Around 10:1 CR works in most engines for 50-60%. But as you can see, the range is wide on different engines. I ran a K-90 on 85%methanol/ 15%oil using 17:1 CR. It worked well. Easy on plugs and could run with all but the very fastest competitors.
Holy cow, is that trapped C/R? What size head button was needed for that?
Hi Terry, Yes, trapped. It's small. I will post a pic later.
I find what Andy said quite interesting. With no nitro your can make a lot of power with just methanol, by using a very high CR (which is normal with methanol only engines). Question for Andy: Was there any reason you stopped, other than the "very fastest competitors"?
I wanted to try 0% nitro, because some years ago I had a lively discussion on here with another well known boater. He contended that running a boat with no nitro would cause nothing but problems. Blown plugs, over heating, difficulty pulling away from the beach are some that I remember he mentioned. He had "been there done that". I had mentioned that I had successfully run 5% nitro in a Rossi 80 powered Mongoose back in 1990. He said that 5% nitro would make all the difference in the world. So when my son started racing and I wanted to move him up to a bigger class, I thought that would be a great opportunity to test out straight methanol and give him a de-tuned boat at the same time. I set up a K-90 in our SGX 80 and installed the 17:1 head at a really high clearance. Maybe about .030", so it really only gave maybe 14:1 CR. It ran fine. Not fast,but not bad. Lacked a little torque. Then we started dropping the head. Each time it got better and better. More rpm and more torque. We got down to .005" and left it there. We only used one plug for all testing and racing. It was a Novarossi #8 Turbo.
We took the boat to the 2012 Fall Nats and Jason raced it in F Hydro. He finished fourth over-all, even after dropping a heat from a buoy hit. We never blew the only plug we used in that engine and the head looks like it just came off the machine. After that, Jason was ready to go faster, so we set it back up for nitro. Right after that he got a girl friend, graduated High school, got married, joined the Army and is now stationed at Ft. Carson, Colorado. They are doing well.
What is nice about the 0% is that it is so easy to burn and easy to get high rpm with a long pipe length. It does not need excessive load on the shaft to burn the fuel. It will run nearly as fast on stock 1667 as 50% nitro will run on a pitched up 1667.
Would it take out plugs? Yes, I am sure it would. But I think the mixture would have to be set extremely lean to the point where the engine was way off on power, so it would be obvious when the mixture is too lean. Not the case with high nitro. It can take out plugs even sounding rich and making great power.
On the other hand! If we would have run the K-90 with the stock head which gives very low CR, I think about 6.5:1, there would have probably been all the problems that the other boater mentioned.
