torque curve" you are infact meaning "peak horsepower"David Redszus wrote:
Now that we have all agreed to use the high end of the torque curve (which by definition makes more power), how do we design an engine to meet the needs of the car and the track application?
In both these examples it's a little misleading due to the time the vehicle is under "full power" (not necessarily WOT)David Redszus wrote:Now that everyone is in complete agreement regarding engine performance, gearing and shift points, I'd like to add another function to the equation.
From actual logged data we find that a sprint car on a short dirt oval, uses full throttle only 44% of the time, the other 56% being at some partial throttle position.
A late model, short asphalt track Chevy 350, uses full throttle only 35% of the time, 25% is completely off throttle and the rest is somewhere between.
A histogram of both engine speeds shows only small amounts of time at speeds well above the torque peak and almost no time at the power peak.
Both cars are at the front and are extremely well driven.
So now we have the real world case of engines being driven at the wrong(?) rpm and mostly at partial throttle. So what difference does more horsepower make (except of course in drag racing)?
