Scott Cramer Motorsports

02/21/2026

A recent string of events got me thinking about writing another informational post. To be honest, I haven't really thought much about sitting down and writing things to put on Facebook since starting at my new job but I think perhaps it may still be worth spending some time doing these.

This all started with a discussion about a 2 stroke "strato charged" chainsaw engine. After listening to some of the comments made about the technology and then having a different conversation about a specific snowmobile engine modification, I realize there may be some misinterpretation of the basic concepts and reasons behind it all.

In the case of normal 2 stroke scavenging, air and fuel is pulled in to the crankcase through the intake port when the piston travels upward toward TDC. As the piston travels downward, this mixture is compressed in the crankcase and forced up in to the transfer ports. The piston continues downward, opens the exhaust port, and then uncovers the transfer ports at which point the air fuel charge is forced up and in to the cylinder above the piston. The piston reaches BDC and then travels upward to compress the fuel charge in preparation of the next ignition event. As it is traveling upward the process starts over by pulling in another fresh charge of fuel and air in to the crankcase.

At the point the piston uncovers the transfer port opening on the way to BDC there is a period of time where some of the fresh charge of fuel and air being pushed up in to the cylinder is inadvertently pulled out the exhaust port by a pressure differential created by the exhaust gasses racing out. This phenomenon is referred to as short circuiting and is considered a problem because it is wasteful and inefficient. It decreases fuel efficiency and increases the emission of pollutants to the atmosphere.

To combat these issues in a world seeking maximum fuel economy and a low carbon footprint, engineering turned to the theories of stratified scavenging for solutions. Stratified scavenging is exactly as it sounds, it is simply the stratification or layering of the air fuel charge. Introducing a layer of air in to cylinder before the fuel charge minimizes or eliminates the waste caused by short circuiting, greatly increasing fuel efficiency and reducing emissions. The portion of the charge that is pulled out of the exhaust port by the pressure differential is either straight air or a very lean air fuel mixture depending upon fuel delivery methods and port configuration designs. As is the case in most modern EFI snowmobile engines, which are direct or semi direct injected, the fuel is either sprayed directly in to the combustion chamber or in to a separate port much closer to TDC and the ignition event. The air entering the crankcase through the intake port is pure air and that air is what enters the cylinder and what gets pulled out the exhaust port before the fuel is injected later, closer to TDC.

This technology is marketed as more efficient and therefore, being less wasteful, one presumes it produces more power as well. This may not be and quite often is not the case. Although wasteful from a fuel economy standpoint, most modern DI and SDI 2 stroke engines may actually produce more power without the stratified scavenging. The technology was implemented for fuel economy and to meet emissions standards not to make power.

From listening to a discussion about how strato charging increased power and efficiency of a chainsaw engine. A brief stop with a short discussion about the old 4 injector Polaris CFI system. Then on to someone asking me the reasons behind a snowmobile being modified by moving the fuel injectors from the cylinder (SDI) back to the throttle bodies. It all leads here. Stratified scavenging is mostly used for fuel economy and emissions regulation requirements. The previously mentioned snowmobile modification of moving the injectors back to spraying fuel at the beginning of the intake tract (at throttle body), afforded performance advantages by allowing more time for the fuel to mix and atomize with the air. They do not care about the fuel efficiency loss.

Don't get hung up on any of this. We live in a time where you receive great performance coupled with incredible fuel economy. Be happy about that and accept there are some unwanted side effects. Just ride.

01/22/2026

04/29/2025

Effective May 1, 2025 SCM will cease it's daily operations. All future services will be conducted by appointment only. We would like to thank all of our loyal customers for a successful past 12 years. We will continue to try to help whenever possible in the future.

04/24/2025

02/15/2025

I happened to notice an uptick in the buzz about the Polaris Dynamix suspension system. I was kicking around sharing my thoughts on this subject but who really needs to hear my opinion. A recent message exchange spurred me to sit down and write this little editorial.

For those unfamiliar with the Dynamix system, the synopsis would be it is a computer controlled suspension system. It uses a data acquisition unit to gather information regarding the motion or "dynamics" of the vehicle (acceleration, braking, roll, pitch, etc.) In turn, a computer adjusts the shock settings in much the same way you do when turning the adjuster dials on your shocks. The difference being all this adjusting happens in real time, in the blink of an eye, and not at your next stop when you get off and turn the dials. This is pretty awesome technology that unfortunately comes at a price. The system obviously adds a substantial amount of money to the price tag of snowmobiles that are already often priced out of the average consumer's range. I fear this is simply another "nail in the coffin" for an industry that is already struggling. I watched a YouTube video recently where sleds with this system were touted as the best handling machines ever produced and, in the same breath, they were talking about changing springs to improve it for their riding style. I'm not going to argue that this system is not straight up, next level awesome. I'm confident it holds a sled flat and likely feels like it's on rails. Unfortunately it clearly is not the be all, end all to suspension tuning. They are obviously making modifications because a shock can't do what a spring does and vice-versa. My fear is that exclaiming this as "the greatest thing since sliced bread" only sells product and fuels the manufacturers to push further. Whether anyone cares to admit it or not, the truth is, a large segment of riders can't even feel a difference when adjusting a shock. As I write this there are likely hundreds of Walker Evans shocks on the trail with the bleed adjuster valve blown out of the end cap and the rider is still adjusting the k***s. No offense to anyone, but those riders do not need a $25,000 snowmobile to enjoy the sport. Making the average snowmobiler think they will have some sort of out of body awakening by purchasing a sled with computer controlled suspension is borderline unethical. As my dad said back in the day, "I know you want that 650, but you don't really need that 650". People have been trying to perfect the suspension forever and that will still be the case with these systems. The only difference will be the type and price of the components the aftermarket company sells you to "improve" your ride and handling. Technology is awesome but it comes at a price, in more than one sense of the word.

03/02/2024

ROD PRESSURE IN GAS SHOCKS

I had a recent conversation about "tuning" the pressure on gas charged shocks. Apparently this has gained popularity and is being used as an adjustment procedure to change suspension characteristics. I'm here to try to explain why this adjustment may not be accomplishing what you think.

First a quick explanation of what the gas charge is for. I won't bore you with a long explanation, you can easily research information on the subject if needed. In short, the gas is there to hold the column of oil stable and allow the piston to process oil through the shim stack. If this gas charge is not present, or the pressure is too low, the shock fluid will cavitate and the shock will not function as intended. That is all the charge of gas is there for... it has NO other purpose.

Now on to the explanation as to why someone would tell you to change the amount of gas pressure in your shock. In short they are likely telling you to increase pressure because it will make the shock feel stiffer. This is pretty much where your interpretation of what you're doing goes wrong.

The gas charge in a shock creates an unwanted side effect called rod pressure. I say unwanted because in some forms of motorsport, rod pressure is despised because it kills the feel the driver has for what the vehicle is doing. Rod pressure is a force that tries to push the shaft back to it's original starting position. Full extension. There is a mathematical formula to derive the rod pressure of a shock involving the shaft size and how much gas pressure you have. It is simply a result of the area the shaft taking up volume in the oil column which displaces the floating piston and compresses the gas charge. This force can be pretty significant, in fact on a typical 5/8" shaft shock that is running 100 psi of gas pressure there will be a force of around 30 pounds. I say around because there is a small variation in actual poundage due to amount of head seal drag on the shaft. If you push the shock shaft in by hand you would need to overcome this 30 pounds of force to make the shaft move. If you raise the gas pressure to 200 psi the force will increase to approximately 61 pounds. This means if you were to push the shock shaft in by hand now you would need to overcome double the amount of force that is trying to keep the shaft extended. You will no doubt be able to feel that difference. This is likely why they're telling you to adjust the pressure... you add nitrogen and when you push down on the suspension it feels like everything is stiffer.

Here's the biggest catch, this force you're feeling is not linear to travel like a spring nor is it velocity dependent like damping from shock valving. It is a one time force added to move the shaft and then hold it at any position from there on. This means the force acts just like preloading a spring. In fact it is a "spring" force. It does nothing to change the damping characteristics of the shock. It simply accomplishes the same thing as adding preload to the spring does... it raises the ride height of the vehicle. It's not making your shocks "stiffer".

Can gas pressure be used as a suspension tuning tool? Possibly, but you now have a better understanding of what it actually does. Although there is apparently a pretty popular belief to the contrary, dont be confused. It doesn't change the damping curve of the shock, it simply alters the ride height which could possibly be useful in some situation.

On a side note, I recently heard some information on gas pressure, rod pressure, and shock feel that may have been misleading. The discussion stated enlarging the gas chamber and therefore the volume of gas would decrease the unwanted effects of rod pressure on driver feel. The size of the gas chamber is not included in the mathematical formula to determine rod pressure. Changing the size of the chamber does nothing to decrease the initial forces created by rod pressure which is solely dependent upon shaft size, and gas pressure (along with seal drag). What changing the size of the gas chamber does is lessens any increase or "creep up" of the rod pressure due to the compression of the gas charge volume. As the shaft enters the shock body the area it takes up in the oil column pushes on the divider and compresses the gas. The farther the shaft enters the oil column the more the divider is displaced, the more the gas is compressed, and the pressure exerted back on the column of oil increases. In a normal shock this is could be on the order of a few pounds extra added to the rod force when the shock reaches full compression. So in our 200 psi example above if the rod pressure is 61 pounds at initial compression travel, by the time you reach full travel compression it may be something like 62 or 63 pounds. Increasing the size of the gas chamber will lessen the effect of this increase or "creep up" but it will not effect the initial rod force.

01/17/2024

In the common configuration most snowmobiles have three rear springs that hold up the weight of the vehicle and it's cargo. If you remove one of those springs by ANY means the suspension will sag and feel softer. Once again, this type of scenario is not a shock problem. Although there is a "supporting" force generated by rod pressure in a gas shock, that force is actually an often unwanted byproduct of shock design... it is not and was not put there to support weight.

10/19/2023

During a short conversation regarding the failure shown in this picture, it was mentioned that it appeared to be just like the lean burn downs of days past. Holed a piston. The question then arose, aren't the EFI systems on today's sleds designed to detect, control, and correct before failures like this occur? In short, my answer was yes, they are. It did, however, get me thinking about the importance of knowing the difference between pre ignition and detonation. I decided to write this to try to inform. I think these situations are far more complex than most of us care to acknowledge. It is simply easier to say (insert brand here) sleds are garbage and these companies don't know what they are doing than it is to research for information and a better understanding.

Regardless the amount of poor information you will find online or from your local watering hole "expert", the phenomena of detonation and pre ignition are distinctly different. In short, detonation is unplanned combustion AFTER the engine's ignition event and pre ignition is well, exactly what it sounds like, unplanned combustion BEFORE the engine's ignition event. For whatever reason these two terms are often interchanged or used in conjunction with each other as though they are synonymous. They are not.

I could probably write a short book here but I'm going to try to make this rather blunt and to the point. The "sophisticated electronics" on your machine are really only sophisticated enough to catch certain things. First, there is an audible "knock" associated with detonation that a sensor is monitoring for. The "knock" comes from parts of the fuel charge "exploding" instead of burning in a controlled manner as the piston goes from TDC downward on it's power stroke. The sensor is basically a microphone attatched to the engine. It listens for noise. If your engine makes more noise than a preset programmed threshold the computer performs adjustments to timing, fuel delivery etc. The noise could literally be from anything. Failed engine mounts, the exhaust system, or from things going wrong inside the engine, like bearings etc. All of those things will obviously not be corrected by adjusting timing or the fuel map. It also could be the noise from detonation, in which case the sensor finally does what it was intended for and corrects to lessen occurance of detonation. Second, here's the catch in the failure shown... it wasn't caused by detonation. This, in my opinion, is a pretty clear cut case of pre ignition. With pre ignition the fuel charge burns on the compression stroke and causes incredible heat and pressure in the cylinder. The weakest part in there is the crown of the piston and well, just like in 1985 before any fancy electronics, it melts a hole straight through it. There is no discernable "knock" for any sensor to detect as the burning of the fuel charge is slow and controlled. It's not the "explosion" that occurs in a detonation event. There's likely an increase in operating temperatures noted by sensors and sent to the computer but it can't act fast enough to put this fire out... your engine is going to fail.

There are several things that can cause pre ignition and I'm not going to go in to depth about them now. The most important thing to take away from this is today's modern snowmobile engines are designed for fuel efficiency and low emissions. These things create heat. If you put enough heat in to metallic objects you can make them glow cherry red. This is an ignition source and a recipe for a pre ignition event. In my opinion many new sleds are likely flirting with disaster every ride. Use your imagination. There's a lot of things that computer can catch but it can't catch everything.

09/26/2023

"That's what she said"
Michael Scott