I have been receiving a lot of emails and PM's lately on the topic of aftermarket silencers in EFI 2 stroke snowmobiles... and thought I would put together the replies into one post.
Here are my thoughts with some BASIC tech explanations. This in no way will cover all aspects of the engineering of a modern 2-stroke snowmobile engine/exhaust Here. There are many out there that have a MUCH better understanding of these principles/application than I... MUCH better
... If you have more to add or even contradict in this topic...feel free to post it up along with your reasons/descriptions. Let me start by saying that I really like aftermarket silencer offerings lately... they drop weight from the sled usually from higher quality materials and construction techniques than the factory can afford to use ...but sometimes from less silencing abilities. (Which is another topic that can be discussed HERE)
When I refer to "Simple addition of a silencer" I am referring to removing the stock silencer and installing a different silencer in it's place with no other changes to the engine or engine controls (eg fuel control boxes, pressure regulators etc)
Many people assume that the computer (ECU/ECM) would make up for any rich/lean condition that may be caused by changing the exhaust.
The fuel injection on ANY of the two strokes (including the E-Tec) is not a closed loop system... It cannot automatically monitor/adjust the fuel/timing.
All the two strokes run, and will probably for some time, what is called "speed Density" control system.... It has a fixed fuel map (the set of instructions that the computer has as it's control mechanism). It monitors such things as intake air temp (IAT) Throttle position (TPS) Coolant temp, crankshaft position, barometric pressure and protective devices like detonation sensors and exhaust temperature (which is often mistakenly thought of as and oxygen sensor). It runs on a specific set of instructions that take into account the information from these sensors and responds in a predetermined way.
If you change something in the system... like modifying the airbox, change the shape/length or type of the tuned exhaust system (which the silencer is part of) or the type of fuel that you are using... the computer will still run the engine according to that predetermined map... the computer (ECU) cannot compensate for changes to the hardware.... (like porting, pipes etc) which is why it is important to match the stock back-pressure very closely when installing an aftermarket can. Now sometimes, the factory has to get a certain pollution component under control like Nitric oxides, carbon monoxide, hydrocarbons etc... and they cannot push the power or get it to meet standards with less back-pressure... BUT more often than not, these days, the factory is not leaving stuff like that on the table.
When simply installing a can adds power on a tuned exhaust on a two stroke engine (evidenced by needing to add clutch weight) this is an indication that it has altered the exhaust characteristics and that changed the way the existing map works on the engine...
Now, if a drop or increase in back-pressure (which is the only thing a silencer can do to effect the performance) increases the power output...it is doing that by giving the exhaust a different state of tune... this affects how the cylinder is scavenged (sucking out the exhaust and unburnt air/fuel mix that is being pulled through the transfer ports and partially out the exhaust before "stuffing" the unburnt fuel back into the cylinder).
If it does this scavenging at a different rate or "timing" this can affect how much fuel is left in the cylinder and therefore the Air/fuel ratio (whether the engine is running lean or rich).
An increase in power could be from leaning the motor out which could be bad if it is done past the engines ability to tolerate a lean burn. OR.. this might just pick up on a margin of power if the factory had to leave something on the table to meet EPA regulations for emissions... There really is no way to know this outside of doing extensive Air/fuel ratio monitoring (piston wash or as actual O2 content in the exhaust stream) and engine teardowns after long term testing (a full season with thousands of miles on sleds in different regions using different riders and different fuels.)
I can say this...EFI has RADICALLY changed the way exhaust mods, including the addition of an aftermarket silencer, work... and affect the overall reliability/durability/power of a given engine.
The days of simply making an exhaust more "free flowing" and "uncorking power" are all but gone. The exhaust system from the engine and out the silencer has to be looked at as a system... a TUNED exhaust system. Heck, even in the old days, changing out the silencer required often that the jetting be changed to accomodate the new setup.
Is it possible that a simple change in silencer can actually ADD power in a durable/reliable way... yes. Is it likely, without causing other issues... Not in my opinion on modern EFI engines without changing/controlling the mapping.
IMO, I believe that more than ever, it is important that the aftermarket silencer, when installed on a stock pipe/engine/control system, has an identical flow rate to the stock silencer. This way.. a lighter/smaller silencer can be had... and even a different exhaust note if you want.
If not flow-matched, extended testing in the field needs to be done to show that this modified exhaust is working within the sleds abilities to cope with the change when you are counting on the sled to get you out of a sticky situation, WFO...Full load, where a tow back to the trailer for new pistons is not an option.
There are some "good" cans out there for sure... just make sure you do your homework and ask questions.
I'd like to hear your thoughts on this...
It will be good conversation.
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