The big difference between the r20 and z40 is the lobe centres. The lift and duration are very similar. The z40 has wider lobe centres and they are set at a greater crank angle. What this means is that the z40 is designed for fast road racing and by spreading the centres and closing the inlet valve later it will make all its power high up the rpm range. The r20 on the other hand opens both valves early and closes them early. And its design is very close to that used in speedway engines which need to pull from about 3000 right through.
This is with advance on the Z40. A few degrees advance on the z40 would be worth playing with. The opening times are similar but the z40 will close both valves about 5 dgrees earlier, which I don't mind at all.
On the other end of the spectrum, if you want to play with your r20 cam and run it a few degrees retarded, you will keep the inlet and exhaust open just a little longer yet have same opening times as the z40. If you have both these cams then there is a fair scope to experiment with.
Sittin at my puta' late at night got carried away with the theory of it all.
So after you get done making the advance anyway you like, slotting would be easiest, you could run either cam and have very similar camming of each by switching to either end of your slot. The r20 is a few degrees fatter which like I said will give you a good range if you own both cams. Just have to make sure you have clearance for the lift and extra degrees of the r20 and you should be able to use both these cams in a racer and be able to quickly and easily select a combo to suit the day. I'm about to try this with an r20 and will be interested to see how she goes (being a bit fatter) degreed to the z40 centres.
Dax here led us to an some cam data on a German site a bit back. My main interest was the excell sheet itself. I cleared it of data and rewrote it in english. This is probably a breeze for anyone who uses excel, but that is not me! I am now entering data I know for a fact has all cams degreed properly and all mapped at zero lash using the same rocker ratio to be accurate for comparison. I am checking the accuracy of the timings I already have and trying to get more one by one, but it is slow and expensive! Be good to have all the diff. cams available for the same head types, or running same rocker ratio's, to add to the data. If anyone reading this is interested in mapping a cam in an agreed head would make this process faster. I was hand graphing cams before I found this. When I am done will gladly forward the sheet so we can all benefit. I have confirmed the z40 and r20 specs above are mapped in the same head so are viable for comparison to each other, but am yet to map both in a Lifan head to match my other data. In about a week I will though. Does anyone already know rocker ratio's for dif. honda heads compared to China?
So far, it seems the only head with different rockers is the 50 head, the rest, 70, DRP Ultra, Honda TB units, GPX, Lifan BVH all seem to use the same rockers. Along with XR75s, XR80s... Honda was shopping from the same parts bin for awhile it seems.
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