Howdy,
I tried searching for this, but could someone give me an explanation of compression ratios and their relationship with turbos? I have read that the smaller t25 isn't the best choice for the 9.5:1 DE. Why is that?
Thanks.

Turbo's and Nitrous need low compression.

Wel, in order to run higher boost, the CR needs to be lower, right? I'm just looking for an explanation of why and hopefully a t25 on a 9.5:1 CR DE will be used:D

My friend NOS loves high compression motors. Turbos will need low compression on pump gas to run high boost on the street. But if you could run 11 to 1 with a turbo and had a fuel with enough of a high octain rating you would have a fast car.

In the most basic sense, it all comes down to heat. The reason to do low compression is to help prevent detonation. Detonation can occur under a good number of circumstances, but one of the more common ones is in-cylinder temps. Detonation occurs after the spark, when most of the gases have been ignited and the charge has been mostly burned. When the temps get too hot, some of the endgases self-ignite. Because the temperature is so high, the endgases burn at the speed of sound, causing a shock front inside the cylinder. This shock front slams the cylinder walls and the head so hard you can actually hear it while standing outside the car, and even in the cabin... that's why some people call it "pinging", and it sounds like a metallic ringing noise. The shock wave hits so hard it will actually take tiny chunks out of your cylinder head, which is why it's imperative that if you ever hear pinging to turn down the boost or retard your timing, put higer octane gas in or believe it or not, increase your RPMs. By increasing your RPMs, you are speeding up the ignition process and not allowing enough time for the endgases to ignite.

So as far as what compression has to do with it, in this case "compression" means exactly that: the stock 9.5:1 ratio of the DE indicates how much the air is being compressed. The harder it gets compressed, the hotter the temperatures get.

So reducing the compression ratio to 8.5:1 will effectively reduce the in-cylinder temps, helping to prevent detonation. This is why intercooling is so important, because the intercooler cools down air that comes through the intake manifold through the valves into the cylinder. And once again, when you have cooler air to start with, your in-cylinder temps are lower.

This is also why not to run a top-mount intercooler without cutting a hole in your hood. Without fresh air, the top mount IC just takes in ambient engine temps (far hotter than outside air, obviously) and pushes it right into your engine, increasing cylinder temps and therefore the chance for detonation to occur.

I thought detonation occured before the piston reaches TDC? That's why it's hard on the bearings...right? If that were the case, wouldn't this be before the plug fires? I'm confused here.
Travis

Your plug fires before the piston is at the top. That's why you adjust your BTDC (before top dead center timing). When you advance it, you're telling the plug to fire farther before when the piston reachs TDC. Huh, guess that's why you get detonation when you advance too much... never thought of that before. But yeah, that's how it works I think.

Khiem