Vernier Pulleys

[julians]

Quick question, what do they do that regular cam pulleys cant do, and why would you need to use them?

 

Julian

[Graham Sewell]

They allow the cam timing to be set at better than 1 cam belt tooth granularity. Assuming that a cam pulley has 36 teeth, it would only allow the cam to be set at the specified point +/- 5' (to the nearest tooth).

 

The vernier pulley is two pulleys connected by a peg. If one pulley has 11 holes and the other pulley has 13, there are 143 positions that can be achieved. In this way, the cam can be timed perfectly (according to the manufacturers spec).

 

[CHRIS CLARK]

Ditto Graham above, + they are normally anodised red or blue and therefore look better!!! Wasted under a cam belt cover though!

[Rob Walker]

They also allow you the option of adjusting the cam timing for optimum power at the rolling road. This has veilded hidden power from cams timed up as per the manufactures figures.

[STEVE GILBERT]

You should only need them if the relationship between Cam and Crank has been reduced/Increased? ie Head Skim, working on the theory that the std timing has been optimised at the time of manufacture. I always understood that alterations from std only 'move' power around the range so it depends what your after on that score. Why not check what you have to see if you need verniers? Just be careful if the Cam boxes are still on the engine and if you do need them the practice with Dial Guage/Protractor/Blobs of red/white paint wont be wasted anyway. The other fun thing is the first time you go for the key and start the car...was the maths right? and did you add 55 deg before or after true full lift on the exhaust cam...? Great game!

[oilyhands]

You dont just need them if the crank/cam relationship has changed, they are pretty much essential if you have changed the original cams for those with a different profile, its common for replacement cams to use a different lobe centre and for the profile to be ground innacurately WRT to the cam dowel.

 

This can lead to valve timing well away from the manufacturers intentions if the standard pulleys are used. It's very rare indeed for an after market cam to be correctly timed when using the manufacturers standard pulleys and dowels even when the new and old cams share the same lobe centre.

 

The vernier allows the timing of the cam to be accurately set to the optimum value in a controlled manner. Anyone who has set up a K engine on a dyno or rolling road will trstify to how sensitive the K is to cam timing, especially on the exhaust, without a vernier it would not be possible to make these changes.

 

Without a vernier the minimum change is 18 degrees of rotation.. which is of course a large variation in timing and in terms of lift around 1.5mm.

 

Oily

 

Edited by - oilyhands on 11 Apr 2001 22:36:34

[Peter Carmichael]

You don't *need* them with new cams, as long as you trust the manufacturers timing spec. It is fairly easy to get offset dowels for the cam sprockets. These can set the timing very accurately but are a bit fiddly compared to verniers.

 

My Supersport cylinder head suffered cavitation erosion damage, undermining the head gasket. We had to skim 20thou (0.5mm) off the head to remove the damage. The timing was fixed by filing a notch into the original cam sprockets and then bolting it all up. Still seems to work OK.

 

On my other engine, at Piper's spec, the cams were producing a fairly lumpy torque delivery and 237bhp. With a little tweak (~1 degree) on the exhaust cam timing (courtesy of vernier pulleys) the torque fattened up and produced the 250bhp figure. Unless we just got very lucky there may be more to come.

[oilyhands]

Offset dowels and keyways are all very well (and encouragingly inexpensive) but you have to remove the pulley to fit them , this usually involves removing the belt and tensioner which can be a ball-aching job.

 

Well I remember filing that little slot in your cam sprocket Mr Windbag..

 

Oily