Cam timing is one of the most misunderstood mechanical subjects and the easiest thing to screw up. To further complicate the issue Ive seen a lot of suggestions posted by people who havent a clue much like the blind leading the blind. Turbocharging often requires lowering the compression, which on the Hayabusa is achieved by adding cylinder spacers, which in turn requires cam retiming. This post is intended to take out some of the mystery and provide a basic understanding of the choices.
In my example a .080-inch cylinder spacer is added with any base gaskets. Since the OEM base gasket is .010 inch this represents an increase of .070 inch. My measurements of the Hayabusa cam sprocket yielded the following numbers.
34 teeth
6.4mm chain pitch
2.731 inch sprocket pitch diameter
8.57534 inch sprocket pitch diameter
Based upon these cam sprocket numbers each .010 inch added to the cylinder base will result in the cams being advanced by .420 degrees. A .080 spacer without base gaskets will therefore advance the cams by 2.94 degrees.
Experience has shown that the exhaust cam can either be left in this new position or it can be retimed back to the stock setting. The reason it can be left advanced is that it will provide a little less valve overlap, which in theory is helpful with FI but no one has tested to see which provides the best result.
The intake cam is normally retimed back to the stock position.
My measurements indicate that stock timing is 105/105 degrees.
There are 3 possible ways to retime the cams but only two that are correct.
1. With adjustable cam sprocket/s.
This is the standard method in which one or both cams received APE adjustable sprockets and cam timing is done using lobe centers as reference points. Lobe centers are used simply because it is relatively easy to determine the center position with simple tools. A degree wheel is attached to the crank shift and piston TDC (top dead center) is located by employing a positive piston stop in a spark plug hole. Once TDC is known the lobe centers can be determined by measuring when a valve is within .050 inch of opening and closing with a dial indicator. The measurements are taken at .050 inch of lift to avoid the very gradual ramp at the beginning and end of the lobe, which would make accurate measurements difficult. The middle of these two points on the degree wheel is lobe center. Intake lobe center is set at ATDC (after top dead center) and exhaust at BTDC (before top dead center).
I am not going to explain the step by step process of cams are timing because I would only be repeating what has already been covered hundred of times elsewhere. If youre interested this is one such write up.
tinyurl.com/a3f95
One point of caution that is rarely addressed is that care must be taken when pressing on the adjustable sprocket hubs to maintain the same sprocket relationship as the OEM position numbers. By maintaining the same position you can use the factory adjustment standard of 15 pins between sprockets, which will make future assemblies simpler.
My suggestion on retiming cams is that if you are unsure of why and how it is done you should leave it to an experienced mechanic. As mentioned before it is easy to do wrong and the consequences can be costly engine damage. Cam timing is well beyond motorcycle mechanics 101.
2. By retiming the crank shaft cam chain sprocket
This is the less known and understood method but it can provide good results without the cost and time associated with using adjustable sprockets. While what youre doing by retiming the crank sprocket is simple in practice it is sometimes hard to conceptualize. If you really want to clearly understand the mechanics you can make a cardboard model of the sprocket and rotate it through the process to visualize what happens.
The sprocket is mounted to the crank with an 18-tooth spline. For OEM installation the sprocket and crank are center punched to show the correct position. If the O.D. of the sprocket also had 18 cam chain teeth there would be no wrong or right way to install it but it doesnt. The O.D. (cam chain) has only 17 teeth. One full revolution of the sprocket equals 18 teeth on the spline but only 17 on the O.D and 1 O.D tooth equals 21.176 degrees (360 / seventeen). The resulting effect is that if you (ADVANCE) the sprocket by one spline tooth it will retard the cam chain teeth by 1/18 of the O.D. teeth, which equals 1.176 degrees (21.176 / eighteen) at the crank. Since the cams turn at half crank speed that equals .588 degrees at the cams.
Therefore using my example of a .080 spacer with out base gaskets that has advanced the cams by 2.94 degrees you can by ADVANCING the crank sprocket 5 spline teeth retard the cam timing by 2.94 degrees, which in this case takes it back to the stock position. Different spacer thickness will results in different numbers but this is the math to determine how close a correction this method will provide.
While this approach is a lot simpler than adjustable sprockets it also can be done wrong (what cant) so beware. If you use this approach I suggest you double check the final cam timing with a degree wheel.
3. By changing the number of cam chain pins between the intake cam (position #3) and the exhaust cam (position # 2).
Stock timing calls for 15 pins between these two sprocket positions but it is possible to retard the intake cam by changing to 16 pins. While I have read that this method has been used before it will not provide the correct timing for the following reason.
The cam chain pitch is 6.4mm (.252 inch) so changing one pin would retard the cam sprocket pitch diameter by .252 inch. That equals 10.58 degrees so unless that is how much the cams were advanced its going to be way off. As explained before a .080 spacer without base gasket only advanced the cams by 2.94 degrees.
The only time that using a pin count other than 15 makes sense is if the adjustable sprockets hub/s have been installed in the wrong position.
In my example a .080-inch cylinder spacer is added with any base gaskets. Since the OEM base gasket is .010 inch this represents an increase of .070 inch. My measurements of the Hayabusa cam sprocket yielded the following numbers.
34 teeth
6.4mm chain pitch
2.731 inch sprocket pitch diameter
8.57534 inch sprocket pitch diameter
Based upon these cam sprocket numbers each .010 inch added to the cylinder base will result in the cams being advanced by .420 degrees. A .080 spacer without base gaskets will therefore advance the cams by 2.94 degrees.
Experience has shown that the exhaust cam can either be left in this new position or it can be retimed back to the stock setting. The reason it can be left advanced is that it will provide a little less valve overlap, which in theory is helpful with FI but no one has tested to see which provides the best result.
The intake cam is normally retimed back to the stock position.
My measurements indicate that stock timing is 105/105 degrees.
There are 3 possible ways to retime the cams but only two that are correct.
1. With adjustable cam sprocket/s.
This is the standard method in which one or both cams received APE adjustable sprockets and cam timing is done using lobe centers as reference points. Lobe centers are used simply because it is relatively easy to determine the center position with simple tools. A degree wheel is attached to the crank shift and piston TDC (top dead center) is located by employing a positive piston stop in a spark plug hole. Once TDC is known the lobe centers can be determined by measuring when a valve is within .050 inch of opening and closing with a dial indicator. The measurements are taken at .050 inch of lift to avoid the very gradual ramp at the beginning and end of the lobe, which would make accurate measurements difficult. The middle of these two points on the degree wheel is lobe center. Intake lobe center is set at ATDC (after top dead center) and exhaust at BTDC (before top dead center).
I am not going to explain the step by step process of cams are timing because I would only be repeating what has already been covered hundred of times elsewhere. If youre interested this is one such write up.
tinyurl.com/a3f95
One point of caution that is rarely addressed is that care must be taken when pressing on the adjustable sprocket hubs to maintain the same sprocket relationship as the OEM position numbers. By maintaining the same position you can use the factory adjustment standard of 15 pins between sprockets, which will make future assemblies simpler.
My suggestion on retiming cams is that if you are unsure of why and how it is done you should leave it to an experienced mechanic. As mentioned before it is easy to do wrong and the consequences can be costly engine damage. Cam timing is well beyond motorcycle mechanics 101.
2. By retiming the crank shaft cam chain sprocket
This is the less known and understood method but it can provide good results without the cost and time associated with using adjustable sprockets. While what youre doing by retiming the crank sprocket is simple in practice it is sometimes hard to conceptualize. If you really want to clearly understand the mechanics you can make a cardboard model of the sprocket and rotate it through the process to visualize what happens.
The sprocket is mounted to the crank with an 18-tooth spline. For OEM installation the sprocket and crank are center punched to show the correct position. If the O.D. of the sprocket also had 18 cam chain teeth there would be no wrong or right way to install it but it doesnt. The O.D. (cam chain) has only 17 teeth. One full revolution of the sprocket equals 18 teeth on the spline but only 17 on the O.D and 1 O.D tooth equals 21.176 degrees (360 / seventeen). The resulting effect is that if you (ADVANCE) the sprocket by one spline tooth it will retard the cam chain teeth by 1/18 of the O.D. teeth, which equals 1.176 degrees (21.176 / eighteen) at the crank. Since the cams turn at half crank speed that equals .588 degrees at the cams.
Therefore using my example of a .080 spacer with out base gaskets that has advanced the cams by 2.94 degrees you can by ADVANCING the crank sprocket 5 spline teeth retard the cam timing by 2.94 degrees, which in this case takes it back to the stock position. Different spacer thickness will results in different numbers but this is the math to determine how close a correction this method will provide.
While this approach is a lot simpler than adjustable sprockets it also can be done wrong (what cant) so beware. If you use this approach I suggest you double check the final cam timing with a degree wheel.
3. By changing the number of cam chain pins between the intake cam (position #3) and the exhaust cam (position # 2).
Stock timing calls for 15 pins between these two sprocket positions but it is possible to retard the intake cam by changing to 16 pins. While I have read that this method has been used before it will not provide the correct timing for the following reason.
The cam chain pitch is 6.4mm (.252 inch) so changing one pin would retard the cam sprocket pitch diameter by .252 inch. That equals 10.58 degrees so unless that is how much the cams were advanced its going to be way off. As explained before a .080 spacer without base gasket only advanced the cams by 2.94 degrees.
The only time that using a pin count other than 15 makes sense is if the adjustable sprockets hub/s have been installed in the wrong position.
