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post #1 of 2 (permalink) Old 07-19-2014, 03:42 PM Thread Starter
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Best Shift Points on a Road Course, Stock 6S

I posted this in another thread about CR's track event, but think it's a worth a thread of it's own.

There is always someone confused by the influence of engine hp vs engine torque in best acceleration:

Engine torque is the amount of static torque that an engine dyno would need in resistance to hold the engine at full throttle, at a specifiic rpm.

Eng Hp rates the ability of the engine to do work in a specific amount of time. An example would be spinning a lock-nut on a stud with an air gun set at 50 ft-lbs and 20 rpm. With a conversion factor to change rpm to arc length per minute, you get Ft-Lbs/min, or Hp gun supplied at 20 rpm.

The Torque Peak simply is the highest ratio of [Torque / RPM] .

with N= rpm

Hp = TN/5252 , T = 5252 x HP / N

looking at units (dimensional analysis) :

T = Const x (ft*Lb / min) / (revs / min) ... minutes drop out

T = Const x Ft*Lbs / Rev

Tpeak = 5252 x Tmax /rev

For any gear, the car will accelerate the hardest when at the torque peak, ignoring wind drag. But, in most cases, you can downshift to a lower gear and accelerate even harder, off the torque peak, due to more hp at higher revs, in he lower gear.
.................................................. ............

Years ago, I created an Excel Spreadsheet called "Shift Points". You enter the gear ratios, tire dimensions, and the HP curve from a dyno run, or curves from the auto maker. In this case, it was a dynojet run for the stock 3.0 Mazda 6. The firstcase is the stock 5 speed manual, and the 2nd case is for the 6 speed available in Australia. I put the results on "smug mug" .

The first case is for the as built 5 speed with CR's last tires at VIR:




Note that the last two points on each gear's curve represent 6250 rpm and 6350 rpm.

Since each curve shows the axle torque with full engine available hp based on rpm, there is no case where you would upshift before max rpm, in any gear. Since the plot is axle torque vs rpm , and rpm is expressed by evenly spaced dots on each gear curve, this is actually a 3 dimensional plot for a family of gear curves.

One main point is that it is hard to find a sweet spot of significance based on the engine's torque curve.


1st --- 3.153
2nd -- 1.842
3rd --- 1.258
4th ---- 0.947
5th ---- 0.733
final -- 4.133
tire 275 35 18, 24.81" dia

.................................................. ...............................


I also tried the 6 speed available from Austrailia:




Note that my base Excel program is based on a 5 speed, so for the 6 speed I input data for gears 2-6 .

You still should hit max revs before shifting, excepth at 125mph, you should barely "short-shift" at 6250 rpms in 5th to 6th.

As expected, more axle torque in all gears (1st?), and less max speed in each gear.


2nd ---1.842
3rd --- 1.310
4th --- 1.030
5th --- 0.837
6th ----0.755
final -- 4.388

tire 275 35 18, 24.81" dia

I know a person that uses shipping containers to ship british parts, and cars, overseas. The way to get a 6 speed would be to find a place that gets your 6 speed into a container in australia, then to the US by container ship, and then to you in the states.

.

04 6S atx hatch
93 RX-7, 75 Trident

Last edited by KevinK2; 08-03-2014 at 05:58 PM.
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post #2 of 2 (permalink) Old 08-24-2014, 02:46 PM
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Headed back to VIR in two weeks with better telemetry. I'll be more conscientious about my shifting near red line and share the results.
Thanks for the analysis!
CR

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