Stock Commando Cam Specs

Specially the intake valve closing ABDC @ .050".

I'm trying to compare the Dynamic CR of the stock 8.5:1 engine with the 10.2:1 static CR engine I am putting together.

Here is the effective/dynamic CR for my planned combination.

Just purloined this from the thingo ,



Was actually suposed to be a Dominator ' S S ' cam timing , which ' they recon ' is the Std. c'do cam . Which might be right , or might not be entirely right .

Cam timing must suit the exhaust.

acotrel said:

Cam timing must suit the exhaust.

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You end up with toomuch noise.

acotrel said:

Cam timing must suit the exhaust.

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Cam timing must suit the static compression ratio.

I don't understand why cam specs @ .050 seem to be non existent fir the stock 850 commando cam.

Hard to compare the dynamic compression compared to aftermarket cams that are all spec'ed at .050. All of the effective/dynamic compression calculators are based on . 050.

Dan1950 said:

I don't understand why cam specs @ .050 seem to be non existent fir the stock 850 commando cam.

Hard to compare the dynamic compression compared to aftermarket cams that are all spec'ed at .050. All of the effective/dynamic compression calculators are based on . 050.

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To get the figures you want you or someone would have to actually measure a known good cam. That is a problem industry wide as various cam makers use various lift points for measurement. As I read it, the dynamic compression ratio should be calculated from the end of the clearance ramp, not at .050 lift. Of course the effects of that .050 lift will diminish as RPM increases but what one likely wants to know is how the engine behaves at low to mid RPM such as coming out of a turn.

And all these figures assume that the cams are ground "to spec," a dubious assumption. A cam I put in my pickup long ago has always had noisy tappets. Last time I had it apart, with degree-wheel and dial-indicator attached, I checked it against the cam card. The lobe-centers and lift are correct as are the .050 specs but the advertised duration is way short, meaning the clearance ramps are short, which explains the noisy tappets. Being a working truck engine, I don't dare set the clearances much tighter but I dislike the noise. Plus, this cam has an early-opening exhaust valve that does, indeed, create a noisy exhaust. And as I use it as an RV (carrying a camper) the noise bothers me and aside from that, the exhaust manifold temperatures are higher and it tends to burn out gaskets. I would like to change cams if I can find one that suits me, which brings us back to the .050 spec issue.

Thomasa said:

To get the figures you want you or someone would have to actually measure a known good cam. That is a problem industry wide as various cam makers use various lift points for measurement. As I read it, the dynamic compression ratio should be calculated from the end of the clearance ramp, not at .050 lift. Of course the effects of that .050 lift will diminish as RPM increases but what one likely wants to know is how the engine behaves at low to mid RPM such as coming out of a turn.

And all these figures assume that the cams are ground "to spec," a dubious assumption. A cam I put in my pickup long ago has always had noisy tappets. Last time I had it apart, with degree-wheel and dial-indicator attached, I checked it against the cam card. The lobe-centers and lift are correct as are the .050 specs but the advertised duration is way short, meaning the clearance ramps are short, which explains the noisy tappets. Being a working truck engine, I don't dare set the clearances much tighter but I dislike the noise. Plus, this cam has an early-opening exhaust valve that does, indeed, create a noisy exhaust. And as I use it as an RV (carrying a camper) the noise bothers me and aside from that, the exhaust manifold temperatures are higher and it tends to burn out gaskets. I would like to change cams if I can find one that suits me, which brings us back to the .050 spec issue.

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The calculators I have found use . 050 + 15°

Dan1950 said:

The calculators I have found use . 050 + 15°

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Adding 15 degrees is a step in the right direction but that assumes facts not in evidence, as the lawyers say. What is the profile as the cam starts it lift? Except for some charts members have provided for various Norton grinds, we don't know. As an example, a Crower cam I have has a .050 duration of 220 degrees and an advertised duration of 252. What the cam card doesn't say is whether the advertised duration includes the .020 and .015 valve lash. By my measurement it does not. Setting the lash at spec produces ~252 degrees, so the difference between advertised and .050 is 32 degrees -- on this cam. Advertised is what I used to calculate my dynamic compression ratio of 7.45 with a static ratio of 9.2.

I was concerned with detonation at around 2400 RPM, just below where this engine really starts to pull. The engine's predecessor, with a static ratio of 8.2, burnt a piston somewhere on a long trip and I was concerned that the full point bump in ratio might repeat that unfortunate event. That said, perhaps I needn't have worried as it had north of 200,000 miles on it.

Matt Spencer said:

Just purloined this from the thingo ,

Was actually suposed to be a Dominator ' S S ' cam timing , which ' they recon ' is the Std. c'do cam . Which might be right , or might not be entirely right .

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John Hudson sat me down one time and told me almost exactly that. He said that the "SS" (or Sports Special) cam was what made the 650SS different from earlier Dominator cams and that that cam grind is the cam grind used as the Standard cam in the Commando. He felt that the 650SS cam was the very best cam for a Dominator twin (of course, unless somebody want to add a sidecar, etc. but for the average sporting rider). He felt that top end power and the power curve at the top end were both very good but his favorite part of this cam's performance was the very strong and wide lower-end torque and the resultant strong low-to-mid power curve.
He was adamant that the stock Commando cam was the best cam -- unless someone was genuinely only interested in top end horsepower (and he thought that most people who think that they need/want/can handle an engine with a cam with a very high end, "peaky" power curb are fooling themselves).
John Baker put his skill and experience into building my 850 Production Racer engine -- he and I talked about this at length and he was strong in his agreement. His take was that John H. was right and not only is the standard cam strong, it gives such good acceleration that it's benefit overall. And I've loved that engine's characteristics for almost 50 years.
BTW - John Hudson always asked that the "Combat" cam be called the "2S" and the "Sports Special" cam be called the "Ess Ess" cam. This was in keeping with the long designation of the Sports Special cam and fit in with the "3S" and "4S" naming scheme at the time.
And for another BTW, he felt that the ramps, lifting and releasing flanks, and the profile of the cam across the high lift areas were too extreme and that they put too much stress on the cam and valve train. I think that the success of the PW3 cam versus the 3S cam is a good example that he was right; after all, Peter Williams liked the 3S cam but once computer-assisted design allowed a camshaft designer to maximize the cam profile performance while it minimized stress and excessive acceleration on the valve train.

I suspect that all the big cams are harder on the valve train than the stock cam.
Jim told me that Fullauto Ken's PW3 cam that died early at about 25,000 miles ( taking everything with it!) was the longest wearing PW3 he had seen.

This may not matter if you only do a few hundred miles per year.

Glen

After the second cam in my MKIII went flat, I hunted through the boneyard in Pokes basement and came up with a 2S cam that looked to be in good shape. I didn't want to put in another soft cam from the Norton supply chain and have it go flat too, so I used that; proven and work hardened, I thought. On that search, I also discovered a XX cam that went into Hot Shoe Les Emory's flat tracker. He loved it and was very competitive, God rest his soul. I don't know much about it but The Boss recognized it. It was radical from the look of the lobes and it did thrash the valve train. I recall replacing loose pushrods in Les's motor at least once a season and valve springs as a precaution, though I don't remember replacing any broken ones. I did Pokes head work and we used lighter valves and Pat at Pat's Top Hat cycle came up with some titanium retainers he acquired from a machinist friend at Boeing who made them as a "Government Job." I also got some similar retainers for my G80cs that had the C1 and CE cams in it which also thrashed the valve train. That motor no longer had hair-pin valve springs and would blow up the armature in the Lucas dynamo, several times, before it ran out of revs. I learn slow. My current Matchless has an alternator and the short-track HS cams, a much better setup. But back to the MKIII...

The 2S is a little doggy at low to mid RPM with its 21-tooth counter-shaft sprocket but these days that keeps me out of trouble lest I hit a deer coming out of a blind corner on full song. That cam has been in the motor 20 odd years now with only a few lash adjustments so I'm confident it's not going flat. It pulls really nicely above 4000 RPM and my gas mileage made a big improvement. I have kept a set of NOS pushrods for it, just in case you know.

On a side note. Yesterday, sorting through parts in my shop I came across a freezer bag with two Norton cams; one is the last ruined MkIII cam, which has two completely flattened lobes, and the other is one I don't remember. It has notches to drive the timed breather, a tacho-drive gear and the tapered end for the auto advance unit. It has no ID markings on it. It appears to be in good shape but I don't remember why I saved it and why it's not in some motor. What era did Commando cams have all those features? Did Norton keep the notches after they binned the timed breather? The ruined cam has no notches.

I enjoyed the 2S cam. The one I have is marked with 2 S's is notch less and taught me a few things about crank case pressure relief. The 2S worked a little friendlier at low RPM after installing a TriSpark ignition and FCR carburetion, but it did not appreciate short shifting with higher gearing and advanced timing. It looked good when I removed it and replaced it with the JS2, but it only has about 15K miles on it. It was a little tough on the lifters. All of them had a little groove in the middle across the smooth flat lifter surface. I suspect that was from using too tight a lash for too long before I knew it should be at least 10/10. The JS2 I replaced it with is a better street cam with more mid-range torque and equivalent top end. The softer ramps single beehive springs and lighter BSA lifters will hopefully make the valve train last. Time will tell.

Excuse me not sticking to the static or dynamic compression discussion. I never bothered to check that. I figured knowing that wouldn't make a fart worth of a difference when twisting the throttle. I'm not an over thinker though and relied on Jim Schmidt doing all the over thinking for me in the JS2 cam design and recommended parts.

Dan1950 said:

I don't understand why cam specs @ .050 seem to be non existent fir the stock 850 commando cam.

Hard to compare the dynamic compression compared to aftermarket cams that are all spec'ed at .050. All of the effective/dynamic compression calculators are based on . 050.

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The plot below is old data from a Comnoz post, which shows IN and EX duration @ 0.050” for a CDO cam to occur in the range of 253-254 deg. IO - 26, IC - 48, EO - 47, EC - 26. It's not a big cam, but its won plenty of races and in a fully optimized state of tune (airflow, compression, pipe) this is plenty of cam to allow an engine to rev freely to 7000 rpm.

Seems that John Hudson was right, the Domi SS cam 22729 went on to be the same grind as the 06.1084 it seems. This 22729 cam was drawn up in 1959. Strangely and sadly we have drawings for a cam called the 'Combat' cam, which is hardly readable but in one place said it was identical to the racer cam, which I assume was the TX 0302. Here it gets cloudy as the the 06.3761 cam is the one specified in the replica proddy racer supplement but what the genuine racers used I don't know, but seems this cam 06.3761 was not used on the Combat as we know the 2S is which is different to the Domi SS cam. The drawing dates for the Combat and 2S cam dated around the same time, a time it seems there was a lot of cam designs about in 1972.
We also have a drawing dated 1963, for part number 24105 which is called and Exp cam, this is very close to the PW3 and has lift between the 2S and 3S.

WZ507 said:

The plot below is old data from a Comnoz post, which shows IN and EX duration @ 0.050” for a CDO cam to occur in the range of 253-254 deg. IO - 26, IC - 48, EO - 47, EC - 26. It's not a big cam, but its won plenty of races and in a fully optimized state of tune (airflow, compression, pipe) this is plenty of cam to allow an engine to rev freely to 7000 rpm.

View attachment 101309

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How long would an 850 last frequently revved freely to 7000 RPM with a stock crank, rods, and cases if it was assembled with care? I get the impression (probably a misinterpretation on my part) that the heavier crank is a gating item on making an 850 wind up that tight all the time. Is 7000 RPM not high RPM on an 850? I've never owned one, so no clue.

Schwany said:

How long would an 850 last frequently revved freely to 7000 RPM with a stock crank, rods, and cases if it was assembled with care? I get the impression (probably a misinterpretation on my part) that the heavier crank is a gating item on making an 850 wind up that tight all the time. Is 7000 RPM not high RPM on an 850? I've never owned one, so no clue.

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Since the 750 and 850 have the same stroke, I see no reason for there to be a real difference in RPM potential.

Does a 302 small block Chevy have a lower RPM potential than a 283? They both have the same 3-in stroke, the 302 is a 4-in bore while the 283 is a 3 7/8 in bore.

Besides, just because an engine will rev to 7,000 RPM, that doesn't mean it's going to spend a lot of time there.

I think the real limiting factor in RPM potential with the 850 lies in the breathing. Both the 750 and 850 have the same size valves.

WZ507 said:

The plot below is old data from a Comnoz post, which shows IN and EX duration @ 0.050” for a CDO cam to occur in the range of 253-254 deg. IO - 26, IC - 48, EO - 47, EC - 26. It's not a big cam, but its won plenty of races and in a fully optimized state of tune (airflow, compression, pipe) this is plenty of cam to allow an engine to rev freely to 7000 rpm.

View attachment 101309

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If I'm reading the text correctly, the stock Commando cam has an intake closing at 48°ABDC.

This would result in an effective/dynamic compression ratio of 6.9: 1 at 8.5:1 static compression.

Compare that to the 7.5: 1 effective/dynamic compression ratio of my 10.2:1 static compression combination with the Web 312a cam running flat followers.

My main concern is cylinder pressure at the bottom of the RPM range and the detonation that could result from that cylinder pressure being too high.. A . 6 raise in dynamic compression should result in an engine that would not be significantly more prone to detonation than a stock engine. Furthermore, it should also have low end torque similar to the stock combination.

Schwany said:

How long would an 850 last frequently revved freely to 7000 RPM with a stock crank, rods, and cases if it was assembled with care? I get the impression (probably a misinterpretation on my part) that the heavier crank is a gating item on making an 850 wind up that tight all the time. Is 7000 RPM not high RPM on an 850? I've never owned one, so no clue.

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One cold October I was heading home with a friend after the last ride of the season. We took off from a traffic light and I got the drop on him and looked over my shoulder, grinning. When I looked back The tach was reading 7,300 rpm as I shifted into second.
That winter I took the head off for an inspection and saw a witness mark in the carbon on top of a piston where a valve had touched so I would say not long. '73 850 btw.

Dan1950 said:

Since the 750 and 850 have the same stroke, I see no reason for there to be a real difference in RPM potential.

Does a 302 small block Chevy have a lower RPM potential than a 283? They both have the same 3-in stroke, the 302 is a 4-in bore while the 283 is a 3 7/8 in bore.

Besides, just because an engine will rev to 7,000 RPM, that doesn't mean it's going to spend a lot of time there.

I think the real limiting factor in RPM potential with the 850 lies in the breathing. Both the 750 and 850 have the same size valves.

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True on the not spending a lot of time at 7000 RPM on the street. However, I thought getting up over 6500 RPM quick was one of the reasons you were freshening up the motor. The need for speed.

On a side note: I'm gearing my little 750 up some more in the primary. It gets to 7000 RPM sooner than I like in 3rd and is wound up more than I like in 4th on the HWY around here. I have to ride in the slow lane at 70mph. Something to think about if you have to ride on the HWY to get to where the fun riding is. A fresh motor should pull taller gears without any trouble. Excuse the useless advice.