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2013-14 Shelby GT500
Why more torque with lower pulley?
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<blockquote data-quote="Bad Company" data-source="post: 13683090" data-attributes="member: 141815"><p>I'm not the best one to answer this question, but I believe it is simply a matter of the fact that the larger diameter lower pulley is spinning the blower to higher RPM at lower engine RPM. Which creates higher boost at lower engine RPM. Higher boost at lower engine RPM = more torque. </p><p></p><p>The stock lower is 7.1" in diameter with a 2.71 diameter blower pulley. 7.1 divided by 2.71 = 2.61 This gives you a 2.61 drive ratio. . </p><p></p><p>2.61Ratio x 4500 Engine RPM =11,745 Blower RPM</p><p></p><p>Lets say we increase the lower pulley diameter to 8"</p><p></p><p>8 divided by 2.71 =2.952 drive ratio</p><p></p><p>2.952 X 4500 Engine RPM = 13,284 blower RPM</p><p></p><p>Now lets try a stock lower 7.1" diameter pulley with a smaller blower pulley. For ease of installation and not requiring an idler pulley in the equation to keep the belt from slipping we will reduce the blower pulley down to a 2.5" diameter. </p><p></p><p>7.1 divided by 2.5 = 2.84 drive ratio</p><p></p><p>2.84 Ratio x 4500 Engine RPM = 12,780 Blower RPM</p><p></p><p>As can be seen by the math the blower is turning 504 more RPM at 4500 engine RPM with the 8" diameter lower pulley and the stock 2.71" blower pulley, versus the 2.5" upper pulley and the 7.1" stock lower pulley. That is a difference of 1,159.2 Liters of air entering the engine with the 8" lower over the 2.5" upper where peak torque would be produced. Higher blower speeds will generate more boost at the same engine RPM, so power output of the engine will increase. </p><p></p><p>Can you go down in upper pulley diameter to match the 8" lower? Yes</p><p></p><p>The 2.4" diameter upper with the 7.1" diameter lower has a slightly faster drive ratio of</p><p></p><p>7.1" divided by 2.4" = 2.958</p><p></p><p>2.958 Ratio x 4500 Engine RPM = 13,311 blower RPM</p><p></p><p>But to install this small of a pulley is now getting into other problems. The blower support may need to be modified to fit the pulley. An Idler pulley should be added to get a higher degree of belt wrap around the smaller diameter upper pulley to counter against belt slippage. You get into more supporting mods with the smaller upper pulley versus the lower for not much gain in blower RPM.</p></blockquote><p></p>
[QUOTE="Bad Company, post: 13683090, member: 141815"] I'm not the best one to answer this question, but I believe it is simply a matter of the fact that the larger diameter lower pulley is spinning the blower to higher RPM at lower engine RPM. Which creates higher boost at lower engine RPM. Higher boost at lower engine RPM = more torque. The stock lower is 7.1" in diameter with a 2.71 diameter blower pulley. 7.1 divided by 2.71 = 2.61 This gives you a 2.61 drive ratio. . 2.61Ratio x 4500 Engine RPM =11,745 Blower RPM Lets say we increase the lower pulley diameter to 8" 8 divided by 2.71 =2.952 drive ratio 2.952 X 4500 Engine RPM = 13,284 blower RPM Now lets try a stock lower 7.1" diameter pulley with a smaller blower pulley. For ease of installation and not requiring an idler pulley in the equation to keep the belt from slipping we will reduce the blower pulley down to a 2.5" diameter. 7.1 divided by 2.5 = 2.84 drive ratio 2.84 Ratio x 4500 Engine RPM = 12,780 Blower RPM As can be seen by the math the blower is turning 504 more RPM at 4500 engine RPM with the 8" diameter lower pulley and the stock 2.71" blower pulley, versus the 2.5" upper pulley and the 7.1" stock lower pulley. That is a difference of 1,159.2 Liters of air entering the engine with the 8" lower over the 2.5" upper where peak torque would be produced. Higher blower speeds will generate more boost at the same engine RPM, so power output of the engine will increase. Can you go down in upper pulley diameter to match the 8" lower? Yes The 2.4" diameter upper with the 7.1" diameter lower has a slightly faster drive ratio of 7.1" divided by 2.4" = 2.958 2.958 Ratio x 4500 Engine RPM = 13,311 blower RPM But to install this small of a pulley is now getting into other problems. The blower support may need to be modified to fit the pulley. An Idler pulley should be added to get a higher degree of belt wrap around the smaller diameter upper pulley to counter against belt slippage. You get into more supporting mods with the smaller upper pulley versus the lower for not much gain in blower RPM. [/QUOTE]
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2013-14 Shelby GT500
Why more torque with lower pulley?
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