I think Yamaha has some sort of variable exhaust port on their street bikes - not sure, but I think it uses centrifigul force of the crank or cam at higher rpm to open up a slide, increasing exhaust port volume as revs rise.

BTW, some Forumula 1 cars have done away with conventional coiled valve springs and use air. Get a computer to control the volume of air quickly and precisely, and you can get different valve lift at different rpm.

Two cycle dirt bike engines have had 'power valve' technology for over 15 years. This varies the size of the exhaust ports at different rpms. The result has been more power and better mileage. With computer controlled fuel injection and variable exhaust ports, the next generation of two cycle engines will also pollute much less.

Essentially, all internal combusiton engines are a compromise. Through better technology the engines will be able to operate at or near peak efficiency over a wider rpm range.

 

I think I am tired from thinking about it now. Positive displacement pumps, backpressure, etc. etc. I went and got a price of $180 for a custom cat back system for my truck but I am scared to death that I will lose some torque. Thanks for the info and clarification on this issue. I think I will have them do it with smaller than standard pipe to help retain the backpressure.

1stSCrew sounds like you got it down pretty well. I'll be checking out howstuffworks.com very soon too.

 

seacrow,
I am uncertain if the theories you discussed apply equally to positive displacement pumps (engines/piston & diphram pumps) as they do to non-positive displacement pumps like an impeller driven clear water pump.

Head pressure has a HUGE impact on non-positive displacement pumps, less so on positive displacement pumps.

Foster:
I don't know if you are "on to something" with the torque vs. HP.

I think I understand your point. But I4s have very different torque curves than V8s and yet they have the same intake/exhaust time limits.

I know there is a lot to it; and that 10 seconds isn't quite enough time to master it.

Regarding back pressure pressing on the valve...Hmmmm reduces pressure on the cam lobe? allowing camshaft to rotate more freely? the price is poor scavenging as discussed above and increased head/crankcase pressures from exhaust gases pushing their way up the valve stem and into the valve cover...thus pressing back against the blow by gasses in the crankcase? which 'wins?' I DUNNO! sooooo complicated!

I am only discussing theory as I analyze it.
I do NOT know the answer. I really want to understand it completely before I buy in to the practice of merging the exhaust behind the cats. (I understand the value of the H-pipe if I do run true duals.)

Not trying to offend or ruffle feathers; but these systems are complicated and the interrelationships force constant balancing of interests; like everything in life.

 

The way I understood that backpressure can improve torque is that it keeps the intake charge in the cylinder at low rpm. This prevents the intake charge from exiting unburned out the exhaust valve during the 'overlap' (when both intake & exhaust valves are open).




[This message has been edited by dirt bike dave (edited 03-25-2000).]

 

Back to the HP vs. Torque thing:

I have a question!

This thing about bigger pistons giving more Torque/power... I suppose thats true based on the fact that you have more weight in motion it takes a higher opposite reaction to stop the motion of the pistons...

Given that torque is the moment of a force, the measure of its tendency to produce torsion or rotation about an axis, equal to the vector PRODUCT of the RADUIS vector FROM THE AXIS of rotation to the point of application of the force by the force applied.....

This said: piston size is NOT the only factor... If you keep all things the same BUT lengthen the RADIUS or distance from the piston to the crank shaft you increase torque as well....

ANALOGY: A person can apply more torque at the "point of application" if he simply applies a 2 foot extension to his wrench as he is attempting to rotate a lug nut!!!

Just my .02

2nd question: How is this new Honda Civic Si able to produce 160HP but only like 115 ft. lbs of torque??? Thats a very strange combo?? I guess the opposite is true for tractor trailer engines that usually produce an average 500 HP but 1500 ft lbs of torque....I haven't pondered those facts yet..... Given that HP is a derivative of TORQUE!

 

Torque AND rpms.
You must rev the hell out of a Honda engine to get the HP out of them, but they are designed to do that.
In a tractor engine, they produce lots of torque but revving them doesn't help Hp much.
The engines are simply not designed to turn at 8,000rpm like the Honda.
A tractor engine needs to push out High Hp CONSTANTLY, Whereas a Honda engine only needs it for short bursts.
Torque tows.
HP races.

------------------
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Kicker ZR360 for the 2 Audiobahn AWC10 subs
regcab54@sc.rr.com

 

Rand,

Yeah, that's the basic nature of torque and the rating used is foot per pound, meaning the force is measured as if using a foot long lever, but no matter how you measure it, it is essentialy a measure of power (strength). The larger cylinder gives you greater force from the combustion, regardless of the geometry of the rest of the engine (unless it affects the air fuel mix). The other parts may effect with what effeciency that power is transfered but they don't affect the initial amount of power.

 

Foster:

I agree "The other parts may effect with what effeciency that power is transfered but they don't affect the initial amount of power." Thats very true. 2 drops of fuel mixed with oxygen will release much more energy than 1 drop of fuel! Thus, a larger VOLUME, hence cubic inches, will allow for greater energy release within the combustion chamber.

I think I under stand the Honda's reving engine producing HP. Its because of INERTIA??

I guess what I was trying to say, what you already said... Its what you do with that release of energy!

I'm sure there's a lot more to it than that!

 

Rand,

From what I've been able to understand, it is a combination of very efficient combustion and high rpms. Honda engines manage to keep the combustion pretty efficient all the way into the 8,000 rpm range, allowing an engine with negligable torque to produce pretty good horsepower. I'm pretty sure their variable valve timing (VTEC) aids in the efficiency at high rpms but someone else probably knows more about that than I do.

 

This all sounds pretty simple and basic, but tell me: if a chevy blows up on the road and no one hears it - does it really blow up?

 

FWIW

The equation relating HP to torque is

HP = T X N /5252

Where T is torgue in ft-lbs
and N is RPM.

For a given HP, torque and RPM are in an inverse relationship. Increase one and the other must decrease or horsepower will go up. This explains the what if not the why of the Honda and the I6.

 

Would this make any sense:
Large bore/ Short stroke = More HP per liter but less torque?
Small bore/ Long stroke = More low RPM torque but less HP?
Desiel engines normally have very long strokes.
Notice also the 5.4L vs 4.6L vs 4.2L.
Not much Hp difference but a lot of torque difference, not to mention a big seat of your pants difference.


[This message has been edited by 54regcab (edited 03-28-2000).]

 

That makes sense. A longer stroke would take longer to complete (limiting your rpms) but the longer stroke would also let you get the most power from the combustion.

 

After reading on and on, I have a few things to add:

1. As posted above by someone else, the amount of TORQUE ouput from an engine is DIRECTLY proportional to the amount of heat (fuel) added to the system. Torque is measured in ft-lbs or N-m, which is also a unit of energy. The average energy (from heat release and pump work) output per cycle can be thought of as the torque output. Therefore, more torque = more energy = more fuel. More fuel per cycle means you need a larger displacement (gasoline engines MUST run near a stoichiometric A/F ratio of 14.7:1). Note that this is only the "starting" basis for other torque "enhancements." An old engineering "rule of thumb" is that the maximum torque you can extract from an engine is 1.3 times the displacement volume (in cubic inches).

2. In ANY engine, you will find that there is a "limit" to the maximum piston speed. No matter what engine (i.e. large bore diesel, small bore 2 stroke gasoline), the maximum piston speed is constant. So, for a given engine displacement, there are a few scenarios: large bore with a small stroke, and a small bore with a long stroke. Knowing that the maximum piston speed must be the same, it can be shown that the engine with the longer stroke can not rev as high as that with the short stroke. Therefore, a short stroke engine will "always" make higher power due to its capability to extend its RPM band. (FYI - passenger car engines have a bore/stroke ratio of about 1, where Formula One engines are around 2:1, which explains why they can rev to extreme RPMs like 18,000 RPM).

3. Exhaust backpressure is NOT the reason for the loss in low-end torque, but is caused by a resonance tuning effect. The IC engine is an air pump, which has distinct cycles. As the piston pushes the exhaust gases out through the exhaust valve, it creates a high pressure zone in the exhaust port. This high pressure "zone" traverses through the entire exhaust stream (at the speed of sound) in a pressure wave. This pressure wave then "bounces" back when it reaches a "wall" or an opening, such as the cat or muffler. (Note that these pulses travel at MUCH HIGHER velocities than the average exahust velocity.) This pressure wave will travel through the exhaust system, from the manifold to the limiting component (which could be the cat, muffler, or even the tailpipe opening). A simple correlation is an organ pipe. Resonance is whay makes the sound. And sound is nothing more than air pressure waves.

If you can "time" the reflected pressure pulse so that the low pressure "portion" correlates to right before the exhaust valve closes, you create a natural scavenging effect (negative pressure). This tuning is determined primarily by the diameters and lengths of the exhaust piping (with a slight contribution from geometry). Note that this tuning effect is only valid for a very small RPM band, and that there is an offset exhaust valve positive pressure at a different RPM band. This is exaxtly the same scenario in the intake system, except that you tune to harness the positive pressure at the intake valve.

Low back pressure is always good, but in the process you may effect the tuning of the engine, which is why there may be a loss in low-end.

I hope this makes sense. If not, please let me know, and I'll try to clarify...

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VENOM 99
www.students.uiuc.edu/~cellariu
2000 F-150 SuperCab 4x4



[This message has been edited by VENOM 99 (edited 03-28-2000).]

 

OK, that's it, I am sticking to jigsaw puzzles.