In the late 60's we were working with a farmer from Minnesota with his G1000 LP tractor. Because it was a spark fired engine (504), we were not limited by the Lanova system of the diesel which seriously limited the rpm it could run. That system is great for low rpm and huge torque rise as they were known for.
This tractor used LP/alcohol combination along with a huge turbo. The engine would turn some scary rpm's but as long as the alcohol held up it ran great. I seem to recall it running well past 4000 rpm. Remember this was 40+ years ago, and he had an 8 mph speed limit. We did have to use double valve springs to prevent valve float.
Oh, by the way, he farmed with this tractor the year around, just never went past 2200 rpm or so, and did not use any acohol.