Map Cycle 850cc 10.5/1 compression pistons

Fast Eddie

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Why not look into JS Motorsports rotating assembly ? I had the same goal as you...I went with Jim's Stage 1 kit...JS Kit included Carillo rods,light pistons,gapless rings his BSA grind cam and followers...all lightened and balanced to work properly, all of his gaskets and without a doubt it is the smoothest free reving Norton I have ever experienced ( I should mention that it is 40 over and the head was "breathed on" by Jim Comstock ) and all the engineering has been done for you...seriously it is a proven kit....just my 2 cents worth...good luck with your build and keep us informed
Can’t argue with that. I did the same in my 850. Then I put JS pistons and rods in the 920. The pistons are lighter than stock 750s. It is uncannily smooth, it shakes around at tickover but smooths out beautifully in the normal riding range.
 
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Combustion conditions are affected by the balance between comp. ratio, fuel/air mixture and ignition timing. If you raise the comp. ratio, you will need to adjust the other two. If I was running high comp., I would retard the ignition curve by a couple of degrees and jet to it - regardless of the cam. At 10 to 1 comp. , you should not get into a situation which is impossible to manage.
 
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In the worst case scenario, if your comp. ratio is too high and you cannot stop the motor from pinging by adjusting the timing and mixture, you can always make a base plate to raise the barrels by a couple of millimetres. But that situation is very unlikely to develop in your case.
 
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When you raise the comp. ratio, you usually use more fuel to keep the combustion temperatures down. - That is where the extra power comes from. If you retard the ignition, you lose a bit. Race cams waste fuel when the motor is not being used 'on-song'.
 
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I run the Mapcycle 8.5 (20 thou over (77.5mm) pistons and their standard length steel rods.
Had Mr Comstock rework my rh10 head with larger inlet valves,porting etc,also had him shave a bit off so i´m also at around 9.5 :1.
also webcam 312a cam with radiused lifters and had the crank balanced.
couldn´t be happier.
 

APRRSV

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I run the Mapcycle 8.5 (20 thou over (77.5mm) pistons and their standard length steel rods.
Had Mr Comstock rework my rh10 head with larger inlet valves,porting etc,also had him shave a bit off so i´m also at around 9.5 :1.
also webcam 312a cam with radiused lifters and had the crank balanced.
couldn´t be happier.
Thanks Blaisestation,
Do you find low end torque absent using the 312a cam?

Ed
 

APRRSV

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Thanks to all who took the time to comment. It's helping me agreat deal.

Ed
 
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UK unleaded will run very high CR, the Yamaha MX shop near me modify and prep two stroke race bikes for races all over the world. They run a forecourt mix at CR 16:1 at high rpm on their dyno to check. Not sure what they mix it with though, they would not say.
Decent modern fuel should handle high CR without any issue these days.
Build it carefully, and not worry about the CR and fuel so much.
 

gortnipper

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Torque is not as pronounced down low with a 312a with rasiused lifters as a stock motor. I think this is due to a few factors In my motor
  1. The cam itself comes on above 3500 rpm. It may not have LESS total torque below that, but certainly makes more above.
  2. My head is also flow ported by Mr. Comstock, and delivers much more to the motor
  3. I had my crank dynamically balanced
  4. The combination of all of this means the motor spins up much, much more quickly. This gives the perception of a bit of effortlessness from a stop.
 
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Ed,
gortnipper and i have very similar set ups and i would also say that the same applies for me as what he has already written.
the low down torque is certainly not absent just different,and it really kicks in around 3500 rpm.
This suits my style of riding and the areas that i ride in.
Maybe if i was confined to inner city riding with constant traffic lights i would consider the webcam 312 instead of the 312a.
but as it is i have no problems with stop starting or town riding.
good luck with your choice
 
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See this for more info
Note this is from 2012. I believe Jim Comstock's bike is now fitted with a stock cam which he said makes the greatest amount of midrange power. It also might offer the least amount of valve train wear when used hard. It will be interesting to see if the early cam wear problems go away.

Glen
 
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I wouldn't use more than 9:1 CR with a stock cam and street gas in hot weather. You can use more CR but beware of detonation.

The Webcam 312 is nearly the same as the Stock Commando cam with about the same duration but with approx .015" more lift.

The Webcam 312A cam is a hotter cam and close to the Axtell #3.

I've been asked what the diff is between the 312 cams and the JS cams. The JS0 cam is the same (lift and duration) as the 312 and the JS1 is the same as the 312A but the JS cams are configured to be used with 1-1/8" radiused lifters and have improved ramps to reduce wear and valve bounce.

Anytime you go to a hotter grind you give up a little bottom end to gain more top end.
 
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gortnipper

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Note this is from 2012. I believe Jim Comstock's bike is now fitted with a stock cam which he said makes the greatest amount of midrange power. It also might offer the least amount of valve train wear when used hard. It will be interesting to see if the early cam wear problems go away.

Glen
Yes, I was simply offering up the 312a comparison from 2015.
 
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The biggest improvement I gained with my 850 was when I fitted the close ratio box behind it. All you do to your motor to get the same level of improvement would probably cost the same as a decent 5 speed gearbox.. With a standard box the motor is more sluggish as you come up through the gears, - probably because of the heavy crank. But the heavy crank might be an advantage if you use it properly.. Five speed - four close and up high, with a low first would be good. Most Commando motors are probably already fast enough., especially if you raise the crank balance factor a bit. - You might believe that a blue-printed motor would be the fastest, but loose motors are often quicker. Good valve seating is important.
 
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What the cam does, can be greatly affected by the exhaust system, and where you set the exhaust valve opening point. I advanced my cam when I fitted the 2 into 1 exhaust. The pulse out of the exhaust port at each firing has to cause the whole system to resonate. Any back pressure can stifle the motor. So being loud is always a problem, if you want more go. My cam is an 850 with a bit more lift, but you can get almost any cam to give you performance - OR NOT.
With any motor, to get max performance, you need to optimise everything. With a different cam, it is wise to START with the maker's recommended timings. But be prepared to move the cam etc. as you test the bike.
With a road bike, you have the advantage that you can wheel it out the door and ride it. Race bikes are more expensive to tune. Some people believe in dynos, but I don't know the torque and top end values which give best performance with a close ratio box on the circuits on which I ride. When you practise on a race circuit, where your bike is quicker or slower is important., and it varies with the circuit.
 
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concours

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I used the Std. CR 850 pistons and this thinner head gasket to liven up my ‘74 a bit.
Works great.
 
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Whenever I have replaced a standard cam with a race cam in a motor, I have got more urge everywhere from bottom to top, but the power band has become more pronounced. If you are doing it with a road Commando, you might have to live with a bit more rush of power as the cam and exhaust system come on song. With a race bike which has megaphone exhausts, when the system starts to work, it can come on with a savage burst of power which can make the bike difficult to ride smoothly. With a 2 into 1 exhaust, the transition to coming on song is much more gentle - so you can do more with the bike if you are battling it out in the middle of a pack.
When I first started racing, I was a pretty reasonable rider. My 500cc short stroke Triton turned me into an instant dud, So I had to fight the cam and exhaust system. The bike ended up doing good lap times, but it was nasty. With megaphone exhausts, it was impossible. My friend built the bike and raced it at Bathurst in the 50s. It stuck him into a guard rail and broke his arm and leg. However when I raced the bike, I was never aware of it's previous history. I only found out what had happened many years later.
 
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