Another cam question

The cam that was in my 73 850 I assumed was stock. After measuring it its aprox. .362 lift intake and ex. at the valve.
It has a T stamped on the lobe. Any ideas what it could be.
Also I just purchased a 4S. After measuring its actually about .359 on both lobes, and seems to be a regrind, as its different that what I found for specs on-line.
Also it doesn't have the small ramp I've read that the 4S is supposed to.
This cam has nicer lobes and I'd like to use it, but not sure what the specs are.

Go here

http://atlanticgreen.com/camsurvey.htm

Maybe if you put up more information about your bike some of the smart guys(not me) here would chime in. No one can really give you any input if they do not know what your engine is going to be used for and what other modifications have been done to it besides a cam switch.

Depending on who grinds a particular cam profile that has a "4s" label slapped onto it, it could have different specifications, so with the information you supplied no one can say much.

If your engine is all stock except for throwing a big cam in it, then there is a good chance you will make the bike slower and less reliable for street use than if it had a stock cam, especially if you have one of the RH4 cylinder heads on your bike and the stock compression ratio.

Any other modification you can do to a Norton short of swapping the cam will have a better return on power and reliability.

A stock-cammed Norton with a nicely done head, well done ports and valves, milled for compression and a good exhaust system is going to be a lot more fun to ride than a stock motor that has had a big cam stuck in it, and almost always faster too. And that is all work done from the cylinder head gasket on up.

A hot camshaft grind is not especially useful unless it is in a bike that is run consistently at high-rpm and/or tracks with longer than average straightaways, otherwise it is not worth the cost or decrease in reliability and low-mid-range power.

My two cents.

I don't find an over cammed full race engine a bother in stop and go or parade conditions, like we see in pits at races, so build as wild as Ya like and putt putt like normal until giving er the gun then hang on.

Fitting a hot cam is a fun thing to do, and the easiest way to get more top end power. However the usual penalty for fitting a hot cam is development of a 'cam spot' where the motor gives a strong surge of power probably about a third up the usable rev range. In a Norton it is not really so noticeable due to the normal torque characteristics due to the long stroke, but if you don't change the balance factor you limit the range you can safely use. ( The comp. ratio is irrelevant, the cam will work with any compression). You will always tend to use lower gears and keep the motor over the cam spot around town,( keeping the revs up above the spot). The police will love you.
When I was a kid we used to fit E3134 race cams to 650 Triumph engines - they made the bikes great fun to ride on the street. I survived it - just !

Soulkitchen said:

The cam that was in my 73 850 I assumed was stock. After measuring it its aprox. .362 lift intake and ex. at the valve.
It has a T stamped on the lobe. Any ideas what it could be.
Also I just purchased a 4S. After measuring its actually about .359 on both lobes, and seems to be a regrind, as its different that what I found for specs on-line.
Also it doesn't have the small ramp I've read that the 4S is supposed to.
This cam has nicer lobes and I'd like to use it, but not sure what the specs are.

Click to expand...

Sounds like the cam you have in it was a stock 850 cam. It has the right lift, and the T stamp is correct.

The 4S you have is definitely not a 4S profile. The 4S has different lift for the intake and exhaust lobes, and yours are both the same. If that's .359" lift at the valve, it sounds like it's been reground to a stock profile. If that's cam lift, then it could be a Norris SS profile (not the same as the Norton SS or 2S), which is specified as .359" lift. The Norris SS grind makes a decent high performance street cam for an 850. If you plan to use the high rpm range with it, you should probably also go with a spring kit (RD, KPMI, etc.). To really know what you have, you'd have to degree the cam to find out the duration and overlap numbers, but the odds are pretty good that its not a really extreme grind. Most of those use more lift. If it is a Norris SS grind, the valve clearance settings are .008"/.010".

Ken

Ken

I think maybe the specs i had found are incorrecr. I thought the stock cam had less lift.
Once i get to that point i will have to check duration etc.
It did run strong, but the tips of the ex lobes are starting to show a little flat spot.
I will most likely use the used cam i prchased, just dont want to put in a cam thats
Milder than what i had. I know a couple thou on lift is minor, guess ill know when its time ti
Degree it.

If you would like to send it to me with return postage I will put it on my machine and map it for you. Then you will know. Jim

If you have .362" lift at the valve on the stock cam, and .359" lift at the cam on the "4S", the "4S" is certainly not a milder cam. With the 1.13 rocker arm ratio, .359" lift at the cam will give you .398" lift at the valve (with .008" clearance), which is a lot more than .362",

From what you've said in your posts, I'm assuming that you measured the lift on the stock cam at the valve, and lift on the "4S" by measuring the cam lobes.

It can be difficult sorting out comparative profiles, because some places list specs with lift at the valve, and some with lift at the cam, and they aren't always clear which they are using. The service manuals list the stock Commando lift as .332" at the cam, and .375" at the valve, but doesn't subtract the running clearance for lift at the valve. .006"/.008" clearance brings lift at the valve down to .369"/.367". Online sources list the stock cam as .330" lift at the cam, .362" lift at the valve, and other values within a few thou of these.

I think you will be fine running the "4S" cam you purchased.

Ken

comnoz said:

If you would like to send it to me with return postage I will put it on my machine and map it for you. Then you will know. Jim

Click to expand...

An even better idea!

Ken

lcrken said:

If you have .362" lift at the valve on the stock cam, and .359" lift at the cam on the "4S", the "4S" is certainly not a milder cam. With the 1.13 rocker arm ratio, .359" lift at the cam will give you .398" lift at the valve (with .008" clearance), which is a lot more than .362",

From what you've said in your posts, I'm assuming that you measured the lift on the stock cam at the valve, and lift on the "4S" by measuring the cam lobes.

It can be difficult sorting out comparative profiles, because some places list specs with lift at the valve, and some with lift at the cam, and they aren't always clear which they are using. The service manuals list the stock Commando lift as .332" at the cam, and .375" at the valve, but doesn't subtract the running clearance for lift at the valve. .006"/.008" clearance brings lift at the valve down to .369"/.367". Online sources list the stock cam as .330" lift at the cam, .362" lift at the valve, and other values within a few thou of these.

I think you will be fine running the "4S" cam you purchased.

Ken

Click to expand...

I measured both at the lobe then multiplied the 1.13 for rocker ratio.
The actual lobes measured .322 on the cam that was in the bike, and .316 on the one I purchased.

comnoz said:

If you would like to send it to me with return postage I will put it on my machine and map it for you. Then you will know. Jim

Click to expand...

I may do that, Thanks!

Sorry about the confusion, SK. When you wrote "Also I just purchased a 4S. After measuring its actually about .359 on both lobes", I assumed that was the actual measurement of the lobes. My mistake.

Ken

Well Ken is a smart guy and he says use it. Raising the compression might be a very complimentary modification though.

I had an old Norris cam that I did not know anything about sitting around here once and a friend of mine that was overhauling his 850 Commando wanted it to try out. The cam looked like a big one to me and I told him this and he went ahead and put it in and ended up really hating it because he had to rev the motor way up to get good power. It was a shame because he did a really nice job of putting the bike together, it was clean and did not leak a drop of oil anywhere, so he did not really want to tear it back apart and switch cams again.

I have seen people with automobiles do the same thing more than once, drop in a hot cam and end up with something that was slower than it was with the stock cam, so as a rule I don't recommend cam changes to anyone unless it is a proven combination that is done with complimentary work, raising compression etc.. A big cam will drop your effective compression at low rpms making the engine soggy down low. Raising the mechanical compression gives the engine back some of the torque the big cam took away, then when the cam reaches it's efficient operating range and is working with the inlet and exhaust it stuffs in enough fuel-air to hopefully bring cylinder pressure up to or above what the engine had at that rpm before.

The 2s cam was used in stock Commandos, but ones with high compression from the factory. Thrown into a stock 850 with low compression, there is a chance that the bike will be no faster down a quarter mile strip as you will lose power down low and maybe gain it up high. Used as the factory did it though, you would effectively make an 850 Combat, which should run better overall.

Good luck.

Beng has it right.
I might add that I have dynoed an 850 before and after the addition of a 2s cam with no compression changes. The result was less torque at all speeds and the horsepower seen at 6200 rpm with the stock cam was not seen until 7200 rpm with the 2s cam. Jim

I know not to over cam.
I plan on ccing the chamber and deck hight once I get new pistons.
Will mill the head to get compression where I want it. Probably around 9 to 1.
I am going to use the nicer cam I have, we'll see how it runs by spring.

I am about to fit a 2S Combat cam into my 850. It has a 2 into 1 exhaust system and I will be advancing the cam by 6 degrees over standard timing. Has anyone tried doing this ?
I've read the recommendation about raising the comp. ratio, however I've fitted race cams to several motors in the past and found that there has been an increase in torque across the whole rev range regardless of the comp. ratio. What usually changes is where the power band is located within the usable rev range..
Does a 750 Combat with separate exhausts pull hard from 4000 RPM to 7500 RPM ? What were the bad effects of the 2S Combat cam in that motor ? Were most of the blow-ups due to over-revving - a 2 into 1 exhaust usually knocks the top end off the power band unless the cam timing is optimised to suit ?

acotrel said:

I am about to fit a 2S Combat cam into my 850. It has a 2 into 1 exhaust system and I will be advancing the cam by 6 degrees over standard timing. Has anyone tried doing this ?
I've read the recommendation about raising the comp. ratio, however I've fitted race cams to several motors in the past and found that there has been an increase in torque across the whole rev range regardless of the comp. ratio. What usually changes is where the power band is located within the usable rev range..
Does a 750 Combat with separate exhausts pull hard from 4000 RPM to 7500 RPM ? What were the bad effects of the 2S Combat cam in that motor ? Were most of the blow-ups due to over-revving - a 2 into 1 exhaust usually knocks the top end off the power band unless the cam timing is optimised to suit ?

Click to expand...

As I understand it, the Combat produces less torque and power than a stock motor below 5000rpm and only kicks in above 5000rpm.

Given your own preference of using the low end torque, I struggle to see how this is a good move for you.

Running the set up with a lower compression will only make this worse.

There are other, less aggressive, cams that deliver more power below 5000 but till pull wrong to 7000 and above.

acotrel said:

I am about to fit a 2S Combat cam into my 850. It has a 2 into 1 exhaust system and I will be advancing the cam by 6 degrees over standard timing. Has anyone tried doing this ?
I've read the recommendation about raising the comp. ratio, however I've fitted race cams to several motors in the past and found that there has been an increase in torque across the whole rev range regardless of the comp. ratio. What usually changes is where the power band is located within the usable rev range..
Does a 750 Combat with separate exhausts pull hard from 4000 RPM to 7500 RPM ? What were the bad effects of the 2S Combat cam in that motor ? Were most of the blow-ups due to over-revving - a 2 into 1 exhaust usually knocks the top end off the power band unless the cam timing is optimised to suit ?

Click to expand...

yes its more like 5000rpm when the combat starts to go !! the one i owned had seperate down pipes no balance pipe and pea shooters

5000 RPM to 7000 RPM is more than the usable rev range I have with the 850 cam and the close ratio 4 speed box. I don't think I ever drop much below 5,500 once the bike is mobile. I usually try to change up before 7000 RPM, however usually see 7,500 on the change. The 2 into 1 pipe makes a difference.

I'm not talking about a heavy bike with a standard wide ratio Norton box - that's when pulling hard from 3000 RPM might be needed - pretty much a waste of time.
However thanks for the info about the 2S Combat cam's power band. I can work with that

How many revs does a standard 850 commando drop on each upwards gear change if it is revved to 6,500 RPM at full throttle ?