The calculation results are summarized. Currently I am focusing on connecting rod reliability, and especially on the connecting rod lower end bearing. My plan is to use the standard Triumph connecting rods and to swap them out for new ones every ten runs. My target rpm is 7,500 and red line is 8,400 rpm. The percentages are all of the options compared to the inertia loads from the 790 cc pistons at 7,500 rpm. I know the engine condition at this rpm with the 790 pistons.
First, I look at the 865 cc cast pistons. They weigh the same as the standard 790 cc pistons and they do not increase inertial loads. The loads at the 8,400 rpm red line are 26 percent higher than they are at the target rpm. I am comfortable with this setup and the boxes are highlighed in orange. The white bearings and chamfered oil holes will make it reliable.
Now, I examine the 865 cc forged pistons. They are slightly heavier than the cast ones. The inertia loads at 8,500 rpm is 29 percent higher than the 790 cc pistons at the target rpm. I will reprogram the rev limiter to lower the red line 100 rpm to 8,300. Now the maximum inertia loads will be the same as the 790 cc engine.
My 947 cc stroker crank idea using the 865 cc forged pistons is next. The inertial loads are far higher. This is a dangerous setup and the boxes are highlighted in red. There is a possibility of broken rods, pistons, etc. I do not want to go there. Some of this higher inertia is due to the piston accelerating harder to travel the longer stroke. The rod length to stroke ratio of the stroked engine is 123 mm / 74.4 mm = 1.65 This is lower than the standard engine rod length to stroke ratio of 123 mm / 68 mm = 1.81 Engines with low ratios tend to have high piston acceleration rates and other characteristics that are not good for a LSR motor. I am not literate enough to explain this. See
http://ftlracing.com/tech/engine/rsratio.html The stroker crank idea will not become reality.
Now the 994 cc big bore kit. This option provides a lot of extra displacement with minimal increases in inertia loads. There is a 9 percent increase at the 7,500 rpm target. I can set the rev limiter to 8,000 rpm. This will limit the inertia load at red line to 25 percent higher than the 790 cc pistons at 7,500 rpm. I can live easily with this lower redline in exchange for a lot more power. I like these big jugs. This will be a future hop-up if I find the money.
My 750 cc small bore screamer is next. There is a small decrease in inertial loads as compared to the 790 cc setup. There will be a lot less power. This idea seemed good when I thought about it, but the calculations show otherwise.
A fellow racer said "wind that sucker up to 9,000 rpm and it will haul a__" The inertial loads at this rpm are much greater than at the lower engine speeds. Carillo rods will be needed, the engine will must be torn apart after every race for rod bearing inspection, and cam chain life will be short. Nine grand is an option for a dedicated racer with lots of time and money.
The white bearings, chamfered oil holes, relatively light 865 cc pistons, 7,500 rpm target, and 8,300 rpm red line will give me the rod big end bearing life that I need. Now it is time to figure out the fourth of five reliability issues, the rod little end.