OT - Restricted internal combustion engines

In general, breathing characteristics of an engine (technically called volumetric efficiency) affect the torque curve mostly in the higher rpm regions. So the torque curve is likely to be about the same at lower rpm, but fall at higher rpm.

Two-stroke engines are much more sensitive to the efficiency of the intake and exhaust systems than four-strokes, so the effect would be similar in shape but worse in magnitude.

Contact me off-line for any further discussion, Enzo.

Enzo Matrix wrote:

In general, breathing characteristics of an engine (technically called volumetric efficiency) affect the torque curve mostly in the higher rpm regions. So the torque curve is likely to be about the same at lower rpm, but fall at higher rpm.

Two-stroke engines are much more sensitive to the efficiency of the intake and exhaust systems than four-strokes, so the effect would be similar in shape but worse in magnitude.

Contact me off-line for any further discussion, Enzo.

Enzo Matrix wrote:

geez enzo, that would take many pages to cover well.

The restriction will limit VE, as the engine cannot breath, and hence max rpm will be reduce. At lower rpm BHP may also be impacted, which is once again a drop in VE. Ultimately peak torque at most rpm points will be lower, dropping dramatically when the exhaust volume exceeds the flow rate of the obstructed exhaust system. It'll run like a dog.

As for the second part which throws in a vague "equivalent two stroke" just what does that mean? It's insufficient information for anything beyond the vaguest answer. What does 'equivalent' mean: rpm, torque, displacement, VE, BSFC....

However....for a vague response.

The impact of an exhaust restriction on a two stroke is highly dependant on design of the engine and exhaust system itself. Is the engine reed valve or ported, is the exhaust tuned pipe/expansion chambered or not, does it have any of the myriad variable port systems (eg. Kawasaki's 'KIPS') ?

Sticking with very simplistic designs, a 2 stroke would see a loss in torque across most of the rpm range (more so in the upper range). If the restriction is significant higher rpm would not be achievable.

That made me think of the endless questions on rec.auto.tech about "i put in a low restriction exhaust and now I have no low end torque" and the replies re valve overlap and the beneficial effects at low rpm of slight backpressure in the presence of same but I thought that was maybe going too far with what the instructor was looking for in an answer but then i started thinking about why the question referenced 2 cycles and the built-in amount of "valve overlap" inherent in the 2 cycle design and how the added backpressure might similarly increase the torque of the 2 cycle in the low rpm range there but when I reread the question it wasn't clear that the instructor meant an unrestricted

2 stroke vs a restricted 2 stroke, or an unrestricted 2 stroke vs the restricted 4 stroke but in the end i just wanted to see how long i could get this sentence to go on.

Maybe. It depends on the design of the "restictor", and the timing in the stroke. I have restrictors for my Harley pipes that supposedly increase torque at lower rpm when the baffles are removed - they insert at the head end of the pipe. And then there's the tuned pipes found on most 2-stroke dirt bike engines, and model aircraft engines which act like a poor man's supercharger (but they typically peak at only one RPM).

Guess what I'm getting at is that it depends how the flow is balanced for the design intent.

Thanks Don. That's exactly the sort of information I needed. It's very much appreciated.

See, I just *knew* there would be people here who could help me.

Thanks very much for the help. It's most appreciated.

You're right about the vagueness of the questions. However, I think that is a deliberate ploy on the part of my tutor in order to stimulate research and discussion in the tutorial sessions.

Many thanks to all who answered. It is most appreciated.