2.5L Jeep TJ Turbo Project
“My TJ wants to be like Fred Williams’ Tubesock ”
It really does….I just want to help it live its dreams to the fullest. I am going to consider myself an enabler.
Im also going to be a bit realistic. As a busy dad there is precious little time to be spent in the garage building a Jeep. Any project has to start with research, even for simple projects. Say you want to add some speakers. You start with some basic research and spirals into the abyss of amazon reviews or 10 year old forum posts. Gah.
The subject of this project is a 1997 2.5L that I’ve racked up 50,000 miles while driving to work. Although not the most sought after of all the TJs, but they are cheap, plentiful and a great place to start out. This specimen came into my life as a rescue…I bought it slightly wrecked and fully intended to fix and sell it fairly soon. The problem is that it grew one me and I just kept driving it and its now at about 200,000 miles.
Im going to try adding a turbo to my poor old 2.5L AMC engine. Its tired and wheezy. It smells like an old lawn mower when it starts in the morning. It is only effective at turning gas into heat and noise. It needs a boost.
I like turbos and have read anything I could on turbo charging the 2.5L Jeep TJ. There are quite a few builds that have been done but the write ups are not super detailed. My goal is to keep the boost low and keep from blowing up the engine. The point is add some power on the (relatively) cheap.
A while back I started collecting a few bits to make this happen. The turbo was first part and I found a TON of Subaru take off turbos on ebay. Mine is from a 2000ish wrx. Is it the exact optimal size? I doubt it…but I really dont care. I estimate that its pretty close. The boxer engine is a 2.0L so the slower revving 2.5L should even out to a similar air flow. The inter-cooler from the same series of engine is fairly undesirable to the Subie crowd but looks to be appropriate for the Jeep.
I also bought a couple of weld flanges for the inlet and outlet to weld the exhaust to.
I wanted to do a quick sanity check on my EBay turbo. It’s a Mitsubishi TD04 from an early 2000’s WRX / Forester and really common. The Subaru engine is a modern 4 valve head design and revs a bit higher so the air flow should be equivalent for a two valve 2.5L spinning slower. I did some quick google searching for a compressor map and ran some hand calcs.
When you look at the compressor map you find the X and Y axis labels are air flow rate and pressure ratio. Pressure ratio is fairly easy, just take the estimated boost pressure you achieve and divide by the ambient air pressure. At sea level this is 14.7 psia. Im going to target a max boost number of 10 psi at 5000 rpm. This turns into 14.7+10 -> 24.7 psia. Your pressure ratio is then (24.7 psia / 14.7 psia) or 1.68:1
The next number you need is the air flow at a specific RPM. The units on the pressure map are in m3/s so we need to work towards that.
The pieces work like this:
A 2.5L Engine displacement converts to .0025 m3
1 revolution on a 4 cycle engine moves half of its total displacement.
That means 1 revolution moves .00125 m3 of air.
At 5000 revolutions per minute that is 83.3 revs per second
83.3 revs per second X .00125 m3 of air = 0.1042 m3/s
This number is the airflow rate at 100% volumetric efficiency. As it is this engine is probably only about 75-85% efficient but considering forced induction 100% is a fair low estimate.
This website is a good reference.
http://hpwizard.com/volumetric-efficiency.html
So we now have two numbers to plot on the compressor map
Pressure ratio = 1.68
Air Flow Rate = .01042 m3/s
That looks like this:
The intersection drops the operating point right in the most efficient region for this compressor. Keep in mind that this is one point (and a fuzzy estimate at that), but it’s on the chart and right in the sweet spot!
The next part of this build is to size some injectors and find a way to tune the whole system. As get that plan put together I’ll be sure to post what I find.