valve timing / cam identification

NickZ said:

Is it likely that both exhaust lobes could wear that much with the inlet lobes wearing virtually not at all?

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I don’t know.

It’s a possibility, ie someone could have shimmed up the exhaust springs too hard.

Another possibility is its a different cam, based on a 2S but slightly different.

But, if this is actually a Combat, and it doesn’t look like it’s been messed with too much, then I think I might be inclined to assume that your measurements indicate it is a 2S. I don’t have experience measuring them, but would not be surprised if a little manufacturing ‘tolerance’ plus a little wear, could equal the difference you see.

Whatever the real issue, I would suggest that your cam measures up close enough to work, so as ILLF8ED suggested, maybe your focus should now be on the timing.

With that you enter another world of debate re the best method! But IMHO opening and closing points are the worst. Lift @ TDC or lobe centre are better.

acadian said:

I recommended that method because with it you can ascertain opening, closing and lift:

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I can send you the spreadsheet that does all the calculations for you once the numbers are plugged in

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I would love a copy of that.

Fast Eddie said:

I don’t know.

It’s a possibility, ie someone could have shimmed up the exhaust springs too hard.

Another possibility is its a different cam, based on a 2S but slightly different.

But, if this is actually a Combat, and it doesn’t look like it’s been messed with too much, then I think I might be inclined to assume that your measurements indicate it is a 2S. I don’t have experience measuring them, but would not be surprised if a little manufacturing ‘tolerance’ plus a little wear, could equal the difference you see.

Whatever the real issue, I would suggest that your cam measures up close enough to work, so as ILLF8ED suggested, maybe your focus should now be on the timing.

With that you enter another world of debate re the best method! But IMHO opening and closing points are the worst. Lift @ TDC or lobe centre are better.

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The opening points set the point in the timing at which the standing waves in the inlet port and exhaust system are set up when the engine is running. They are more important than the closing points,. If there are recommended cam timings to suit a particular exhaust system, the timing of the exhaust opening is really the only one which needs to be achieved. When you advance a Commando cam to get more go by opening the inlet valves earlier, you also open the exhaust valves earlier which can affect noise levels. With twin exhausts, advancing the cam might change the way power is delivered as the revs rise. Because of my 2 into1 exhaust, my cam is advanced 12 degrees. It is loud, but it works. More back pressure needs more advance. A stronger pulse makes the pipe resonate better when there is back pressure.
You usually have restrictive mufflers to reduce noise levels . - Advancing the cam defeats that purpose.

S

NickZ said:

I agree that the lift should be a good way to differentiate the 2S cam. The problem I have is that my measurements show the lift of my cam is like 2S for intakes but not for the exhaust. Exhaust lift measures closer to non-Combat cam spec.

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Sounds About right. Intake lift is .390. Exhaust is I believe .341. Standard is .330 for both.

NickZ said:

I think my next step will be to use this method and get duration measurements at .050" lift and see if those fit with any of dynodave's profiles. After that, I might as well get the rest of it calculated from your speadsheet, but I still don't have a reference for lobe center spec and don't believe I can get that accurately from the dynodave profile graphs.

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As others have noted and your comments indicate, you are planning to abandon the low lift cam timing procedure and look at the 0.050” lift data. The ramp on the 2S cam is a peculiar one, and in light of this, trying to measure opening/closing at such low lifts when there is significant lift occurring over a short amount of duration is a bad idea. See the link below for a closer look at the 2S ramps

https://www.accessnorton.com/NortonCommando/tappet-clearances.35294/page-2#post-583576

I’m sure you’ll resolve your issue quickly with the 0.050” lift data sets. The plot below, courtesy of a past Comnoz post, shows valve lift for a 2S cam. The 0.013” lift numbers in the plot are not in good agreement with the factory data, nor are the maximum lift values, but it is nevertheless data from a 2S cam. The factory 0.013” data suggests IN and EX centerlines of 105 and 104 respectively, resulting in an IN/EX x-over point very near TDC. The plot shows a x-over point about 5 deg before TDC. I believe that if you get your cam timed with a x-over occurring anywhere in the 0-5 deg BTDC range you’ll be pleased with the outcome. Keep us posted on your progress. Thanks.

Plugging in the 2S numbers published by the NOC (https://www.nortonownersclub.org/support/technical-support-commando/camshafts) this is what I get.

I appreciate all the help and valuable references provided.

I believe I now have enough useful information to draw a conclusion. I changed my measuring technique by anchoring the dial gauge to the head steady rather than the frame, which resulted in much more repeatable and I think accurate measurements. Because of the Isolastic engine mounts, when the gauge was anchored to the frame, it could move relative to the valve being measured as I turned the rear wheel to rotate the crankshaft. Also, as suggested, I changed from measuring valve opening and closing at the specified 0.013" lift and comparing to the numbers in the manual to measuring at 0.050" lift and comparing to the dynodave profile graph.
Here are the results:



So I conclude my cam is closest to a 2S, as Fast Eddie suggested. Still don't know why the exhaust lobes are off a bit, but as several have suggested, probably not an issue.
I can also conclude that my valve timing is set correctly for the 2S cam. So I can rule that out as the cause of the performance issue. I was hoping ILLF8ED had solved it for me.

So, mystery #1 solved, it's a properly timed 2S. Mystery #2 still ahead, the cause of performance issue. I'll focus on ignition timing and carburation next in that order. I have already ruled out fuel starvation.

Of Note: I couldn't set the inlet valves to zero clearance for the measurements because when set to 0 the valve contacted the piston before full lift was reached.

Good work sortingthrough this, but something seems off with your numbers, the LC's should not have a spread of 8 degrees (if a 2S cam). If your intake piston to valve clearance is resulting in contact, seems to me that the cam may be advanced. I've never measured a 2S cam, so maybe the figures posted by the NOC are incorrect, I dunno

For comparison, these are the numbers for the 312A cam in my bike (note the 8.5 degree LC spread)

Almost without exception moderate Norton performance cams have a lobe separation angle (LSA) ~102-104 degrees, and as cam duration increases further the LSA increases to ~ 105-106 deg. The fact that both the 2S, reported here by NickZ, and the 312A, reported by “acadian” both have LSAs of 104 degrees sets well with me since they both conform to the norm in moderate performance Norton cam design. I wouldn’t pay much attention to the actual lobe center numbers, but rather take comfort in the fact that with a LSA of 104 these cams are well designed for a Norton and can be timed to provide whatever lobe center the owner thinks is best for his/her engine.

Below are various Megacycle grinds to peruse and observe LSAs. You can see similar info here - https://jsmotorsport.com/camshafts/ - for JS Motorsport cam offerings, all of which have LSAs falling the range of 104-106 range.

Very interesting discussion of cams. Per my comments in the INOA discussion of cams ca. 1998, I have found that the SS cam in an otherwise stock 850 w/ 8.5: compression a little lacking on the bottom end. This is most noticeable in city traffic but out out the road it's fine. One interesting item is that the SS cam improved my gas mileage on the road a lot, like 10+ mpg compared to the stock cam with no other changes except perhaps weather. On the trip I first noticed this, I was running 65-75 mph, 3500+ RPM This could be due to a couple of factors, lower vacuum resulting in lower pumping losses and perhaps running closer to the engine's torque peak or the resonance in the exhaust system it doing it's negative pressure thing. This is strictly conjecture as I don't really know. Perhaps one of you tuning wizards knows.

acadian said:

Good work sorting through this, but something seems off with your numbers, the LC's should not have a spread of 8 degrees (if a 2S cam).

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acadian said:

I've never measured a 2S cam, so maybe the figures posted by the NOC are incorrect, I dunno

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I have some difficulty with the NOC numbers in that reference. I am not certain what lift value they are using for what measurements in their table. They have .040" "clearance at lift" for 2S cam, but the valve open/close values listed are the same those in the Workshop Manual specified at .013" lift. Then they list .010" clearances before the duration column. For what measurements are each of the mentioned clearances used? They mention the rocker ratio before the table but don't state whether the readings are cam lift or valve lift. While some of that my not affect LC calculation, I can't be sure.

In the 2nd column of my table in post #27 I have what I measured off dynodave's 2S profile. Best I can tell, that shows the 2S LC spread of that cam to be 4.5 degrees, which is about half way between my number and the NOC number. I have to keep in mind that there are plenty of potential sources of measurement error in all of this.