RWHP vs. HP at the Crank

[m3f_97M3CCC]

ok, I probably have a question that most of you guys have the answer too. First off my 97 M3 has 198 RWHP right, not 240 (that's the crank HP) right? And second all of these different performance sites like AA and UUC and so on always give the amount of HP that a component will give you, but I don't know whether they are talking about RWHP or Crank HP. Do you guys have any input?



For example, on UUC they said that the System U exhaust puts out 6.2 HP, now that sounds like it could be RWHP but i'm not sure. But what really gets me is every site says that the Power Programmer (Shark Injector) puts out like 34 HP so that's probably HP at the crank right?



I'm just really confused.

[m3f_speagle_641be587f7]

240 hp is the crank hp listed that our cars put out. If yours is making 198 rwhp, then its safe to say your making about 230 to the flywheel. I typically assume about 17-18% loss to calculate hp to the wheels.



And as far as claims go from manufactures, i'm not sure. If they do dyno their product, i'd imagine that they are dynoing it at the wheels. However, i wouldnt doubt that if some exhaust company claims 8 hp, that they dynoed it at 5 or 6 whp and made the conversion for us to make the product seem better...i dunno..maybe thats not right, but it seems logical to me

[m3f_VTRick01_f4e20f34c1]

yeah, you've really got to take what the manufacturers say with a grain of salt. There are some chip manufacturers for the e36 M3 that claim like 25 rwhp gains, which means about (25 X 1.17) 29.25 hp at the crank. For a NA car, especially ours, this is bogus from all I have ever seen. Anyone seen gains like this for real? :hmm:



Most of the time, they are talking about crank horsepower unless they say RWHP. But even then, the probably inflate their numbers ,or don't tell you that you get those claimed gains in combination with cold air intake, exhaust system, whatever.

[m3f_M3AN_5f4dc0ba80]

Basic rule for an NA car... if the mod sounds like it gives too much HP... it's BS!



I think your RWHP numbers are sound, most mods are quoted at the rear wheels... exhausts almost certainly are because I think crank HP is calculated with the engine on a rig and only the manifold attached (stand to be corrected). Either way, it would be hella expensive for any co. to rig an engine up to measure HP so I think they're all meant to be RWHP.



IMHO... at the rear wheels, a great exhaust will yield 2 hp and a CAI possibly another 2. Not much more, however much you spend you will not feel it on the butt dyno.

[m3f_astan100_7ef66fbe89]

I thought US e36's put down closer to 210-215 rwhp stock.



Am I wrong?

[m3f_desmo001]

I'll jump in and state the obvious in case the original poster is still confused... rear wheel horsepower is just crankshaft horsepower minus the friction loss/mass effects of the rest of the driveline (tranny, differential, various bearings, tire friction, etc). Horsepower is horsepower, just depends where you measure it.



I think it's probably safe to assume most hp measurements from aftermarket companies are rear wheel hp. And it's also safe to assume that most aftermarket companies are lying to you about power gains. Also remember that different types of dynos give different results, and even the same dyno will vary from run to run.



Desmo

[m3f_fivefreshfish]

^^^ Also, dyno results can vary depending on ambient conditions.

[m3f_97M3CCC]

hey thanks guys for the input i appreciate it. I definatly think they inflate some of those numbers though, some are just rediculous. And I did know about HP is HP and it's lost through all the components on the car until it reaches the wheels, but thanks anyways for all the info. I've just been getting anoyed with all the different numbers I don't know which part to get first, because i want the biggest bang for the buck.

[m3f_95blkbimm3_9842002abc]

97m3cc, on your AA SSK does the shift knob screw on, or does it 'attach' the same was as the stock knob?

[m3f_Realhotguy_80d06ef0ad]

I'll jump in and state the obvious in case the original poster is still confused... rear wheel horsepower is just crankshaft horsepower minus the friction loss/mass effects of the rest of the driveline (tranny, differential, various bearings, tire friction, etc). Horsepower is horsepower, just depends where you measure it.





Desmo

How can you reduce the loss of horsepower to the wheels from the engine?Make stuff lighter and less friction?Where can i find such products?

[m3f_desmo001]

Two ways I suppose, one is to actually reduce the friction losses of the system, and the other way is to reduce the mass of the driveline components so the engine can spin them up faster.



The most common friction improvement is probably synthetic oil in the transmission and differential. I can't think of much else, other than running really hard and skinny tires that are inflated to really high pressures to reduce rolling resistance (think road bicycle tires, like Lance Armstrong uses). That wouldn't do much for your handling though.



Mass reduction would be things like lighter wheels and flywheel/clutch asssemblies. Perhaps a carbon fiber driveshaft as well.



Desmo

[m3f_dyermaker]

You can increase wheel horspower by decreasing "unsprung weight". Making your wheels lighter and a smaller diameter will give you more power to the wheels, light flywheel, pulleys, or really anything that has a rotating force moved by the engine.



That is the only downside to an all-wheel drive car. They lose a lot of power to the wheels since the engine now has twice as many tires to spin. They may take many a RWD car out of the hole, but if you ever catch a WRX on the highway...he'd better look out. :ratatat:



Also does anyone know what one of the most effecient street cars is, in terms of putting power to the wheels with lack of drive train loss. The M3 seems to be about 17%, which is right on par with most other cars out thier. I imagine it would be a very expensive car that uses very light materials to reduce that unsprung weight...Ferrari maybe?

-Chris

[m3f_M3AN_5f4dc0ba80]

Also does anyone know what one of the most effecient street cars is, in terms of putting power to the wheels with lack of drive train loss. The M3 seems to be about 17%, which is right on par with most other cars out thier. I imagine it would be a very expensive car that uses very light materials to reduce that unsprung weight...Ferrari maybe?

-Chris



Any front wheel drive car will lose less than a rear wheeel drive car... so I suppose you start with your high output NA Hondas etc... :???:

[m3f_desmo001]

You can increase wheel horspower by decreasing "unsprung weight". Making your wheels lighter and a smaller diameter will give you more power to the wheels, light flywheel, pulleys, or really anything that has a rotating force moved by the engine.



That is the only downside to an all-wheel drive car. They lose a lot of power to the wheels since the engine now has twice as many tires to spin. They may take many a RWD car out of the hole, but if you ever catch a WRX on the highway...he'd better look out.



You're sort of saying some things that appear to be on topic, but overall you're not quite on track.



Unsprung weight is a term used when talking about suspension, not about drivetrain losses or horsepower. More unsprung weight means your suspension has to be stiffer and more damped to control the wheel motion, but that doesn't have anything to do with spinning the wheels. Obviously lighter weight wheels and tires will help both handling and "available road horsepower", but the term unsprung weight shouldn't be used to talk about rotating mass. For example, you can reduce unsprung weight by replacing steel suspension components with aluminum ones, but that won't increase your wheel horsepower.



And the comments about AWD cars are way too general to be of any use. Saying that the only disadvantage of AWD cars is the fact that they have to spin all four tires is like saying peanut butter sucks because it's made from peanuts. I would say that some of the disadvantages are a higher drivetrain mass and sometimes quirky handling characteristics.



Desmo