Roundcase cam specs

[machten]

Hi all,

Myself a few other interested parties are currently dial gauging our cams on various Sport and GT 750s. This is a bit of an investigation to see the effect of cam timing on engine performance of some freshly rebuilt engines - including why some engines really "bark" through the contis, and some are more restrained despite everything else being "equal". We've done a Sport and a GT and have a number of each yet to do, and whilst I have VeeTwo and some other third party specs, I can't find any definitive original Ducati 750 springer camshaft specifications to compare against.

Does anyone know where to find them/know them/have them?

Kev

[Duccout]

Well, there were two different grinds of GT/Sport cams. The originals had timing figures of: inlet 65 84; exhaust: 74 58. Later, a higher performance cam was produced by grinding the base circle down to give higher lift, but the same timing figures.

[wdietz186]

The owners manual lists the specs Duccout provided, IO 65 btdc,IC 84 abdc and EO 74 bbdc,EC 58 atdc. The figures are with the valve clearance at .010 mm and the specs allow a + or - of 5 degrees.
I suspect there was a difference between the shim type and screw type adjusted cams as the older ones seemed to be the ones that "barked".
I do know it is easy to get confused timing the cams in on the roundcase models what with all the fiddly bits spinning around in there and having to remember that the crank spins backwards to what you would expect!

[machten]

DOH! Thanks guys. Clearly they are not using a cam checking clearance of 1mm as is usual. Those figures collate to a 0 clearance. Re two grinds, yes I understand that to be the case too. I'll need to check some more to get a good picture, but at this stage we have one grind with an extra 1.1 mm of lift on the inlet although exhaust is only a very marginal difference increase and inside our margin of error. Interestingly, that ain't the one that barks!

What we have found so far though, is that the cam alignment on the "barker" between front and rear cylinders is perfect (I'll check the actual timing compared to the figures you've provided later), whereas the sport cam aligment is 11 (crank) degrees out between front to rear cylinders. We'll definitely be looking at trying some offset woodruff keys on that one to see what difference it makes.

Kev

[Duccout]

Highly regarded English tuner Anthony Ainslie told me once that a, 750 Super Sports were no faster than Sports, because the cam timing was so far off between the two cylinders as standard; and b, he preferred to use Megacycle cams when tuning valve spring 750's.

[Desmo46]

In my experience, most of the cam timing differences are due to poor shimming of the timing gears, rather than errors in the cams themselves (which aren't perfect but usually close.) The gears are helical and raising or lowering the gears with the shimming screws one gear into or out of the other, thus advancing or retarding the cams.

[machten]

Yep, shimming can certainly impact cam timing, but the one that has the 11 crank degree variation has all bevel checking surfaces properly aligned and meshed. Any shimming changes to acheive what is (I think) 0.44 of a tooth difference (not even sure if that big a change could be acheived by shimming) at the top bevel would be to the detriment of the meshes.

As mentioned earlier, Ducati does specify +/- 5 degrees - so this isn't far past that, it's just the worst possible spread!

Kev

[Duccout]

This is a problem unique to v twins, I believe??? A vertical twin has one camshaft serving both cylinders, so any error would at least affect both equally, whereas on the Duc, the error can be at opposite ends of the timing with both cylinders 'fighting' each other.

[machten]

A vertical twin has one camshaft serving both cylinders, so any error would at least affect both equally, whereas on the Duc, the error can be at opposite ends of the timing with both cylinders 'fighting' each other.

Correct. Or at least, they sure ain't helping each other like they should be.

Kev

[machten]

Well, we've put the offset woodruff keys in to the cams of the 73 Sport. We had an 11 degree difference across the horizontal and vertical cylinders. Using a "barker" as a reference point, we used a +6 degree offset on one and a -6 degree offset on another. Guess what? It barks like a dog. I think we're on to something.

Kev