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Is all HP created equal
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CMWS
Posted 11/29/2012 00:14 (#2722704 - in reply to #2722096)
Subject: Re: Is all HP created equal


Every engine will have a different HP/torque curve. The exact same engine used in different applications will even have different curves (tractor vs combine). You would have to get the curve for the specific engines you are looking at. It's all about where the power is at and what RPM it is rated at. Torque is simply a function of HP and RPM. Having the torque and HP in the RPM where you need it is as important as the number itself.

Practical example: Let's take a gas engine that has a HP rating of 300, but it occurs at 6,000 RPM (peak HP) and ramps up from around 125 @ 2,000 RPM. The torque available would be incredibly low. Torque= HP*33,000 / (2*pi*RPM) or 262ft/lbs of torque at 6,000 RPM and 328ft/lbs of torque at 2,000 RPM. So really, you have no torque rise, no low end, no lugging, etc... As soon as the engine began to lug, it would just fall on its face.

Now take a diesel rated at 300hp at 2,100 RPM. It likely has a peak HP of 340 around 1900 RPM (from tractor HP and torque curves I've seen). So your torque at 2,100 RPM would be 750ft/lbs and your torque at 1900 would be 949ft/lbs of torque. Now when you are pulling your ripper at 2,000 RPM and get into a tough spot, it will pull down to 1,800 to 1,900 RPM, where it finds all the torque it needs to pull through, and then it ramps right back up.

Hope that makes sense and helps.

Edited by CMWS 11/29/2012 00:15
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