The physics of an engine

[Gismo R]

If you look at a dyno slip for our cars, you'll notice that the torque peaks out before the power does. How exactly does that happen? I know that torque is the dot product of the force applied and the radius of the object, and that power is the dot product of the force and the velocity. But how does the hp continue to rise after the torque begins to fall? And what velocity is measured. Is it the ?tangental? velocity of the wheel/or flywheel? Maybe I would better understand it if I new exactly how a dyno test worked. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/confused.gif" border="0" valign="absmiddle" alt='???'>

Thanks to anyone that can answer this. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>

[DuffMan]

The relationship of torque and power is:

power = torque x RPM x C
where C is a constant

In the case of HP and ft. lbs, i think the constant is 1/5225. So basically, at any given RPM

Power= Torque X RPM / 5225.

This is why the HP is still increasing after torque has peaked out. In order for HP to start decresing the torque has to be decreasing at a faster rate than the RPM's are increasing.

This is also why in dyno graphs you always see the torque and HP cross at 5225.

Whew, feels like i am teaching a physics class.

[AceInHole]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (DuffMan @ Mar. 08 2002,05:38)</td></tr><tr><td id="QUOTE">This is also why in dyno graphs you always see the torque and HP cross at 5225.</td></tr></table><span id='postcolor'>
Actually, on a lot of dyno graphs, the cross point is usually before 5225. &nbsp;The drivetrain loss isn't generally accounted for on a chassis dyno, which would explain the % of drivetrain losses causing the cross point to seem as though it happens earlier.
That said, I'm pretty sure there's a way to figure out your exact drivetrain losses by comparing the cross points... although I haven't figured it out (not that I tried... yet).

[William]

read this, you'll be smarter by the end...

http://www.vettenet.org/torquehp.html

[Sick240]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (DuffMan @ Mar. 09 2002,04:38)</td></tr><tr><td id="QUOTE">Power= Torque X RPM / 5225.</td></tr></table><span id='postcolor'>
isnt it 5252?

[William]

yes, it's 5252. &nbsp;the artcile I posted a link to explains why.

[Gismo R]

Yeah it is 5252. I read the article and it did help me understand. Thanks William

[DuffMan]

Yeah i couldn't remember which way to arrange the last 2 and 5, and I was too lazy to look it up. &nbsp; <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/sleeping.gif" border="0" valign="absmiddle" alt=':zzzz:'>

[Ni5mo180SX]

HA! Thanks a lot guys, I remember learning this a couple years back but not having my car and not talking about cars much, has made me a little rusty.