The physics of an engine - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

Gismo R · 03-10-2002, 02:36 AM

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 · 03-10-2002, 03:38 AM

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* · 03-10-2002, 10:00 AM

</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.  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 · 03-10-2002, 11:01 AM

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

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

Sick240 · 03-10-2002, 09:03 PM

</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 · 03-10-2002, 09:25 PM

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

Gismo R · 03-10-2002, 09:29 PM

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

DuffMan · 03-10-2002, 10:22 PM

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

Ni5mo180SX · 03-11-2002, 01:07 AM

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.

03-10-2002, 02:36 AM	#1
Gismo R Zilvia Member Join Date: Feb 2002 Posts: 167 Trader Rating: (0) Feedback Score: 0 reviews	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:'> __________________ '97 240sx SE 5-Speed: Injen CAI ; GReddy MX

03-10-2002, 10:00 AM	#3
AceInHole Autox Technician Join Date: Dec 2001 Location: New Haven, CT Age: 43 Posts: 3,961 Trader Rating: (0) Rides Gallery: AceInHole Feedback Score: 0 reviews	</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.  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). __________________

03-10-2002, 09:03 PM	#5
Sick240 Leaky Injector Join Date: Jan 2002 Posts: 114 Trader Rating: (0) Feedback Score: 0 reviews	</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? __________________ 90 240sx Coupe

03-10-2002, 09:29 PM	#7
Gismo R Zilvia Member Join Date: Feb 2002 Posts: 167 Trader Rating: (0) Feedback Score: 0 reviews	Yeah it is 5252. I read the article and it did help me understand. Thanks William __________________ '97 240sx SE 5-Speed: Injen CAI ; GReddy MX

03-11-2002, 01:07 AM	#9
Ni5mo180SX Zilvia Junkie Join Date: Dec 2001 Location: Orange County Posts: 423 Trader Rating: (0) Feedback Score: 0 reviews	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. __________________ Mazda RX-7 FC3S: Jspec motor, ported turbo & wastegate, JIC FLT-A2 coilovers, Mazdatrix adj. rear camber link, TB mod, Koyo 53mm Radiator, Corksport Cat back/midpipe/downipe, 3" Al TID, Apex'i air cleaner, Mazda Comp. mission/motor/differential mounts, Mazdaspeed Strut Bar(R), FD torsen differential, HKS SSQV, HKS elec. boost gauge & elec water temp, Cusco Oil catch, 680cc secondary's, Walbro 255, Banzai Racing FCD, RB DTSS Eliminator bushings, Exedy Clutch & PP, Exedy chrome moly flywheel, RB oil baffle, RB dual alt. pulley, 5 lug conversion, FC3S.org thermal bypass pellet, KVR Cross drilled rotors, complete Energy PU bushings, stainless steel brake lines/clutch line, Aerodyne shroud panel, Azenis RS, and some other stuff. A hybrid of sorts I guess

03-10-2002, 03:38 AM	#2
DuffMan Post Whore! Join Date: Oct 2001 Location: in a cardboard box Posts: 2,517 Trader Rating: (0) Feedback Score: 0 reviews	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.

03-10-2002, 11:01 AM	#4
William Leaky Injector Join Date: Feb 2002 Posts: 90 Trader Rating: (0) Feedback Score: 0 reviews	read this, you'll be smarter by the end... http://www.vettenet.org/torquehp.html

03-10-2002, 09:25 PM	#6
William Leaky Injector Join Date: Feb 2002 Posts: 90 Trader Rating: (0) Feedback Score: 0 reviews	yes, it's 5252.  the artcile I posted a link to explains why.

03-10-2002, 10:22 PM	#8
DuffMan Post Whore! Join Date: Oct 2001 Location: in a cardboard box Posts: 2,517 Trader Rating: (0) Feedback Score: 0 reviews	Yeah i couldn't remember which way to arrange the last 2 and 5, and I was too lazy to look it up.   <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/sleeping.gif" border="0" valign="absmiddle" alt=':zzzz:'>