Why torque drops at higher RPM ?
Thread Starter
Burning Brakes
Joined: Sep 2004
Posts: 1,117
Likes: 0
From: San Jose CA
Why torque drops at higher RPM ?
I keep wondering what is the most significant factor to cause
Torque to drop at highend RPM. Ignition timing, or valve operation or
any others ?
Torque to drop at highend RPM. Ignition timing, or valve operation or
any others ?
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,271
Neither - the two primary reasons - VE drops off past the torque peak and the ever present friction forces increase parasitic power loss, which increases with the cube of engine speed.
Valve timing (greater duration and overlap to a point) can improve VE at high speeds, but you loose VE at low speed. There is no free lunch! The relative efficiency of the inlet system can be summarized in a dimensionless parameter known as the Mach index (see Taylor's IC engine textbook or the Engine Analyser simulation program.) Once it reaches about 0.55 the inlet system is essentially choked and power will drop off. If you've already reached a practical limit on valve timing the only alternative left is to increase inlet flow efficiency with larger or better shaped ports and manifold runners.
Friction power can be reduced with greater clearances, smaller piston skirts, reduced accessory load, or reduced accessory drive ratios, good dry sump systems to reduce windage losses, but friction is very tough to control when you get over 4000 FPM mean piston speed.
Duke
Valve timing (greater duration and overlap to a point) can improve VE at high speeds, but you loose VE at low speed. There is no free lunch! The relative efficiency of the inlet system can be summarized in a dimensionless parameter known as the Mach index (see Taylor's IC engine textbook or the Engine Analyser simulation program.) Once it reaches about 0.55 the inlet system is essentially choked and power will drop off. If you've already reached a practical limit on valve timing the only alternative left is to increase inlet flow efficiency with larger or better shaped ports and manifold runners.
Friction power can be reduced with greater clearances, smaller piston skirts, reduced accessory load, or reduced accessory drive ratios, good dry sump systems to reduce windage losses, but friction is very tough to control when you get over 4000 FPM mean piston speed.
Duke
Last edited by SWCDuke; May 22, 2005 at 10:24 PM.
Racer
Joined: Nov 2002
Posts: 359
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Originally Posted by SWCDuke
Neither - the two primary reasons - VE drops off past the torque peak and the ever present friction forces increase parasitic power loss, which increases with the cube of engine speed.
Valve timing (greater duration and overlap to a point) can improve VE at high speeds, but you loose VE at low speed. There is no free lunch! The relative efficiency of the inlet system can be summarized in a dimensionless parameter known as the Mach index (see Taylor's IC engine textbook or the Engine Analyser simulation program.) Once it reaches about 0.55 the inlet system is essentially choked and power will drop off. If you've already reached a practical limit on valve timing the only alternative left is to increase inlet flow efficiency with larger or better shaped ports and manifold runners.
Friction power can be reduced with greater clearances, smaller piston skirts, reduced accessory load, or reduced accessory drive ratios, good dry sump systems to reduce windage losses, but friction is very tough to control when you get over 4000 FPM mean piston speed.
Duke
Valve timing (greater duration and overlap to a point) can improve VE at high speeds, but you loose VE at low speed. There is no free lunch! The relative efficiency of the inlet system can be summarized in a dimensionless parameter known as the Mach index (see Taylor's IC engine textbook or the Engine Analyser simulation program.) Once it reaches about 0.55 the inlet system is essentially choked and power will drop off. If you've already reached a practical limit on valve timing the only alternative left is to increase inlet flow efficiency with larger or better shaped ports and manifold runners.
Friction power can be reduced with greater clearances, smaller piston skirts, reduced accessory load, or reduced accessory drive ratios, good dry sump systems to reduce windage losses, but friction is very tough to control when you get over 4000 FPM mean piston speed.
Duke
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Whats the problem? Did u expect something else?
Originally Posted by GroundPounder
I keep wondering what is the most significant factor to cause
Torque to drop at highend RPM. Ignition timing, or valve operation or
any others ?
Torque to drop at highend RPM. Ignition timing, or valve operation or
any others ?
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Whats the problem? Did u expect something else?
Originally Posted by GroundPounder
I keep wondering what is the most significant factor to cause
Torque to drop at highend RPM. Ignition timing, or valve operation or
any others ?
Torque to drop at highend RPM. Ignition timing, or valve operation or
any others ?
I'm bored but not this bored.
cardo0
Race Director
Joined: Oct 2000
Posts: 14,064
Likes: 1,166
From: N. Alabama
GroundPounder, that is a very good question. The same question that cam grinders/engineers have to deal with each time the architecture of the engine changes. Perhaps someone with a little first hand knowledge will chime in as I would be interested myself. The dynamics of such things can really be fascinating. That is one reason I enjoyed Dave Hill's book 'Corvette from the inside out' so much. He touched on a bunch of issues that engineers wrestle with in vehicle development. Such questions as yours usually require more than superficial thought. 
