More thoughts on the RGM 920 and related stuff
As I recall, the original question was an inquiry into the efficacy of the RGM 920 kit. I apologize for getting side tracked in my previous post.
The last time I talked to Roger at RGM he told me that his pistons were flat topped and, when installed, the ‘crown’ or ‘flat top’ of the piston came to more than .060” below the top of the cylinder barrel. I would be inclined to phone Roger and see if anything has changed since I talked to him several years ago. With a standard head gasket thickness of .030 and a cylinder head volume of 50cc you will get about 8.2:1 compression. I would check your head volume. This can be easily done with some 5W oil and a graduated cylinder. When you do this I recommend that you do an independent measurement at least 3 to 4 times to get a sense of your repeatability and hence accuracy. A Norton head is typically around 50 to 55cc. Again, I will email you the spread sheet to aid in your measurement and calculations.
I, personally, like to get at least 9.5:1 compression. I will have to go through my notes to find a reasonable estimate of the relationship of Compression Ratio to Torque. As I recall a 15% increase in compression will net you less that 40% of 15% in torque. (~ 6%) I will get back to you on that. As an aside, I am currently on an extended trip through the Middle East and am currently writing this from Muscat, Oman, and I do not have access to my files where I have squirreled away such gems of information.
An obvious fix to the Compression Ratio issue is to have the base of the barrels planed before the new barrel sleeves are installed. The only issue here is valve clearance. Use plasticine to verify that you have a minimum of .060” valve to piston clearance.
My next set of pistons will be some type of protruding crown design. I am going to work with Johnny Rocket on the specifics.
Back to the question at hand. The RGM kit would be OK if you don’t mind a lower Compression Ratio. Roger produces many fine Norton performance upgrades and I don’t have big concerns on his quality. (my experience only)
If I were to offer my opinion, or tell you what I would do if I were going to start over; I would buy the spun cast sleeves from RGM and have my own pistons designed.
Allow me to digress once again and tell you of my first 920 kit. As itemized in my previous post it was a Norvil kit with a big valve head conversion. I did the standard items of re-inforcing the left-hand crankcase bearing journal with a shrink fit piece of 3/8 T6 aluminum and went to a belt drive system to improve transmission life with that extra torque. I bought some 1-1/2” pipes from Kenny Dreer (his old stock) and fitted 1-1/2” std style pea-shooter silencers.
Although I had more bottom end torque, the overall experience was quite disappointing. (I will send you the Dyno charts, if you are interested.) I had nearly a ‘dead-flat’ 50 ft-lb torque curve from 3,000 rpm to 6,500 rpm. This was not the excitement that I had shelled out all those dollars for! When I changed out to a custom build 1-3/4 exhast system the performance change was dramatic!! With a straight through after market silencer on this system I gained a full 10 ft-lbs of torque at beyond 4,500 rpm. Talk about a seat-of the pants difference!!
So the moral of the story is that, even if you increase the displacement, if flow-through resonance characteristics are not favorable, you will likely be disappointed in your results. In my experience, flow-through resonance is largely dependant on your exhaust system. My advice? Before you begin your project invest in a base line dyno test. After your build up, go back to the same dyno and test again. If you do not see the results you are looking for, DON’T GIVE UP!!!!
The answer is almost certainly in flow through resonance. You can change your intake length and diameter (difficult) or change your exhaust system (relatively easy). By the way, I have made 3 different exhaust systems by buying Mandrel bends from the local auto performance shop and cutting and welding till I get the shape I’m looking for. Relatively easy to do, trust me…
Once again, I strongly suggest you invest in dyno tests. If you don’t you will always be playing a guessing game!
Cheers...