1993 1.8 Liter Impreza Turbo FAQ

Disclaimer: This article is written for information purposes only. I have done a lot of research on these subjects and though to my belief it may be true, I do not take responsibility for any misinformation. These are the most common questions I get and so I thought I would write a FAQ. If you have any questions or comments please fill out the feedback form.

How much boost will my Impreza handle?

This of course depends on the setup. I didn’t originally have an intercooler so I could only run about 7 pounds before hearing any kind of detonation. Realize this is on a 1.8L with 9.5:1 compression. Your results will vary depending on location. To keep it safe, I would recommend  keeping it at 5 pounds until you can get some sort of electronics or SAFC to change the fuel ratio and an intercooler to get the intake temperatures down.

How much Was it?

The initial setup for me ran about $1000 for everything described in the turbo build up page. Expect to spend about $1500 for everything if you shop around and you’ll be a little more safe. Or if you like, all the necessary parts are listed, call around. It’s said that speed cost money, how fast do you want to go?

How does a turbo work.

If you look at the picture, it shows 2 sections. The Turbine section is the part that connects to the exhaust. When you accelerate, the force of the exhaust spins the wheel. The wheel is connected by a center piece that spins the compressor wheel on the compressor section. That is the wheel that sucks air in and forces it through the compressor housing and ultimately into the motor.

How does a Wastegate work?

A wastegate is a bypass for the exhaust. When boost reaches a predetermined level, the wastegate opens up and allows the exhaust to go around the turbo instead of through it and thus maintaining boost at a desired level.

How does a Turbo Timer work?

All a turbo timer does is tell your motor to run for a set amount of time before shutting off. So if you pull into work, you can take the key out, lock the door, and your motor will still run for a little while before turning off. What this allows is for the oil to continue to flow through the turbo and prevent “cokeing.” In other words, the turbo gets red hot from working and if you just shut the motor off after a hard drive, the oil sits in a red hot turbo and just bubbles up which causes it to stick to the internals on the turbo and can damage it.

How does an SFC or Fuel Computer work?

SFC stands for Simple Fuel Computer. It’s a piggy back computer which means it wires into and works with the factory computer. What it allows you to do is adjust the amount of fuel going to the motor at a given RPM by increasing the duty cycle or the amount of time the injector sprays during a pulse. This allows you to keep a constant 14.7 to 1 (is that right?) air fuel ratio throughout the RPM range even with the added air from the turbo. The actual ratio you would want is more like 12 to 1 to be safe on a boosted engine.

What’s a RRFPR and how does it work?

A rising rate fuel pressure regulator or (RRFPR) raises the fuel pressure a predetermined amount based on the amount of boost. For example, a 4:1 setting would raise fuel pressure 4 pounds for every 1 pound of boost. What this does is force more fuel in through the injector during each pulse. Again this is related to solving the fuel starvation issue.

Won’t you blow your motor?

I usually answer this with an enthusiastic yes. When the turbo went on, the car had about 120,000 miles. Now it has 150,000. The motor will go either way, but my belief is with a relatively low amount of boost, and some proper precautions such as an intercooler, the motor will not be overly effected by it’s newly found life.

UPDATE: Sure enough, it blew. The motor had 153,000 miles when it went. Apparently I’ve only been running on 3 cylinders for a while now. When I lost another one, the car just wouldn’t start. I was able to get another motor though for $600 which only has 30,000 miles so it’s worth it. As for why it blew, I think that’s because of my wastegate problem. As stated earlier, it’s best to keep it at 5 pounds until you have the electronics to control the fuel starvation issue. I would spike at 10 pounds.

Update2: The 3rd engine is now in. Pulling a heavy trailer in 100+ degree heat over mountains is simply too much for a non properly intercooled and fueled 1.8L engine.

Can I tell a difference in power?

Yes. The first time I took it out after putting the turbo on, I had four people in the car. When I accelerated from a stop and around a corner, all four tires broke loose. Normally it would have just slowly inched forward. So yes, I can tell.

Will a turbo effect my gas mileage?

With five pounds of boost, not really. I can’t even tell a difference in gas mileage. With an SFC, yes it will because the whole point of an SFC is to increase the amount of fuel going to the motor.