Exhausting The PT-425

[SnowRidge]

While I had my engine out, I took a close look at the PT built muffler--see attached sketch. I discovered it has a rather restricted outlet diameter--enough so, that I thought it warranted discussion.

I took measurements of the Kohler and the muffler. The Kohler's exhaust ports have an I.D. of 1.04 inches. They appear to be designed to mate up with Sch. 40, 1" pipe, which has an I.D. of 1.049". The muffler's two exhaust port manifold pipes appear to be Sch 40, 3/4" pipe, with an I.D. of 0.824". On mine, the mounting flanges do not align well with the pipe.

The manifold pipes enter the muffler chamber where the gasses are bounced off of some sort of curved deflector (or perhaps the pipes themselves curve inside the chamber).The muffler has a large exhaust pipe constructed of what appears to be Sch. 40, 2" pipe, with an O.D. of 2.375" and an I.D. of 2.067". There is also a cold air intake pipe constructed of what appears to be Sch. 40, 1 1/2" pipe, with an O.D. of 1.900" and an I.D. of 1.610".

The muffler is constructed in such a manner that the cold air intake pipe enters through the left side of the chamber, and is concentric with the exhaust pipe, which exits through the right side of the chamber. In other words, the cold air intake pipe goes into the exhaust pipe. Exhaust enters the exhaust pipe by flowing around the outside of the air intake pipe. That is the only opening into the exhaust pipe, except for the inside of the cold air intake pipe. I hope that makes sense.

My math (always suspect) calculates the following areas:

Kohler exhaust port 0.86 sq. in.
Muffler inlet pipe 0.53

Exhaust outlet pipe (I.D.) 3.36 sq. in.
Cold air inlet pipe (O.D.) 2.84
Effective outlet area 0.52

The effective outlet area is represented by the xxxx's on the sketch.

These numbers concern me. The choking down of the exhaust ports into small diameter pipes doesn't seem like a normal way to design an exhaust system, and while the effective outlet area is only 0.01" less than the choked down exhaust inlet pipes, it still seems awfully small to me.

Beyond the design concerns, there is the manner of actual construction. As I noted earlier, the exhaust mounting flanges are poorly aligned with the 3/4" pipes, which will cause an additional restriction. Worse, the inside end of the cold air intake pipe doesn't look like it was ever cleaned up after being cut and before being welded into place.

The rough edges from the cut clearly increase the O.D. of that pipe, which in turn reduces the effective outlet area. The inside end of the exhaust outlet pipe may well have the same problem, but I have no way of seeing that point without tearing the muffler apart.

The thrust of all this boils down to this: I think these mufflers (at least the one on our PT-425) are choking the Kohler engines, and may be making them run hotter than necessary, and may be reducing available power to a significant degree.

I am seriously considering either modifying the muffler, building an entirely new one along the same design, but with different pipe sizes, or installing a Kohler muffler instead.

So, what do you all think?

SnowRidge

 

[smartguyz]

</font><font color="blue" class="small">( I am seriously considering either modifying the muffler, building an entirely new one along the same design, but with different pipe sizes, or installing a Kohler muffler instead.

So, what do you all think?

SnowRidge
)</font>

Wow. That's a good analysis, my friend. I suppose the thing to do is to simply bolt up a Kohler muffler and try some empirical testing. Now how to test the PT - not like you can just hook it up to a dyno. Or can you?

Hmmm.

-Rob /forums/images/graemlins/grin.gif

 

[duane]

Snowridge,

Great observations. The thing sounds fairly heavy, is it all supported off of the kohler engine?

I believe by measuring intake manifold vacuum at various RPMs you can tell if the exhaust is a constriction to normal engine breathing. Try an internet search on about catalytic converter plugging, and it will yield some tests to determine if the exhaust is a restriction. Just a thought, let us know what you find out.

Duane

 

[SnowRidge]

Further info that got omitted in the original post:

The two pipes aren't perfectly concentric. This doesn't reduce the effective outlet area, but it does mean the gap isn't uniform. At a couple of the narrow points, I can see bits of carbon caught in the restriction. Of course, they do reduce the area.

In one of the other threads, there is mention of watching the interior of the muffler glow in the twilight. Perhaps this is the reason why.

SnowRidge

 

[SnowRidge]

</font><font color="blue" class="small">( The thing sounds fairly heavy, is it all supported off of the kohler engine? )</font>

It is very heavy. I wish I had a scale to measure it.

It is supported by braces from below as well as the maniflold bolts.

</font><font color="blue" class="small">( I believe by measuring intake manifold vacuum at various RPMs you can tell if the exhaust is a constriction to normal engine breathing. )</font>

The references to that all give figures for automotive engines--and I don't have a vacuum gauge. Still, it would be interesting to see what a gauge says. I may pick one up if I can find a place to hook it up.

SnowRidge

 

[davesisk]

Engines are my area of least knowledge on these contraptions, so what I say may sound totally ignorant! Don't you want some back-pressure? Isn't that necessary for torque or something like that (up to a point)? A neighbor told me that not enough back pressure causes the fuel mixture to become too lean, causing a loss in hp and/or torque. Can anyone explain this to me (and everyone else)?

TIA!
Dave

 

[Charlie_Iliff]

Dave:
Backpressure is almost invariably a bad thing. Basically, the exhaust system needs to get the hot gases out of the cylinder as quickly and thoroughly as possible. In race or high output engines, a lot of study and experimentation is done to get the entire system, from beginning of intake to end of exhaust, to pack as much air in as possible and get as much exhaust out as possible. Valve overlap even gives you time where the outgoing exhaust pulses scavenge and lower the cylinder pressure so more cold air/fuel mixture comes in the intake port/valve. If the length, diameter or shape is wrong, and a high pressure pulse coincides with exhaust valve opening, the power is reduced.
Often, on two-stroke motorcycles, for instance, the engine develops a lot more power when a can with a small appearing outlet is put on, rather than a large straight pipe. This is due to resonant waves in the can being properly tuned to the frequency of the engine exhaust port openings in the power band, reducing exhaust pressure when the port is open, not to back pressure.
What does this have to do with the low RPM 4-strokes on the PTs? /forums/images/graemlins/confused.gif Not a whole lot, /forums/images/graemlins/grin.gif but a proper length and diameter exhaust will deliver more power than no exhaust pipe, or a mismatch. In these engines, the difference won't be a lot, and considerations of noise and heat shielding are probably more important.

 

[MossRoad]

Is that why you don't see expansion chambers on 4 cycle engines? /forums/images/graemlins/wink.gif

(I loved the sound of mine on my RD400)

 

[SnowRidge]

Moss, do you have any problems with that rear exhaust Kohler muffler on your PT? No melting the battery or anything like that? Any disadvantages to the rear exhaust that you are aware of?

Do you know the Kohler part number for it?

Thanks,

SnowRidge

 

[Charlie_Iliff]

<font color="red">Is that why you don't see expansion chambers on 4 cycle engines? </font>

We have some tuned 4-stroke race exhausts that increase in diameter. They don't choke back down, but do go to a collector to try to pulse match with other cylinders. I don't know if you could get a high peak at some rpm with a 2-stroke type chamber. (The guys who do know are engine developers who won't tell anyone but the people paying their salaries. - /forums/images/graemlins/tongue.gif)