2T Exhaust on 4T Engine?

BigDaddy SnakeOiler · Post by **BigDaddy SnakeOiler** » Fri Jan 22, 2016 2:37 am

I was reading an article in Scootering which compared all the stock-looking exhausts available for PX 2 strokes. Other than fit issues, could you use these on a 4 stroke?

az_slynch · Post by **az_slynch** » Fri Jan 22, 2016 5:05 am

Probably won't fit. The 4T exhaust fits to the cylinder with a two-bolt flange. The 2T engines have a metal stub poking out the bottom of the cylinder that the exhaust downpipe clamps on to. The downpipe diameters are very different in diameter (2T is bigger) and the downpipe mounting position is very different ( 2T fits to the cylinder, 4T connects to the cylinder head).

The exhaust design is quite different as well. The 2T exhaust is designed to return a pressure wave to the exhaust port, where a 4T has an exhaust valve and doesn't require backpressure as much as it requires a vacuum pulse to work efficiently

jimmbomb · Post by **jimmbomb** » Fri Jan 22, 2016 3:10 pm

az_slynch wrote:Probably won't fit. The 4T exhaust fits to the cylinder with a two-bolt flange. The 2T engines have a metal stub poking out the bottom of the cylinder that the exhaust downpipe clamps on to. The downpipe diameters are very different in diameter (2T is bigger) and the downpipe mounting position is very different ( 2T fits to the cylinder, 4T connects to the cylinder head).

The exhaust design is quite different as well. The 2T exhaust is designed to return a pressure wave to the exhaust port, where a 4T has an exhaust valve and doesn't require backpressure as much as it requires a vacuum pulse to work efficiently

Let's say that someone who really wanted to, and had a bunch of stock exhaust laying around. If that person did some cutting and welding and fitted the exhaust onto his 4T. Now he has a stock 2T exhaust mated and fitted to his four stroke, do you think he would have any problems running the motor and it not doing any serious damage to the motor?
And, he would have to choose between a 150, & A 200 2 stroke exhaust which one to put on his four stroke.

Johnny O · Post by **Johnny O** » Fri Jan 22, 2016 5:48 pm

So I spent some time pondering this question today and I came up with my own explanation as to why I believe installing a 2-stroke exhaust on the 4t will work, but with diminishing returns.

The 2-stroke exhaust uses the initial shock wave of the gas exiting the cylinder on the down stroke to reflect a return shock wave back to the cylinder contributing to some additional compression and prevent loss of unburnt (new) fuel when the 2-stroke cylinder begins on its return stroke up. This is done in the expansion chamber typical to the 2-stroke design.

Good diagram here:

https://en.wikipedia.org/wiki/Expansion_chamber

I believe applying this design feature to a 4 stroke engine will cause reduced performance. Here's why: The return shock wave is a result of the design of the expansion chamber in the 2-stroke exhaust. Its function (the shock wave) is to push some of the air fuel mixture that normally escapes out during the start of the compression stroke. This will inhibit full release of all combustion products in the exhaust stroke of the 4t. I would suspect that during the intake stroke there would be at less of a vacuum then normal due to the pressure wave from the 2t exhaust system and less air/fuel drawn by the natural draft seen during the 4t intake stroke.

I think your engine will turn, I think it will run. I just think it will run almost choked off. Like its not getting enough air. In time I would also suspect fouled plugs to be a hassle resulting from an incomplete combustion.

In this circumstance you could adjust when your exhaust valve closes to prevent the pressure wave from bouncing exhaust back in but this may prove difficult to get a nice balance during the compression stoke.

If funds are no option and you like to weld, build a custom exhaust. Just not an expansion chamber. My thoughts. Thanks

BigDaddy SnakeOiler · Post by **BigDaddy SnakeOiler** » Fri Jan 22, 2016 6:34 pm

I appreciate the great answers. I was thinking along those lines. I already have a 4T Sito plus that I'm getting ready to install, but was curious.

jimmbomb · Post by **jimmbomb** » Fri Jan 22, 2016 6:39 pm

Johnny O wrote:So I spent some time pondering this question today and I came up with my own explanation as to why I believe installing a 2-stroke exhaust on the 4t will work, but with diminishing returns.

The 2-stroke exhaust uses the initial shock wave of the gas exiting the cylinder on the down stroke to reflect a return shock wave back to the cylinder contributing to some additional compression and prevent loss of unburnt (new) fuel when the 2-stroke cylinder begins on its return stroke up. This is done in the expansion chamber typical to the 2-stroke design.

Good diagram here:

https://en.wikipedia.org/wiki/Expansion_chamber

I believe applying this design feature to a 4 stroke engine will cause reduced performance. Here's why: The return shock wave is a result of the design of the expansion chamber in the 2-stroke exhaust. Its function (the shock wave) is to push some of the air fuel mixture that normally escapes out during the start of the compression stroke. This will inhibit full release of all combustion products in the exhaust stroke of the 4t. I would suspect that during the intake stroke there would be at less of a vacuum then normal due to the pressure wave from the 2t exhaust system and less air/fuel drawn by the natural draft seen during the 4t intake stroke.

I think your engine will turn, I think it will run. I just think it will run almost choked off. Like its not getting enough air. In time I would also suspect fouled plugs to be a hassle resulting from an incomplete combustion.

In this circumstance you could adjust when your exhaust valve closes to prevent the pressure wave from bouncing exhaust back in but this may prove difficult to get a nice balance during the compression stoke.

If funds are no option and you like to weld, build a custom exhaust. Just not an expansion chamber. My thoughts. Thanks

The OP was questioning a 'stock' exhaust,
Ive seen many proformance (-1 sp) pipes for the 2t. And they are look like expansion chamers with the Sito+ being the exception.
Are 2t stock piles expansion chambers?

az_slynch · Post by **az_slynch** » Sat Jan 23, 2016 12:55 am

jimmbomb wrote:The OP was questioning a 'stock' exhaust,
Ive seen many proformance (-1 sp) pipes for the 2t. And they are look like expansion chamers with the Sito+ being the exception.
Are 2t stock piles expansion chambers?

The stock 2T pipe would be hard-pressed to qualify as much of an expansion chamber.

If you want a form expansion chamber behavior from a stock-looking 2T pipe, try a Sito+, SIP Road 2 or a BRM Big Box (on order from least to most effective).

BigDaddy SnakeOiler · Post by **BigDaddy SnakeOiler** » Sat Jan 23, 2016 1:07 am

jimmbomb wrote:The OP was questioning a 'stock' exhaust...

No, was asking about "stock-looking" units.

jimmbomb · Post by **jimmbomb** » Sat Jan 23, 2016 8:56 pm

BigDaddy SnakeOiler wrote:
jimmbomb wrote:The OP was questioning a 'stock' exhaust...
No, was asking about "stock-looking" units.

I stand corrected.. I hope you get the answer you're looking for.. The folks on this board are very knowledgeable and share. Very helpful!!!
good luck

BigDaddy SnakeOiler · Post by **BigDaddy SnakeOiler** » Sat Jan 23, 2016 9:37 pm

I did. Which is basically what I thought. The exhaust waves are literally tuned differently for a 2 or 4 stroke.

Johnny O · Post by **Johnny O** » Mon Jan 25, 2016 2:15 pm

jimmbomb wrote:
Johnny O wrote:So I spent some time pondering this question today and I came up with my own explanation as to why I believe installing a 2-stroke exhaust on the 4t will work, but with diminishing returns.

The 2-stroke exhaust uses the initial shock wave of the gas exiting the cylinder on the down stroke to reflect a return shock wave back to the cylinder contributing to some additional compression and prevent loss of unburnt (new) fuel when the 2-stroke cylinder begins on its return stroke up. This is done in the expansion chamber typical to the 2-stroke design.

Good diagram here:

https://en.wikipedia.org/wiki/Expansion_chamber

I believe applying this design feature to a 4 stroke engine will cause reduced performance. Here's why: The return shock wave is a result of the design of the expansion chamber in the 2-stroke exhaust. Its function (the shock wave) is to push some of the air fuel mixture that normally escapes out during the start of the compression stroke. This will inhibit full release of all combustion products in the exhaust stroke of the 4t. I would suspect that during the intake stroke there would be at less of a vacuum then normal due to the pressure wave from the 2t exhaust system and less air/fuel drawn by the natural draft seen during the 4t intake stroke.

I think your engine will turn, I think it will run. I just think it will run almost choked off. Like its not getting enough air. In time I would also suspect fouled plugs to be a hassle resulting from an incomplete combustion.

In this circumstance you could adjust when your exhaust valve closes to prevent the pressure wave from bouncing exhaust back in but this may prove difficult to get a nice balance during the compression stoke.

If funds are no option and you like to weld, build a custom exhaust. Just not an expansion chamber. My thoughts. Thanks
The OP was questioning a 'stock' exhaust,
Ive seen many proformance (-1 sp) pipes for the 2t. And they are look like expansion chamers with the Sito+ being the exception.
Are 2t stock piles expansion chambers?

Again just my thought.....An exhaust like this i.e. "Stock" 2T exhaust shouldn't cause much trouble IMO:

http://www.scooterwest.com/items/?_page ... P200E/2120

Until you try to modify a 2t exhaust you don't know for sure how it will run. At this stage of the game, there are so many different options for the 4t exhaust I can't imagine the time it would take; plus the hassle of getting flanges to fit and tuning the engine all over would really justify not just buying a replacement exhaust designed to fit the 4t. In theory, yes it can be done. I wanted to discuss the difference between the 2t and 4t designs and why I believe the expansion pipe design won't work and add a little science discussion for thought. At first glance the basic stock 2t exhaust appears it doesn't have any design features that would necessarily inhibit use on the 4t. Performance designs my prove to be a challenge. Good Luck!