Here is what i have learned from countless de+t setups. All 9.5:1 and 10:1 motors btw.
1. Especially when your running a small turbo like the t25, i have learned that no matter how good your a/f ratio is. When you get to the point where your running out of breath on the turbo and again this can be affected by many things on the setup, intake mani, cams, exhaust size and so on. You can only push it soo far. If thats 11 psi and its making crazy good power then leave it at that level. I noticed with my T25 that it ran almost as good at 9-10psi as it did when i had it hitting 17 and dropping to 14 by redline.
Why? Because for the simple reason of backpressure. It builds a lot of heat inside the cylinder and on the tops of the pistons because during the exhaust stroke its not getting out all the heat because of excess backpressure.
2. So if the car is making really good power at 10 psi and not detonating then leave it there. You wont run into these problems with larger turbos. You need something to cool off the pistons and intake charge significanly to keep detonation from happening. The pistons get hot very fast on the smaller turbos when pushed to their limits. The manifold and turbo can only flow soo much power through them before it just becomes overkill for it which only causes problems.
That is where you have to be very careful. I went through many motors before figuring this out. A t25 in my opinion with everything done to it is only safe and reliable to roughly 250whp or so on the de motors. Anything more than that you are risking detonation, again i dont care how good the tune is. Like you said Ben, you are at a good a/f ratio and still possibly detonating. Thats because your probably right there at the limit of what that turbo and manifold can flow together. I bet if you were to take backpressure readings off your mani, they would be quite high at your peak power.
You dont run into these problems on bigger turbos for a couple reasons. One being you shift your powerband up a couple thousand rpms, Instead of it making full boost at 2700-3k your making your peak torque at 5500 (full boost) depending on size of turbo. This unless your tune is way off and you run it lean as hell is very hard to get detonation on. The faster the piston moves the less likely it is to detonate.
Second thing larger turbos do is they flow a ton better than the smaller setups. Especially paired with a good tubular manifold and good wastegate. You will have very very little backpressure.
This is why i gave up on the smaller turbos. They are responsive as hell down low but i wanted more power and to do that reliably you need a larger turbo. When you start pushing the limits is where you run into problems. All my motors that i melted a piston on, cracked ring lands on, my a/f was spot on and even on the richer side. 11.3-5:1 all the time under WOT. Didnt matter. Too much boost and power for the setup. i think what occurs is when your backpressure starts getting above the boost pressure your running is when you run into problems with heat soaking of the pistons. And again this is instant. When you have more boost than backpressure, during your overlap period you have nice cool fresh air that also pushes heat out of the cylinders as well. Now if you have more backpressure than boost your will be left with a ton of heat still left in the cylinders and causes hotspots which will again lead to detonation.
Sorry for the long post but just sharing my experience if it helps someone out then cool if not oh well. haha
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Originally Posted by nismo94tuner 
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