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Some analysis on ideal shift points

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siliconcenturion

Go Kart Champion
I posted this over at golfmkv, and everyone seemed to think it was useful, so I thought I'd re-post it:
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Common wisdom on this forum suggests that GTI owners should shift well before redline since the turbo runs out of breath so early. I did analysis on some some dyno logs to show that this is not the case.

Here's the dyno output from a friend's APR stg1 2009 DSG TSI, with requested/actual boost superimposed.



In order to find optimal shift points, torque*gear ratio curves need to be found for each gear. These curves represent the transmission output torque of the car. Where these curves intersect is considered the best place to shift. You can read some discussion on why this is the case here.

Anyways, I wrote a MATLAB script to use the raw output file from my friend's dyno log and also used the gear ratios for the DSG to compose a graph of the different transmission torque curves for each gear. The intersection of these curves gives the ideal shift points based on this dyno information.



The graph shows that contrary to popular assumption, you really shouldnt shift more than a couple hundred RPM's before redline.

3rd gear 91oct run, same car





Low-end torque dropped compared to the 4th gear pull, so all of the shift points are higher than that run.

Here's 'stock'. Either his intake is good for 10+hp, or his 91oct tune is underperforming.





From this graph, in a stock car like this one, it seems like you should be shifting at redline all the way up until 4th gear.

DSG and 6MT gear ratios are pretty similar, so it makes very little difference as to shift points. Maybe -50rpm or something inconsequential.



Of course, this is just one set of runs from one car with a specific set of mods.
However, even given just this info, I think it's clear that you should generally shift fairly close to redline.

If you have your own dyno data (dynojet .drf output files), send it to me and I'd be happy to do the same analysis.

Hope this clarifies some things.
 
B

BAM its mitch

Go Kart Champion
Personally I would say that 6k RPM is quite a bit before redline, especially when you are in the higher gears and your RPM's aren't climbing quite as fast. I did the same analysis except I just used numbers from APR's torque curves and found roughly the same thing. Carry 1st out to redline and then shift a little earlier each time from then on out. 5th to 6th should be at around 6k.
 
DRedman451

DRedman451

Go Kart Champion
ok so basically there are too many outside factors to base this purely on a dyno and gear ratios

grambles423 said:
When driving a vehicle you're air drag increases exponentially with every MPH. With constant Cd and frontal area, its not hard to calculate. This air drag directly coorelates to traction force which is calculated by your driveline.

THAT then leads to temperature variation and fluid viscosity of the oil in the gears, which leads to oil shear across the mechanical parts, which then leads to heat transfer principles, which then leads to convection parameters which then affects underbody drag, which ultimately affects our traction force again.
Iterative I know.......but doable. We also havent accounted for driveline losses with pitch angles and force transfers between gears. Lets not even get started about human factor (which is actually capable of being calculated, but I havent been able to talk to those guys about it. They stay locked in a room doiong ergonomic principles for manufacturing design)

We can even go into air density variation around the hood with increasing engine temperatures during a 0-60 or 0-100 run if you'd like. A little bit of thermo for some people, but again doable.

Fuck it, lets do variable coefficient of drag accounting for your vehicle not being washed and waxed properly.

This is all microscopic I know, but every tenth to hundredth of a second adds up. We had an issue with acceleration when we were designing the Acura TL. Someone didnt account for some of these things and we almost let go wrong specs for 0-60 acceleration which is illegal, FYI.

This just goes to show you, theres MANY things to account for rather than a dyno and some gear ratio numbers.





FYI if hp= (torque*(RPM/5252)) then:
5200RPM Torque = 207 (assuming constant tq) hp = ~204hp Dyno verifies.
6100RPM Torque = 172 (following dyno characteristics) hp = ~200hp. Dyno says 186hp.

Its a good estimator tool to use, but its not accurate enough for forced induction vehicles. Pressure ratio has too big of a play in here.

Use this instead:

Brake Horsepower (Watts) = N*Displacement(in cubic Meters)* (RPM/2)* nc*nm*nth*nv*Qhv*(F/A)*Air Density

N = Number of cylinders
Displacement = Bore*Stroke*pi/4
RPM/2 = Must be placed in Revs/Second = RPM/2 * (1/60 {s/min})
nc = Combustion efficiency = about .98-.99 on newer cars (can be calculated further, but these estimations are fine)
nm = mechanical efficiency = Function of RPM/heat/etc range of about .8-.93 (can be calculated further, but these estimations are fine)
nv = volumetric efficiency (what percentage is your TB open?)= 100% at WOT (Possibly a little more depending on the flow. Most I've seen is 108%)
nth = thermal efficiency = 0.8*(1-[compression ratio^(-.35)])
Qhv = Heating value of the fuel = 43,000,000 J/kg

Air Density = Pressure Entering / (287.2 J/kg-K * Intake Temperature)
Pressure Entering = Atmospheric Pressure * Pressure Ratio (This is a function of RPM) - J/m^3

Torque is then function of power. But can be calculated based off of explosion rates of the ignition and the force generated on the piston. There will be some mechnical loss due to the rod and journal bearing action, but should be close.

Now you can see pressure ratio variation is a BIG factor in this. Most NA vehicles follow a linear graph because of this reason. Pressure entering is just atomosphereic for an NA vehicle.

All of this though is a function of heat and material dynamics. Iron and aluminum tend to move a lot more once heated up so that must be calculated too.

Waaaaaaaaaaaaaaaaaaayyyyy off topic....sorry. Just trying to stretch some brain thinking. :D

Shift at 5700RPM
Click to expand...
 
S

siliconcenturion

Go Kart Champion
DRedman45 said:
ok so basically there are too many outside factors to base this purely on a dyno and gear ratios
Click to expand...

Heh, I'll go post in that thread, thanks.

edit: I agree in theory that there are many factors at play, but I would bet the most important ones are the curves above and the available traction. Obviously no one is going to shift at redline in first. 5700rpm is too low, however. I'm not sure how grambles arrived at that number.
 
Last edited:
McQueen77

McQueen77

Banned
if you get a feel for your engine, you don't a graph. good drivers for eon's knew when to shift. shift too close to redline, you run out of steam and its diminishing returns. shift too soon and you don't land in the sweet spot. shift in the right place and you'll always be in the sweet spot/meat of the powerband. 4000-4500 rpms is a nice place to start.
 
kthor7031

kthor7031

Go Kart Champion
Did you reflect the two different final drive ratios in this car in your graph? We have two different ones.
 
M

mully

Ready to race!
to quote voltaire, perfect is the enemy of good. Those really are words to live by. If you strive for perfection over better you will fail almost every time.
 
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