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Discussion Starter #1
i loved physics, but i took the calculus-based series nearly 100 years ago. the subject says it all. it seems tranny losses (which i assume are frictional) should be a constant (the tranny's coefficient of friction) rather than a function of the applied force (which is what we're saying when we talk about losses as percents).

in a stock setup, i can see that we would say tranny losses are x%; say 25%. for example, a stock wrx with 227bhp produces 170whp; 57hp tranny loss.

but if we then add mods and get 250whp, it *seems* that people will reverse the logic and say it's making 333bhp (333bph - 25% = 250whp).

if my supposition is correct (that tranny losses are constant), then these mods would actually result in "only" 307bph (250whp + 57hp tranny loss = 307hp).

put another way. if we add did mod that added 100bhp (note the "b"), i would expect all 100 extra bhp to make it to the wheels.

i'm missing something fundamental. enlighten me.
 

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where is 91TB78?

Probably because people don't know any better.

But remember that driveline inertia also plays a roll. The less load your dyno puts on the car, the more of an effect inertia has... Especially on a dynojet, where ALL of the load is just the inertia of spinning the drum. Some dynos can actually decelerate the car at WOT, I'd imagine that would take driveline inertia out of the picture. On a dynojet lighter driveline components should show up as a % improvement, so in that sense its % based.

Since different chassis dynos read differently, it stands to reason that engine dynos do too. IMO thats just way too many variables to try and figure out. The formula for drivetrain losses probably includes tire type, tire pressure, weight over the drive wheels, oil viscosity and temp (for engine, tranny and diffs), driveline inertia, speed, RPM and other stuff I don't know about.
 

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Sacriledge?

Well, think about this from a practical standpoint rather than theoretical. Would you pull your engine out and have it put on a dyno after adding mods to get bhp? Probably not. That's the only real way to measure it after adding mods. Anything else is just a guess. You can really only measure the whp before and after. It's just an educated guess as to how much of the difference is due to mods and how much is more or less from the drive train loss. Maybe if you email Shiv or Cobb they might have done somehting like this.
 

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First we need to understand what the difference is with BHP, Net Horsepower, and wheel horsepower,

-BHP- we know this is measured at the flywheel, however lots of people forget that this is measured without any load (drivetrain) and engine accessories attached to it, things like p/s pump,alternator,a/c compressor etc..
-Net Horsepower- It is measured at the flywheel and with all acessories attached to the engine, this is more ideal to use and more accurate when you are talking about "Horsepower at the fly wheel".
-Wheel Horsepower- is measured obvisously at the wheel with load from drivetrain and all accessories. this is actually the true horsepower measurement when you are talking about horsepower, because the reading reflects the actual horsepower needed to move the car.

why is the powertrain loss is expressed in percentage?

Well, when you are measuring WHP you need to take all thing into consideration, because horsepower loss is due to "Friction" between the flywheel to the actual drive wheel. example, front wheel drive cars has less components between the flywheel to the wheel, it goes from trans. to axle to wheel(least amount of friction). rear wheel drive cars has single axle to differential,axle and to the wheels (moderate amount of friction). finally AWD/4WD cars axle, center diff. drive shafts, rear diff.(most friction) Friction is not constant in automobiles because theres too many moving components that can effect friction, think about your a/c compressor, when on its taking HP away to make that cool breeze, power steering, power brakes etc.. when on or off its adding or removing load to the engine therefore effecting your coefficient of friction. remember typical powertrain loss is between 15-22% but it will vary from car to car. its not so much about theory dont get me wrong you need that too but having a good understanding about how horsepower is made is more so relavent to this.
I hope this had help to clear thing up alittle....

Billy
 

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OnefastWRX said:
why is the powertrain loss is expressed in percentage?

...remember typical powertrain loss is between 15-22% but it will vary from car to car. its not so much about theory dont get me wrong you need that too but having a good understanding about how horsepower is made is more so relavent to this.
I hope this had help to clear thing up alittle....

Billy
Your saying that driveline friction goes up with power, as a percentage? BS, and here is why:

Friction causes heat, all energy lost to friction is converted into heat. Take a stock Supra making ~300whp, and say its got 15% driveline loss, which is 50hp worth of heat thats going into the transmission and rear differential (engines are usually dynoed with the accessories attached, although the alternator is usually under less load). Neither the transmissor or diff has any kind of active cooling at all.

Now take Titan Motorsport's Supra, which dynoed 1150rwhp. They run the stock 6-speed transmission and a stock rear diff with a modified spool. Assuming a 15% loss, the would be putting 172hp into the tranny and diff, thats more than most cars have to dissipate in their radiator! If that were the case, the fluids would quickly boil and things would break. If we go by the general rule that ICEs are 33% effecient (1/3rd power, 1/3rd into the radiator and 1/3rd out the exhaust), then that would be like trying to cool a 516hp engine without a radiator. With the exception of automatic cars with high stall converters, friction just doesn't go up linearly with power. Lots of cars out there road race with double the stock power output and I've never heard of one boiling tranny or diff fluid.

Its not like BHP matters anyways, and with all these different dynos, good luck finding 2 brands that read the same with whp, let alone wheel vs. crank hp.
 

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How much less wheel-driven horsepower will depend on how many MECHANICAL parts exist between a vehicle's engine and its driven wheels. This is usually measured as a percentage loss due to the "friction" (and yes i know friction causes heat) of the intermediate components between the flywheel and the actual wheel. For a Rear Wheel Drive car, engine power has to travel through a transmission, driveshaft, rear-differential, and two axle shafts (one for each rear wheel). That's four separate mechanical components taking a bite out of the car's horsepower before the rear wheels even begin to turn. Front-wheel drive cars with transverse-mounted engines usually have a lower frictional loss because horsepower only has to travel from the engine, through the transmission and down two short driveshafts before reaching the wheels. with this said again i hope this will clear things up alittle because theres been a misunstanding some where in here. when did we start to compare bhp to whp in the message??? :)
 

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Discussion Starter #7
thanks for the responses, but the spirit of the question remains unaddressed (i think). maybe the "subject line" was too general. i know why they use percentages in the *stock* case. what i wanted to know was if it is correct to continue to use those percentages *after* mods. i'm motivated to ask this because i have heard people either explicitly or implicitly use the stock %loss for a modded car. as in my example above, using the stock percentage significantly overestimates the modded bhp. <shrug> maybe the point is too esoteric or irrelevant.
 

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Maybe you're just too smart for the rest of us!!!

;)
 

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Heres my opinion about this now that the subject seems to be clear, we know drivetrain loss is due to friction and in term causing heat, as you Mod your car to produce more horsepower you are placing more load on the drivetrain thus more heat is being generated due to friction (loss of horsepower). For example, if you have two identical cars with the same drivetrain set up, ones got 600 horsepower and the other has 300 HP, obvisously the 600 hp car is going to be faster than the other but the 600 hp cars drivetrain is being placed under twice the load as the other causing all the fluid to be heat up faster, this is true because we know the increase in acceleration, and the same work is performed in less time, the result is that more horsepower was required Since the force required to spin the gears grows with speed, and since increased horsepower is causing the rate of acceleration to increase so the actual amount of work required to spin the gears is greater than with lower horsepower. Therefore as horsepower goes up the percentage of loss power loss through drivetrain actually increases. so you cant apply the drivetrain loss number to a car after mod. now this is just my opinion I look at it this way, having the 1150 HP supra is great on the strip but how great is it once you put the car on the streets in terms of reliability, adding additional coolers/fins to the drivetrains is definitly something i will think about.
 

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Gauges

Lets get some equations going. Aren't there any ME's that can help with this? Common sense theories are not going to help here.
 

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I was just reading up on this topic last night...how timely that it came up again today!

I was visiting the vishnu website and here is what Shiv has to say:
http://www.vishnutuning.com/dyno_myth.htm

Basically, the two ways that 4MLA1FN mentioned are the extremes of calculating flywheel hp. In his example then, the wrx produces at least 307 hp but no more than 333 hp. You wanna be conservative, just quote the lower number. That way you know you have at least that.

In the next article, Shiv talks about the various ways tuners can "fudge" dyno numbers:
http://www.vishnutuning.com/dynos_dont_lie.htm

There are all sorts of correction factors as well as how measurements are taken that can greatly affect the outcome of the dyno numbers.

Hope this settles some of the controversy :) No equations were given in the articles, possibly due to the many factors that lead to drive-train loss.
 

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Discussion Starter #13
Re: Dealer Mark-up List

great find SnowbordrWRX. thanks for posting. i'll check out his stuff tomorrow.
 
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