Had a very good time testing the modified TRX 2.5 engine last weekend. Temp was in the lower 60’s I was well dressed for the weather and my hands did not hurt until latter in the day...
I was running the TRX 2.5 with the modified exhaust timing. Following Prestons suggestions I increased the exh another 5-deg to bump up from 150-deg to 165-deg. It’s my understanding that 165-deg exhaust timing on a nitro boat engine is still very low, but my intent was to cut and do some testing and to continue cutting until I started to notice a drop on the lower-end of the engine. My next mod will be to increase it to 175-deg.
The whole thing that started this was after I put a highly cut-back Prather S215 SS prop on my Nitro Aggressor cat hull. The engine was turning higher RPM’s –but the speeds did not seem to improve by much.
I put a wire tie in cooling line to cut some of the water cooling from the engine. I noticed that after I started reducing the water cooling the engine would continue to accelerate hard after it left the shore. Before that the RPM would go down after it got into the open water.
I moved the strut up and boy the engine looked like it got a second wind. The rooster tail was way up, the nose of the boat went down and it just flat out hauled a$$. I’ve never seen the boat go that fast. The engine was operating in a total different RPM range it was finally starting to sound like a real heat racing boat! The funny thing was that I was making very sharp corners to see if the engine would sag or slow down like before, but it just kept on humming like it was going straight!
I kept shortening the pipe a couple of times by 1/8”. I did not notice much change in RPM and it was going around the corners without missing a beat. Then I slid the pipe all the way down, till the coupler was on the elbow of my exhaust manifold and still it was running fast and it did not loose a beat in the corners. I ran out of fuel on that last run ( I sort of spaced out cause it was going so freaking fast! ). When I got the boat back to shore the plug looked brand new.
I’m running 33% Nitro fuel with 165-deg exhaust duration. I got a set of thin feeler gauges, still need to grind on them some more to check what my deck height is. It makes sense that I’m going to end up with a very short tuned pipe. Low- nitro and low exhaust numbers will tend to go that way…
I cut half and inch from the header last night to continue to shorten the exhaust to see if I can go over the edge. I’m still looking for my piece of soft silver solder that I use to measure my pipe length with, I will give out the numbers the next time out.
One interesting thing was that I swapped out my flex-shaft with another that has a 1/8 step-down stub-shaft. I had a Y535 prop on it and the engine was loaded more than with my hacked down Prather S215. I’m going to do some testing with some 1/8 props to see what happens with smaller higher-pitched propellers in the 32-35mm range.
I know that some of you guys are pushing 38-40mm props with your small-block riggers. This is a 24” cat and the hull does not ride as light as a rigger, remember that if you are comparing my numbers to yours. As a matter of fact I don’t see any development occurring on racing small-block boats for the exception of riggers and the couple of .12 monos that grim adapted from a .21 John Finch wild thing design. I’m sort of a lone-star in my quest to development a small block tunnel hull that can handle race water and break the 40 mph barrier. I hope that this will be the year that I reach my goal!
Thanks to all for the tips and tricks that have made me get much closer to my goal