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03012010, 07:22 AM 

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Kieth,
You are correct..some clarification W = speed in RPM.. Also for anyone interested to convert Kw = HP x .746 So it's important not to confuse torque at wheels vs torque at engine due to losses.
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03012010, 07:48 AM 

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Thx Todd.
This helps me understand KW where I just used to ignore it. KW sounds like a different measurement unit for 'P', Power, or HP. If this is true, then I suspect or our P formula above might just have a different factor under (TxW) and or T might be something other than ft lbs. Just guessing. Simplistically, I suspect the way an Inertia Dyno works is it times rate the large drum accelerates. This measures power at the wheel and gives them load by time measurement. Yes, the sensors probably compute this in Ft Lb's. And it dose meet the definition of and is TQ. However, this is not motor TQ. Much more needs to be done. They also monitor the motor RPM. From this they can compute an effective gear ratio between the wheels and the motor. From there they can compute or back into the wheel HP by RPM. Then I suspect they should be able to by timing how long the mass of the roller and the wheel drive train combined take to run down and they should be able to calculate an estimate of how much the drive train loss is. From they should be able to come close to a number for HP at the crank to. Once we have WHP or CHP we can multiply that by 5220 and then divide by RPM to compute the motor's TQ values. So I am guessing motor TQ, as we know it, is the last conclusion in an Inertia Dyno's computation. Never really thought about this before. 

03012010, 09:06 AM 

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Quote:
Yes, I can calculate crank torque from crank hp and crank rpm. Duh. What I'm saying is that it's WHEEL torque that matters and I get WHEEL torque with just two pieces of information. 1. current crank hp 2. road speed Just think about that for a moment. I understand why one would think crank torque matters so much because of f=ma. However, the f is WHEEL torque divided by wheel radius. If you want f from crank torque you NEED to know gearing. Not true with crank hp. Again: 1. we can all agree that wheel torque is what matters 2. if you can calculate wheel torque with only road speed and crank hp, and NOT road speed and crank torque then it is crank hp that gives you a better idea of wheel torque and therefore acceleration. If you want to compare the acceleration of two cars you want to fix road speed because you want to compare if they BOTH start at 10 mph, or 20 mph, not if one car starts at 20 mph and the other 10 mph. Then with that fixed, it's only current hp and weight.... BTW if our high school physics is failing, then so are our masters degrees in physics ('hint,' 'hint') Edit: I thought about it more and I think the problem here is maybe you think we are trying to say that crank torque doesn't matter or something strange like that. You can get power from torque and rpm, so of course it matters. It's just that if you consider hp you already have a quantity that gives you the information you want, if you use crank torque, you need to know rpm or other parameters in order to get the information you want, acceleration.
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Keith 1979 930 2007 WRX wagon Last edited by Yargk; 03012010 at 02:21 PM.. 

03012010, 10:50 AM 

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The thread title is "Best HP for a Stock 4 Spd". Therefore gearing is a given and road speed is assumed to be the same for any comparison.


03022010, 03:23 AM 

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Yes  we have gotten off base with the debate over TQ v HP. This might be considered part of the foundation for such a discussion but it would be nice to get past this as the transmission can be a significant impact on performance and or driveability.
As the title suggests: How do we build a 930 to work better with the unique wide ratio, tall gear, 4 speed . Would probably be reasonable to expand this to: What makes the 4 speed a challenge? What is the opportunity to fix the 4 speed's gear ratios. A cost benefit of a trans mod v more motor HP mods. 

03022010, 07:35 AM 

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Pardon me for injecting the idea of the need to do a cost/benefit analysis of mods on the engine vs. mods on or replacement of the trans in a previous post. I know dollars spent on transmissions don't generate impressive dyno charts. I see this as a somewhat new phenomena. As short as 15 years ago, before plentiful dynos, vehicle performance was judge by the overall vehicle package. Still today, you pick up a car magazine, they don't announce: "The (XZ) makes 400HP/350TQ, the (YZ) makes 350HP/350TQ  XZ WINS........(add some pictures)......end of story." It seems absurd when put this way but I often get the impression many think this way. 

03022010, 09:50 AM 

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