Quote:
Originally Posted by ReDestructDead
Follow up question for you sir. Which setup would result in less stress to the engine: 12 psi pulley on 93 fuel with a lowered redline (6.8-7k), or stock pulley with flex fuel? I have done a lot of research to understand the changes that would occur but at the end of the day I am not a tuner so I would like some education from an expert. From what I understand however, there is an increase in torque on E85 resulting from increases ignition advance. However I don't seem to fully understand the relationship between stress in the combustion chamber, boost pressure, ignition advance, and torque. I understand that higher torque results from more expansion force from combustion, so is it a direct relationship between stress on the system and measured torque? Here is a theoretical example: let's say that two identical FA20s are tuned to both produce 200 lb-ft of torque at 7000 rpm, one with E85 at 10 psi and one with 93 at 12 psi. Are these engines experiencing identical levels of stress (putting aside the differences in combustion temperature and possibility of knock)? Or is the engine with a higher boost pressure subjected to more stress purely because of the volume of air it's compressing? I also understand that the valve system will be subjected to higher stress from the higher boost pressure, but I am curious about the stress that would be delivered to the rods and developed into torque in this particular case.
The final question that I have is regarding tuning - can the ignition advance and other parameters be safely decreased to reduce the power output on an E85 tune?
If anyone else outside of Mike has some good knowledge of this subject feel free to chime in - I'm an engineer and should be able to grasp whatever you present pretty well so the more detailed of an answer that anyone can provide the better.
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It depends on the impulse.
In the FA20's case, the E85 is easier, because the amount of **time** spent pushing the piston down during combustion is greater, rather than the amount of **force** against the piston. This is courtesy of the ignition timing advance, rather than exploding more gas with more air to get a bigger boom.
Another persoective: more timing advance allowed by ethanol allows you to harness more of the energy in the fuel used. You have the same energy density, because stoich hasn't changed, and your air intake hasnt changed. You are able to use more of it because you can advance timing without knocking. More boost = more fuel = bigger kaboom = more torque. However, because it's MORE boost, you (long story short) get LESS ignition timing, so while the boom is bigger, you are able to harness less of it.
Remember, engines don't deliver a constant level of torque like an electric motor, but rather, deliver pulses. These pulses are smoothed out by things like heavy pulleys, flywheels, clutches, transmissions, etc. E85 = longer pulse. More boost = stronger pulse. Net torque output = sum of the pulse, whether longer and/or stronger.
Hopefully that made sense. I can elaborate more. There's many other factors as well, but this is just a very surface level version.