Please bare with me as I am a novice when it comes to engine dynamics. I am looking for opinions and/or advice. I am building a n/a motor. And fully understand that FI is the biggest bang for the buck, concerning HP/$$ figures but I have my own reasons for staying n/a. With that said, I had some questions and was hoping you all could help. Maybe you can send me in the right direction as to what or what no to do.

My concern lies in cylinder pressure and c/r; how it works, n/a vs fi..

What effect an aggressive cam will have?

And what you think/know the highest c/r that could be achieved from n/a??

There are quite a few things I'm worried about, detonation being at the top of the list and close behind is what type of fuel. I want to street the car so 116 octane is out of the question for me.

What I want to do is raise the c/r. That's essentially what FI is doing right? Most of you guys who add FI to stock motors are around 8-10 psi right? So that brings you pretty close to 18:1 for an Effective Compression Ratio (ECR). On the n/a side, if I have 14:1 h/c pistons, the ECR is 14:1? Does this mean I could "hypotheticaly" run 18 or 19:1 c/r via h/c pistons ?

Now on the other hand, those that are running 20+ psi with l/c 8.5:1 your talking an ECR of 20:1. How would that compare to running a c/r of 20:1 on an n/a application? And why are the HP figures so differnt if the ECR is so similar?

We're burning lots more air and fuel. You cant force enough air into your n/a engine to use any more fuel or make any more power. Unless your compress it. With FI.

14:1 will detonate prolly even at idle. The compression is still the same with FI, if its 8:1 your still compressing what the engine took in 8 times to 1. ( I cant really think of the right words to explain it over the internet) From what ive experienced, about 11:1 is the most ive seen work on 93 octane gas. E46 M3s run higher than 11:1. Running cams with a lot of overlap can help too, bleeds off some cylinder pressure at higher load, low rpm situations.

So.... 11:1 barely works with 93 octane. How in the hell does my supercharged, stock cam, 10.5:1 CR motor live on 91 octane then? :confused

we have RON98 in australia which is eqiverlant for memory to the MON93 stuff u have there . My S50b32 with 11.3:1 compression ratio runs fine on the RON98 stuff. from the few FI examples that i know of for the s50b32 run as high as 8psi with the high compression with no dramas

FI isn't really a compression thing, get's complicated but really all it does is effectivly give a larger cubic inch displacment for any engine by increasing air flow to something that would normaly breath that much air while being NA.
Even NA is kinda under FI already since atmospheric pressure is 14.7 PSI @ sea level

FI isn't really a compression thing, get's complicated but really all it does is effectivly give a larger cubic inch displacment for any engine by increasing air flow to something that would normaly breath that much air while being NA.
Even NA is kinda under FI already since atmospheric pressure is 14.7 PSI @ sea level

No. FI is nothing BUT compression. No. There is no "effective larger displacement" when talking FI. We have positive displacement superchargers, but that, again, is another topic.

Honestly, I think that the biggest bang for bucks staying NA is either the camshaft/ECM, eihter the complete rebuild of the whole engine to tune every components for a higher RPM efficiency range : beefier rods/piston/fasteners, reduced stroke, high comp pistons, high tension valve springs, solid lifters, aggressive cams, bigger valves, bigger ports, bigger and shorter headers, bigger exhaust line, bigger and shorter intake, bigger throttle body, bigger MAF, custom tune....but BMW already put that in your car and pretty much stretched it to the practical limits. I guess you'll barely feel the difference at all unless you build the engine to rev in the five digits range.
My .02$

Honestly, I think that the biggest bang for bucks staying NA is either the camshaft/ECM, eihter the complete rebuild of the whole engine to tune every components for a higher RPM efficiency range : beefier rods/piston/fasteners, reduced stroke, high comp pistons, high tension valve springs, solid lifters, aggressive cams, bigger valves, bigger ports, bigger and shorter headers, bigger exhaust line, bigger and shorter intake, bigger throttle body, bigger MAF, custom tune....but BMW already put that in your car and pretty much stretched it to the practical limits. I guess you'll barely feel the difference at all unless you build the engine to rev in the five digits range.
My .02$

I plan on rebuilding the entire engine; that will include a slight overbore, and of course some aggressive cams along with a SEMS. I would certainly hope there's a difference that I can feel, I would anticipate making somewhere very close to 400 hp at the wheels.

In reguards to c/r.... the s50b32 is 11.3. s54 is 11.5 and if memory serves me correct, there are those in the honda community that run 14:1.

How are they avoiding detonation???? on pump gas.....

we have RON98 in australia which is eqiverlant for memory to the MON93 stuff u have there . My S50b32 with 11.3:1 compression ratio runs fine on the RON98 stuff. from the few FI examples that i know of for the s50b32 run as high as 8psi with the high compression with no dramas


how could this compare to running 14:1 c/r on the same engine without FI?