Why you need to be careful when choosing an exhaust silencer/can.

How elevation effects your engine (internally).

 How to  adjust for higher elevation riding?


NOTE: There are several "articles" incorporated into this one page.. They address different areas and aspects of the exhaust system.. Please check out  AND ENJOY, ALL the information in this page.

There is a bit of confusion as to why most aftermarket silencers are not working well with mod engines..

FIRST and foremost... It has nothing to do with a silencer and a piston or head, or porting, working well together..
What it is related to is TOTAL ENGINE POWER!

Nearly all aftermarket silencers are more restrictive in air flow than the OEM muffler! Since the main jet on ALL stock sleds is too large, then you can effectively lean down an engine via restricted exhaust flow and no jet change.. OR you could reduce the main jet to proper size and accomplish the same thing for a lot less $$

Let me explain.. If you have a exhaust system or exhaust component (like a pipe or silencer) that is designed to work and perform well on an engine that is making 140HP.. Then that is a matched component for that 140HP engine.. Now if you take that same component (pipe or silencer) and install it on an engine that is making 20-35 MORE HP (i.e. flow more air) than the engine it was designed to run on (140HP) then you have a mis-match in the component to the engine...

So, it does not matter if you are running RKT pistons, RKT head, or some other piston or head ... IF the engine is making ANY SUBSTANTIAL amount of increased power.. then the exhaust system is mis-matched.

So whether it be the RKT drop in kit, RKT 172HP 827 kit , RKT 860R, RKT 925 or anybody else's big bore or piston kit.. if the aftermarket silencer is working well on it.. there is a very good chance that the performance package (what ever it may be) is not making big power (like you may have been lead to believe when you bought it).

This is why the stock exhaust components work so well with the modified engines (at least the ones that are making power)
The OEMs build their engines to survive on low octane fuel for pro-longed running and high EPA standards/regulations.. So, they design an engine and exhaust system that is a bit handicapped so they can meet these warranty and EPA demands... One easy way to meet these demands is to make an exhaust system that is a bit "loose" so that there is not too much heat put back in the engine under heavy running. Another way is to add a single ring piston design that will lose power the longer it is run.. and then they always have it jetted up 1-3 sizes larger than it really needs to be. So, basically, you have an exhaust system that is too loose and an engine that will making less power the longer it is run and the harder it is tasked.. and Oh, the excessive main jet installed by the factory... Pretty smart heh?? you start off with 800 power and after a bit of hard running you have 700 power...

So, NOW you add an engine package like the drop in (+18-20HP) or the 827 (+30-32Hp) and your engine is able to make more power and , better yet, SUSTAIN, this power during repeated running..

OK, now that loose OEM exhaust system is suddenly not so "loose"! Make sense?? So, now you have a stronger running engine that is flowing more air and you Stuff a restrictive exhaust can on the end on your pipe... WOW.. what happens??

YUP.. the O'LE Banana in the tail-pipe syndrome YE59.gif headbang.gif try stuffing a potato in your truck's tail pipe someday and see how she runs! dry.gif headbang.gif

So, the jist of all this.. IF your engine is making SUBSTANTIALLY (like more than 10HP) more power you can not run a restrictive exhaust or you RISK placing too much heat back in the engine..

The "Key" is to determine which exhaust components are more restrictive than stock or less restrictive than stock..

To add even more confusion to the whole "can" issues.

Most cans are what is called a straight passive bleed system.. Meaning it is no more than a bleed valve for the main exhaust.. This is why you could have the center dump pipes.. It did not really matter where the bleed was located, you just had to have one and one that was not restrictive (there's that word again YE59.gif )

Some (like the Arctic Cat) silencers are actually a resonator silencer.. without going into too much detail... it is a "tuned" silencer and will resonate at certain frequencies and provide pulse assisting to the main pipe.. So, it is not a passive bleed system but rather a tuned system...

I have my suspicions that the DOO cans may also be this type (resonator). But have no verification of this. I DOO know that the CAT cans are this type and are VERY sensitive to aftermarket cans

So, if you consider putting a passive bleed can (like all aftermarket) on an exhaust system that is designed around a resonating can system.. You can see how this would hinder performance..


Just a thought: But do you EVER see a silencer on any of the Drag Sleds?? If the silencer was just a player in performance, then all drag sleds would surely have them attached. Most drag sleds will have no silencer attached and this just strengthens the theory that silencers are not and should not be an active "player"  in the tune of the exhaust system!



The main relationship between the head and the exhaust is how much heat it (the head) puts into it and how much pressure (not to be confused with back-pressure AT ALL) or more appropriately VOLUME of F/A and air is returned to the cylinder (and eventually the head) via stuffing pulse from the TUNED PIPE.. NOT THE Y PIPE OR THE CAN!...
These "tuned" stuffing and suction pulses are a direct result of the cones in the TUNE PIPE. They (the cones)  are the major players in the control these pulses. The pipe's back-pressure is determined by the STINGER outlet that is directly after the convergent cone.  The silencer is nothing more than a bleed valve and does should not be used as a method to control back pressure. What it does effect is how fast it can dump HEAT out of your engine. Do not confuse heat with back pressure.. They are not the same... Think of the silencer as the drain in your bath-tub and think of the tub water as the exhaust HEAT.... Now, the bigger the drain (silencer) , the faster the water (HEAT) drains...and the vice-versa is obviously true...

Keep in mind.. the back pressure has already been determined via the STINGER..BUT.. can be altered via a restrictive silencer.

So, what really happens is that if you have a tighter silencer is: The returning pipe stuffing pulse is hotter and since heat effects the speed of sound. The timing (via speed) of the pipe pulse can be wrong and this can cause you performance issues and possibly reliability issues.
Is the pressure increased from this?? possibly..

Again, the combustion process (head) is a player in how much HEAT is put into the pipe and less heat in the pipe means a MORE efficient combustion process (giving all things equal). MORE HEAT in the pipe means a LESS efficient combustion process ( ie. LESS push on the piston and less work being performed on the crank train)

Of course this is a simplified explanation of the system.. but it should get the concepts across..

NOW.. So, where does this leave the idea that a silencer and a head have to work together?? Or a head and a Y Pipe working together?? Hmmm....they do not.. The silencer is nothing more than a bleed valve. When you have an engine that makes MORE power than a silencer is designed for.. you can not "dump" the HEAT fast enough and CAN get too much HEAT back into the cylinder OR change the "tune" of the exhaust system via excessive HEAT.. Does this added heat increase the pressure?
Gas laws will tell you YES... Is this added pressure going to cause your engine problems?? IMHO.. no (read above) I believe it to be more related to the heat and out of time exhaust pulses vs. the actual pressure rise..
Now onto the Y Pipe...Is the Y Pipe and the head related?? IMHO.. NOT IN ANY WAY! The Y pipe is not a critically tuned component.. Its length and diameter , IMHO, are the only critical portions. Changing either one of these dimensions can effect the heat transfer and pulse timings.. Are these related to the head?? IMHO.. NO..
So, Y pipes adding big power?? Mainly  if there is a mis-match with exhaust outlet diameter and Y pipe diameter.. then yes, this will help a bit.. Can you fix the mis-match and achieve the same results?? Most likely, yes..

You can alter the length of the pipe via Y pipe shortening or lengthening and THIS can change performance for the better  and also for the worse.
Are these aftermarket Y pipes for these engines smaller or larger in diameter? Longer or shorter in length? If so, then they could very well alter the timing of the pipe and this can add or lose power!



Sound waves are pressure waves, but pressure is not sound.


To accurately measure the pressure of any oscillating wave one needs sophisticated high speed transducers. These are very expensive and are generally computer controlled.


It should be noted that waves in a medium (pipe for example) are oscillating. With an oscillating waveform, the amplitude of the wave will be constantly varying. There is usually a negative and a positive component (value). The shape of the wave can be uniform or very non-uniform (like I suspect an exhaust pressure wave would mimic)


When measuring any wave with a mechanical device (such as an analog pressure gauge) this gauge is very over-tasked for this sort of a measurement. The result would be a RMS (root mean squared) value, AT BEST, and would not accurately represent the actual value.


Let’s assume you have a steady frequency sinusoidal wave in the pipe (which we do not but we will assume for the moment) Taking a measurement with a gauge will give you the RMS value of this wave. The RMS value will be an “average” at best.


Now assume a pressure wave that is constantly varying it frequency, and amplitude (like a true pipe pulse) and now, you get a pressure that can not be measured correctly with a simple analog gauge-type device.


As we rise in elevation, atmospheric pressure decreases. This is a given. So, if you do NOTHING but rise in elevation, you drop pressure. So, ANY pressure measurement in a non sealed medium will be LOWER just from the rise in elevation.


Now the pipe pulses are treated as sound waves and the speed of sound is used in these calculations. The speed of sound (in a non-sealed medium) is temperature dependant. As we rise in elevation, temperature usually decreases and therefore; the speed in which the pipe waves travel are slower and this why we need to have increased pipe heat as elevation increases. You can increase the pipe heat via several methods. 1) Minimize radiation losses via wrapping the pipe with a TRUE heat barrier. 2) Increasing the internal gas heat via added power (engine enhancements etc.) these are a few. OR, you can alter the tuned length of the pipe to compensate for the slower travel.


So, one would surely benefit from a pipe that is designed around the lower atmospheric pressures that are present at the higher elevations.

BUT, let’s say you increase your engine’s internal heat and pressure via some true power increasing modifications. Now the internal pipe pressure may still be lower than if you were at sea–level BUT, it would be higher than if your engine did not have the power enhancements and you would have more rapid travel in the pipe.


So, the higher in elevation we go, the less power the engine makes unless something changes to process more air through the engine (Engine enhancements etc.). The naturally aspirated engine loses power because it can't process any more air. The engine can't magically grow displacement or increase rpm


So, what is the point to all this?? ONE point would be that the increase in elevation will produce less internal engine pressure and, as a result, less pipe pressure. Increasing the pipe’s internal pressure via stinger choking or restrictive silencers CAN allow for more pressure to be placed back in the engine via the “plugging pulse” of the pipe.

BUT, this can come at a cost.

The pulse’s strength and length are determinant mainly by the cone sections of the exhaust, NOT the stinger. The diameter of the stinger is a critical component. WHY? Because it has some control over the pipe’s operating temperature (internal heat). Too large of stinger and you can lower the pipe heat to a point of power-loss. Too small and you get an exhaust that can not bleed off pressure effectively and the engine will be the recipient of this “non-exhausted” heat and can cause engine failure, especially on long WOT pulls).

Get the stinger diameter just right and the engine acts like a tad shorter pipe and you can gain some power.


 The silencer is located AFTER the stinger. Yes, the silencer does effect the rate of exhaust bleeding but having too large of a silencer does not effect the stinger’s operation... having too small of silencer diameter can effect overall pipe operation due to internal heat and can cause engine failure.


OK, so what do we know?

1) Increasing elevation will decrease pressure EVERYWHERE.

2) Pipe pressures are determinant on the power of the engine.

3) Internal pipe heat effect the speed of the waves in the pipe.

4) The proper way to increase pipe heat and pressure is to design a pipe that accounts for the higher elevation.

5) The Stinger section of the pipe has a strong effect on the RETURNED internal pipe heat to the engine.

6) Increasing the returned pipe pressure via stinger or silencer choking can cause severe engine failure but can also aid in some cases.

7) Internal pipe pressure can not be accurately measured via ANY mechanical gauge.

8) More power creates more internal pressure/heat and can allow a pipe designed for low elevation to work at high elevation.



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