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Old 07-22-2010, 10:19 AM  
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Arrow Boosting 1990-2002 USDM Subarus: A Chronological Guide

I'll start from the very beginning as far back as I've ever touched an aftermarket boosted Subaru. We didn't need much in the way of electronics at all way back when. You still don't. All of this info pertains to the now older 90's-02 cars. It doesn't seem like 10+ years ago!

Some folks are making a simple thing way too complicated for the older cars. You don't really need "all the JDM components you can find." I need to put my simplistic view of swaps and the work that is being done out there. Basically, don't argue with me junior. Sit there and read and shut the hell up. This works for superchargers and turbos both. I used all of this same knowledge later with ethanol fuels. With no boost and all the fuel system flexibility, I was off and running down a different trail. The same limits apply but in different respects. Don't outrun your AFR with boost, just like you don't outrun your AFR with too much ethanol.

There's 2 kinds of boosted classic Subarus.

1.) 300hp and less that lives for 300k. There's a few still on the road but not many.
2.) 300hp+ that blows something up frequently (by that I mean yearly, not weekly...)

Most of you working on older scoobs that want to have a little fun are in category #1. That is who this is written for. If you just want to boost your older Subaru and have a little fun then this is a short and concise guide for you on using stock parts. Fab work and mild tuning does you right as opposed to a swap. There's not much tuning to do and you leave the electronics alone. If you like to build stuff and aren't such an electronics wizard, then you are why I wrote this. Stop throwing away good equipment. At the same time, noone ever told you how simple it is to use when done correctly. You don't need a WRX crossmember and never did. It's nice to have, but not required. $2k-ish on quality components and labor goes a long way.

Rules thou shalt not breaketh!

#1) Always pay attention to FUEL! These motors are very thirsty because they are oversquare like our powerful domestic gashogs.

A.) get a walbro 255
B.) An RRFPR is your friend!
C.) That's enough fuel mods to handle 8psi comfortably. Don't worry about big injectors and electronics because this is still just mild boosting. The airflow isn't there to need all that extra fuel. 565s are too big!

Fuel quality is a separate issue. Subarus run lots of timing, not lots of boost. That's why bolt-on turbos were still fast on stock vehicles--they used just marginally more air with just enough fuel from a fuel pressure bump. We augmented fuel and quelled knock with charge temp reduction and plugs. Subarus respond so well to better fuel quality because of their aggressive timing curves. That also gets them in trouble with knock so quickly.

I'm the spoiled bitch that hates using unleaded gas and noodles with his fuel . I noodle with the fuel specifically because it fixes problems like knock and lets me run aggressive timing. The other 99% of you that don't have that option have different tools at your disposal.

#2) Charge temp reduction is your best friend outside of fuel quality. Most of you are still on unleaded gas. Charge temp reduction is probably weapon #1. Intercooling is a requirement, not an option. On alcohol fuel it's a different story, but for the rest of you petrolheads it's a requirement. Get a good quality intercooler!

#3) Cold plugs and a slightly tighter gap. Removing more of that heat in the combustion chamber is a good thing. Colder plugs also help top end performance. Charge cooling, low boost, and a happy compressor lets you run lots of timing SAFELY on pump gas. Colder plugs remove heat from the combustion chamber and transfer it to the cooling system. You will notice your heater running a lot warmer when in use. Cold plugs may also be a little grumpy below freezing but just until it warms up. Just dropping one heat range can reduce combustion chamber temps that much.

#4) Don't shoot for sky-high boost levels. This means don't overwork a small compressor you found in a junkyard. Stay at 8psi or less and you'll always be happy and never left stranded. A VF turbo is probably the best bang for the buck on home made combos. These engines breathe better than you think, even on older heads. You are selling your combo short with a td04. They still run hot even on a modded older motor. A 2.5-3in DP also works wonderfully. If you want to build something from scratch, then do it right.

#5) Sorting electronics can be a pain, but then again you should see a Honda swap. Where do you go with all of the options and model changes? Sometimes, the answer is nowhere. Older cars are one of the vehicles you can do that with AND get away with it! Keep the MAF in front of the compressor and everything else falls into place. Fuelling is handled mechanically and timing is left alone. Don't run a stupid boost setting and you'll drive happily. Most new WRX owners jump too quickly into the electronics. There's actually a heck of a lot of flexibility in there already, even on OBD0! 300hp on a stock ECU is a no brainer IF you know the mechanical side of things that supports it.

#6) Do not pressurize the emissions lines! Those vent tubes from the valve covers and straight out of the crankcase need to see vacuum and only vacuum. The PCV valve is fine but everything else needs to see vacuum, preferably after running into a catch-can.

#7) Specifically for superchargers.
A) 180D of belt wrap
B) Use tensioners!
C) Make sure the belt alignment is spot on.
D) Make sure the supercharger alignment is spot on. (C+D are for longevity and maintenance sake.)
E) Always use a recirc valve.
F) Relocating the TB is worth it!
G) When you add an exhaust, get ready to add fuel. Boost may drop but they get even thirstier at WOT! Instead of 2-1 on the RRFPR, you might need 4-1 or 6-1 depending. There's enough wiggle room here for <50% ethanol blends, too.

8.) Mind the little things! When you change your fuel pump, change the filter sock, too! Don't boost an engine on old spark plugs. BTW, you will go through plugs marginally faster. 100k on plugs is "possible" but isn't smart. Older Subaru engines are awful about carbon. Tune up the car before you boost it with new filters, o2 sensors, and an oil change. Make sure you have a good timing belt and pumps.

9.) Don't use an N/A Borla header on a bolt-on turbo kit. They're ok on superchargers but EQ headers are always the way to go.

10.) A stage 1 clutch will always do fine for 8psi setups. Don't do 4wd burnouts or hard launches and your clutch will be fine. Too stiff of a clutch plate kills the hydraulic clutch cylinder quicker ($300+ part).

Boosted USDM combinations that work.

You can actually save some money here but strategy changes. A good welder is your friend. Swapping the crossmember and exhaust manifold can work on dual port engines, allowing for a motor upgrade later still under the older stock electronics. This guide covers all 1990-2002 Subarus.

Turbos--do it right and get a VF or 16g. The stock 2.0WRX turbos are too small. T3-T4s are almost too big for such low boost levels. t28s and 13ts are to small. VFs are the happiest, IMO.

Superchargers--don't go smaller than a 300hp capable blower. No M45s. Stick to M60, M90, 1L+ twinscrews, or smaller centrifugals. Rotrex's felt good, but a roots or a twin screw just feels better. All the same tuning rules still apply. Always use a recirc valve and relocate the throttle body for better response. 6-8psi will definitely do you right. Clearing the ABS with a sidemount is a pain. I went for the throat and got new P/S lines. Between us, poking the blower out of the hood just isn't a big enough fad yet but it's coming . I <3 u Spud!

1.8T--the little sleeper that could.

It used to be cheaper and easier to build a turbo system to fit these older cars. The 1.8s were high compression and dual port. High compression and boost is a different ballgame. 8psi with intercooling, cold plugs, and plenty of fuel was a hoot in AWD or FWD. It made about 200whp in a 2600lb chassis that was fun and quick. An SAFC or Field worked fine for bigger injectors. SDS secondary injectors were just two pots to adjust like a radio dial. You could pass emissions easier with secondary injectors instead of big primaries.

These motors like good fuel because of their aggressive stock timing curves and compression. Remember that spark timing and fuel play more of a role in power than boost on a Subaru. E85 didn't come into the picture until much, much later. SOHC heads, a 2.5L STI block, and plenty of fuel with still run on a stock 1.8L ECU.

Mounting a supercharger to this block and manifold is a PITA! Phase 2 heads and manifolds let one literally FALL on the car when done right.

2.2--single port.

More worth the effort to turbo than you might think. Just remember the above listed rules with low boost and proper tuning. $1500ish can do a lot and complete the entire system. The electronics are just as basic as the 1.8. 180whp is not out of the question. 12psi is workable because of the lower stock compression.

'98 DOHC

Same rules still for the electronics. The computer is still just dumb enough to let you get away with boost. Higher compression becomes a problem because of fuel quality that limits boost. 8psi is still plenty of fun that gets down the road. Timing retard is helpful for 8-12psi.


Same rules, still! Keep the MAF in front of the turbo and keep it fuelled. RRFPRs work great with low boost clear up until +01. This is also where supercharging gets easier. The manifold is the key part and lets you mount anything on the side of the motor. Clearance issues pop up like power steering lines, injector bodies, and ABS lines. Timing retard is helpful for 12psi.

00-01 SOHC MAP

THIS is where the ECU rules change and get progressively more complicated. 10:1 compression is pretty much "THAT" limit for boost and pump fuel. You also have to get around the MAP sensor that hates seeing boost. 8psi is again the happy zone for the motor. Timing retard will let you get to 12psi safely.

02+--why you shouldn't do it and you're on your own from here...

Simply put, 2002+ N/A engines have pistons that can't handle much boost. Those of you that have managed to get them to live are all under that strict 8psi limit and managed to fool the electronics just right. 4-6psi is about all you can really get at safely. That's less of a return for the effort. I got out of boosting older Subarus around 2002 because of it. Who wants to repair all those failures? It's best to just NOT boost these. That doesn't mean other avenues aren't open like E85 and nitrous.

I'll always advocate a fuel swap for 02+ N/A cars. They respond better to the E85 fuel change than older cars because of their higher compression and aggressive timing curves. Get a Walbro and rrfpr OR 440s and you're good for E85. Colder plugs does these right, too.

Beyond 8psi, the hairy edge.

Like taking chances? Good. Me too.

8-12psi is about as far as I ever went with old school scoobies and boost. 8psi is 50% more air, that's why it's so healthy on a high compression engine. You can also work below 8psi on stock injectors with very little work. Fuel quality and quantity becomes paramount at this level. 440s-565s or secondaries are required. This is how you break 300chp. 8psi or less is still a healthy 220-250chp and won't break your clutch or transmission. Everything I listed here also tunes for torque. 12psi can incinerate your clutch or break a diff. It's also the very limit of 91 pump gas on these older motors.

That's all I've got for now. I wanted to write all of this down before I forgot more of it.

Last edited by James555; 09-18-2017 at 02:27 AM..
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