Project 68mm – Pt4

Well here we go again, the new RE274 cam installed. The cam itself is 274 Duration on the inlet and 270 on the exhaust, but it may come out a few degrees more with the 1.5 Ratio rockers. My Re13 cam is 276 duration on the inlet and 236 @ .050″ The 274 cam is 274 duration and 248 @ .050″ The exhaust lobe is 270 Duration and it is 244 @ .050. This grind really needs to be ground on a wide lobe cam (1275 type). So you can see that it is a couple of degrees smaller @ Advertised duration but it is a lot bigger from .050″ onwards, So you get the advantage of a smaller duration cam, but bigger duration once you get it open to fill the cylinder. So in theory we should get the advantage of a small cam for low down torque, but the bigger duration once the valve is open. So it is going to help carry the torque further through the RPM and should make more power in the mid to upper RPM range.
As we saw the last time the big head lost horse power and torque through the range as the head was just a little too big for the RE266SS cam. So hopefully the RE274 cam will bring the head to life.


Click image to enlarge

After running the cam in we gave the string a pull and things looked very promising, a little fiddle with the timing saw it use a couple of degrees more advance.Then back to the carbie, I went up one jet bigger in the main, I didn’t loose a thing down low but it improved the top end. As you can see by the dyno sheet the little motor made really good horsepower, but I had been hoping to make around 125HP and at least a solid 100ft/lb of torque. But it wasn’t to be.


Click image to enlarge


Click image to enlarge

The down side to this cam is that we lost all the low down bottom end. Where the RE266SS would take full throttle at 1500RPM with the smaller head, The RE274 cam and bigger head I was struggling to get a solid reading down at 3500RPM. This doesn’t mean that it won’t pull away from 2000 RPM at part throttle though.
So if you are going to build a small bore motor (the little ones are great fun) Think about what you are going to use it for and what RPM you are to work in. Have a look at what we have done to achieve the results with this motor. With the RE274, it is going to be a little lumpy in the small bore motor, but if you want a lively cam that has to be driven hard and gives good midrange to top end, then this could be the cam for you.
This cam in a big bore would be a lot tamer. As you go up in capacity, it will make the cam a lot smoother. Another alternative would be to grind the exhaust lobe 270 duration on both inlet and exhaust. This gives you something in between the RE13 and the RE274.
Which ever way you go you are going to have a lot of fun with these little motors, and they will be more than capable of keeping up with the traffic.
One other thing that happened while dynoing, while testing the RE274 cam, the motor started to get a little miss, mainly in the higher RPM. After trying the plugs, points and jetting it was time to change the head gasket and have a look at the head. The gasket looked ok, so lets look at the ports. No sign of water getting in the chambers, all looked ok, just carboned up. All the same colour, so a new head gasket and try it again. The miss was still there, turn off the motor, scratch head, give it a rev to 8000RPM and see what happens.The miss was alot worse. Time to check the tappets. Taking the rocker cover off, it was very milky, try to turn the motor over, locked solid…. Pulled no.1 spark plug out and out comes water! Not Happy! So the water was drained out of the motor, crank the motor over with no plugs to get rid of the water. Plugs back in and a quick rev to 6000RPM on full load, no miss. It had cleared all the water out of the cylinder. The motor was taken off the dyno for a customer motor.
Aaron the internet lemming, comes around the next day and pulls the head off and on very close inspection, right in the corner of the combustion camber, a tiny little hole was found. It was getting bigger the more it was run. This is the problem with 50 year old cylinder heads, they rust through from the inside out and we wind up being in this situation.


Click image to enlarge

So without the water leak, we could have seen more torque and horsepower if we had kept playing around. So we will try and fix the head and have another go. I also have another cam to try, it will be TOO BIG for the road, but I like to play with new cam designs. I still have a lot to learn about cams and designing new profiles. Just gotta keep playing.
We will be back after time for another cuppa and a bikkie.

Pt5, I think we will try going back to the 266SS cam and the 998 head that we modified in this article. Stay Tuned….