Rotary Valves 159 vs 147

[louis914]

Greetings,

I have a question. In a highly modified racing engine, what difference would it make using a 147 vs a 159 rotary valve. Timing them both the same 65 degrees closing. Does one make more torque and one make more top end power? I have the 159 in this engine now, timed at 65 closing, and it revs up and over 7800rpm, making great power, but the low end throttle response is not as good as I had hoped. It is not in the carbs, as the carbs are very sensitive to adjustments, and I would say I have them very close.

thanks

[Bill O'Neal]

Louis,
Is this your modified 650 based motor? Are you using a boozoka pipe? If so, the pipe is a top end rpm type pipe and the way to increase the holeshot lies in the impellor and nozzel combo, although using the 147 degree valve will add to the bottom end torque, taking away a bit from the top end power.

[louis914]

Bill,

Yes this is the 650 / 800 stroker, and I am using the bazooka pipe and manifold. I have 2 of these motors, and just changed them out, and the only difference is the porting in the cylinders, and the RV (this one has the 159) The other motor had better hole shot, but this one screams, I mean screams top end almost 400 more RPM with the same prop and nozzle, same 44 BRM carbs, etc...

This motor also has all 3 exhaust ports open in the cylinders. DOn't know the effect of that.

The power is clean, and smooth, just weaker in the bottom end, it dosent just leap out of the water like the other one did, you kind of have to pull the throttle slowly.

Would you reccommend the 147 valve for a try?

Thanks

Louis

[louis914]

I just took the cover off to have a look, and I don't know how I did it, but I had the RV timed about 5 degrees retarded. I dont know if this is a big deal or not. I will wait on your opinion of the 147 valve before I put it back together.

Like I said the top end is Mega... I GPS'd it in chop yesterday at 61mph, and this is the carbon hull boat, that is not known for top speed. I would gladly trade 2 or 3 mph for the torque back.

I don't want to open the nozzle, or go down with the prop pitch, as the RPM is already so high, I can't see turning it over 8000.

I also have the compression dropped in these motors down to 175psi, @ .065" squish. I know this is not ideal with that pipe, for making torque, but the other motor was setup the same, and it made great torque.

Thanks again

[Bill O'Neal2]

My guess is the exhaust port timing is much higher on this motor, taking away a bunch of bottomend torque. I really do not think installing a 147 valve will get much of it back. You can try it though.