PROJECT GNCC 200XC

PART 4 INTERNAL MOTOR MODIFICATIONS

In Part 3, we covered bolt-on items that help make the 200 motor stronger, with better bottom end pull, and a real midrange instead of a non-existent midrange, and even more top end as well, plus gain tractability in rough terrain. Now we are going to take it a step further with some internal motor modifications, keeping with our original goals of increasing low RPM pull and adding midrange while not adversely affecting reliability. This is not going to be a full-on, big dollar build up that no one can afford. We will selectively do the things that will give the most bang for the buck, keeping the costs low and the results high.

Our Project GNCC 200XC is once again up on a lift in the KTM World service dept, cleaned up and ready for some 'magic', so off I go to the parts dept for all the top end gasket and seals, and a piston kit with wrist pin and bearing. Pulling the top end on the 200 motor is a pretty quick procedure, and in a short while the head is off and I'm looking at the top of the piston and the combustion dome in the head, both of which have almost no carbonization present. Taking a quick measurement, I find the piston has a slight amount of negative deck height, which means that when the piston is at top dead center, it does not protrude past the top of the cylinder, and in fact does not quite reach the top of the cylinder. Next off is the cylinder and piston, and a close inspection shows the cylinder wall and piston look great, and after measuring everything, I find the piston has very little wear, and the cylinder is the same. The rings are OK, but there is a bit more ring end gap than I want, so new rings will go in when the motor goes back together. There is very little axial play from the lower rod bearing, and no discernable play in the crank bearings, so everything is pretty darn tight. I know this customer used Motorex oil, so kudos to Motorex: almost no carbon present, and very little measurable wear. Very good!

I spent some time looking at the cases, and I can see places that could use some minor 'stuffing', which is epoxy work in the case. Epoxy is used to fill any extra space in the crankcase area, making the area or volume of the case smaller. In a two-stroke, the piston comes down and creates pressure in the crankcase, forcing the fuel/air mixture up the transfer ports and into the combustion area of the cylinder. Less volume in the bottom end of the motor means the piston will generate more pressure as it comes down, forcing more fuel/air into the cylinder at a faster flow rate, and that helps make more power in a 2-stroke motor, particularly helping improve throttle response and acceleration at low to mid RPM 's, which we definitely want. KTM is pretty good at keeping close tolerances in their 2-stroke cases, and this 200 is no exception, so although there is some gain possible, it's not going to be an amazing difference, so I elect to leave the bottom end together and not stuff the cases. If I had to take the bottom end apart anyway to replace crank bearings, I would do the case stuffing then.

Now it's time to take a look at the cylinder ports. I have several decades of time spent working with a flow bench, testing and modifying ports in 2-stroke cylinders and 4-stroke heads, and even I can learn a few things given that many years! It did not take long after purchasing my first flow bench to learn that most of what I thought I knew about porting and air flow was wrong. The flow bench showed me that what I was doing with porting tools, and what other engine tuners were doing with porting tools, was all just a guess, and most of the time, the guessing was not so good. This is still true today. Most engine tuners you know do not have a flow bench and have never used a flow bench. They are still guessing, and they are still guessing wrong most of the time. How about a little true or false test to see how you would guess?

1. Protruding metal the size of small marble would impede air flow in a port more than an equally sized indentation along a port wall. True or False?
2. For best power, the transfer ports in a 2-stroke need to give as straight a shot of air/fuel into the cylinder as possible. True or False?
3. An increase in port size increases flow. True or False?
4. Polishing the intake ports on a 4-stroke produces more power than leaving them with a rough, sandpaper-like appearance. True or False?

There are a zillion questions like these that I could ask, but let's see how you did on the 4 questions in our little test. You didn't answer false to any of them did you? Did you answer true to all of them? If you guessed true to any of them, you guessed wrong, just like a lot of people out there that are getting paid to do port work on your motor, but have never tested anything they do on a flow bench to see if what they are doing is better or worse. When you don't know what you are doing, it's easier to do more harm than good in a cylinder or head. Much easier.

The Flowbench tells no lies.

In keeping with our 'most bang for the buck' modifications, I looked the cylinder over with an experienced eye, checked a few things on the flow bench, and rather than go for Stage 5 porting, I'm going to fix the worst of the problems without spending too much time or money. We'll call it Stage 2 porting, essentially 3 hours ($195) spent working with the cylinder. More time spent will give more benefit, but at a decreasing rate of return. The best gains are made in the first few hours of work, and after that you are getting less benefit for each dollar spent. So what does the 200 cylinder need most? Work in the exhaust port and exhaust power valve area, and in the transfer ports.

Before...

After...

The exhaust ports on most 2-strokes are just not as good as the rest of the cylinder casting because they are quite difficult to cast and machine for all the power valve components that reside there and move around as RPM 's increase and decrease. The exhaust power valve on the KTM 200 consists of 3 distinct valves - 2 counter rotating barrels that open and close the secondary exhaust boost ports, as well as a main port valve that moves up and down to increase and decrease the effective main exhaust port height. Lots of stuff that moves in different directions (see photo), but with the common goal of keeping the exhaust port area small and the exhaust port height low for good torque at lower RPM 's, then opening up the exhaust port and raising the effective port height as RPM 's increase, giving the motor what it needs to make good high RPM horsepower. Everything has to match just right between the cylinder casting and all the moving pieces, or you don't have an efficiently working exhaust port, and the exhaust port is a very critical area. Time was spent to match everything and rework casting problems that were hindering efficient exhaust flow at low RPM 's, then in the transition zone as the 3 different power valves were opening, and at high RPM after the power valves were fully opened.

Chinese jigsaw puzzle

The transfer ports at the base of the cylinder were quite a bit larger than the engine cases where they mate together at the base gasket, leaving a sort of reverse step (void) going from the cases into the cylinder. Not good. A sudden increase in volume within a port decreases velocity, and flow velocity is very important to making good power in a motor, 2-stroke or 4-stroke. We removed material from the exhaust port to achieve better flow and increased power, but we are going to add material to the transfer ports to speed velocity and reduce tumbling of the air in the port, which will give us another gain in power, especially in the low to mid RPM range. We did not stuff the cases, but we are about to stuff the ports a bit, which will increase flow velocity, and also increase bottom end compression as we talked about earlier. Don't try this at home with household epoxy. You need a special epoxy made for internal engine use, and I always thoroughly clean, then deeply etch the metal surface to give the epoxy a rough and clean surface on which to bond. After the epoxy hardens, it has to be worked to match the case surface and blended to the curvature of the transfer port.

The special epoxy is smoothed and feathered well back into the transfer ports, and combined with the rest of the porting, will make a tremendous difference in the 200's power output

Now that the cylinder is finished, it's time to assemble the engine, but remember we talked earlier about the negative deck height? We are going to change that and give the motor some positive deck height and get the piston to slightly protrude from the cylinder at top dead center. This will increase compression giving us more pull and better throttle response, particularly at low to mid RPM. The change to positive deck height will also lower the cylinder, thereby lowering the port timing for better low to mid RPM power, plus this will tighten up the squish band. The squish band is the area between the piston at top dead center and the outer ring portion of the cylinder head, before the head opens up into the main combustion dome. The amount of clearance and the angle of the head relative to the piston crown, and the radius of the bend where the squish band opens into the main dome, are all part of what makes a 2-stroke motor burn fuel more efficiently and make better power. As the piston comes up to top dead center, it 'squishes' the fuel/air mixture up into the combustion dome, and if the squish band is too big, or the relative angle is too great, you get a less efficient burn. A smaller squish band generally will produce more turbulence in the combustion dome as the piston nears top dead center and the spark plug fires. Anything you can do to create turbulence generally increases bottom end and midrange power.

So how do we accomplish all this positive deck height/squish band voodoo magic using our 'best bang for the buck' approach? Have I mentioned yet that the KTM World parts dept is awesome? We have 8 different base gaskets in stock ranging from .07mm to .75mm thickness, with .75mm being the thickest offered by KTM. The base gasket that came out of the motor was a .75mm, and there is lots of room available between the piston and cylinder head (squish band) to use a thinner base gasket, but if we go too thin and bump the compression too high, the motor will require a strict diet of race gas to keep detonation at bay, along with the motor destruction that always follows detonation. Detonation occurs when the fuel ignites and burns too quickly, before the piston reaches the ideal moment before top dead center. Then the ignited charge in the cylinder is forcing down hard on the piston as the piston is trying to reach top dead center, and over a little bit of time, this will destroy a motor with the added force and stress on the internal parts. You might be asking yourself why race gas would prevent that? Doesn't high octane race gas burn faster than pump gas? Wouldn't that make detonation worse if the race fuel burns faster than pump gas? The answer is no, contrary to what most people think. Higher octane fuel actually burns slower than lower octane fuel. This gives a longer burn, a more efficient burn, and a longer push on the piston, all of which makes more HP, but not in the way most people think race gas makes more HP. We could start playing with ignition timing as well, but backing off the ignition timing to control detonation can also have an adverse effect on the 200's nice low end throttle response, and I don't want that.

After a little consideration, I selected a .50mm base gasket, which is .25mm thinner than the base gasket that came out of the stock motor. This will bump the compression a little, but not so much that we need to run race gas all the time, and the base gasket change will lower the port timing by .25mm, helping our bottom end and midrange pull, and it will also tighten up the squish band by .25mm. All of this with a little bit of measuring and a simple base gasket change.

OK, a very light hone on the cylinder, new rings on the piston, a fresh wrist pin bearing, new gaskets, and the top end is assembled and ready for a new spark plug. The stock plug from KTM is a NGK BR8EG, which is not a crappy $2 plug, but I prefer the BR8 EIX Iridium plug. The BR8 EIX has a pointy thin wire center electrode, plus a tapered and angle cut outer electrode. For those of you that have some background in physics, you may recall that an electrical spark jumps more easily between sharp or pointed edges of a metal object than from a flat metal surface. The thinner electrode also burns off carbon and oil better.

Big difference in electrodes

Now it's time to adjust the power valve (PV). Many KTM 200 owners do not realize the 200 PV is adjustable, and more than a few KTM dealers don't know this either. It's fairly common knowledge that the 250 and 300 KTM's have adjustable PV's since they come with 3 different springs and an external adjustment, but somehow the 200 seems to get overlooked. Adjusting the PV on the 200 makes a big difference in how the power comes on, and on the 200, this often affects how much traction and drive you get from the rear wheel. Remember that one of the big problems with the 200 is the lack of drive available on rough ground when the motor reaches the flat midrange, then suddenly hits the upper RPM power band. The motor sings to you at that point, but the refrain is the same: the rear tire spins, bouncing around on the rough ground, and you are not driving forward, especially if your suspension is still factory stock and/or has the wrong springs. So what I want to do with the PV is adjust it so it opens very late, extending the good low end pull as long as possible, and letting the RPM 's get to a higher point before the PV opens fully and the resulting HP hit does not start the tire spinning nearly so early. With the modifications already made, this motor is going to have very strong low end pull and a lot better midrange, so this PV trick will make the 200 feel like a lot less like a 200 and a lot more like a 250. Take the big aluminum nut off the right side of the motor and you will see this:

Turn the outer threaded piece to adjust the PV. Don't turn the inner bolt

On Project 200XC, the PV adjustment I liked best had the PV line (on the right) just a bit lower than the case line on the left. This makes the PV open late, which works great with this strong pulling motor.

After bolting down the seat, tank, etc, we have a complete motorcycle again, and it's time for a little ring break-in and testing! In order to seat rings in a motor, you need a fair amount of cylinder pressure, so I am not a big fan of super easy break-in procedures that end up not seating the rings well. A little warm up, then moderate throttle, followed by some healthy doses of throttle, and away we go for a ride with generous throttle but no extended high RPM use for the first few miles.

Even before any jetting changes, my first response was 'WOW'. Double and triple WOW! We have taken an ordinary mild mannered Austrian 200 and turned him into the Arnold Schwarzenegger 200! This motor is positively muscular! It rips everywhere now, bottom to mid, mid to top, plus it hooks up! This baby pulls really, really well at low RPM 's (more than you are thinking), and now it has a real midrange with guts (more than you are thinking), then it hauls the mail on top even better than I ever thought it would. I was never really trying to make a big top end horsepower increase, but we got it anyway. Find a place to pin the throttle and row the gear box through 4 th , 5 th , 6 th , and it blows you away with the new found acceleration in the upper gears, and remember, we took 2 teeth off the rear sprocket in Part One, so it's got taller than stock gearing.

After a few jetting changes, the 200 is just fantastic and now pulls one gear higher on trails it never could in stock trim, and the bottom to midrange boost from tree to tree in 3 rd and 4 th gear through the woods is nothing short of phenomenal for a 193cc bike. The motor mods in combination with the VForce3 reed cage, Factory Hard Parts pipe and silencer, plus the added flywheel weight, has all come together to make a tractable and powerful 200, and it feels a lot like a lightweight version of the KTM 300: same power characteristics, same amazing low end and midrange, just not quite as much as the 300 has available. With the suspension working really well now, and the new found muscle from the motor combined in a light weight package, this bike makes you feel like Superman - or at least Ahhhnold.

Part three of our Project GNCC 200XC.
Bolt-on performance.

Part five of our Project GNCC 200XC.
Final testing & conclusion.