MPG Quest

[wildBill83]

What transmission are you running? Get a 5spd from an LX and pop that on there.

[H311RA151N]

Don't get confused with the 'power up high' and 'torque down low' thing. Power is power and torque is torque, and both exist at any rpm. Torque however is a 'real' thing that can be directly measured, whereas power is 'only' a calculation that includes time (the 'minute' in rpm). If you have X power at Y rpm, and want X+ power at Y, then the only way you will get it is to increase torque at Y rpm. This applies across the whole rpm range.

Very simplistically, what you feel increasing the velocity of the car is torque, an actual force, whether it's lower or higher in the rpm range. What you feel as 'torque down low' is the torque produced at lower rpm, what you feel as 'power up high' is torque produced at higher rpm. What you perceive as 'power' at any rpm is probably the rate at which torque is increasing as the rpm rises over time.

Power vs torque is an argument that confuses even very knowledgeable people, and smarter ones than I am. The previous paragraph is just my take on it...



The volume of air (i.e. quantity) entering the cylinder is what it's all about at any given rpm.

At higher and higher rpm the air flow through the port (and to a lesser extent the induction tract as a whole) will at some point become cross section limited, you can only flow so much air in a short time period through a given sized port, unless you push it in harder than atmospheric pressure can do (turbo anyone...). With a larger cross section more air can flow in the same short time. At higher rpm the time available from valve open to valve closed is relatively short, so a bigger port will tend to flow more air in a shorter time. If the port is too small for a given rpm then the air flow becomes 'choked'.

At lower rpm the airflow isn't cross section limited, but is velocity limited. Air will pass at a slower speed through a larger port, faster through a smaller port. There is more time at lower rpm to fill the cylinder, and the faster the air moves the more will get into the cylinder. Increasing the port size results in the air speed slowing down. For a given port there will be a point on the graph where at a particular rpm the air velocity is at it's maximum, and not slowed by 'excessive' port size. This will be the point at which maximum flow rate for that port at that rpm will be optimal. Any larger and the air speed will slow too much resulting in less inducted air.

This is all affected by cam timing, valve size, port length etc. What may be an ideally sized port (no such thing exists really, since the rpm is constantly changing) for one camshaft etc. may not be for another. It's all about choosing the best compromise for intended purpose.

A couple of months ago, when I asked that question, I went and found the in depth explanation of what I was after. What you hit on about:

Power vs torque is an argument that confuses even very knowledgeable people, and smarter ones than I am.

Is probably why I was not able to get a thorough grasp. It cannot be correctly explained in the way people commonly express it and how I have heard about it every since I can remember.

Thank you for taking the time to explain your take on it. You explain it much simpler than the information I found on the subject and your aspect parallels my findings really well.

[wildBill83]

Power is how hard you hit the wall, torque is how far the wall goes with you.

[johnl]

Power is how hard you hit the wall, torque is how far the wall goes with you.

Lets not confuse the 'torque / power' thing with the 'mass / inertia' thing...

[wildBill83]

It's an old movie quote I can't remember the movie though....