To my other GT6 pages.

November 24, 2020

Cam Timing

Since the crankshaft determines the positions of the pistons, and the camshaft determines the positions of the valves, there has to be a very specific relationship between the crankshaft and the camshaft rotations to synchronize the four-stroke choreography.  On the GT6 engine, the two shafts are coupled by a timing chain running on sprockets on each shaft. 

After some cleanup, these are the sprockets, chain, and some other nearby parts.  The sprockets and chain were replaced in my earlier rebuild, so are close to 30 years old, but still young in terms of miles.  They show little or no sign of wear, so I'll re-use them.




For long chain life, the sprockets need to be aligned to be co-planar.  This can be adjusted by using shims under the crankshaft sprocket, but I didn't need any.




Chain stretch is measured by the gap between the slack side of the chain, and a straight edge.  The factory workshop manual allows 0.4", but I had less than half that.




So, to set the cam timing, we need two datum points:  For the crankshaft, it is Top Dead Center (TDC) of the #1 (front) piston, and for the camshaft, the position of the #1 or #6 cam lobes.  Since pistons #1 and #6 share the same TDC, it's the cam, which rotates half a turn for each full turn of the crank, that determines which one is on the compression/power or the exhaust/intake stroke.

The stock GT6 cam and many of the popular replacements are "symmetrical" cams.  This means that for each cylinder, the max lift points of the intake and exhaust valves are set to "straddle" TDC for that cylinder.  That is, the number of crank degrees between max exhaust lift and TDC is the same as the number of degrees between TDC and max intake lift.  So a common method for symmetrical cams is to set the cam so that the #1 (or #6) intake and exhaust lifters are at equal lift (intake rising, and exhaust falling), set the crank at TDC, and install the chain to hold that relationship.

However, my new BP270 cam from BPNW is not symmetric.  Max inlet and exhaust lift points are not to be centered around TDC, and the opening and closing ramps are not the same, so another method of timing is necessary.  It is actually simpler.

For this cam, the data sheet instructs to set the cam such that the #1 intake lifter is at max lift, set the crank to 112 degrees After TDC, and install the chain.

Obviously, some way of measuring degrees of rotation on the crank is necessary.  This is a generic degree wheel, with a simple pointer bolted to the block.  The wheel is set at an arbitrary position to start.




A dial indicator is used to find TDC.  Since motion of the piston is very insensitive to crank rotation near TDC, one way to do this accurately is to pick a piston position around 1/2" below TDC on the rising side, and note the indicator reading and the degree wheel reading.  Rotate the crank clockwise (facing the front of the engine), past TDC, until the indicator reading is the same as before, but on the falling side, and again note the degree wheel reading.  TDC will be halfway between the two degree wheel readings.  Set the crank there, loosen the degree wheel, set it at 0 degrees (TDC), and tighten it.  The pic shows doing this with the chain installed, but it normally would be done without the chain.




To measure cam lobe position, I made this little slug to fit in the back of one of the lifters.




With the lifter installed for the #1 intake valve, the dial indicator was moved to measure lobe lift, and the cam rotated to maximum lift.




The crank was then rotated clockwise to 112 degrees ATDC, and the chain was installed.




A word about how close you can get to the specified setting:  The cam sprocket has 42 teeth, which puts them about 8.6 degrees apart.  So the error from some intended setting could be as much as 4.3 degrees.  To the crank, which turns twice as fast, this error would be 8.6 degrees.  On the positive side, Triumph engineers included two pairs of mounting holes on the cam sprocket 90 degrees apart.   This 90 degres represents 10.5 teeth on the sprocket, so that by moving to the alternate pair of mounting holes, the sprocket effectively advances 1/2 a tooth, or 4.3 degrees.  This has the effect of cutting the potential error in half, which brings us back to 4.3 degrees at the crank.  Fortunately, small errors in cam timing don't usually make a huge difference in performance.  Even so, some tuners will intentionally set cam timing a few degrees off either way, depending on what they are trying to optimize.  The error in my installation was pretty small--maybe a degree or so advanced.




So, with all the brain work wrapped up, it was nice to turn to something less taxing.  The timing cover was pretty rusty, with some remnants of paint, but it cleaned up and powder coated pretty well.




The tensioner was well scored, so I installed a new one, along with a new oil seal.




While thinking about stuff at the front of the engine, I dug out the harmonic damper.  Though appropriately crusty on the outside, the damper appeared to be intact, with no detectable play between the outer ring and the hub.  I'm not a subscriber to the notion that some deterioration of the exposed rubber is an indication of damper failure.  That rubber on the surface is exposed to a lot of things bad for rubber--mainly oxygen, ozone, moisture and various noxious hydrocarbons. The rubber inside the damper that actually dos the work is not exposed to these things. 




Beyond this, my skeptical nature frets about some aftermarket parts or "rebuilding" outfits.  A harmonic damper is part of a tuned system, and though deceptively simple, there is a lot of science inside.  So, barring any evidence of slipping or other damage or degradation, I don't have reservations using ths original damper.




The front seal sleeve showed a slight groove where the seal rubbed.  Since the sleeve is almost symmetrical, I just flipped it over, which gave a fresh surface for the new seal.  I did enlarge the chamfer on the leading edge to match the one on the other end, to help it slide into the seal.




All buttoned up.  I'd checked it before, but note that with the engine at TDC, the indication on the damper is correct.  I'll probably run an electric radiator fan, so that fan mounting hub may go away.




This was one of the more expensive parts of the project--about $400 for the cam and lifters, but was good for a few enjoyable afternoons in the shop.

Comments to Ed at elhollin1@yahoo.com
 
To my other GT6 pages.