stiffsimon
21-06-2011, 08:03 AM
For those of yo that visit Oldskoolsuzuki.com I'm sure that Speedy Steve Racer (SSR) needs no introduction. For those that haven't heard of him he is the ultimate knowledge base of the old Suzuki GSX motors, and engines in general it has to be said, having drag raced these beasts successfully for decades. Running his own company SK Motorsports Engineering Steve has very kindly allowed me to post an article he wrote on degreeing in cams. As this is something I'll be doing myself sometime in the future I thought that it would be useful for those of us contemplating this task but not so sure on how to do it.
VALVE CLEARANCES
Many engine builders take lobe centre measurements with zero valve clearance so that all movements can be detected. In fact, the valve clearance can actually be slightly negative, that is the valve can be held slightly open by the cam with the valve in the closed position. You may also do the calculation with the normal clearance that you have when the engine is running. The amount of pre-load or clearance on the valve has no effect on the lobe centre number but will affect the opening and closing numbers. What IS important is that, for future comparison purposes, you always do it the same way with the same clearance value. It is also very important that an accurate top dead centre TDC reference be used when degreeing cams. Note also when degreeing in cams DOHC engines exhaust cam first, SOHC engines intake valve first.
Degreeing Camshafts
You will need the cam timing specifications, these are printed in workshop manuals for stock bikes or are included with performance camshafts, take care not to lose the spec sheet if you buy a performance camshaft. Without these specs nobody will be able to time the camshafts accurately or reliably. The ramp of a camshaft lobe is designed to gradually open and close the valves, if the ramp was not gradual, the valves would be banged open and slammed shut, this would result in valve bounce and high wear and be mechanically noisy. The problem is that this ramp makes measuring the exact point at when a valve is opened almost impossible to establish. This is why valve opening and closing times will be stated at a certain amount of valve lift in performance camshaft spec sheets, the amount of valve lift (movement of the valve from its seat) is easily measured with a dial gauge placed to read from the top of the valve follower, bucket or spring retainer (but not rocker arm). Valve lift should also be measured with no valve clearance. The exhaust camshaft should be degreed in first, as the exhaust cam will change the timing of the inlet cam, the inlet cam should be degreed last. Mount the dial gauge so that the plunger is in line with the valve and reading zero lift when the camshaft lobe is pointing away from the valve, as you rotate the engine the camshaft lobe will start to move towards the valve and then start lifting the valve from its seat, the dial gauge will also start to measure the lift, when the dial gauge has measured the amount of lift specified in the cam spec sheet, stop, you now need to jot down the figure that the pointer is pointing to on your degree disc, this is your Exhaust cam lobe opening figure and is usually X of degrees Before Bottom Dead Centre (BBDC). Continue to rotate the engine until the valve has fully opened and is now on its way back to the valve seat, the gauge needle will have reached a certain point and be returning towards the amount of lift you took your opening figure at, if for example this was 0.30, stop and jot down this figure, this figure is your Exhaust cam closing figure and is usually X degrees After Top Dead Centre (ATDC). Continue rotating the engine and make sure the dial gauge reads zero again when the valve has finished moving. Now you have 2 cam timing figures, you can work out the Lobe Centres.
Lobe centre = the larger figure + 180, then subtract the smaller figure and divide this total by 2.
For example, if your exhaust cam lobe opening figure was 60 Before Bottom Dead Centre and your Exhaust cam lobe closing figure was 21 After Top Dead Centre
60 (the larger number) +180 = 240, take away 21 (the smaller number) 240-21 = 219,
Half of 219 = 109.5, so now we know that 109.5 is our current lobe centre figure.
If you are dialling in stock camshafts, the workshop manual may not state at what amount of valve lift the cam degreeing figures are taken, in this situation you could you could use this equation to work out the lobe centre figures using the manuals opening and closing figures.
For example;
Exhaust opens @ 40Before BDC
Exhaust closes @10 After TDC
40 (larger number) +180 = 220, take away 10 (smaller number) =210, 210 divided by 2 =105.
105 would be the lobe centre figures to aim for.
Take your degreeing figures at 040 of valve lift and when you have the opening and closing figures, run them through the above equation, if you end up with, for example, 109.5 lobe centres, 109.5 would be 4.5 from the 105 you are aiming for.
To adjust cam timing slacken off the exhaust cam sprocket bolts and rotate the crankshaft (without moving the camshaft) in this example 4.5 degrees, then tighten the cam sprocket bolts with the sprocket in its new position (the cam sprocket would have rotated but the camshaft should have stayed still). Repeat the measuring of opening and closing to make sure that your figures end up equalling 105.
Once you have the exhaust cam giving the right lobe centre figure, torque up the cam sprocket bolts and use loctite. Move on and duplicate the process on the inlet cam, the inlet cam may have a different lobe centre figure to the exhaust camshaft.
For example;
Inlet opens @ 15 Before TDC
Inlet closes @ 35 After BDC
35+180 = 215, 215 15 = 200, half of 200 = 100, so the inlet lobe centre figure is 100 degrees.
If at any time during the procedure, the engine does not rotate easily, do not force it, you could damage a valve or a piston if you do.
I have consistently found that when it comes to setting up camshafts that you should begin with the INTAKE cam, since the intake valve is the most likely to be damaged by insufficient valve to piston clearance or incorrect timing.
The common range of lobe centre values for SUZUKI engines is only about 10 degrees wide from about 102 to 112 degrees, a change of one degree can have a considerable effect on the power delivery characteristics of a SUZUKI engine.
The effect of moving lobe centres is that by advancing the intake and retarding the exhaust, known as CLOSING UP THE CENTRES, it will increase the valve overlap and will move the power UP in the RPM range, although at the sacrifice of LOW- RPM power. The result would be LOWER numerical values on both intake and exhaust lobe centres.
If you retard the intake and advance the exhaust, known as SPREADING THE CENTRES , valve overlap will decrease and will result in a WIDER power band whilst sacrificing HI RPM power. This is indicated by higher numerical values on both intake and exhaust lobe centres. If you move only one cam the results are not as predictable, traditionally it is the INTAKE CAM that is moved to change power characteristics since small changes here seem to have a greater effect.
Speedy Steve Racer - SSR
VALVE CLEARANCES
Many engine builders take lobe centre measurements with zero valve clearance so that all movements can be detected. In fact, the valve clearance can actually be slightly negative, that is the valve can be held slightly open by the cam with the valve in the closed position. You may also do the calculation with the normal clearance that you have when the engine is running. The amount of pre-load or clearance on the valve has no effect on the lobe centre number but will affect the opening and closing numbers. What IS important is that, for future comparison purposes, you always do it the same way with the same clearance value. It is also very important that an accurate top dead centre TDC reference be used when degreeing cams. Note also when degreeing in cams DOHC engines exhaust cam first, SOHC engines intake valve first.
Degreeing Camshafts
You will need the cam timing specifications, these are printed in workshop manuals for stock bikes or are included with performance camshafts, take care not to lose the spec sheet if you buy a performance camshaft. Without these specs nobody will be able to time the camshafts accurately or reliably. The ramp of a camshaft lobe is designed to gradually open and close the valves, if the ramp was not gradual, the valves would be banged open and slammed shut, this would result in valve bounce and high wear and be mechanically noisy. The problem is that this ramp makes measuring the exact point at when a valve is opened almost impossible to establish. This is why valve opening and closing times will be stated at a certain amount of valve lift in performance camshaft spec sheets, the amount of valve lift (movement of the valve from its seat) is easily measured with a dial gauge placed to read from the top of the valve follower, bucket or spring retainer (but not rocker arm). Valve lift should also be measured with no valve clearance. The exhaust camshaft should be degreed in first, as the exhaust cam will change the timing of the inlet cam, the inlet cam should be degreed last. Mount the dial gauge so that the plunger is in line with the valve and reading zero lift when the camshaft lobe is pointing away from the valve, as you rotate the engine the camshaft lobe will start to move towards the valve and then start lifting the valve from its seat, the dial gauge will also start to measure the lift, when the dial gauge has measured the amount of lift specified in the cam spec sheet, stop, you now need to jot down the figure that the pointer is pointing to on your degree disc, this is your Exhaust cam lobe opening figure and is usually X of degrees Before Bottom Dead Centre (BBDC). Continue to rotate the engine until the valve has fully opened and is now on its way back to the valve seat, the gauge needle will have reached a certain point and be returning towards the amount of lift you took your opening figure at, if for example this was 0.30, stop and jot down this figure, this figure is your Exhaust cam closing figure and is usually X degrees After Top Dead Centre (ATDC). Continue rotating the engine and make sure the dial gauge reads zero again when the valve has finished moving. Now you have 2 cam timing figures, you can work out the Lobe Centres.
Lobe centre = the larger figure + 180, then subtract the smaller figure and divide this total by 2.
For example, if your exhaust cam lobe opening figure was 60 Before Bottom Dead Centre and your Exhaust cam lobe closing figure was 21 After Top Dead Centre
60 (the larger number) +180 = 240, take away 21 (the smaller number) 240-21 = 219,
Half of 219 = 109.5, so now we know that 109.5 is our current lobe centre figure.
If you are dialling in stock camshafts, the workshop manual may not state at what amount of valve lift the cam degreeing figures are taken, in this situation you could you could use this equation to work out the lobe centre figures using the manuals opening and closing figures.
For example;
Exhaust opens @ 40Before BDC
Exhaust closes @10 After TDC
40 (larger number) +180 = 220, take away 10 (smaller number) =210, 210 divided by 2 =105.
105 would be the lobe centre figures to aim for.
Take your degreeing figures at 040 of valve lift and when you have the opening and closing figures, run them through the above equation, if you end up with, for example, 109.5 lobe centres, 109.5 would be 4.5 from the 105 you are aiming for.
To adjust cam timing slacken off the exhaust cam sprocket bolts and rotate the crankshaft (without moving the camshaft) in this example 4.5 degrees, then tighten the cam sprocket bolts with the sprocket in its new position (the cam sprocket would have rotated but the camshaft should have stayed still). Repeat the measuring of opening and closing to make sure that your figures end up equalling 105.
Once you have the exhaust cam giving the right lobe centre figure, torque up the cam sprocket bolts and use loctite. Move on and duplicate the process on the inlet cam, the inlet cam may have a different lobe centre figure to the exhaust camshaft.
For example;
Inlet opens @ 15 Before TDC
Inlet closes @ 35 After BDC
35+180 = 215, 215 15 = 200, half of 200 = 100, so the inlet lobe centre figure is 100 degrees.
If at any time during the procedure, the engine does not rotate easily, do not force it, you could damage a valve or a piston if you do.
I have consistently found that when it comes to setting up camshafts that you should begin with the INTAKE cam, since the intake valve is the most likely to be damaged by insufficient valve to piston clearance or incorrect timing.
The common range of lobe centre values for SUZUKI engines is only about 10 degrees wide from about 102 to 112 degrees, a change of one degree can have a considerable effect on the power delivery characteristics of a SUZUKI engine.
The effect of moving lobe centres is that by advancing the intake and retarding the exhaust, known as CLOSING UP THE CENTRES, it will increase the valve overlap and will move the power UP in the RPM range, although at the sacrifice of LOW- RPM power. The result would be LOWER numerical values on both intake and exhaust lobe centres.
If you retard the intake and advance the exhaust, known as SPREADING THE CENTRES , valve overlap will decrease and will result in a WIDER power band whilst sacrificing HI RPM power. This is indicated by higher numerical values on both intake and exhaust lobe centres. If you move only one cam the results are not as predictable, traditionally it is the INTAKE CAM that is moved to change power characteristics since small changes here seem to have a greater effect.
Speedy Steve Racer - SSR