Exhausting The PT-425

eambrose · May 11, 2004

Hey Snow Ridge - I agree with Charlie - I have not worked that much on small 4 cycle engines like the PT, but I kave worked extensively on race engines (a long time ago) and I have to believe that (as in regular engines) partially blocked exhaust ports are a bad thing. As Charlie says, it will retain heat, create backpressure and reduce power - ALL BAD

Call Kohler Tech support and get their input - they were very helpful when I called

I think it would be best (and look cool) to take a pipe right out the side and then up through a muffler - kind of like the muffler pipes you see sticking out of the hood of some CUTs.

I thought someone posted a recent picture on a new PT that had the pipe come out the side and into a muffler (but it didn't go up)

I have to believe that the situation you describe may be causing some problems - Who Knows, maybe the added heat contributed to the push rod failure - lots of heats makes those things much softer and easier to bend.

smartguyz · May 11, 2004

This looks like the appropriate muffler - I'm sure other places have them...

Sincerely,

Rob /forums/images/graemlins/grin.gif

FOURTEEN · May 11, 2004

"" This looks like ""

Just what does it look like!!????!!

SnowRidge · May 11, 2004

( This looks like the appropriate muffler - I'm sure other places have them.. )

I believe that is it, Rob. Thanks for the link. /forums/images/graemlins/cool.gif

SnowRidge

blackwell · May 11, 2004

The Koher 25 and Robin 25 allow up to 28" water as max back pressure at 3600 RPM. I ask PT about this and was told it was around 24" on the PT transvector muffler. I tested my 425 and found it to be only 21" water at 3600 RPM.
I agree too much back pressure can cut HP or burn exhaust valves, but PT does not have a problem with this muffler. The heat is held in the muffler with the wrap and with the fresh air entering the oposite side moves the heat out of the engine compartment. When the engine is turned off and no fresh air moving the heat inside the wrap and muffler causes heat soak which causes the muffler temp to raise very high which is what you see. The temp probally doubles for a few minutes soon after the engine is turned off.
Remember, all the new carb regs that recently went into effect forces engine manufactures to run all engines a lot leaner which increases exhaust gas temps. I have seen a 50% increase in exhaust gas temps in the last two years.
Enjoy you PTs,
EB

SnowRidge · May 11, 2004

( I have to believe that the situation you describe may be causing some problems - Who Knows, maybe the added heat contributed to the push rod failure - lots of heats makes those things much softer and easier to bend. )

The pushrod failure was due to the valve seat departing the head. What caused that is unknown, but heat may have been a factor.

Even if it wasn't a direct cause for the valve seat failure, it can't be doing the engine any good in the long term. I agree it is probably causing some problems. I just need to decide how I'm going to rectify it.

I should have the engine back in the PT tomorrow, if all goes well. I really don't want to run it hard with that muffler, so I guess I'm going to have to do something pretty quickly--the grass here is getting loooooong.

SnowRidge

SnowRidge · May 11, 2004

( The Koher 25 and Robin 25 allow up to 28" water as max back pressure at 3600 RPM. I ask PT about this and was told it was around 24" on the PT transvector muffler. I tested my 425 and found it to be only 21" water at 3600 RPM. )

I'm curious as to how you were able to test your machine. Where did you hook up the manometer? Could you describe the procedures for us?

( The heat is held in the muffler with the wrap and with the fresh air entering the oposite side moves the heat out of the engine compartment. )

Our version of the PT-425 did not include the muffler wrap. I believe one of the group with a like machine queried Terry about this and was told it wasn't needed.

( Remember, all the new carb regs that recently went into effect forces engine manufactures to run all engines a lot leaner which increases exhaust gas temps. )

Yes, and that is why I am concerned about the design and construction of this muffler. The engines are already running very hot, and I don't think there is any room for excessive exhaust restrictions--if in fact, that is what I have.

In particular, I'm not sure how I could accurately measure back pressure when the first restriction is right at the exhaust port itself (the necking down into 3/4" pipe).

Any ideas?

SnowRidge

MossRoad · May 11, 2004

No problems with the rear exhaust. It points straight back.

I'll try to get the part number tomorrow. It's late. /forums/images/graemlins/smile.gif

smartguyz · May 11, 2004

( Our version of the PT-425 did not include the muffler wrap. I believe one of the group with a like machine queried Terry about this and was told it wasn't needed. )

SnowRidge,

I was the one who called and asked Terry about the muffler wrap - he mentioned that the new design - with both pumps in front, steel tank in the lid, etc. - necessitated the addition of a thermal wrap - I wanted to know if I could buy one, as I noted that the muffler does get mighty hot, and he said "The older design, like yours, doesn't need it - there isn't the concern with muffler temp on yours - besides, the wrap is really expensive, like over $100 dollars - believe me, you don't want it - it won't do anything for ya."

I figured Terry was being honest, and took him at his word.

Sincerely,

Rob /forums/images/graemlins/grin.gif

duane · May 12, 2004

I quess I'm curious what the actual benefit is for the cold air intake. Snowridge mentions that grass will burn on the exhaust, so it does not sound like the muffler shell is running that much cooler than a normal muffler. I suspect the expelled gases might be a little cooler, but the older vintage PTs with the normal mufflers seem to expel the gases to outside the engine cover. I thought the cold air intake was to reduce the shell temperature to reduce the overall heat input in the engine compartment.

Duane