Unusual domi race cams need identifying

hybridracer

hybridracer

Joined
Feb 21, 2013
Messages
121
Country flag
www.dropbox.com

Extra special cam.png

Shared with Dropbox
www.dropbox.com www.dropbox.com
www.dropbox.com

Special cam.png

Shared with Dropbox
www.dropbox.com www.dropbox.com

The special cam I measured assuming a flat face follower. It has 3 keyways and the one in the standard tdc position was used for the degree measurements. Before anyone asks, yes I double checked that I was rotating it in the correct direction. It runs in standard bushes and has no pressure oiling of the lobes Although the lift is the same as the 650ss cam it is not that. Does anyone have an idea of what it might have been used for? If I ever rebuild my 88 race engine I might use it but it would be nice to know if I should be advancing or retarding the cam somewhat.

The Extra special cam I know exactly what it is as I got the follower lift data directly from the creator. I just want to know if anyone can guess what it is.

I have another race cam that has the Dunstal type pressure oil feed to the lobes and runs in needle bearings and also has 3 keyways. I'll post details when I get around to measuring it.

Non of the cams have any identifying marks.
 
The cam timings need to be optimised to suit the exhaust system. A 2 into 1 system can need the cam to be advanced. With a megaphone exhaust system, recommended cam timings can cause the bike to be difficult to ride smoothly. It depends on how you intend to race - faster down the straights and slower in corners might not be the way to go. Dropping the bike in a corner might not be as bad as blowing-up a motor on a straight ? Race cams usually tend to increase top end rather than the middle.
To work properly the whole exhaust system must resonate - with a 2 into1 system, there is more of it. Opening the exhaust valve earlier gives a bigger bang.
 
Last edited:
hybridracer said:
Thanks, I already have that. I suspect the "special" cam was a factory one-off.
Click to expand...
It's got a nice name. Very Millennial.

That abrupt opening ramp is interesting with the relatively mild closing. What do you think the effect would be with that combination?
 
I think I have cracked what the "special" cam is!
Measuring the timing at 0.013" (as per the stock commando 1s cam) I get IO 52, IC 92, EO 80, EC 68 .
Comparing these timings with the Commando camshafts data sheets the timing matches the 4s cam retarded by 8 deg. within the error of the measurements.
I don't believe it is a coincidence so why would someone make a 4s cam, retard it by 8 degrees and use the same lift as the standard cam? For what purpose?
 
Thanks to all who took the time to read this thread and, I like to think, also thought a bit more about Norton camshafts and perhaps asked themselves why are there so many?. If anyone ever has more information on the "special" cam, please inform me.

The answer to the matter of the "extra special" cam is that it is the actual 1962 factory Norton Domiracer cam. (any registered design rights have long since expired). Any one claiming to have a genuine or replica '62 Domiracer motor must have this cam in it!

I
 
hybridracer said:
I think I have cracked what the "special" cam is!
Measuring the timing at 0.013" (as per the stock commando 1s cam) I get IO 52, IC 92, EO 80, EC 68 .
Comparing these timings with the Commando camshafts data sheets the timing matches the 4s cam retarded by 8 deg. within the error of the measurements.
I don't believe it is a coincidence so why would someone make a 4s cam, retard it by 8 degrees and use the same lift as the standard cam? For what purpose?
Click to expand...
I think I may be able to suggest why the cam is retarded by 8 degrees. About 20 years ago now I developed a 500 Dommie for NZCMRR clubmans class racing down her in New Zealand. I did many dyno runs with cams and cam timing.

I tested various cam timings but one thing I settled on was the following.


My thoughts were that these days most cams are made for the Commando. And if you buy a Domi cam from any of the suppliers it will be a Commando cam with just the timing sprocket end modified. The timing will be Commando timing.

But a Dommie and a Commando have very different rod stroke ratios. Surprisingly the Dommies rod is actually 6 inches versus the 5 7/8 of the Commando. Plus of course the stroke is longer on the Commando.

The rod stroke rato on a Dommie is about 2.1 from memory while a Commando is around 1.7 ??? . Something like thst. It's 15 years since I did this.

So of course for a given crankposition the Dommie and Commando will have the piston located at a different percentage of the stroke.

My calculations suggested by proportionality you should retard a cam manufactured for a Commando by around 7 degrees if you wanted to match the piston location on a 500 Dommie.

I have never seen this discussed by anyone else and it's totally my own thoughts but it's interesting my noes seem to match your cam.

My Dommie was dynoed at 51 BHP rear wheel at 6100 rpm and at the time 15 years ago was the second fastest clubmans bike in NZ. So retarding the cam worked. I was using a PW3 cam but the same principle applies I think.
 
Last edited:
The fastest bike in clubmans was an ES2. Developed by the same team that ran the ES2 at the Manx for the past two years. Two third places and one lap I think at 111 mph.

So I'm not going to catch those guys. Peter Lodge is a pretty damn good engineer.
 
johnm said:
I think I may be able to suggest why the cam is retarded by 8 degrees. About 20 years ago now I developed a 500 Dommie for NZCMRR clubmans class racing down her in New Zealand. I did many dyno runs with cams and cam timing.

I tested various cam timings but one thing I settled on was the following.


My thoughts were that these days most cams are made for the Commando. And if you buy a Domi cam from any of the suppliers it will be a Commando cam with just the timing sprocket end modified. The timing will be Commando timing.

But a Dommie and a Commando have very different rod stroke ratios. Surprisingly the Dommies rod is actually 6 inches versus the 5 7/8 of the Commando. Plus of course the stroke is longer on the Commando.

The rod stroke rato on a Dommie is about 2.1 from memory while a Commando is around 1.7 ??? . Something like thst. It's 15 years since I did this.

So of course for a given crankposition the Dommie and Commando will have the piston located at a different percentage of the stroke.

My calculations suggested by proportionality you should retard a cam manufactured for a Commando by around 7 degrees if you wanted to match the piston location on a 500 Dommie.

I have never seen this discussed by anyone else and it's totally my own thoughts but it's interesting my noes seem to match your cam.

My Dommie was dynoed at 51 BHP rear wheel at 6100 rpm and at the time 15 years ago was the second fastest clubmans bike in NZ. So retarding the cam worked. I was using a PW3 cam but the same principle applies I think.
Click to expand...
DynoDave has mentioned it to me several times, but I don't recollect it being said in these pages.
 
johnm said:
I think I may be able to suggest why the cam is retarded by 8 degrees. About 20 years ago now I developed a 500 Dommie for NZCMRR clubmans class racing down her in New Zealand. I did many dyno runs with cams and cam timing.

I tested various cam timings but one thing I settled on was the following.


My thoughts were that these days most cams are made for the Commando. And if you buy a Domi cam from any of the suppliers it will be a Commando cam with just the timing sprocket end modified. The timing will be Commando timing.

But a Dommie and a Commando have very different rod stroke ratios. Surprisingly the Dommies rod is actually 6 inches versus the 5 7/8 of the Commando. Plus of course the stroke is longer on the Commando.

The rod stroke rato on a Dommie is about 2.1 from memory while a Commando is around 1.7 ??? . Something like thst. It's 15 years since I did this.

So of course for a given crankposition the Dommie and Commando will have the piston located at a different percentage of the stroke.

My calculations suggested by proportionality you should retard a cam manufactured for a Commando by around 7 degrees if you wanted to match the piston location on a 500 Dommie.

I have never seen this discussed by anyone else and it's totally my own thoughts but it's interesting my noes seem to match your cam.

My Dommie was dynoed at 51 BHP rear wheel at 6100 rpm and at the time 15 years ago was the second fastest clubmans bike in NZ. So retarding the cam worked. I was using a PW3 cam but the same principle applies I think.
Click to expand...
Very interesting and very logical.

And 51 rwhp from a 500 Dommie is seriously good. In fact it’s a chunk more than 99% of all Commandos out there !
 
  • Like
Reactions: baz
Fast Eddie said:
Very interesting and very logical.

And 51 rwhp from a 500 Dommie is seriously good. In fact it’s a chunk more than 99% of all Commandos out there !
Click to expand...
Power band was nasty under 3500 but from 5 to 7000 it went well. Difficult with a 4 speed gearbox good with 6 speed. But the rider was very good and ex 2 stroke competitor so it was normal for him.
 
johnm said:
I think I may be able to suggest why the cam is retarded by 8 degrees. About 20 years ago now I developed a 500 Dommie for NZCMRR clubmans class racing down her in New Zealand. I did many dyno runs with cams and cam timing.

I tested various cam timings but one thing I settled on was the following.


My thoughts were that these days most cams are made for the Commando. And if you buy a Domi cam from any of the suppliers it will be a Commando cam with just the timing sprocket end modified. The timing will be Commando timing.

But a Dommie and a Commando have very different rod stroke ratios. Surprisingly the Dommies rod is actually 6 inches versus the 5 7/8 of the Commando. Plus of course the stroke is longer on the Commando.

The rod stroke rato on a Dommie is about 2.1 from memory while a Commando is around 1.7 ??? . Something like thst. It's 15 years since I did this.

So of course for a given crankposition the Dommie and Commando will have the piston located at a different percentage of the stroke.

My calculations suggested by proportionality you should retard a cam manufactured for a Commando by around 7 degrees if you wanted to match the piston location on a 500 Dommie.

I have never seen this discussed by anyone else and it's totally my own thoughts but it's interesting my noes seem to match your cam.

My Dommie was dynoed at 51 BHP rear wheel at 6100 rpm and at the time 15 years ago was the second fastest clubmans bike in NZ. So retarding the cam worked. I was using a PW3 cam but the same principle applies I think.
Click to expand...
That's great info. I acquired the cam about 30 years ago and I had left it, mislabeled as a 650ss, with my cam collection.
 
Fast Eddie said:
Very interesting and very logical.

And 51 rwhp from a 500 Dommie is seriously good. In fact it’s a chunk more than 99% of all Commandos out there !
Click to expand...
Bob Reynolds once told me that the standard Commando 750 produced 49 hp on the dyno.
 
worntorn said:
Would that be 49 hp at the rear wheel ?

Glen
Click to expand...
Good question. I don't recall. The engine mounting frame for the Heenan & froude dynamometer was for direct drive but I don't know if they had a rolling road.
Gearbox losses in the top gear are very low owing to the direct drive in the AMC box. A well setup and lubricated chain drive is assumed to be 98% efficient (96% for primary + secondary).

Bob
 
It is common for cams to have a slower closing rate than their opening rate. When the valve snaps shut, that is when the stem is more likely to break.
I usually use a dial micrometer and degree disk, and plot the lift curve when comparing cams. I have not bothered to do it with my 850, because it is quick enough without playing with cams. I do not know what is in it now. Fast enough is good enough.
 
You must log in or register to reply here.
Back
Top