<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Paolo Castellano:
I kind of overlayed the Roe #'s onto the Heffner graph on paper and it seems that the Roe Hp is higher until about 3650 RPM and the torque is higher until about 4,000 RPM. [/b]<HR></BLOCKQUOTE>
This sounds right. Sean's kit with the autorotor SC will provide a lot of boost right off the line whereas the centrifugal unit will need to spool up - hence Roe's SC is more of a "flatter line" whereas the Heffner kit will have a higher peak but take some more RPM's to get there.
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I have the Orbital Gears chart of MPH/RPM in each gear with the stock 3.07. Shifting at 5500 RPM 1-2 will yield 3750 RPM in 2nd, 5500 RPM 2-3 will yield 4,000 RPM in 3rd, 5500 RPM 3-4 will yield 4250 RPM in 4th, 5500 RPM 4-5 will yield 4050 RPM in 5th. <HR></BLOCKQUOTE>
Ok, so your power range when you're racing is going to be 3,750 to 5,500RPM's. Concentrate on this area of the graph - this is where you want to look if you're looking for the absolute best acceleration.
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Now the Roe package Hp is still on the rise from 4,000 all the way to 5800 RPM but the torque is falling 4,000-5800 RPM. The Heffner car's HP and torque are both rising 4,000-5250 RPM, albeit the slope of the Heffner HP curve is steeper, it is also running higher boost so it is unfair to compare apples to apples for the absolute slope of the HP curve.
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This makes sense. To get horsepower you multiply torque by RPM (and divide by a correction factor), so every 100RPM more on the graph, the HP is going up by 100 times whatever the torque went up. If the torque went down, as long as it didn't fall 100lb-ft in that 100RPM range, the HP will still be climbing. It's like if you were in your car going 70mph then you go in neutral... then you go up a gradual slope. You're still moving forward and going up the hill but more and more slowly, until you eventually come to a stop and start rolling back. If you were in 3rd rather than neutral, you could keep accelerating up the hill and you would get up there faster. Sort of like the graphs... the Heffner kit is still rising in torque at that RPM so the HP is going to be going up quicker. The Roe kit will still make HP so long as the RPM's rise quicker than the torque falls, so as long as the torque isn't going down by more than 100lb-ft for each 100RPM, the HP is still going up.
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Mike, you said the Twin-Screw car will feel as it is dropping off compared to the Centrifugal one. Is this because total acceleration is a function of both the HP and Torque through that range? I still do not exactly understand. Sorry, Paolo
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Not so much a function of both HP and torque... you can't feel horsepower. Nobody ever felt an ounce of horsepower, they only feel torque.
Torque is a force applied times the distance on a lever.
Power is defined as the amount of work done over a period of time.
Work is the force applied in relation to the distance.
If you're taking off your lug nuts, you want to use a long bar to get more leverage (torque). If I gave you a 3" long ratchet to take off your lug nuts, you would have to push REALLY hard (remember torque is force times distance, so if the distance goes down, the force must go up to produce the same amount of torque). If I said you had to take your lug nuts off and you had an hour to do it, you could turn them really slow. If I said you had 1 minut to do it, you would have to work very quickly and turn them very quick to make it in time. Even though the amount of energy you output might be the same, you would have to use more "power" to take them off quicker. Like running a 100 yards.. if you take 10 minutes to do it, no problem. If you do it in 30 seconds, you gotta be moving fast and you are using more power to do it more quickly.
So, you can't really measure "power" because it's force and time. It's sort of like "intensity" if you will... I mean, if I have to travel a mile, it's more "intense" if I do it in 5 minutes than if I do it in 5 hours. So in your car, if the crank is turning with a force of 500lb-ft but it's going at 5,000RPM, it's making a lot more power than if it was making 500lb-ft at 1,000RPM. Does that help?
So if your car can make 500lb-ft of torque at a faster rate (higher RPM) then it's more "intense" and it's making more power than if it did it at a slower rate. Horsepower is "how much turning force can the engine output and how quickly can it do it". If it can only produce 300lb-ft, but if it can do that at 20,000RPM then that's very "intense" and hence makes a lot of horsepower (this engine would be making 1,100hp from it's 300lb-ft of torque because it can produce that 300lb-ft at such a high RPM).
Every engine has a usable RPM ranges. The only way to make more horsepower is to make more torque or make the same torque but at a higher RPM. All the things we do to "open up" our engine for air flow helps stuff air in and allows the engine to make a little more torque in the RPM range which means more horsepower. Once you consider gears, horsepower gets important. In the example of the 20,000RPM engine.. well, it's "only" making 300lb-ft of torque right? Well let's say I want to turn a wheel on a car with that engine... I would need to gear it down. Let's say I want to turn the wheel 1000 times a minute and I want to do that at it's maximum power level. Well, on a car that makes max torque at 5,000RPM, I need to reduce the speed by 5 times. I could do that with a 5:1 transmission gear and a 1:1 rear end, or I would use maybe a 3:1 rear end and a 1.5:1 transmission gear. So, we know gears multiply the torque right? So I am reducing the RPM's by 5 which means I am multiplying the torque by 5. So my 5,000RPM engine that makes 500lb-ft of torque is putting 2,500lb-ft of torque to the wheels.
Now, our 20,000RPM engine is sweeter! It makes a maximum of 300lb-ft but at 20,000RPM. To get our wheels moving at 1000 times a minute, we need to reduce the power by 20 times. We might use a 5:1 transmission gear and a 4:1 rear gear. Now, we reduced it 20 times but we multiplied our torque 20 times as well. So our whimpy 300lb-ft of torque engine is putting 6,000lb-ft of torque to the rear wheels.
So we have car A that makes 500lb-ft at 5,000RPM. It has more torque than car B. But car B makes 300lb-ft at a HUGE RPM level (meaning BIG horsepower) which means we can gear it and get a TON of power to the wheels!
This is getting long
The only thing you ever feel is torque. If two identical cars have the same setup (gears, wheels, weight) but different torque at a given RPM, the car with the higher torque WILL be accelerating quicker. The RPM ranges on both superchargers are the same, so forget about gearing and horsepower, and just concentrate on torque in the RPM range you will be using the car. For maximum acceleration, you will be going from 3,750 to 5,500RPM. Whichever supercharger has the higher torque numbers between those two RPM's WILL be the quicker accelerating car - no if's and or but's. Getting it to the ground is another matter.
On the street, you spend most of your time between 1,500 and 4,000RPM... so a twin screw with it's instant torque will feel real nice on the street. On the track, the one with the most average torque (area under the curve) will be the quicker one.
That's why I would never want a Supra with 900HP. It has a huge turbo that takes forever to spool up. The torque curve is so peaky... it might make 700RWTQ at 6000RPM, but it might also make 200RWTQ at 2,000RPM. It's a weak car until it 'spools up'. For street driving, lots of torque low in the RPM range feels nice - thats why a Viper "feels" faster than a Ferrari 360. When you get on it, all that counts is the average torque in your acceleration RPM range.
If anyone is still reading at this point, I hope it helped
