Rex Sucks
Stage one of mods to AutoSpeed's MY94 Subaru WRX - the air intake...
By Michael Knowling
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It had to happen. After dropping a Japanese replacement engine into our MY94 Impreza WRX it's come time to start looking at ways to extract a little bit more power...
Where to Start
Basic modifications to a factory turbocar typically involve a high-flow air intake and exhaust, increased boost pressure, enhanced intercooling and often some kind of engine management tweak. The '94 WRX leaves a lot of room for improvement in each of these areas - the standard intake and exhaust are terribly restrictive, boost pressure can be safely upped a couple of psi, the top-mount air-to-air intercooler is effectively an 'interheater' and the ECU runs the engine bulk rich at maximum load.
So what should be the Number One modification?
I'm kicking off proceedings on the MY94 WRX by 'opening up' the air intake. Why? Well, a revised intake is an extremely cost-effective way to improve acceleration and throttle response, it comes with only a slight noise penalty and - using the approach outlined here - the changes are likely to go unnoticed during a quick police inspection.
A Look at the Factory MY94 - 96 Impreza WRX Air Intake
The '94 - 96 WRX uses essentially the same air intake system as the older Subaru Liberty (Legacy) RS. Air is drawn from within the inner guard cavity (between the metal inner guard and the plastic guard liner), through a resonant snorkel assembly, an airbox with a hot-wire airflow meter, another pre-turbo resonant chamber and - finally - into the turbo compressor.
Okay, so the standard intake is obviously fairly complex - but how restrictive is it and where does most of this restriction occur?
Using a manometer, we measured the pressure drops (ie restriction) at four different points along the factory intake path. (for reference, check out our series on 'Eliminating Negative Boost')The first position was in the base of the airbox (the atmospheric side), the second was in the top of the airbox, the third was the elbow joining the airflow meter to the pre-turbo resonant chamber and the fourth was just downstream of the pre-turbo resonant chamber. Here are the results, obtained on the road at wide-open throttle in second gear:
Item Peak restriction (inches of water)
Resonant snorkel assembly/lower half of airbox 36
Air filter element 1
Top half of airbox/airflow meter Approximately 9
Pre-turbo resonant chamber Approximately 5
Total intake restriction (measured at PCV junction prior to turbo) Approximately 51
Note - Restriction went off the scale of our 29 inches of water manometer at more than about 4500 rpm. Some of these figures were measured using a Magnehelic gauge that is scaled up to 40 inches of water while, for restrictions higher than this, figures were extrapolated from measurements made at slightly lower rpm.
As you can see, the resonant snorkel assembly and lower half of the airbox caused the vast majority of overall intake restriction. The reasons are pretty obvious once the intake is pulled apart; the snorkel pipes are just 58mm diameter and the draw-through resonant chamber causes massive flow turbulence. This photo shows the internals of the chamber - note the foam layer on the inside walls, which probably serves to reduce intake noise.
Interestingly, the standard air filter - which had already been used for a few thousand kilometres - posed only around 2 percent of the overall restriction. An aftermarket drop-in filter (with unquantified filtering performance) wasn't worth bothering about in this case.
The second most restrictive section of the system was the top of the airbox and the airflow meter. There are a couple of reasons why this section caused up to 9 inches of water pressure drop - the entry to the airflow meter is quite poor (butted up against the airbox wall) and the airflow meter is fitted with a wire mesh screen on one side.
The pre-turbo resonator caused a comparatively mild pressure drop - just 5 inches of water, or around 10 percent of the overall flow loss. With the chamber removed from the vehicle, you can look through it and see its inlet and outlet pipes aren't joined; the gap in the middle allows air to swirl around inside the body of the chamber. There are also two hose fittings that connect into the chamber - the feed for the idle air bypass system and return for air bled from the factory boost control solenoid.
Having comprehensively identified the airflow characteristics throughout the standard intake, the next critical part is to look at intake air temperature...
As mentioned, the factory intake system draws air from inside the inner guard cavity. But from where does the inner guard cavity find its air? Well, what appears to be wasted space between the intake snorkel and the body cutout is actually where the inner guard cavity sucks a lot of its air; that means the engine bay has preheated much of the air that enters the airbox. With a probe positioned in the lower section of the airbox, we learnt intake air temps were as high as 48 degrees Celsius while driving in stop-start traffic - and that was with an ambient temperature of 19 degrees Celsius! There was certainly some room to lower the intake temps.
The Modifications...
Instead of bolting in an aftermarket pod filter we've improved the performance of the factory intake arrangement. Yes, yes, a pod filter - which replaces the entire airbox assembly - does flow very well, but by the time you shield it from underbonnet heat and run airflow ducts, you're up for a lot of money and/or work. A pod filter is also an obvious aftermarket fitment that can cause problems when the police are combing over your car for defects. The AutoSpeed approach is to retain much of the standard under-bonnet scenery and keep things simple - it's a process any reader could do on a Sunday.
Our intake modifications focus on two areas - the highly restrictive snorkel into the airbox and the pre-turbo resonant chamber. Obviously, the airbox snorkel is where we can remove the biggest portion of the overall restriction and, well, the pre-turbo resonator is a relatively easy section to alter...
A New Intake Snorkel
The first step in this process is to jack the front right of the car, place it on chassis stands and remove the front wheel. Next, prise out the clips holding the lower section of plastic guard liner and remove the single retaining screw. Don't be too annoyed if - like us - you break the plastic clips trying to get them out; replacements are available from Subaru for a couple of dollars each.
Once you've manoeuvred the lower section of plastic guard liner from the vehicle you'll see the resonant snorkel. To remove the snorkel, you'll first need to take out the airbox assembly and undo the nut that secures the top of the snorkel to the body (the nut immediately forward of the cutout that feeds the airbox). Next, slide under the car and you'll see a second nut holding the lower half of the snorkel in place - undo it and you'll be able to rip the resonator from the vehicle. Put it in the bin.
Next comes creating our replacement snorkel...
Not wanting to cut or file any of the bodywork, we decided to feed the airbox using the full cross-sectional area of the cutout that the original snorkel passed through. Of course, the bigger we make the cross-sectional area of our new snorkel the less restriction. The approach we've taken involves a short length of 90mm plastic storm pipe (bought for the AU$5 minimum charge at our local hardware), a heat gun and a fair bit of patience...
Before any pipe work is done you must first outline where the body cutout lines up against the lower half of the airbox. Then, remove the lower half of the 'box from the car, drill a series of holes along the outline, punch the centre out and tidy the edges with a file. The box can now breathe.
Next, we set about moulding a short length of plastic pipe to the new shape of the airbox intake hole. Soften the end of the plastic pipe using the heat gun and - wearing gloves - hand mould its shape to match the airbox hole. Once you can fit the end into the airbox hole, very slowly pull the pipe through the airbox while focussing the heat gun around the outside of the pipe as it passes through the hole. This creates a snorkel moulded into the shape of the airbox hole and - assuming you did a good job with your earlier outline - matched to the adjacent body cutout. Note that it may also be necessary to reheat and push out any sections of pipe that have drooped or caved in.
If everything has gone well you should be able to bolt the lower half of the airbox into the car and slide the duct snugly through the airbox hole and the body cutout. If it doesn't want to pass through, you can always reshape the appropriate sections of the snorkel using the heat gun; lucky that plastic is so workable!
To ensure maximum airflow you should bell-mouth the pick-up end of the snorkel. This can be achieved by heating the end of the snorkel and rolling the edge outward using a spark plug socket. Note that - once the pick-up end is bell-mouthed - you can no longer push the snorkel through the from the airbox side out through the body cutout; you must now insert the snorkel from the opposite side.
To mount the snorkel we drilled a hole through both ends of a metal strip, bolted one end through the snorkel and - after bending our metal strip to the desired angle - bolted the other end through the small body hole that was originally used to secure the factory resonant snorkel. Oh, and you might want to give the new snorkel a hit of black paint to finish it off - not that it's very visible.
Now there is only one thing left to do - make sure the inner guard cavity can breath air (remember, thanks to our new snorkel, the inner guard cavity can't suck air from the engine bay anymore). Fortunately, the MY94 - 96 WRX has front brake cooling ducts that receive air from alongside the factory driving lights and direct it into the wheel arches. Because the ducts aren't attached to the back face of the discs they're of limited use and - therefore - we decided to let the intake snorkel steal air from driver's side duct. This was a simply a matter of cutting out a large section of the plastic that divides the brake cooling passage from the inner guard cavity. Make sure the section you remove has a cross-sectional area at least as large as the new intake snorkel.
Reinstall the plastic guard liner, drop the vehicle to the ground and - congratulations - the free-flowing intake into the airbox is complete.
Off-the-Shelf PVC Snorkels for Later Model Subies...
Michael South from MSE (Michael South Engineering) has assembled ready-to-go PVC cold air snorkels to suit MY97 - 00 WRXs plus the Liberty B4. MY97 - 00 WRX versions can be bought through the AutoSpeed Shop (at Subaru Cold Air Ducts) for AUD$99.99, while the later Rexy and B4 pipe costs just AUD$15. MY94 - 96 WRXs are not catered for.
A New Pre-Turbo Pipe
The second - and last - phase of our intake modification is in relation to the factory pre-turbo resonant chamber. This chamber is easily removed; loosen the clamps on nearby induction plumbing and the screws on the airbox lid and manoeuvre it out of the engine bay. Oh, and make sure you slide off the idle air bypass hose, unclip the throttle cable support and release the hose that connects to the boost pressure solenoid (which is hidden around the back of the chamber).
There are two approaches to replacing the pre-turbo resonant chamber - have a pipe custom fabricated or buy one off-the-shelf from MRT (Middleton Rally Team). We were initially going to go for a custom job, but the replacement pipe is deceptively complex - it cones down from 78 to 66mm OD, incorporates a very slight bend as well as the necessary fittings for the idle air bypass and boost control system. In the end, we elected to spend a very reasonable $60 (RRP plus freight) for the powder coated and ready-to-go MRT part.
Installation of the MRT replacement pipe is as straightforward as it gets - it's simply a reversal of the procedure to remove the factory part.
Going Further?
As mentioned, the lid of the standard '94 - 96 WRX airbox looks pretty bad from an airflow perspective so, therefore, we investigated swapping to the lid from a MY94-ish Subaru Liberty 2.2-litre. While it appears the Liberty lid will drop on perfectly, it doesn't fit snug against the lower half of the WRX airbox. Furthermore, metal clips are used to hold the Liberty's lid down rather than the screw retainers found in the WRX. Still, with some tricky custom fabrication - perhaps some plastic welding - it looks like the smoother flowing Liberty lid could be adapted to fit. It all depends how much time you've got...
You may notice that - unlike the previous intake mods we've made to the AutoSpeed Audi S4 and Holden VL turbo - we've left the wire mesh screen fitted to the WRX's airflow meter. This screen can easily be removed but, in the case of our high compression import engine, we were reluctant do anything that gave a 'random' change of ignition timing and air-fuel ratio. Removing the screens shouldn't cause any major concerns on a locally delivered engine, but - regardless - we'd advise checking timing and AFRs before an after.
Results
After half a day working under the bonnet, our intake mods have paid dividends. It only takes a few seconds of driving to notice the much improved throttle response, the added tractability at low rpm (the engine is now quite happy to accelerate away from 1500 rpm) and, yes, the extra all-out go. Hand timed 60 - 90 km/h splits (in second gear) show an improvement from 2.0 to 1.9-seconds, while 20 - 100 km/h runs fell from 6.1 to 5.9-seconds - not surprising considering total intake restriction has been slashed from around 51 to 21 inches of water (a terrific 59 percent improvement)... The highest intake temperature in the airbox is now in the high 20 degree Celsius range - a considerable drop from 48 degrees Celsius while driving in the same 19 degree ambient temperature.
So how much extra power are we talking here?
Well, on a Dyno Dynamics four-wheel-drive chassis, we conducted some before and after power runs. In this graph you can compare the standard vehicle (as shown by the red plot) versus the car with its modified intake (plotted in blue). Interestingly - despite the considerable improvement on the road - the new intake gave only a slight top-end power gain. In third gear, the stocker made 121kW at the wheels and our intake mods gave another 2-3kW at the wheels, or - put another way - around 2 percent more power. At this stage, we can only assume that excess exhaust backpressure is to blame for this disappointing measured improvement. In contrast, the biggest gains are seen in the lower half of the rev range, where the new intake delivers a solid 10 percent grunt.
So - out on the street - are there any trade-offs?
As mentioned, there is more induction noise under load but it's not obtrusive when the plastic guard liner is in place. We also noticed that the idle became slightly rougher immediately after the mods. This roughness might, however, disappear as the ECU learns in its closed-loop feedback mode - we'll keep you posted.
Bottom line; here's how to give your early WRX a very real on-road acceleration gain along with improved throttle response and tractability - all for less than $100.
Contact:
MRT (Middleton Rally Team)
+61 2 9809 2110
info@mrtrally.com.au
www.MRTrally.com.au
