cam choice? stock, SS or 4s

Fixing to finish mock up & about to start building the motor... I'm putting a atlas crank in a 73 850 case & was wondering what choice of cam I should go with... I want to have highway rider but I want to dust my harley buddys also... (which they are in envy about this project)... the motor in the chassis is a MK3 with 8.5-1 compression with a R10 head which now is 30mm & on the bench which I'm doing the Dunstal pocket porting & opening up the intake to 32mm..carbs will be a mukni TM 32mm flat slides I know that I will have to bump up the compression either by shaving the head or upgrading my pistions... If you have any imput it would be helpful have several choices stock... SS or the S4... cam's..... Also would I need to change the cam followers ?.....let me know what you think...

Hi, when you said you had put an Atlas crank did you change it's balance factor (84%) to the commando (63%), cause the Atlas flywheel is heavier .......? bear in mind that the rotor key of the Atlas is not in the same place as the Commando (90° apart, but don't remenber which way...!!)
Otherwise , about the cam , a stock one should be enough to blow the Massey Fergusson up ! better put a mild one (if you insist) than a poorly made 2S or 4S........my thoughts! more to come .........

the frame is a matchless we are going vertical is why we chose a atlas crank... I was thinking to stay with the stock cam with the custom dunstall head work i'm doing it should be a fire breather..... not trying to build a better mouse trap just efficent clasic

Jim Schmidt sells an. 003 copper head gasket that will raise your compression one full point to around 9.5 without need to alter the head or change pistons.

Glen

A couple things I would consider are the cam choice and the need to open the 30mm ports to 32mm.

Although the 2s may be close to what you want, there are so many better choices. The JS stage 1, 560nr, and the PW3 come to mind. I think anything more than that will hinder functionallity in the city. These cam offer a much wider range of operation and the midrange to blow by the HD"s without a lot of downshifting.

I had a Combat head and although it functioned to spec, it had an issue with rocker geometry and besides that, it was just beat. I found an old RH1 with the old guides the would not except the valve seals. I had them replace and the seat brought to match. FYI, the seats in the head were like factory fresh. This old head was so very close to NOS.

I then machined .040 off the face, ala Combat head, and opened the port only about 1/2" in the match the 32mm manifold. I know that you are doing much more to your head, but I would ask in more details about that whole porting thing. It seems I have read and seen where people actually weld up the ports to reshape for flow.

All I am saying is that you should do some research before doing some thing that may hinder rather than help performance.
FWIW, this head run as well if not better than the old Combsat head. I feel some of this is do to the 30mm ports rather then 32.

pete.v said:

A couple things I would consider are the cam choice and the need to open the 30mm ports to 32mm.

Although the 2s may be close to what you want, there are so many better choices. The JS stage 1, 560nr, and the PW3 come to mind. I think anything more than that will hinder functionallity in the city. These cam offer a much wider range of operation and the midrange to blow by the HD"s without a lot of downshifting.

I had a Combat head and although it functioned to spec, it had an issue with rocker geometry and besides that, it was just beat. I found an old RH1 with the old guides the would not except the valve seals. I had them replace and the seat brought to match. FYI, the seats in the head were like factory fresh. This old head was so very close to NOS.

I then machined .040 off the face, ala Combat head, and opened the port only about 1/2" in the match the 32mm manifold. I know that you are doing much more to your head, but I would ask in more details about that whole porting thing. It seems I have read and seen where people actually weld up the ports to reshape for flow.

All I am saying is that you should do me research before doing some thing that may hinder rather than help performance
FWIW, this head run as well if not better than the old Combsat head. I feel some of this is do to the 30mm ports rather then 32.

Click to expand...

This is happening with Vincents for sure, there are more badly ported used heads than stock used heads available. One way to get performance back is to weld them up then clean them up back down to stock size or thereabouts.

If a bike is a great performer, like the Commando was when it came out, or the Vincent in its day, what do people automatically do to it?
They alter it in an attempt to get more from it. Often the result is a totally screwed up engine that underperforms the original if ever put to the acid test. Even those tuners who really know their stuff and build great engines will admit that something is generally lost somewhere in order to make a gain somewhere else.

The best part of the Commando engine is the terrific midrange power. You will probably lose some of this to pick up a few hp on top. It might be worth it for your situation, but for most road riders the extra midrange power is what actually gets used, and it gets used a lot. Doug Mcrae posted a graph showing the results from his Herb Becker race engine after getting "the works" . Thiscwas superimposed on the stock engine dyno from before the race mods were done. As I recall, the race engine had considerably more hp on top, but also considerably less than the stock engine down in the 3-4 thousand RPM where most road riding is done.
So I agree with Pete, do lots of research before removing metal from the ports.
The Commando is already a hot engine as Norton built it. A little bump from 8.5 up to 9 or 9.5 compression might be all you need.

Glen

I have heard a preference from several independent sources that for a 750, the early 28mm port heads with the bumped up compression is the hot tip.

pete.v said:

A couple things I would consider are the cam choice and the need to open the 30mm ports to 32mm.

Although the 2s may be close to what you want, there are so many better choices. The JS stage 1, 560nr, and the PW3 come to mind.

Click to expand...

pete.v, I am interested in any more comments you have on this, I am inclined to think it is true, but why are these other cams better?, what do they do that the 2S doesn't?

Steve

I have ported the head on my 850 to match up 34mm carburettors, however only over the first 25mm of the port. It is easy to over-port a motor and destroy it's mid range. If you open the ports too big, they become effective higher up the rev range and it is not wise to go there with a suspect bottom end. Better to keep the ports small, play with the cam and exhaust system, then gear to suit the extra grunt ? It can be very deceptive when you increase the gearing after improving the midrange - you might expect your bike to accelerate slower however find it all happens much quicker. If you build a top end motor out of a commando engine, I suggest you are heading for the big bang. I know all the theories and that is why I never believed in my bike, however I found that the strength of the commando engine lies in it's midrange torque.

As far as the cam is concerned, I suggest you will have to find out for yourself what is best. The first thing I did was soften the cam sprocket and broach two more key ways into it at random to make the timing adjustable. I then tried advancing the cam progressively and riding the bike to see the effect - what works depends on your exhaust system, so you need to establish what you are going to use. I suggest skinny header pipes and preferably a two into one. What is most important is the exhaust cam opening point, if you advance the timing too much the bike often gets louder and goes slower. I've found that moving the inlet opening point can move the cam spot up the rev range - a disadvantage of the Norton camshaft arrangement, where the lobes all move together. Triumphs are easier to play with, however otherwise not as good. When you are doing this stuff you will have to watch your gearing, and try short-shifting to detect torque increases. Better to do this on a race circuit, where you always know what gear you should be pulling.

SteveA said:

pete.v said:

A couple things I would consider are the cam choice and the need to open the 30mm ports to 32mm.

Although the 2s may be close to what you want, there are so many better choices. The JS stage 1, 560nr, and the PW3 come to mind.

Click to expand...

pete.v, I am interested in any more comments you have on this, I am inclined to think it is true, but why are these other cams better?, what do they do that the 2S doesn't?

Steve

Click to expand...

Without going too far out on a limb here, for someone will sure knock me down, the key word is midrange and usable power.

From what I have read, the 2s will be rather unsung until a certain rpm (higher than normal riding) then it's balls to the wall all the way to redline.

With the others, the midrange is wider and becomes effective at a lower rpm. I have the JS stage 1. The low end is as good as any and when the rpm hits around 3500 to 3750, it's a real nice boost. As mentioned in another thread, if cruising at 80 at 4500 rpm, a twist of the throttle and I'm doing 90 (5000rpm) just like that.

I am sure others (hobot) could chime in on the particular characteristics of the 2s so I don't have to hang my ass out there to far.

Yoose guys might want to get Dunstall's Tuning Notes and Jim Schmidt's Race Manuel to get the whole scope in at once to plug in the factoids listed piece meal here. There ain't a lot of cam degreeing available before something clashes so creep up on the trial error with clearance checks before nail the snot of of it to top outs. Big cams are more exhaust set up sensitive as Acetrel notes and the widler the cam the more fussy the exhuast volumes, lenght and exit size. All cross over connections in exhaust systems help the low end w/o taking off top end, but a tuned 2>1 for cam extraction can shove one's butt harder start to finish. Alan's note about louder w/o more power is revealing tune up gem, similar to spark time too, which of course must be done first before the above can be dealt with, after the feul system about dialed in after spark set.

My P!! had some wild drag only cam in it and at least 11:1 CR and idled with a lope at 600 rpm but could take WOT snap after about 1200 rpm to simply roar to redline in N or with a slick no difference so more than enough torque down low to deal with normal use but OMG thought twice before pulling trigger hard. My Ms Peel Combat with 2S cam and 2>1 was similar in low down to over redline response but like 1/3 rd less powerful than the P!! and heavier too, so even though Peel could out leap sport bikes backing off d/t their stupid wheele proness up 90 mph, Ms Peel never impressed pull to the bones apart whip lash neck like the P!! has.

Best wishes on Norton Power, its sure lurking but tricky to get to kind of response me and some others have with some shock for me and angry strangers beyond expectations.
The recently late Ken Augstine of World's Straightest Commando Phatom Oiler fame, wrote about some head work , cam and exhaust combo that two of his race pilots absolutely could not stay in saddle in 1st two gears, no mater what they did the Cdo ran rigght out from under!!! I've had two Norton do that to me too. So when I say shock I mean huncing down, scooting back hard into seat, locking arms white knocked grip, body low forward aroud 2000 rpm and snap WOT, only for an instant though as rips ya grip about, feet off pegs and butt on back side of seat slope!!!

Get some weight off and go run yourself straight into THE Iso Hinge limits on such eager power that only gets worse if letting off or staying on power. I know what I did to get beyond that non-sense when power to spare. Tires become a monthly or bi monthly hassle of course - if getting most the sidewalls worn before center cords fuzzness interferes with grip. W/o a lot of leaning tire might last a tank full or two. Its really fun to have enough response ya can set throttle to set tire spin exactly so much ahead to predict break out then wait a tad for bike to catch up while in mean time the skippage helps aim the thrust soon hit to leap bike into a better line out of there.

zaaarrrrRROOMMM vs BLATTTT*>

I would highly recommend a Mega Cycle 5600 cam, for all around excellent street performance

Three years ago I had Jim Mosher of Performance Indian completely rebuild my motor and the 5600
was his suggestion to be put in my 73 850, I am very happy with it

we also sent my lifters to mega cycle to have them matched with the new cam

Webcam #12a grind has been recomended before by Jim. 2s and 4s are not good for street use.

tripower said:

Fixing to finish mock up & about to start building the motor... I'm putting a atlas crank in a 73 850 case .. the motor in the chassis is a MK3 with 8.5-1 compression with a R10 head ...let me know what you think...

Click to expand...

I'm not clear on what bottom end you have here. Is this a 850 Mk2 motor ('73) or a MK3 ( '75)? Because if its a MK3, then the crankcase / crank dimensions are different to the earlier 850's and 750s....which indicates the Atlas crank is going to have a huge end float issue.

Just to clear the air...The motor that I am building is going to be a 73 MK 2 850 cases with a Atlas crank... he head is a RH 10 which I just started porting the inlet out to 32mm & exaggerating from the bend down to form a pocket behind the guide... hoping to get extra down flow... the exhaust is big bore 1 1/2 ... being a car & ex harley guy, (I'm telling my buddies that the British are comming) HaHa ...I'm just looking for options.... I know if I go with a bigger cam I will need to change the valve speings ...And need to know if I have to change the lifter location plates as well.....all the cams that have been brought up, I have not seen a link where I can view cam specs & spring requirements... I do appericate all the responses though.....I want to keep the original geometry of the valve train . What I'm looking for is a fun country/ city hellcat that I can ride long distance also... ..... Anyway...To get back to the motor that is in the frame that I mocked up ed is a 75 850 MK3 the bottom end will be up for sale in just a couple of weeks, as I'm just about to finish the mock up... I have some early pic up now & will post updated pictures before I tear it down to send the frame over to be power coated.....TriPower

:? :?:

My 850 performs great with the SS cam, it kicks in at about 4,500 RPM and keeps reving, i even give some fast moden bikes a run for their money, but even better with a hand full of throttle throught the corners I get them all the time and Harley riders just eat my dust, they don't even come close.

Ashley

My 850 performs great with the SS cam, it kicks in at about 4,500 RPM and keeps reving, i even give some fast moden bikes a run for their money, but even better with a hand full of throttle throught the corners I get them all the time and Harley riders just eat my dust, they don't even come close.

Ashley

Click to expand...

Above matches my experience trying to follow a camp loaded 850 on soft air tires while on my spiffed up race tired SV650, put me into scary half race mode in hi rpm lower gears to hang with him at his just sightseeing pace! Seriously, so disscussed his low looking rear tire to be told better traction cushy ride. This was going up to 75 mph in Mt tights so right in its sweet zone or torque zooms. A lifted HD that can lean some with a decent rider can strain anyone to keep up with too. Because of real life performance Commandos should last forever in real life use over the rest of the British vintage pickens.