Mark Series Cam profiles

[aggettd]

Does anyone have any opinions as to the advantages and disadvantages of going to the "hotter" cam profiles and timing used on the Plus Series bikes? Are they harder to start, peakier, is there any advantage to using them unless you change the compression ratio, combustion chamber shape, intake tract, carburettors? What kind of performance/economy advantage/disadvantage is seen when using them? Is there any other experience out there using other profiles? Recommendations?

I'm in the planning process of building up a spare motor, and if I have to have the lobes on my spare cam repaired, was considering going with something punchier, or more modern. Any comments?

Happy New Year to all, and best wishes for many happy miles in the saddle of your Douglas!

[Chris]

Hi aggettd     
        Many years ago when restoring my first Dragonfly I found a firm who said that they had a Douglas profile and could restore my camshafts. When they arrived they looked very good and I installed them. When I came to turn the engine over it would not turn and I found the reason to be that the valve springs were coil bound. Checking the camshafts I found that they had been built up and ground to a "Plus" profile. The lift was too high. I have never owned a Plus and cannot speak with authority but I assume it must have different valve geometry to cater for the higher lift. I also believe that the geometry is the same between the Mk. and the Dragonfly as I subsequently fitted Mk. camshafts as I never got the original cams back from the firm after I had sent them back.   Chris.

[trevorp]

Dave have another look at the size of the pushrods if its a Mk1 motor and the question is simple i don't think id start modifying a Mk 1 engine. If u look to start playing the Mk3 series engines would be a better prospect
i think somewhere there is an article about Mk3 sport camshafts with a s on them that were supposed to be the ants pants in its heyday
Freddie Dixon's design on the Mk3 was a vast improvement over the Mk 1 engine

[eddie]

Dave,
           Some years ago - just out of curiosity - I set up various cams with a clock gauge and degree disc to compare the profiles. The clock gauge was fitted with a flat pad to represent the cam follower. The results obtained were surprising in that the 'T', 'S' and Plus No. 3 cams all had the same main profile - only the base circle diameter was reduced to obtain the increase in lift. This, of course, meant that when working from the base circle, the valve would open earlier and close later - giving a wider, as well as higher opening - thus, just a small difference in base circle results in a noticeable power increase. From memory, I think the difference in base circle diameter was something like .040" from the 'T' to 'S' cam - with another .040" to the 'Plus No.3'.  The 'T' and 'S' cams both have the exhaust lobe leading the inlet by 108 degrees whereas on the No3 cam this is increased to 112 degrees.
      Something else that has to be taken into account is the fact that there is a multiplication on the lift resulting from the different lengths of the arms on the rocker (about 5:4?).
     When experimenting with higher lift cams, careful checks will have to be made with respect to; 1) Springs becoming coil bound. 2) Valve spring caps hitting the top of the valve guides. 3) The valves not hitting each other as a result of any increase in overlap with the new cam.
     If you decide to get your cams re profiled, let me know and I can come up with the original data regarding the profile of the 'T' cams (from the original drawings).

                          Regards,
                                     Eddie.

[aggettd]

Trevor:

Good advice. I've been gradually acquiring parts for a MK 3 motor. (Shipping has cost more than the parts themselves) I have crankcase with cams (serviceable, but could probably use some work, hence the question), oil pump, crankshaft (thanks to you), two sets of Japanese pistons( Honda CB750 and Suzuki GSX 650), barrels, heads (with a broken intake outer valve spring), two magnetos, and most of the timing case gears, timing chest cover, and assorted bits of the flywheel and clutch. It's a spare motor so I don't mind mucking about with it, including gas flowing the heads, upping the compression, etc.

I'm not in a big rush, so am just picking up bits and refurbishing them as time permits.

We're in the middle of a snow storm here. It's not too cold (-2 degrees) but we're in the middle of getting about 20 cm of snow, then perhaps some rain tomorrow. Perfect for pottering indoors.

Best regards,

[aggettd]

Eddie:

Your comments intrigued me. Even if I ignore the rocker arm ratio and take the valve lift to be equivalent to the intake cam lift (.25 in.) the curtain area of the fully open valve (the total area of the space between the valve and the seat at full open) would be equivalent to the cross-sectional area of an intact tract with a diameter of approximately 22 mm versus the current, stock 19 mm. If the rocker arm ratio is 5:4 then the matching intake tract diameter would be 24 mm. Were the intake ports larger in the Sport and Plus heads? Or was there some other magic at play in the Plus motors? I believe that one of the manuals I have (not here unfortunately) shows significantly different profiles and overlap for the Plus cams. Do you think these are optimum or would it be likely that I'd see better starting, tractability and performance with more modern profiles?

[eddie]

Dave,
         My comments regarding the cam profiles referred to the 'Plus' No 3 cam which had a less aggressive profile than the earlier cams. Evidently, with the earlier cams, it was almost impossible to keep the followers in contact with the cams at higher engine speeds (hence the triple valve springs and requirement to shim them up to exact fitted lengths). I haven't checked it, but your calculation of the curtain area against the port area may be correct but it doesn't take into account the fact that the valve is only open for a short duration of the full cycle - so would need to be somewhat larger.
     Whilst the larger ports of the 'Plus' engine give an increase in performance - what is more important is the shape of the port. When George Easton was developing his Mark based racer, he found that reworked heads from a Mark gave better performance than 'Plus' heads (because it was possible to rework them to a better shape).
    In my opinion, there was not a lot wrong with the early Mark engines - the Mark 1's had a reputation for rough running, but I put that down to the poor 'pool' petrol of the postwar years. The Mark 3's were generally sweet running engines that would 'sing their hearts out' all day - whether fitted with the sports silencers or a waffle box. Performance of the later Marks tended to be a bit of a lottery - which I feel was the result of the factory being short of cash and the tooling being allowed to get 'very tired' - and quality control being adversely affected. Also, some of the later 'improvements', with hindsight, proved to be a retrograde step.
                            Regards,
                                          Eddie.