Ok, help me do the motor math. (Please just help me with the math and save your opinions on the overall concept/my goals for other threads... This is just a for-instance...)
Let's say I want to build a high compression 3.0L (0.5mm overbore) that runs on CA 91 pump gas, using an AEM FIC and 350-400cc injectors to retard the timing and make that safe.
The maximum amount of timing that the FIC is able to retard is -10 degrees. Stock compression is 10.2:1. What other inputs to the equation do I need to determine the relation between timing and compression? I am trying to determine the highest compression that can safely run assuming the FIC is performing the maximum amount of possible timing retardation.
I am capable of doing math (goodness knows we all have the spreadsheet for figuring out tire sizes that will appease our traction control) but I am asking what math to do.
I pulled 13:1 out of hat. Can a 3.0L 13:1 motor safely run on CA 91 pump gas with the timing 10 degrees behind stock? If not, how to determine the max safe compression?
What about if I plug in 100 oct instead of 91?
Thanks all in advance.
You are talking about static compression calculation. It is much more complex than that. Dynamic compression also takes into account cam profiles, etc that should be accounted for. Everything should be designed to work together as a well-thought-out package that takes into account your intended usage (street, track, dyno queen, etc).
For example, let's say you keep all other engine parameters the same and switch to a "high compression" piston. Unless you specifically request it, pretty much all conventional piston makers will give you a bit more valve relief to account for larger-diameter valves or camshafts that give you a bit more lift since they assume you're into that (you are, right if you're buying their aftermarket pistons). Therefore, right out of the box with an aftermarket piston designed for the stock compression ratio, you have a domed piston that protrudes more into the combustion space than the OEM piston with the same compression ratio!
If you want to increase CR, how do you do it? Increase the piston dome height further. However, this impedes combustion flame front propagation and therefore burn efficiency (and power). On top of that, you're pulling timing too. Not an ideal setup really, especially with the old 1980's combustion chamber technology in our NSX's.
Also, your engine tuning will have to account if you get a bad batch of gas. Putting in these safety factors on fuel and timing with such a radical compression ratio may ultimately lead to a poor-performing engine. Certainly wouldn't be great in the HP/$ ratio compared to other options.
Not trying to be a downer here and say not to do it, but if you do, certainly talk with others who have gone down this route to give you their feedback on driveability, etc. I honestly haven't driven a 13:1 CR NSX engine so I don't know. My experience comes from big-block 2-valve Chevy engines with wild cams, high CR, etc. If you wildly perturb one parameter, there is a tradeoff in something else.
My $0.02.
Dave
