[HavockWK]

Juma and Phoebisis, thank you for your replies.

Phoebisis:
I understand that in drag racing, you rev up the engine before take off to make sure you start the race in the higher end of the power band. But what about other kinds of racing like Cart, Formula 1, etc where there is a lot of racing done throughout most the RPM range due to things like hair pin turns where they gotta downshift and down the RPM’s to get low end torque to blast ‘em back into the straight away to the next turn without breakin’ the back-end loose like you can get away with in a drag race? And for those of us with a 5 speed auto tranny, big tires and no clutch?
As for the dyno thing, I can understand that there are flaws in the ability for a dyno to test the full range. This just means we need a better dyno! But, what about the engine dynos that motor manufacturers use? For example, Nissan says “Fully 90% of its torque kicks in below 2,500 rpm for power you'll have at your beck and call. Even more so, 80% comes in at 1,000 rpm, putting it among the top of its class.” There’s gotta be a way to measure and track this kind of information to verify claims such as a mod increases the ability for the engine to make horsepower, but reduces its ability to generate toqure. No, I don’t want to go pull my engine every time I make a mod change, but I would like to see numbers/charts from the big mod manufacturers (or anybody) to verify the claims. (seeing is believing whether proving or disproving) If all we ever get is a HP/torque chart above 3000 RPM, then how can we be sure of how the low end is being affected?

Juma:
From what you are telling me that because my engine doesn’t have to work as hard, it can’t work as well? I’m not really sure where you are trying to go with the inertia thing. There are too many real life factors that play with inertia inside the engine, especially since part of Newton’s laws specify that an object in motion tends to stay in motion, in the same direction it is traveling. This would fit it we look at the entire mass of the truck being accelerated down a straight dragstirp, but the physics gets weird when you start talking about circles. Also when you think about the pistons, they have to come to an absolute stop at both the top and bottom of their swing. Also, the faster you have to accelerate the piston, the more power it takes and consequently the more force it takes. If we look at these equations:

Work= Force * Distance
Power = Work / Time
Then combine to get: Power = (Force * Distance) / Time

Distance is the linear measure of how far the piston travels from the bottom to the top (or vice versa) of its swing. Time is how long it takes to move the piston from the bottom to the top (or vice versa) of its swing. So basically it takes more power to keep the engine at a higher speed that a slower speed. However, what I am looking for is the low end info. Same equations come into play at the low end, it is just that we are focused more on decreasing the Time variable and consequently Power is increased if the Work value stays constant. Of course if the time stays constant, but the Power is increased, then the Force component of Work will increase because in our case we know that the Distance is constant. Crap, I’ve gotten off track!

Let me get back to the basis of my question… If I use a mod, say big pipes on my exhaust, some people claim I will lose low-end torque, but gain horsepower. Let’s also mention things like scavenging and backpressure, free-breathing and strangulation. If we look at the engine as an air pump as you said, then let’s consider the intake to be capable of a constant max volume ( standard cfm so we can ignore the temp factor) for the entire discussion and only the exhaust volume (cfm) will change through pipe restrictions or lack there of. OK, let’s say that the natural flow balance for the pump is 1:1 (1cfm in, and 1cfm out) for every 1 unit of work. Now if we put an exhaust pipe on the pump that is capable of 2cfm, the pump will still output 1cfm, with 1 unit of work because the exhaust pipe allows the pump to breathe freely. If we put an exhaust pipe on the pump that will only handle .5cfm, then the pump will be required to increase the amount of work to be able to continue to output 1cfm because of backpressure. If it is not able to continue to output 1cfm, then the pump will be slowed and perform less work because the exhaust air passage is constricted causing the pump to be strangled. And if we remember that work is based on force, which in our case is torque, and the constant distance, then you cannot say that back pressure increases low end torque because of backpressure. You can only say that the back pressure will either reduce the output of the engine, or that it requires more torque to do the same work as the free-flowing exhaust. Don’t confuse potential for work with actual work. Just because an engine doesn’t require as much torque to generate power, that doesn’t meant that the engine cannot generate that torque, it just doesn’t have to.

As for scavenging versus backpressure, what I have read is that scavenging is good because it helps draw exhaust out of the cylinders. How does it do this? By the same method as you clean your carpet, be creating negative pressure or a vacuum inside the exhaust system between the exhaust valve and the X-pipe, H-pipe or whatever is your source of scavenging. Backpressure is positive pressure is this same space of the exhaust system. Because the backpressure does not allow the piston to easily push all of the exhaust gases out of the cylinder, and consequently not have as clean of a burn on the next fire, the engine loses its potential energy because it can’t get as much fuel/air mixture in the cylinder. Then when the spark ignites the fuel/air mixture, the bad gases from the remaining exhaust will not allow the burn to be as efficient and therefore you lose the ability to produce kinetic energy that actually generates the force to make torque in the engine.

Basically, if we put on mods that allow our engine to produce more power without requiring as much force, it doesn’t mean the engine is not capable of producing that force. Just think of the difference between driving the truck on the road vs jacking it up and letting the tires be accelerated without having to push the truck. When the truck is on the road, it takes more force to make the tires rotate, but with it jacked up, it takes very little force from the engine to make the tires rotate. I could come up with several scenarios where the amount of force required to do work changes based on alterations of the efficiency of the machine and/or load conditions with a constant maximum force capability.