Car performance calculator

[gliding_serpent]

In my studies of gear ratios and the pros and cons, i found this calculator. You put in your ratios, weight, and model your torque curve, and it gives you optimal shift points and such. Has a decent model of acceleration over time. Could be better, but it was useful plugging my car in, and checking benefits of different ratios. You can do side by side comparisons of different setups.

With my car and a 3.4l engine with flat torque curve, the short ratio acceleration benefits were offset by time lost to extra shifts. It was near a wash vs oem gearing to 120mph. Shortening the shift times made more of a difference to make short gear ratios faster... But it was not night and day... Maybe half a car length from 20-120mph. As is often said, it is best to upgrade between the ears first.

Car Performance Calculator

[gliding_serpent]

Time for some analysis. Warning: this is old news to some, but if anyone has been following my posts, I have been using the rebuild of my car as a means to learn about the theory behind the car, and thus help me choose parts wisely.

Hypothesis: Short ratio gears will improve your acceleration times.

Using the calculator above, I plugged in some info. Of note, this program considers factors like car power, drag, engine weight, driver weight, time taken to shift (500ms), etc. You need to plug in your own torque curve. For the stock format I used a torque curve from Steve Wong's website for a OEM car with stock chip making 217hp (at the crank). Note, this was modeled by eye, so is not perfect. For the 3.4L (my build - 250lbs weight loss, 964 cam, SSI, light intake mods, 9.8:1 single plug 3.4L pistons) I used the torque curve from a recent 3.4L MAF thread posted recently with very similar specs to my car, but with the max hp corrected to something more in tune with what Steve Wong sees for 3.4L SSI cars as I think their dyno was optimistic by about 20hp given historical SSI build results. NOTE: you do not see the short ratio 3.4L car data here as it did not fit on the screen. It is the same as the stock ratio 3.4L but with the same gearing as the 3.2 short ratio. Also note that the data ends at 6500rpm as I did not have good dyno data above that... and it is close to redline anyway. Going up to 7000rpm did not alter the data (trust me, but I programmed a further power drop after 6500).




Here are the dyno curves computed. If you visually check them out... I think they look pretty close to reality, but it is not perfect I am sure. Take note that both the 3.2 and 3.4 have pretty flat torque curves.


Here is the raw performance data the program spits out by default. Much of the calculations are from a rolling start for simplicity of calculations (tire grip, wheelspin, etc). Track types can just interpret things as coming out of corners of various speeds to see benefits/performance gains of stock vs short ratio. Apparently the program computes the fastest times and shift points that give this. Note that short ratio is not showing an advantage. Yes you pull harder, but you also shift more, losing time. In short sprints without shifting, the short ratio wins. OEM longer gearing wins the rest.


I wish this program would not use two near identical shades of blue. Long story short, the lighter more powerful 3.4L engine has a huge advantage on the stock car (surprise surprise). As you can see, the short ratio vs oem gearing in the 3.4L and oem 3.2L cars is a wash. They are neck and neck. Again, extra shifting kills the short ratio car's advantage. Also note that I suspect the short ratio car would eventually surpass the long ratio car above 120mph... but where in the real world will you get to do that? Besides, the short ratio car will top out at around 133mph anyway... and it will be 5-600rpm higher for cruising.


Same data but in a different format... distance traveled. The 3.4 is ahead of the 3.2, but the short vs oem gearing are neck and neck again.


All the blue makes this figure a bit of a mess.


Here is is with only the 3.4L short vs oem (long) gearing.


This data I think is the most telling. This is a 15 second sprint from 20mph. Each line compares two configs, and says who wins the sprint in the end. Long story short, 3.4L stock ratio, beats 3.4L short ratio... by maybe a foot, two at most. 3.2 short ratio beats 3.2L stock ratio (by maybe a foot. I call that a tie. All 3.4L options demolish the 3.2 stock rivals by about 195 feet.


So what have I (and hopefully a few of you) learned? Short ratio in cars with flat torque curves and near OEM rev limits do not significantly benefit from short ratio gearing, ESPECIALLY if you are not shifting fast. And even if your shifts are Lightning quick, the advantage short ratio has is not huge. Also remember that for every extra upshift on a track, you are also adding a downshift. Mind you, over a long race, ever little second counts, but for chumps like me... not so much. Upgrade what is between your ears.

I did not post the data here, but I can tell you that my 250lbs weight loss was responsible for about half of the performance gains (assuming my results match other similar 3.4L builds, outliers aside). That cost me a ducktail, SSIs and new exhaust, AC delete, Heater blower delete, spare and toolkit removal, rear seat removal, and race seats. The 964 grind, 3.4 pistons, engine rebuild costs, exhaust costs... cost me far more and gave about the same gains.

I also did not post the data here, but the 71 SC/RS gear ratios performance was at a wash or was slightly (a foot or two) subpar to the close ratio I posted above. In other words, it made no significant difference from stock gearing.

Finally note that the program was for the most part modeling shifts as high as possible. This reinforces that going to redline in these cars is ideal... In some cases an increased redline would be a help also (assuming your engine is built to stand the torture).

I would appreciate thoughts of others on this data - even attackers. I do not claim to be an expert, in fact I am far from it, and figured this all out in the last 24hrs using a program from the internet. But the data looks compelling. Obviously things are not so simple in real life, and the track is made more complex by turns/acceleration/braking , and you often want to gear to your track. There has to be a reason that race teams have been using short ratio hear boxes for years!!! Mind you, When audi ran the torque heavy R10 TDI at LeMans, they had very long gearing to suit the power curve of the engine, and they did just fine. ;-) But, for the average guy with a slow shifting 915, it is probably best to stick with stock gearing than it is to invest in short ratio.

I would love to hear opposing views from the pro-short-ratio camp. I am sure they have more pros to discuss for short ratio gears. I can think of a few myself.

That being said, I will reiterate that I think short ratio has a place... and that pace is with high rpm cars with a very narrow power-band that is produced high in the rpm range. They NEED short ratio gears.

My next project will be to model the replacement of the SSI's (Known restriction in 3.4L cars, but does presumably flatten the torque curve) with oem heat exchangers and sport exhaust. Steve Wong advises they should give about 10-13hp over the SSI setup without much effect to mid-range. If only I had a dyno curve...