ET: Engine (Twin) Effects of Cam Timing

[Monkeypants]

The 105 cam David Tompkins installed in his racer was from Godet. I degreed it prior to his installation, well before I built an engine for him. David installed it to Godet's specified timing and went to the dyno. It made about 26 BHP, which was lower than the unknown cam he was running. When I degreed the cam with Godets specs it timed out at 4 degrees equal lift, so there was no mystery with the setting. David was a bit depressed.

I told him to get a Megacycle Mk2 and put a Gold Star megaphone on the pipe. It made 34 BHP at the rear wheel, which was a better starting place. Small changes to the carb and exhaust system and the engine now has 39 BHP.

Carleton Palmer got a 105 cam from Ian but ran it only once because the cam was down on power. I do believe that the 105 cam can make good power as some of the figures quoted by others have been quite good. We were not able to cash in on it. Carleton and I stuck with the Renwick cam, which made 43 BHP. However, Carleton and I ran with megaphonitus even without using a megaphone. The extra 4 HP made a big difference. David Tompkins prefers the low torque and I think Greg does too. That is just one of those decisions you have to make for yourself.

David

Such a massive difference, just with cams!
There is still a lot of black magic surrounding cam profiles and one can see why.
A small change in profile makes a big change in output.

Regarding the touring bikes-
In the 1980s Phil Irving wrote an MPH article on Vincent cam profiles. He did not understand why so many folks were installing his Mk2 Lightning cams in touring bikes. He saw this as a performance downgrade for a tourer as he said his MK1s make more power to 4500 rpm. The MK2 makes equal power at 4500 then gradually creeps up above the MK1 to 5500. After 5500 it drops off in power.
That may be where some of these other profiles work well, if they can hold power above 5500. But will that wobbly twin crank hold together at that rpm? History says no, not for long anyway.

Reality is that most of the road miles are covered at considerably less than 4500 rpm.


Glen

 

[royrobertson]

If you want a proper Race cam the John Renwick designed cam, with the many modifications to the the phasing that Gary Robinson developed for me, take some beating but would be horrible in a road bike. Note they are Super high lift and the valve gear and crankcase need serious modification. I have been using them since 1997 and never had any problems. They are built up with Stellite as are the followers. Unfortunately Gary is no longer able to make them.
Roy

 

[Monkeypants]

I thought Terry's MK5s might be like that, horrible for road use, especially when combined with large carbs, high compression and his big port heads.
For some reason that combo works fine for the road. I guess the extra displacement helps.
The engine makes smooth power just off idle. You can ride it quite sedately if necessary.
Throttle response is good too. Sometimes too good!
The idle is a bit lumpy.

Same routine with those cams, there is quite a bit of timing case aluminium to carve out for the extra lift.
The biggest problem with the MK5s seems to be premature wear due to inadequate hardening in the later batch or batches.

Glen

 

[bmetcalf]

I suppose hardening a batch of two cams isn't practical?

 

[greg brillus]

My experience with Terry's cams, not so great.........basically, wrong type of bushes, wrong cam material, machined not ground finish, base circle eccentric not concentric.........and no quality control........so basically rubbish........This is common issue with loads of Vincent parts. The Vincent cam does not allow for much performance, the surface areas on the lobe and followers are way too small.......(Rolls Royce Merlin cams suffered the same issues) This means that performance upgrades are limited........so in a 500 single you are looking at around 40 odd Hp max, 1000 twin about 70 to 80 Hp max........bigger engines only gain the extra HP due to the extra capacity........most 1200 to 1300 cc engines seem to be around the 90 to 100 Hp figure at most....... Lets go to.......symmetrical cam lobe with roller followers, with say 400 lift at the lobe.......then ratio the rockers and add rollers in the pin/bush........larger say 2" or more inlet valve and even go smaller on the exhaust.......Inlet lift of around 600 less on the exhaust........Obviously a decent oil pump system and almost certainly a one piece crank........lets shave a load of weight off the crank and run much lighter short skirt modern pistons........ideally you want a big diameter piston with short stroke........starting to sound more like a modern engine isn't it........so now you are looking at putting out some better HP figures........just like the Horner's engines........It's all doable but costly and very time consuming.......If you look at a lot of the repro classic engines in racing, this is exactly what these engines have.......that's why they go so much better than the originals from years ago. The Godet Flash 500 engine puts out about 53 HP roughly, these go to 9000 RPM easily........big bore short stroke........full oil pressure one piece crank........nothing like an original, but go much better........This bike still way down on power verses some of the trick Norton's and G 50's out there.........Is there anyone out there who will carry out these mods........a bit slim these days......If your up against modern classic race bikes, then your 12:1 specialoids and a set of Mk 2 cams just ain't gonna cut it........Just having the extra capacity of a twin isn't enough........well from the racing i've seen down here anyway.......there are too many trick engines out there that perform very well.

 

[davidd]

MK2 makes equal power at 4500 then gradually creeps up above the MK1 to 5500. After 5500 it drops off in power.
That may be where some of these other profiles work well, if they can hold power above 5500.

The Renwick cam peaks at 6800 RPM, which is why we liked it so much. Carleton found that he had trouble keeping up with the Mk2 in the engine because it shut off at 5500 at 6000 and would really drop off. The Renwick cam did not drop particularly fast above 7000, which gave us a real edge at the flag stand. The Megacycle Mk2 peaked at 5800. But, the difference is about 15 mph at their respective peaks. If you have two Comets with the same HP and gearing, the one with the Renwick cam will be able to go about 15 mph faster on the straight.

David

 

[greg brillus]

I set my limiter to 7200 and the cam easily pulled to that and probably beyond on the dyno. Unfortunately my engine specs are the wrong way in that I have a 100 mm stroke and 92 m bore, so i need to be mindful of max rev's due to piston speed........in reality on the track i shift much lower than this probably about 6200 to 6500 max........this is because the engine has such good torque which is great, that i don't need to rev it so hard. On the dyno my bike put out just over 48 Hp which is quite reflective of David's engine at around 43 Hp.......mine is simply due to the extra capacity.......this makes good sense........however, my cylinder head which is one of Terry's last fully CNC heads was stock out of the box........This head is a revised version of the earlier ones, so these have a smaller port that almost tapers from the manifold face towards the inlet valve, the throat below the valve head is quite restrictive and also around where the guide comes through into the port........we have opened this area up quite a lot and running a near 2" inlet valve now and gone smaller on the exhaust valve........This should improve the breathing side of the head quite a lot......the exhaust port shape is quite good how it is. We are making changes in stages so as to not have the bike off the track for too long.......But the end result will be to run ratio rockers giving 600 at the inlet, and eventually running a modern cam profile with roller followers........probably lighten the crank and run a much more modern lightweight piston as well........We are running experimental Titanium valves, and using LS Chevy beehive springs much longer than standard ones with much taller caps.........living in the Norton frame, there is no height limit from the frame around the head. We have also gone larger in the pushrod size, with 5/16" balls either end, opened the cup size in the followers and different tappet adjusters.........Just so there is no confusion, this bike runs in the unlimited class, not the 500 cc class........ I realise some of you might see all these upgrades as "Cheating" and once upon a time i would agree with you........However, when they allow modern classic's to race, or a Triumph racer with new billet crankcases or similar........it seems that you can go as far as you want if you have the money and the interest to do it.........For me, I like the challenge.........It is not something you just go out and buy, it is something you create, and that is where the fun is.

 

[Monkeypants]

I'm not racing with this bike, it's just in road use, so that's quite a different world.
It does feel considerably stronger than the 100 hp Triumph 1200( well 98bhp/84 ft-lb torque). The Triumph has a 60-80 mph top gear roll on that puts most litre Sport bikes to shame.
The Vin also comes on harder (intially) than the 161 bhp Daytona, seat of the pants. I suspect the Daytona would pull away above 100 mph, but who cares?


Those bikes are both already a bit much for this old guy, so I sure don't need any more output from the T Prince parts. They have worked out really well for me, once the crank balance was sorted.
Will have to report back on the longevity question. The motor has about 7,000 kms on at the moment, no issues and there is virtually nothing on the magnetic drain at oil change.

Glen

 

[Bill Thomas]

My Twin light flywheels !, Many years ago, 1/4" off each side of the wheels,
They made the front wheel come up a few yards after the start of a standing start 1/4 mile sprint,
I didn't like that .

 

[greg brillus]

Crank weight on a twin and the singles for that matter, way too heavy for racing really. The mechanical forces to speed up and slow down rapidly just insane........and having the flywheels constantly immersed in an oil bath.......no wonder the engine/oil breathing can be so bad........if you ran smaller flywheels with an oil pickup at the base of the cases........this is the way to go.......I know there is little you can do about the flywheel area around the bigend, but on the counterweight side you can reduce this quite a bit, then run modern type shirt skirt pistons and even smaller gudgeon pins as well........The best way to look at improving an engine is to simply look at other engines that have done it all before, and some long ago........as far as cams go, the earlier Mk 5 cams Terry made years ago where obviously made and hardened correctly.......the later ones like i had in my racer were rubbish.