The KLR650 MC mod is one of those mods that has become the “de facto” standard on the Gen2. Although there is only a single dyno sheet in existence to support the mod everybody blindly follow and performs it. No questions asked. The data was looked at, tested on a couple of KLR’s and results were very different from other claims.
Please understood that this is not an attack on those that performed the original MC Mod. In fact hats of to them for trying and putting them self out there. During our careers we have made many mistakes. Some of which we refused to except until data that proved us wrong was provided.
It is said that a mistake that makes you humble is better than an achievement that makes you Arrogant!
As human beings we expect an award when we invest time and or money in something. More so when it comes to our rides. Be careful, the bud dyno will make you believe things that never existed.
The MC mod requires one to advance or move forward the the exhaust camshaft timing by one full tooth on the exhaust camshaft sprocket. There are those that claim that they have greatly benefited from the MC mod and then there are those that claim they have not. From increased overtaking ability at 70 mph to so much better through thick sand and climbing out hill sides. Even better and crisper engine response. All unsubstantiated because of the bud dyno.
Original KLR650 MC mod data
Data made available show that the original test bike on which the MC Mod was first performed gain 0.52 HP and 1.17 ft lb of torque. Runs 3 and 9 were done late in the afternoon and within one hour of each other. Six runs were made between the original 36.56 HP run and the 37.08 HP run. During this period the temperature in the dyno room raised slightly and humidity dropped by 3%, from 54% to 51%.
Would a 0.52 HP and 1.17 ft lb of torque gain really be that noticeable on a bike that weighs on average 440 lb wet and a rider that weighs 185 lb?
Kawasaki KLR650 exhaust Cam Specifications:
|Gen 1||Gen 2|
|Lift||8.33 mm (0.328″)||8.33 mm (0.328″)|
|Opens||57 BBDC||57 BBDC|
|Close||31 ATDC||37 ATDC|
|Cam Center Line||103 ((57 + 180 + 31)/2)-31)||100 ((57 + 180 + 37)/2)-37)|
|Peak Combustion||19 ATDC||19 ATDC|
Peak combustion occur fairly late due to the large piston, moderate compression, poor squish area and relative moderate ignition timing.
Note: Exhaust cam on both Gen 1 and Gen 2 opens at “57 degrees BBDC”. The Gen 2 has 6 degrees more duration to allow for slightly larger overlap.
The relationship between the crankshaft gear and the camshaft gear is 2:1. Meaning that the crank has to make two full rotations (720 degrees.) to accomplish one full rotation (360 degrees) of the camshaft. Or otherwise put the camshaft only runs half the speed of the crankshaft. If the crank does 1000 RPM the camshaft does 500 RPM.
Since the camshaft gear has 48 teeth it means that each tooth represent 7.5 degrees 360/48 = 7.5 on the cam gear but 15 degrees on the crank.
Advancing the exhaust camshaft by one tooth or 7.5 degrees will correlate to a 15 degree advancement on the crankshaft. Advancing the exhaust cam timing by one tooth will cause the exhaust valve to open 15 degrees earlier and close 15 degrees earlier.
What does the KLR650 MC Mod do?
KLR650 MC Mod exhaust cam timing for the Gen 2 looks like this:
Open 57 + 15 = 72° BBDC
Close 37 – 15 = 22° ATDC
Cam Center Line 115 ° ((72 + 180 + 22)/ 2)-22) = 115°
Peak combustion pressure = 19°
The effective power stroke from the point where peak cylinder pressure occur is reduced from a 104° duration to 89°. This cause EGT’s to substantially increase. In some cases as much as 320°, an indication that combustion has not fully completed. Again, large piston moderate compression, poor squish and relative moderate ignition timing.
Overlap decreased by 16°, from 57° to 41°. This will also reduce cylinder fill. Reduced cylinder fill will also cause lower peak combustion pressure.
No evidence exist that someone other than the author has actually taken the time to take a degree wheel and dial indicator to the engine to verify the cam timing before and after the advancement of the exhaust cam. Not verifying modifications is the same than target shooting in the dark.
Dyno test the KLR650 MC Mod
Dyno tests on a couple of KLR’s in a well ventilated environment and a properly calibrated dyno, show that not a single machine benefited by advancing the exhaust cam by 7.5 cam degrees or one full tooth. Yes there are those that says that a dyno test mean nothing but it sure will predict how the engine will perform and more important there is a before and after results.
All the KLR650’s that was put on the dyno had the air box mod, KLX needle and jetting done at the same time the MC mod was performed. It is most properly the combination of the airbox mod and jetting that cause the power increase that was felt and not the effects of the MC mod. An immediate increase in power was recorder when the exhaust cam was moved back to the original factory location Additional power can be found by advancing the exhaust cam, but not by 15 crank degrees. Adjustable cam gear that will allow for an exhaust lobe center somewhere in the area of 106 – 108 degrees and an intake lobe center of 110 – 112° will increase power.
TDC = Top Dead Center
BTDC = Before Top Dead Center
ATDC = Top Dead Center
BDC = Bottom Dead Center
BBDC = Before Bottom Dead Center
ABDC = After Bottom Dead Center
EGT = Exhaust Gas Temperature.
AFR = Air Fuel Ratio
CCC = Cold Cranking Compression.
An idle jet in combination with a properly set air screw that will produce an idle with an AFR of 14.6 : 1, in combination with the correct exhaust cam timing will produce an exhaust gas that when you put a bare hand 4 – 6 inches behind the exhaust at idle, the exhaust gas will feel relatively cool to the hand.