ET: Engine (Twin) Twin Cam Timing Conundrum!

Rob H

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Pulling my hair out a bit, well whats left of it.

I am putting in a new set of cams and looking to get the timing as best as I can. When all set up as per Richardson I find the difference between the front cylinder cam and rear cylinder cam on intake lift is 5 deg. The front cam trailing by 5 deg compared to the rear.

In a bid to get them closer thought that if I advanced the front cylinder cam by one tooth (as the pinion has 48 teeth = 7.5 deg) then the difference between the two cams would be only 2.5 deg with the front cylinder cam now being slightly in advance.

However when I have done this and I have done it several times to make sure, I find the intake lift on the front cylinder cam advances 15 deg not 7.5 deg making the difference between the two worse at 10 deg.

Am I missing something? Any help to stop me going crazy would be very much appreciated
 

Rob H

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Additional information - I am using 4 dial gauges mounted on the valves for ease of reading with timing disc in the big end quill. TDC was measured with both a dial gauge and using a piston stop to make sure it was correct. Also ensuring no play on the tappets.
 

timetraveller

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I've done this many dozens (possibly hundreds) of times, both for myself and others. It should work but measuring the exact time of the start of the lift is difficult. What I always try to do, although it is time consuming, is to plot a graph of the lift at every ten degrees of crankshaft rotation. I like to do this for the whole of the opening and closing including checking the base circle as they are not always round. You do not need to do the whole of the base circle at ten degree intervals and as an absolute minimum plot a graph for the start of the lift for at least 50 to 60 degrees of engine rotation. Remember that Vincent opening and closing times are taken at five thou lift, Let us know how it goes.
 

passenger0_0

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Pulling my hair out a bit, well whats left of it.



In a bid to get them closer thought that if I advanced the front cylinder cam by one tooth (as the pinion has 48 teeth = 7.5 deg) then the difference between the two cams would be only 2.5 deg with the front cylinder cam now being slightly in advance.

Am I missing something? Any help to stop me going crazy would be very much appreciated

This is because there is a 2:1 reduction between the crankshaft and camshaft. Since you're measuring crankshaft angle you'll always get twice the angle change when change one tooth on the cam pinion.

Why don't you leave the cam phasing the same so that there is 5 degrees difference between front and rear and then split this difference by moving the keyway on the crank pinion so that one inlet open 2.5 degrees early and the other 2.5 degrees late from factory figures? Unless highly modified, most Vincent twin motors won't notice this small variation.
 

Rob H

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This is because there is a 2:1 reduction between the crankshaft and camshaft. Since you're measuring crankshaft angle you'll always get twice the angle change when change one tooth on the cam pinion.

Why don't you leave the cam phasing the same so that there is 5 degrees difference between front and rear and then split this difference by moving the keyway on the crank pinion so that one inlet open 2.5 degrees early and the other 2.5 degrees late from factory figures? Unless highly modified, most Vincent twin motors won't notice this small variation.

mmm, pretty obvious when you say, just could not get my head round it.

I will check the closing times but will probably end up splitting the difference as you say. Shame they are not a bit closer matched
 

Martyn Goodwin

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mmm, pretty obvious when you say, just could not get my head round it.

I will check the closing times but will probably end up splitting the difference as you say. Shame they are not a bit closer matched
If you want them closer matched then the solution is to press the drive sprocket off one of the cams, reposition it to match the other (NOT an easy task) then press it back on.
 

passenger0_0

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mmm, pretty obvious when you say, just could not get my head round it.

I will check the closing times but will probably end up splitting the difference as you say. Shame they are not a bit closer matched
I wouldn't get too stressed about +/- 2.5 degrees. Theoretically this is an advantage giving you a better spread of torque as one cylinder will have a higher volumetric efficiency at lower speed and the tgher at a higher! :)
 

davidd

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If you want them closer matched then the solution is to press the drive sprocket off one of the cams, reposition it to match the other (NOT an easy task) then press it back on.

The bits to do this are not difficult to make. This is one that Pat Manning made. He copied a design that Peter Barker told us about.

Cam Index Tool.jpg


You need a cam spindle size spindle to go into a lump. You need a pusher that will hold the cam shaft (right) with a hole that is a little less deep than the cam shaft so you can see it home when you press it in. The harder part is the small spindle to the right of the large spindle. This needs to fit in that groove on your cam. It sits in a hole all the way through the base. It is sitting on a spring, like a ball point pen spring, and put a screw in the bottom to hold the spring, spindle and a spacer if you need one, in place. This spindle should be able to push down flush with the base. The little flange on the bottom of this retracting spindle needs to allow the spindle to poke up higher than the cam gear when it is flat on the base. (The hole is counter bored a bit bigger from the bottom.) When the retractable spindle is all the way up it catches the groove in the camshaft before the cam is pushed into the base. This is what is doing the indexing.

What I did next was to slide on a Gary Robinson cam. With the cam locked in place by the retractable spindle, I marked the hole for the drift pin. I figured that Gary knew how to index the cam on a Vincent. With this you are mostly done. I could add another pin where the arrow on the left is pointing for a Comet cam, as that is where the two dots are on the gear for a Comet. You could insert other pins for the twin cams.

I would think the repeatability on this is good. But, I also agree with Passenger0_0 that splitting the difference is fine. The indexing tool would be a good item for a Section to own and lend out.

David
 
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