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SCAT C-35 measurements and valve lash
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E4ODnut
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Joined: August 06, 2013
Posts: 168
Location: Gibsons BC Canada
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PostPosted: Thu Nov 21, 2013 10:20 pm    Post subject: SCAT C-35 measurements and valve lash Reply with quote

For any of you who may be interested in this sort of stuff . This is the advertising information we got on the Scat C-35 from CIP:

Lift at cam: 0.381” (intake and exhaust)
Advertised Duration: 285° (intake and exhaust)
Duration @ 0.050” cam lift: 246° (intake and exhaust)
Intake Opens @ 0.050”cam lift: 17° BTDC
Intake Closes @ 0.050” cam lift: 51°ABDC
Intake Lobe Centre Line: 107° ATDC (calculated)
Lobe Separation Angle: 107° (calculated)

These are actual in engine measurements in a Type 4 with less than 2000 miles since build.

EO @ 0.001” cam lift: 145° BTDC
EO @ 0.020” cam lift: 75° BBDC
EO @ 0.050” cam lift: 54° BBDC
Maximum @ 0.374” cam lift: 110.5° BTDC (Exhaust Lobe Centre Line)
EC @ 0.050” cam lift: 14° ATDC
EC @ 0.020” cam lift: 30° ATDC

IO @ 0.020” cam lift: 34° BTDC
IO @ 0.050” cam lift: 17° BTDC
Maximum @ 0.374” cam lift: 105.5° ATDC (Intake Lobe Centre Line)
IC @ 0.050” cam lift: 49° ABDC
IC @ 0.020” cam lift: 72° ABDC
IC @ 0.001” cam lift: 124° ATDC

Doing a bit of math I come up with:

Exhaust Duration @ 0.020” cam lift: 285°
Exhaust Duration @ 0.050” cam lift: 248°
Intake Duration @ 0.020” cam lift: 286°
Intake Duration @ 0.050” cam lift: 246°
Lobe Separation angle: 108°
2.5° advance built in

Apart from the lift not quite making specifications and the built in advance, things are really quite close. I doubt that the cam is worn because the lift numbers are so similar but after the complete tear down a close inspection should determine that.

Now we come to the interesting part.

Note the EO point @ 0.001” at 145° BTDC. Also note the exhaust Lobe Centre Line at 110.5°. The nose of the cam will be at the centre point of its lift at this angle BTDC on the exhaust stroke. The heel of the cam will be at the centre point of base circle at this angle BTDC on compression stroke. As the crank turns around its direction of rotation, the cam will be into the opening ramp before it reaches the Lobe Centre Line.

There is a similar situation with the IC point @ 0.001” at 124° ATDC. Also note the intake Lobe Centre Line at 105.5°. The nose of the cam will be at the centre point of its lift at this angle ATDC on the intake stroke. The heel of the cam will be at the centre point of base circle at this angle ATDC on power stroke. As the crank turns around its direction of rotation, the cam won’t reach the closing point of the ramp until after it has passed through the Lobe Centre Line.

Why is this interesting? Well, the traditional method to set valve lash on a stock engine is to bring the cylinder in question up to TDC on compression and set both valves. The logic is that at TDC on compression both lifters must be on base circle, which is where the lash must be set. This logic doesn’t work with a high lift long duration cam because of the amount of ramp that must be used with that combination. In this case at TDC on compression both lifters are not on base circle. The exhaust lifter is just starting into the opening ramp and the intake lifter is just nearing the end of the closing ramp. If the lash is set at this point it will be excessive because the lifters are not on base circle.

From the numbers I estimated that at TDC on compression I could have several thousands of lift on the ramps. Actual measurements showed 0.003” on both lobes.

What would be the result? Excessive lash has the same effect as retarding the cam timing slightly. It also makes for a noisy valve train and can cause premature wear. Probably the best way to set lash on any engine is the “EO/IC” method which has been mentioned in previous threads.

This is all the proof I need to use the “EO/IC” method to adjust lash. It’s easy and will work for almost any engine.
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