camshaft sprockets

[CBUEB1771]

the only difference other than the dowel holes being in a different spot.

Gary's photos show exactly what I was describing. The dowel holes for the jackshaft, inlet and exhaust cams are all in the same locations, Lotus just used sprockets as supplied by Ford and then added the registration marks.

[robertverhey]

Interesting thread. I guess when all is said and done, it's the position of the cam lobes that's important at tdc, not the mark on the sprocket. On most of the t/c motors I've come across (often without an "EX" embossed on either wheel), there have been more than one TDC mark on the sprocket perimeter, suggesting that there's been a bit of mix'n matching over the years. The use of a dial gauge to the piston crown plus offset dowels should pretty much enable you to use either wheel on either cam, no?

[garyeanderson]

I think Lotus wanted to keep it simple and they probably have kept the position with the "stock" sprockets to be close enough to keep the power within their spec. That said they may be off a "couple" degrees and to put it "spot on" you will gain an hp or two. If you are building an engine to at high spec then by all means get it dead nuts, a stock engine with a cast iron manifold and some of the other performance sapping problems you may not care a whole bunch. On the other hand, being off a whole tooth is something like 10.6 degrees and you will feel that difference and possible cause some damage.

Gary

[CBUEB1771]

suggesting that there's been a bit of mix'n matching over the years.

I was thinking the same when I was looking at the photos in Jeremy's post on the first page of this thread. An extra dowel hole! Very non-standard.

[andyelan]

Hi Everyone

Just a thought but does anyone here think it's possible that at some stage during the Twin Cams development (std. to SE to Big Valve) it might have been that Lotus made a change to the cam shaft phasing by the equivelet of one tooth. This is somthing I've seen done on high performance varients of other twin cams engines notably for bikes. Although I can find no evedence to support this theory, it wouldn't be the first time Lotus failed to keep their manuals up to date with later specs (after all the setting proceedure using the marks on the sprockets would apply just the same) and it would explain why not all sprokets are marked IN and EX if originally they were both identical.

It's just a theory but if it were true, then that would mean that the symetrical cam timing we're all used to might not actually be correct

Andy

[rgh0]

A whole cam tooth is 20 crank degrees. The biggest shift in timing from standard to the most radical full race cam is less than half this so Lotus did not need to shift timing for the 3 cams they used. The timing on the 3 standard cams is I believe the same and symetrical though I have never had the opportunity to measure up an orginal factory timing setup for each of the cams to verfiy this if the published data is in fact wrong. You can play with the timing of each cam a little to try to improve power or torque and this is common on race engines to vary from the published cam timing to optimise an engine. John McCoy does this depending on the track and whether he is aiming for max power and top end speed or max torque for best acceleration out of corners.

cheers
Rohan.

[billwill]

You can of course, if you wish, replace the standard cam sprockets with vernier adjustable ones, as Rob Morley did to my engine some years ago.

These are easier to adjust than using offset dowels for the same effect. They 'move' the camshaft relative to the chain links.



elan-f15/clay-test-checking-clearance-between-valves-pistons-t26441.html


If you haven't thought about it before you need to realise that because the chain is taut and straight on the left side of the engine, the position of the chain (give or take a bit of wear & stretching) is fixed by the CRANKSHAFT sprocket when piston 1 is at top dead centre.

So all timing of the cams is done by moving them relative to the chain. Initially this is done by design i.e. the precise position of the dowel in the end of the camshaft. That is why it is possible to use identical camshaft sprockets in the design.

Adjustment therefore requires either moving the position of the dowel by using an offset dowel or by moving the position of the dowel hole in the sprocket relative to the teeth of the sprocket wheel as in the vernier adjustable wheels.

[billwill]

Which also brings up the point that if you discover that a previous owner or engine builder has fitted offset dowels in your camshafts, be awfully careful to ensure that you photograph the bare ends of the camshafts, and be sure to mark the dowels IN & EX, to make sure (if they come out) that you put them back in the correct camshaft and also not 180 degrees wrongly positioned.

[andyelan]

Hi Rohan

You're right of course. I hadn't got my head in gear and was halving the 10 degrees at the cam sprocket to get crank degrees instead of doubling it

Andy