Twin Valve Timing

[timetraveller]

Gentlemen. some years ago I published an article in MPH about cams. I am away from home at the moment so cannot give you the reference. The thing about flat lever followers is that, it is my belief, that one cannot get a symmetrical lift with such a system. Now that MartyG has his measurements in a spread sheet it is relatively straight forwards to go ahead and calculate the velocity and acceleration of the valve. Take the difference from measurement one to measurement two and divide by the number of degrees between those measurements. Then do the same for measurement two to three and so on. That will give you a graph of the velocity of the cam. Then do the same with the velocity calculations. Plot those values out and you will have a graph of the acceleration. It is the acceleration which knocks the stuffing out of the bits and pieces. What you will find is that first of all there is a lot of noise which is due to measurement errors but more importantly the lower part of the acceleration curve, which should be flat, has a slope. What this means is that the valve springs, which control that part of the curve, have to be much too strong as they are only working for about half of the time that they should be. If one designs a cam for use with a curved base lever follower then one can get a symmetrical lift curve etc. and the valve springs can do their job for the whole of the duration that is available to them. Additionally there is no need for a ramp at the start or end of the lift as that happens automatically with the curve based followers.
The value for the lift of about 0.45 inches was also independently determined by John Emmanuel when he worked with a chum who had a modern tuning shop with a lot of good gas flow instrumentation. John and his chum also developed a new shape for the inside of the cylinder head which to the naked eye looks to be identical to that developed by Terry Prince so it is encouraging that two independent routes have lead to the same shape.

 

[Monkeypants]

Was that the article which compared the Somerton cams to Vincent MK2s? I found that online not long ago, it was a good read.
Small correction to my earlier post, the Prince Mk5s give 11 mm of lift at the inlet. 11 mm is .433 " , not .440. .440 was the number Terry gave over the phone, maybe a rounding up. I know that mine matched the graph as measured using Dan Smith's dial indicator valve set, so it is strange that Greg found .400" with his. Must be a different profile?

In regards to the OP's timing question, I see the 4 degree BTDC figure again in a posting on thevincent.com from some years ago. The posting is by Roy Cross, whom I recall contributed a lot of good information to MPH over the years. He also uses the 4 to 6 degree BTDC exhaust stroke crossover point for cam timing.

 

[timetraveller]

Yes, I think that is the one. I bought the cams on the understanding that they were Picador cams but was told later that Tom Somerton had copied the cams using a slightly non accurate diameter on the cam follower. Whatever, they timed up nicely and the bike did 135 mph at the end of a standing start half mile, having been ridden to the meeting and then ridden home again. The figure of 135 came from the rev counter which was at 7,000 rpm as I crossed the line and working back through the gearing. As the chronometric rev counters lag a bit it is possible it was a bit more but I was certainly surprised. The MPH article was done when I was designing some new cams which were eventually made but when I came to fit them the front exhaust lower valve guide was missing and several other things needed doing and then my personal circumstances changed and I took on more projects than was comfortable so the bike and cams have still to be tried in anger. To give some idea the new cams, with ratioed rockers, will give up to 0.45 lift, with Mk I (that does say one) timing and yet nowhere does the acceleration exceed that of a Mk I or III. This means that if one wants to get more air/fuel into the cylinder than standard there is only the Mk I duration to do it in so we will have to see what will happen. I've bought 32 mm Mikunis and opened up the ports so I have to get round to it at some stage.
Incidentally, mentioning Mk IIIs, I have yet so see any evidence of 'quieting ramps' on any Mk IIIs that I have measured so whether modern copies are incorrect or whether it was hype at the time I do not know. The story was that they had underground the base circle by twenty thou and then blended that into the original profile over tens of degrees. It might sound like a good idea but when you get involved in cam design it does not look to me as though that would do what was required.

 

[SteveF]

Following on from this thread, I've spent a bit of time measuring the list on my C Rapide. The previous owner claimed that it had 'Lighting Cams', whatever that means.
Once again, I've found nonsense in this engine. The lift on both inlet valves is about 9.4 mm. The exhaust lift is about 9.1 mm on one cylinder and about 8.5 on the other. Clearly one of them is either not a Lightning cam or is worn out. When I stripped the engine all the cams look to be in pretty good condition so I can only assume one of them doesn't belong.
I'm guessing it's the one with the smaller lift.

Can anyone point me at a table of lift values for the different cams so I can decide what to do.

Cheers - Steve

 

[bmetcalf]

See p.18 for the Megacycle versions: http://www.megacyclecams.com/catalog/catalog_optimized.pdf

 

[davidd]

David

 

[SteveF]

Well, drat. Although I measured lift on the valve stem it woudl seem that regardless of which cams I have, they should basically have equal lift on both inlet and exhaust.
More digging required, it seems.
Thanks, guys.

 

[davidd]

Steve,

It appears that you have Mk2 cams, at least by the lift figures. The equal lift, or crossover point is where you can line up all the marks on the gears, at least theoretically. That illustration in Richardson's showing all the marks aligned on the gears is drawn at 4° BTDC exhaust. So, although 4° is not mentioned in the caption, it was well-known at the Factory.

David

 

[mercurycrest]

View attachment 16039
David

Hi David,
OK, where do my Andrews Cams fit into this? What's different from Megacycle stuff?
Cheers, John

 

[SteveF]

Thanks David, I guess I need to remove the cams and measure them. Then if teh cams themselves measure OK I'll need to look at the rockers to work out why the lift is different for the 2 exhaust valves.