E: Engine Valve Timing

[Monkeypants]

These days I'm happy if the engines will pull well to 5500 , for the twins at least. Ignorance is bliss and Im sure I happily went higher more than that a few times in the past, unaware of Vincent history , specifically Picador testing.
In any case, above 5500 rpm HP drops off with MK2 cams. That fact is mentioned somewhere in Irving's writings . Not sure about MK1s but most likely that peak bhp point is lower than 5500.
Peak HP may be higher with some of the aftermarket MK2 styled profiles, but really, it's getting pretty risky to spin much above 5500 with that 70 year old stock crank in those 70 year old stock cases. Very late in the day, the factory found out that even new engines came apart quickly with sustained high rpms. I think that testing was at 5800, not 100% on that, need to reread the articles.


The Prince MK5s make peak torque at 5500 and peak BHP at 6500. I would not use those cams in an engine with stock crank and cases. 5500 rpm crank and 6500 rpm cams is not a good match.
I'm just referring to info gleaned on twins, singles seem to handle more revs.

Glen

 

[bmetcalf]

5500 rpm is 120 mph, so that is really tramping for a Rapide!

 

[Michael Vane-Hunt]

Glen really tramps.

 

[Monkeypants]

5500 rpm is 120 mph, so that is really tramping for a Rapide!

That's true, and neither of mine will pull that in top, although the one with small carbs comes close.
I was thinking about possible over revving in intermediate gears, saying pulling a long hill etc.
After fitting a tach, it's clear that you have to work at it, but they will certainly keep revving well past 5500 in the first two gears and even 3rd gear on the flat.

Glen

 

[Albervin]

Glen, what balance factor do you use and do you have a vibration point? My Rapide with 48T rear sprocket vibrates at 70 mph in 3rd then smooths out. I have never taken it above 90 mph in top as I have other bikes to do that.

 

[davidd]

For some reason, Irving never tested the cams with megaphones Thus, the statement for a 6000 rpm limit is limited to straight pipes. The Mk2 will deliver power up to 8000 with a megaphone, but as far as I know, this should only be done on a single. The single crank can do 7000 RPM readily, but the wider twin crank should be limited to much lower RPM for all the reasons Glen cites. New flywheels made of better material will allow you to do high RPM on the twins.

Coburn Benson set the redline at 8000 for his Grey Flash with a Mk2 and a 10" Velo megaphone with a single taper. The chronometric was 300 rpm slow at that speed due to lag. He changed to an ATC tach, which was quicker, to eliminate his riders shifting at 8300. However, Coburn was racing the Flash in modern road races at the time and competing against factory machines even in the amateur classes. The races were often an hour long. Not really vintage racing.

The other limitation is piston speed. At 7000, on my racer, it is 4133 ft/min, at 8000 it is 4724 ft/min, which is fast enough to cause problems. Coburn did have some problems with pistons breaking and a few valve heads coming off.

David

 

[Monkeypants]

Glen, what balance factor do you use and do you have a vibration point? My Rapide with 48T rear sprocket vibrates at 70 mph in 3rd then smooths out. I have never taken it above 90 mph in top as I have other bikes to do that.

Alyn, Im not certain what bf was used for these Rapides. One was fitted with a new Maughans dynamically balanced crank. That engine was built by John McDougall. I know he used a wide variety of balance factors as he was always experimenting to find the magic number.
The other engine was purported to have had its crank rebuilt by T Prince. He does static balancing and likes to use 60%.
Both are quite smooth at 70 mph and I'm happy for that. As the revs go up the vibrations go up a bit with these two. Not bad, but enough that if you sit on 80 for an hour it feels quite nice to drop back to 65-70. The whole world goes smooth and quiet.

Glen