Hi, i'm currently running a EP91 4efte in a 99 tercel and I want to upgrade boost to 20 psi and even more if possible. I know i will have to change the fuel pump and the turbo for a bigger one. Does someone know if i need something else? Are the internals of the engine strong enough to handle that level of boost? Cuz I heard the ep91 internals and head gaskets were less strong than previous generation. What pieces would i need to replace for better? And if somebody can tell me how much hp and torque it would be at 20, I would like to have that to.
You need to decide what you want to use the car for and when you want the power to be delivered. A CT9 is severely limited in total flow (HP potential) but can deliver that power from 2000rpm with a very early spool up, however due to the restrictive nature of the turbo it will run out of puff up high in the rev range, 5500+rpm. On the other side of the coin a much larger turbo such as a td05 will deliver much more power potentially, however it won't really happen till 4000+rpm and will continue on to rev limit.
Power figures don't tell the full story, you can have a VERY powerful car with a peaky power band that will get thrashed in a drag race by something with very little power but with it delivered across a broad range of engine speeds.
Case and point is finx, running a standard EP91 CT9 turbo (same one you're running presumably) on virtually standard boost (9psi) running a 13.2 1/4 mile compared to many other cars on here and other forums with some huge power figures that run similar or worse times. His car doesn't have huge power (no more than 150whp or 170bhp) but can deliver that power across a very broad range, ideal for a street driven car.
For a daily driver I would suggest you to decide an approximate power figure and then select the smallest turbo that will achieve that power figure while retaining the best possible turbo spool and response. 20 psi on a ct9 will be VERY different to 20 psi on a Garrett GT3076R, boost pressure is a very poor measure of potential performance.