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Parasitic Driveline Losses


MidNiteFury01

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MidNiteFury01

So i keep seeing various figures for parasitic driveline loss, 13%, 20%, 18%, etc., however, I don't get how it can be expressed as a percentage rather than a straight HP loss throughout the power curve;

(i.e. it takes 22.5HP to overcome the parasitic frictional losses within the driveline regardless of the HP output).

I'm probably missing something...? :blur:

Hypothetical #'s here;

2000 RPM @ 200 HP - 15% Parasitic Loss = 30HP Loss

3000 RPM @ 250 HP - 15% Parasitic Loss = 37.5HP Loss

4000 RPM @ 325 HP - 15% Parasitic Loss = 48.75HP Loss

5000 RPM @ 415 HP - 15% Parasitic Loss = 62.2HP Loss

6000 RPM @ 475 HP - 15% Parasitic Loss = 71.25 HP Loss

I can't see it changing that much throughout the power-curve considering nothing about the driveline really changes from 2000RPM to 6000RPM except the rotational speed of the driveline components (maybe some heat and minor, minor expansion requiring just a bit more TQ to turn).

Has anyone actually dyno'd a driveline? Or is it one of those where you have to dyno'd the engine, then dyno'd the RWTQ and figured out that the driveline takes x amount to turn?

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You are correct. Percentages are just numbers people pull from their asses or calculate from stock SAE ratings to actual wheel ratings on a stock car. There are way too many factors. There is no set percent. Some losses go up as power do, others are set.

My car showed a 12.7% loss when it was stock (505 HP SAE crank, to 448rwhp SAE). That is 12.7%. So now that is made ~1200rwhp, that doesn't mean my motor makes 1350hp at the crank.

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Good topic. Not to throw another variable in, but I've always been interested, or perplexed, by the added factor of a high stall torque converter in automatic equipped cars and how they dyno.

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Stalls can definitely change dyno numbers a ton depending on lock up, and how efficient they are. If a stall is 10% slip at WOT, that is 10% not making it to the wheels, but a lot of the more modern triple disc are in the 5% range.

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Just for fun, I dynoed (is that a real word?) my '08 when it had a 3600 Yank and it showed 411 rwhp. A few months ago, I did another pull (this forum dyno day) and it peaked at 394 with no other changes than swapping to a 4000 stall Yank.

However, now I can run 10's in 1500' DA rather than needing something negative.

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And to add more confusion, 411 to 394 could just be dyno to dyno, day to day differences.

Dynos vary, brands of dynos vary, weather, traction, etc.

Hell, my friend gained 20rwhp from swapping to base c6z wheels from his heavy baller 20" 3 piece wheels. Motor wasn't making more power, just took 20rwhp more to spin the big ass wheels. I'm sure as he adds mods it isn't a percent thing, just a set 20rwhp or so.

Tire pressure can make dynos vary too.

Tom basically nailed it, dyno numbers don't matter, performance does.

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