I almost put this in the technical section but changed my mind, as it has a technical element but unfortunately is somewhat hypothetical!
Yesterday I was Sitting in the shed looking at the engine of my Australian Crab wondering what to do to it next. As seems to be the case sometimes with most of us.
I have it in bits to inspect it mainly and to fix up some damage to the gearbox due to the previous owner! Mumble mumble curse curse!!
Anyway I managed to get hold of a good set of S carbs and a set of roller rockers recently. Being a power hungry fellow and considering I intend to give the old girl a lick of red white and black go faster paint, thought I might need a couple more horses in there.
So I started thinking about a suitable cam. Then moved on to thinking about the adjustable cam gear and timing chain setups you can get and then it occurred to me.
What about Vairiable cam timing.
A quick search online revealed a plethora of types etc etc. Having had a look at the modern varieties and what the basic principles are I was a little disapointed to note that it is generally only effective on twin cam engines. Apparently this is so you can vary inlet against exhaust valve timing and get the most out of the different engine speeds. They also vary valve lift but I wasn't considering this.
So as a few of you have used various cams and done some dyno testing, what if any gains might there be if you vary the basic cam timing at various the engine speeds either as two stage or variable adjustment?
The only practical VVT that I am aware of are the vernier adjustable timing gears here. They basically shift the power up and down the rev range. Andrew A here has one on his car and they have to be adjusted manually - not a 5 minute job. A true VVT is complex and expensive and would take a lot of ingenuity to adapt to a BMC B series engine, maybe impossible. If you had lots of bucks you could possibly have a system with no camshaft example but again it would be a serious challenge and would also need to be run by computer!
If you have 2 HS6 Carbs and roller rockers (very expensive) you should look at Andrew A's experience with the 285 and 270 cam. You need to decide how you are going to use the car. The standard cam gives a good balance with plenty of torgue from low speed to around 80mph if tuned properly - and many are not. The 1800S cam (16,56,51,21) which is milder than the 270 really only come into its own above 70mph and loses around town with higher fuel consumption and less top gear flexibility.
If you haven't had a look at the Crab tuning section here have a read, it may help.
I think it would be doable to shift the cam position by up to a couple of degrees Maybe 3. I would use a tensionser on the opposite side to the normal position that shifts the chain across and thus adjusts by a small amount against a modified standard tensioner. The chain would run around a curve instead of straight as the tensioner shifts. Then use the vernier type set up as you said to get the base timing right. All depends on how much I would need to move it! I think if 3-4 degrees is enough that would be possible and might be worth a go. I will have a look at the tuning stuff. Like I said hypothetical at the moment. Im after a best of both worlds setup.
I got a set of roller rockers at a good price amongst some othe bits.
The cam rotation change from the early Mk1 to Mk2 /3 cars was 5 degrees to move the power up the RPM range, a similar change in the opposite direction was made to MGBs to give better low down power as emission equipment was added in the USA.
You would need some automatic mechanism to adjust the cam rotation relative to the crank as the RPM increases and this would probably require different ignition timing and fuelling to get the best out of it.
I have a bit of experience with microcontrollers and have done a couple of projects that drive stepper motors. I was thinking of a stepper motor to operate the cam adjusters. I did make a start on turning a distributor into an crank angle sensor. I was going to use the ignition coil packs from a ford that go direct onto the spark plugs and do away with the coil and leads altogether. I haven't got very far with that though. This could just be intergrated with that. It is very time consuming unfortunately and all sorts of stuff comes up that gets in the way of me fiddling about in the shed with it!!
When I work out what on earth I did with the modified distributor I will post a picture!
Ok so having had a look at the cam gears and chain it is clear that only 2 ish degrees is possible with the original chain set up as I suspected.
Several problems arise from the original setup.
1- The space between the cam gear and crank gear is very limiting.
2- The lack of space between the double chain and the plate.
3- The lack of space under the original cover.
So the first problem can be solved by running the chain around idlers either side that retract into the space between the two gears. Then when cam timing adjustment is commanded it moves the chain out past its normally straight route and on the opposite side in, which compensates for chain length. I will draw it to give you a better idea what I mean but not today! So by moving one side out and one side in this will adjust the relative position of the cam and crank gears on the chain and thus the cam timing. This setup will also be able to set the tension and the oil fed device won't be required.
Second the lack of gap between chain and plate will either require adaptation of the plate, (likely required anyway) or by using a single chain. For the moment I would like to stick will double chain as I have only seen the vernier adjusters in the double chain. These I think will be required for setup!
Third the cover is going to need to be remaid no matter what I think, so the only posible mod is to shim the pullys for extra space.
I have descovered the chain is British Standard 06B-2 chain 3/8 pitch and removable links are available for it. A couple of which I have ordered from Mr E bay.
If you look at the torque curve for the Mk1 1800 (sorry don't have Mk11) and Andrew A's 270 cam after tweaking the carb needle and programming the ignition timing you will see both exhibit fairly flat torque cuves. Mk1 curve 270 cam
By altering the cam timing as you describe you will be moving this flattish curve up and down the rpm range I guess by somewhere between 500-1000 rpm so in theory more at the top end without losing the low down torque.
I think to actually achieve more top end power you may also need to consider changing the cam profile, compression ratio and maybe moving to twin carbs, quite a project but fascinating.