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SVT Shelby GT500
Overdrive Balancer
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<blockquote data-quote="Bad Company" data-source="post: 15369325" data-attributes="member: 141815"><p>Joewee</p><p></p><p>You have to do a drive ratio calculation to answer this question</p><p></p><p>Now the big question is what manufacturer's lower are you using in the calculation?</p><p></p><p>IW and STI diameter of their products. For some unknown reason each has different diameters listed for dampers for the GT500 engines. </p><p></p><p>Stock Diameter</p><p></p><p>IW = 7.225" diameter</p><p></p><p>STI = 7.5" diameter</p><p></p><p>IW and STI list 2 different damper diameters for the 10% OD lower. </p><p></p><p>IW = 7.95" diameter</p><p></p><p>STI = 7.81" diameter</p><p></p><p>Yet when it comes to the 15% OD dampers, both companies have very close to the same diameters listed for their products.</p><p></p><p>ATI = 8.13" diameter </p><p></p><p>STI = 8.12" diameter</p><p></p><p></p><p>So we have quite a few different math calculations to perform to see what each pulley combination does to the drive ratio of the SC versus the engine, depending on which brand of lower pulley that you'd purchase.</p><p></p><p>Lets do the math using IW as your choice for the lowers in this first calculations</p><p></p><p>15% lower with a 2.5" upper is</p><p></p><p>8.13 divided by 2.5 = 3.252 drive ratio</p><p></p><p>10% lower with a 2.4" upper is</p><p></p><p>7.95 divided by 2.4 =3.3125 drive ratio</p><p></p><p>So the larger diameters between these two would actually being spinning the SC slower</p><p></p><p>Always multiply the drive ratio number by engine RPM to achieve SC RPM for the airflow exiting the SC, if everything was at 100% efficiency as far as airflow was concerned</p><p></p><p>Now lets do this with the STI dampers and the different diameters listed by STI for each</p><p></p><p>15% lower with a 2.5" upper</p><p></p><p>8.12 divided by 2.5 = 3.248 drive ratio</p><p></p><p>10% lower with a 2.4" upper</p><p></p><p>7.81 divided by 2.4 = 3.25 drive ratio</p><p></p><p>These two STI lower and upper pulley combinations have nearly identical drive ratios. The 10% lower with the 2.4" upper would yield a very slight advantage in airflow due to the drive ratio of 3.25 versus 3.248. Yet the STI 15% lower with the 2.5 upper pulley with its much larger diameter pulley combination would result in a lot less chance of belt slippage as the engine is being accelerated at a fast rate of speed per second. </p><p></p><p>Drive ratio is how you determine exactly how fast you're trying to spin the SC. It is something everybody should learn how to calculate as you modify these engines. It is the only way to determine if you've achieved maximum RPM for the SC with a given pulley combination versus what engine RPM you are spinning the engine. </p><p></p><p>A good example is this. </p><p></p><p>A STI 10% lower with a 2.4" upper has a drive ratio of 3.25</p><p></p><p>To determine SC RPM you multiply engine RPM by the drive ratio to determine SC RPM</p><p></p><p>So you've modified the engine and are turning it 7000 RPM. At 7000 RPM what is the SC RPM?</p><p></p><p></p><p>7000 x 3.25 = 22,750 </p><p></p><p>This is how fast the SC is turning with that pulley combination at 7000 engine RPM</p></blockquote><p></p>
[QUOTE="Bad Company, post: 15369325, member: 141815"] Joewee You have to do a drive ratio calculation to answer this question Now the big question is what manufacturer's lower are you using in the calculation? IW and STI diameter of their products. For some unknown reason each has different diameters listed for dampers for the GT500 engines. Stock Diameter IW = 7.225" diameter STI = 7.5" diameter IW and STI list 2 different damper diameters for the 10% OD lower. IW = 7.95" diameter STI = 7.81" diameter Yet when it comes to the 15% OD dampers, both companies have very close to the same diameters listed for their products. ATI = 8.13" diameter STI = 8.12" diameter So we have quite a few different math calculations to perform to see what each pulley combination does to the drive ratio of the SC versus the engine, depending on which brand of lower pulley that you'd purchase. Lets do the math using IW as your choice for the lowers in this first calculations 15% lower with a 2.5" upper is 8.13 divided by 2.5 = 3.252 drive ratio 10% lower with a 2.4" upper is 7.95 divided by 2.4 =3.3125 drive ratio So the larger diameters between these two would actually being spinning the SC slower Always multiply the drive ratio number by engine RPM to achieve SC RPM for the airflow exiting the SC, if everything was at 100% efficiency as far as airflow was concerned Now lets do this with the STI dampers and the different diameters listed by STI for each 15% lower with a 2.5" upper 8.12 divided by 2.5 = 3.248 drive ratio 10% lower with a 2.4" upper 7.81 divided by 2.4 = 3.25 drive ratio These two STI lower and upper pulley combinations have nearly identical drive ratios. The 10% lower with the 2.4" upper would yield a very slight advantage in airflow due to the drive ratio of 3.25 versus 3.248. Yet the STI 15% lower with the 2.5 upper pulley with its much larger diameter pulley combination would result in a lot less chance of belt slippage as the engine is being accelerated at a fast rate of speed per second. Drive ratio is how you determine exactly how fast you're trying to spin the SC. It is something everybody should learn how to calculate as you modify these engines. It is the only way to determine if you've achieved maximum RPM for the SC with a given pulley combination versus what engine RPM you are spinning the engine. A good example is this. A STI 10% lower with a 2.4" upper has a drive ratio of 3.25 To determine SC RPM you multiply engine RPM by the drive ratio to determine SC RPM So you've modified the engine and are turning it 7000 RPM. At 7000 RPM what is the SC RPM? 7000 x 3.25 = 22,750 This is how fast the SC is turning with that pulley combination at 7000 engine RPM [/QUOTE]
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