14B or T25...

[upshiftN]

Plain and simple, I have a stock fuel system and don't plan on upgrading it anytime soon if at all. What is better to have, a 14B with low miles or my T25 that came with the car at around 97K. I have a profec b and can set it to max psi, so what would be more powerful at around 15 psi and carry all the way through??? This is probably written somewhere, just can't seem to find it at the moment...

 

[kenamond]

Here's my take on "Knock 101"

When you compress a gas, it gets hotter. In your combustion chamber (CC) you get compression from three sources: 1) Boost 2) Piston sliding up the barrel and 3) spark detonating fuel/air. In the case of the 3rd source, the fuel/air starts burning at the spark and moves away like a growing bubble. The unburned air/fuel outside the bubble is getting compressed by the bubble.

Timing ties the location of the piston at any crank angle to the location where the spark goes off. If you advance your timing, the spark happens earlier, and the piston and bubble will both be compressing the remaining charge. If you retard the timing, the bubble might be growing after TDC and the piston starts dropping releiving some of the pressure on the unburned air/fuel.

Knock is when the air/fuel outside the bubble gets compressed so much (and heats up due to the compression just like all gasses do) that it burns on its own. So instead of a gradually growing bubble that smoothly burns all of the air/fuel mix, the bubble grows and all of the sudden, BANG! - the rest burns all of the sudden. This is bad for your motor.

The knock sensor is just a microphone screwed into your motor that "listens" for this knock. The ECU hears the knock and retards the timing.

EDIT: And if you are sending hot air into the CC to begin with (inefficient intercooler or running a turbo way out of its efficiency range) you start the air/fuel out closer to the ignition temperature, so you get knock easier. If you have an efficient intercooler or can otherwise cool the incoming air/fuel charge significantly, you can run much higher boost, but you'll eventually get knock. Then you move to higher octane fuel.