Another cam question

acotrel said:

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 ?

Click to expand...

Quite a lot - thats why its considered to have a fairly wide ratio box.
But in std road bike format, it will pull strongly from less than 3000 rpms.

In fact, if you are gentle on the throttle, in road bike format, it is quite possible to ride it around town and never exceed 3000 rpms.
And still break the speed limit. !

You have all the details, draw up a chart to show the gearing, and rpm changes.
Surprised you don't have all this already ?
Esp with that TT 6 speed box ??

I haven't had the opportunity to test the TTI box yet. My wife has promised me a practice day in the new year. I've raised the overall gearing substantially and I think with the 6 speeds I should get faster towards the ends of the straights. I ordered the box with the first gear ratio halfway between that of a standard commando and a manx. I'm still wondering if I should play with the camshaft before the next practice, or limit the changes to two stages.
With the Combat cam, I have to grind the scroll into the bearing surfaces before I can use it in the 850 motor, so a rebuild is necessary. I always like to make only one change at a time, then test after each one. The practice days are fairly expensive.

acotrel said:

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 ?

Click to expand...

OK, if you want to keep it between 5,500 and 7,500 I would suggest a 2S will suit you very well. It will produce more power whilst in this rev range than the stock cam and I would say that a 5,500-7,500 powerband will actually work very well with your TTI box, in fact, it sounds like fun!

I must have miss-read some of your past posts though, as I thought you preferred 'short shifting' and 'using the torque' and that 'revving a Commando is a waste of time'.

But I would still suggest you re-think the compression ratio issue. The cam is designed to run with a higher CR. your 2:1 pipe is unlikely to substitute this design requirement, no matter how good it is. With a race bike, you have no poor quality fuel issues to worry about and, within sensible reason, there is no down side to raising the CR. Without raising the CR, your motor will most likely disappoint I think.

The reference to short-shifting was about finding out whether the bike is under-geared. How do you know unless you make the motor pull hard ? I found fairy early that with the commando engine, it didn't seem to matter what you did, it tended to always spin up at the same rate. In addition if you didn't have the very close box, the bike was difficult to ride smoothly.
I always believed that lower gearing gave faster acceleration, the commando is different to other bikes in that respect. I tend to keep the revs well within the 6000s, so effectively riding the top of the torque curve. The other thing is that even with the low comp.ratio, the methanol fuel still works well.
I'm still thinking about whether to go ahead with the combat cam. I've use plenty of race cams in low comp. motors, and got the same distinct power band regardless of the ratio. From what you guys have described, I think I can live with it.
I suggest that what you believe about the cam affecting the compression, is when the timings haven't taken effect because of low revs and haven't created the resonance in the inlet and exhaust tracts ? - Should only ever be a problem with road bikes. When my old 500cc Triton had 4 inch megaphones, there was nothing under 5,500 RPM, then you got it all at once. The commando engine could never be like that.


The silly thing is that I never believed in the Seeley 850, so it sat unraced for yonks. Now I believe it is a very good thing - capable of winning.

The reference to short-shifting was about finding out whether the bike is under-geared. How do you know unless you make the motor pull hard ? I found fairy early that with the commando engine, it didn't seem to matter what you did, it tended to always spin up at the same rate. In addition if you didn't have the very close box, the bike was difficult to ride smoothly.
I always believed that lower gearing gave faster acceleration, the commando is different to other bikes in that respect. I tend to keep the revs well within the 6000s, so effectively riding the top of the torque curve. The other thing is that even with the low comp.ratio, the methanol fuel still works well.
I'm still thinking about whether to go ahead with the combat cam. I've use plenty of race cams in low comp. motors, and got the same distinct power band regardless of the ratio. From what you guys have described, I think I can live with it.
I suggest that what you believe about the cam affecting the compression, is when the timings haven't taken effect because of low revs and haven't created the resonance in the inlet and exhaust tracts ? - Should only ever be a problem with road bikes. When my old 500cc Triton had 4 inch megaphones, there was nothing under 5,500 RPM, then you got it all at once. The commando engine could never be like that.


The silly thing is that I never believed in the Seeley 850, so it sat unraced for yonks. Now I believe it is a very good thing - capable of winning.

acotrel said:

The reference to short-shifting was about finding out whether the bike is under-geared. How do you know unless you make the motor pull hard ? I found fairy early that with the commando engine, it didn't seem to matter what you did, it tended to always spin up at the same rate. In addition if you didn't have the very close box, the bike was difficult to ride smoothly.
I always believed that lower gearing gave faster acceleration, the commando is different to other bikes in that respect. I tend to keep the revs well within the 6000s, so effectively riding the top of the torque curve. The other thing is that even with the low comp.ratio, the methanol fuel still works well.
I'm still thinking about whether to go ahead with the combat cam. I've use plenty of race cams in low comp. motors, and got the same distinct power band regardless of the ratio. From what you guys have described, I think I can live with it.
I suggest that what you believe about the cam affecting the compression, is when the timings haven't taken effect because of low revs and haven't created the resonance in the inlet and exhaust tracts ? - Should only ever be a problem with road bikes. When my old 500cc Triton had 4 inch megaphones, there was nothing under 5,500 RPM, then you got it all at once. The commando engine could never be like that.


The silly thing is that I never believed in the Seeley 850, so it sat unraced for yonks. Now I believe it is a very good thing - capable of winning.

Click to expand...

My reference to cams and compression is quite straightforward really. Cams with a greater duration hold the valves open longer. So, some 'effective compression' is lost through the open valve as the piston ascends on the compression stroke. The greater the duration, the greater the loss. Period.

Yes, this can be minimised at certain rpm ranges by tuning the harmonics, but it cannot be eliminated completely.

At the end of the day, the whole purpose of an internal combustion engine is to compress and ignite that gas to extract maximum power from each bang. Higher compression = greater power output and efficiency plain and simple. Even modern family hatch back cars have compression ratios at 12:1.

A stock 850 is normally around 8:1. No matter how good your exhaust is, it will produce more torque / power if your raise it. Mine is perfectly fine on pump gas at 10.5:1. If you are racing, and are allowed to run methanol, I'd go for 12:1.

But... its your bike, matey and you should, of course, do what you think is best and will give you the most enjoyment... Cos that's what it's all about ultimately !

I'd much prefer to run 12 to 1 comp with methanol, however what you pick up on the merry-go-rounds , you lose on the swings. Tangling the valves is not a good strategy, and methanol works extremely well as long as when you are jetting, you do it as you would for petrol. Also at the low comp. a lot more ignition advance should be used. Running methanol rich to get more power is bullshit. At 8 to 1 comp. it can still work extremely well and in the commando engine, it is really lovely.
I still cannot get my head around your comment that with more radical cam timing you get loss of compression. I suggest that once the motor is running at revs above the cam spot, there is no loss. The simple fact is that if you fit E3134 cams to a 650 Triumph after it has been fitted with road cams without changing anything else, - unless the exhaust system is restrictive, what changes is where the power band is in the rev range. However there is always an overall boost in power. Below the cam spot the motor still pulls harder than with road cams. Megaphone exhausts amplify the effect of the cam spot and can make the bike more difficult to ride well.
What I might expect with the combat cams is the bike might become radically different low down in the rev range, however with the 6 speed box, should not be a problem. A lot depends on getting good starts in races, I think the TTI box will be able to cop a beating without self-destructing.
I would not enjoy push-starting in races these days, however in the old days I would have won with the 4 speed close box. Once the bike is rolling, it is excellent everywhere.
I still have a problem with judging what the overall gearing should be - seems that until you raise it, you don't know what is best. With the 4 speed box, first gear is a killer.

At what revs does the cam spot occur with the 2S combat cam in the 750 commando ? From what you have said previously, it sounds as though it might be at 5000 RPM which is far too high for a road bike.

You cannot compare the E3134 with the 2S, it is more comparable with the stock Commando cam, which makes sense, as they were both initially designed as race cams in the 50s and 60s, but cam design continued to evolve, and manufacturers continued to chase HP figures, and the 2S intake is WAY more radical than the E3134.

Yes, the 2S comes 'on cam' at 5,000 ish I believe, and yes, many agree with you that this is way too high for a road bike, but it is what it is (and it has its fans / supporters too).

12.5:1 is doable on an 850, even if you don't go for the full lightweight package, the crown design remains the same: http://www.jsmotorsport.com/technical_rodspistons.asp

Have read this thread with great interest.. back in the day , cant say that Combat 750s were that bad lower down the rev range . I do remember an article perhaps in Classic Racer or some such in which it was said that setting up Dunstall cams on Dunstalls own bikes was nothing like might be supposed from his Tuning notes Advance it something rotten.. the phrase that sticks in mind ... Now why would it be necessary to do this , have always wondered if it is to compensate for lift being lost by the push rods flexing ? Mine has a standard cam and it will rev quite happily . Did try carbon
fibre push rods .. unlike the dural they are completely rigid - the motor seemed sharper so i assume that flex was causing loss of lift and delaying effective opening. Then one broke! So older sadder and wiser went back to dural standard pushrods Have some steel ones to fit when i get round to it . Would have thought this was worth thinking about for out and out racing

The other was an observation by the late John Hudson in a Roadholder thats the NOC comic.
on timing the standard SS cam ... not to be confused with the 2s or combat cam... He paid a great deal of attention to the actual timing and getting
a lead of 13 degrees. Of what over what I do not remember but he suggested that as built rarely did this come out right and that compromise was necessary - the vernier effect of playing about with the cam wheels. Might be worth chasing this down if you want performance from a standard cam.

8 degrees lead not 13 sorry - thats of inlet opening over exhaust opening

acotrel said:

At what revs does the cam spot occur with the 2S combat cam in the 750 commando ? From what you have said previously, it sounds as though it might be at 5000 RPM which is far too high for a road bike.

Click to expand...

I just read back through this thread as I thought I can't be the only person saying this stuff... And there, way back in Jan 2103 Jim Comstock wrote:

"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"

Nuff said...!

Pick a subject, any subject, and its (nearly) all been said before.
Groundhog day, almost every day...

??

P.S. Somelocally here has just had fitted a 3S cam to an Atlas.
Said to produce much more low down torque, and less top end.
Intended for racing, but not yet tried.

'"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"'

Eddy, I haven't forgotten Jim's comment and I know he is a reliable source of information. However I cannot understand how the 2S Combat cam used that way did not demonstrate a cam spot with increased power above it. Perhaps the cam is so radical that the power band occurs above 7000 RPM ? If that is the case, it would never have worked in the 750 motor.
With the timings it has, I would expect it to cut in at well below 5000 RPM . In timings, it is not radically different to the standard 850 cam.
I am still of two minds whether to try the 2S cam in my 850 - it requires a rebuild to fit it.

acotrel said:

'"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"'

Eddy, I haven't forgotten Jim's comment and I know he is a reliable source of information. However I cannot understand how the 2S Combat cam used that way did not demonstrate a cam spot with increased power above it. Perhaps the cam is so radical that the power band occurs above 7000 RPM ? If that is the case, it would never have worked in the 750 motor.
With the timings it has, I would expect it to cut in at well below 5000 RPM . In timings, it is not radically different to the standard 850 cam.
I am still of two minds whether to try the 2S cam in my 850 - it requires a rebuild to fit it.

Click to expand...

It seems there is only one way to find out! You have a clear hypothesis in your mind. That ain't gonna go away! Are you gonna be happy thinking 'I might have been on to something... But never bothered to find out'?!

Life's too short for that. Do it! And let us know what you find and 'add to the body of knowledge'...

I've had a couple of thoughts about Jim's comment. If the exhaust system is too restrictive, it can stop a race cam from working. Also I cannot rationalise your idea that raising the compression ratio affects the way a cam works. Raising the compression ratio affects the pressure rise within the combustion chamber. Race cam timings have to do with gas inertia and resonance within the inlet and exhaust systems. Increasing the lift rate affects how much mixture is induced through the inlet valve on each pulse and can have an adverse effect on the life of the valve train.
It makes me wonder how many guys re-jet their carbs after raising the comp. ratio ? If the motor is jetted slightly rich before the comp. ratio is increased there will be a bigger increase in performance after the head has been machined.
My brother has two bikes which run 17 to 1 comp. ratio. One is a modified 500cc (now 600cc) Speedway Jawa, the other is an 880 JAP. They are both rocket ships, however I don't know how much of that is due to comp. ratio. They certainly use more methanol, if that is anything to go by. When we play with two strokes, comp. ratio is the one thing we never change, even though we reshape the heads. If you run lean enough, the power is usually there regardless of the comp ratio.

acotrel said:

I've had a couple of thoughts about Jim's comment. If the exhaust system is too restrictive, it can stop a race cam from working. Also I cannot rationalise your idea that raising the compression ratio affects the way a cam works. Raising the compression ratio affects the pressure rise within the combustion chamber. Race cam timings have to do with gas inertia and resonance within the inlet and exhaust systems. Increasing the lift rate affects how much mixture is induced through the inlet valve on each pulse and can have an adverse effect on the life of the valve train.
It makes me wonder how many guys re-jet their carbs after raising the comp. ratio ? If the motor is jetted slightly rich before the comp. ratio is increased there will be a bigger increase in performance after the head has been machined.
My brother has two bikes which run 17 to 1 comp. ratio. One is a modified 500cc (now 600cc) Speedway Jawa, the other is an 880 JAP. They are both rocket ships, however I don't know how much of that is due to comp. ratio. They certainly use more methanol, if that is anything to go by. When we play with two strokes, comp. ratio is the one thing we never change, even though we reshape the heads. If you run lean enough, the power is usually there regardless of the comp ratio.

Click to expand...

I look forward to reading your posts on the before / after findings.

Eddy, that is pretty much the conclusion I have come to. It I don't try it, I will never know. I've had a good look at the 2S cam and it has a much bigger share towards the nose of the lobes than the 850 cam, so the lift rate and the actual lift must be much more dramatic, however the timings make sense to me. I sometimes wonder how much the Norton factory did to make their Commandos quieter and more tractable - more like their main competition the CB750. Performance is in a different direction. I have to wait until the weather cools down a bit before pulling the motor apart. I should be able to get the bike onto the circuit by about March next year.

acotrel
I might think a cam suited to a broader band and increase of torque would be much more beneficial on and off the track for an 850. Sell your 2s to a needy 750 owner and buy yourself something more useful, not to mention proven. JS2, PW3
After that, beef up your gearbox and final and you will probably be a happy man.

Although it's been a couple years since the author posted, it seems he may have had on old Axtel 3 in his hands. Too bad he couldn't get used to it, still suite to a 750 I feel.

pete.v said:

acotrel
I might think a cam suited to a broader band and increase of torque would be much more beneficial on and off the track for an 850. Sell your 2s to a needy 750 owner and buy yourself something more useful, not to mention proven. JS2, PW3
After that, beef up your gearbox and final and you will probably be a happy man.

Although it's been a couple years since the author posted, it seems he may have had on old Axtel 3 in his hands. Too bad he couldn't get used to it, still suite to a 750 I feel.

Click to expand...

How does one 'beef up' a race spec TTI box...?