Specifications

Horizontally opposed 6-cylinder engine

4 cam, 24 valve

96.9mm x 75mm (3.815" x 2.95")

230 HP @ 5400

228 lbs/ft @ 4400

10:1 compression ratio

Overview

The SVX engine is a 3.3 liter flat 6 engine that is virtually bulletproof. Imagine, it has seven (yes, count ‘em, seven) main bearings. All supporting a crankshaft that is only slightly longer than three cylinders. The main bearings are supported by a cross-bolted aluminum block. Although no one seems to be racing the SVX, IMHO, this engine has as much performance potential of any six-cylinder engine of a similar size that is currently available. The rods and crankshaft are forged steel and the pistons are forged aluminum. With a shorter intake port, a larger plenum, a longer exhaust header primary, and a higher rev limit, this engine is easily capable of making 300hp in a naturally aspirated state. Add a turbo to each bank of the engine, a pair of intercoolers, and lower the compression ratio and you could approach 550+ horsepower on pump gas.

Modifications

The first modification I recommend would be to create a complete dual exhaust from the individual cats to the rear of the car. This would eliminate the single secondary cat. The three cylinders on each bank of a flat-six engine fire evenly every 240 degrees of crankshaft travel. Since there is no overlap during this period, this eliminates the problem of one cylinder exhausting while another cylinder’s exhaust valve is still open. So keep the two sides completely separate. This will result in the best flow with no reversal. Don’t go too large on the pipe diameter, or you will slow the velocity of the exhaust and ruin the mid-range scavenge effect. I would recommend 2" exhaust as a maximum. Definitely do NOT use the ‘most advanced new’ method of using sewer pipe to conduct exhaust gas away from the vehicle. In some quest for the least amount of backpressure, these folk are missing the point that harnessing the velocity of the exhaust gases creates a scavenge affect that is better than zero backpressure.

The technically minded may be asking, "My V-8 Mustang runs a balance tube." or "Why not join the exhausts?". Remember that on a V-8, if you look at each bank as a separate engine, they don’t fire evenly (well Ferrari does, but that’s a different story). Anyway, in this case, the balance tube is presumed to have some ‘balancing effect’. My personal opinion is that if the tube is doing anything dramatic, it’s because it’s connected before the cats or mufflers. (Although there may be a discernable pressure drop caused by the venturi effect). In this case, an outgoing exhaust pulse actually sees twice the cat and muffler area, so there is less restriction. If you are planning to join the SVX exhaust with a balance tube, don’t. If you do it anyways, then do it before the cats or mufflers.

This is the cheapest way to make horsepower in your SVX, and it has the benefit of being removable. You should upgrade the exhaust as mentioned earlier to handle the increased amount of exhaust volume that Nitrous will provide. Racers many times forget that while the intake capacity needn’t change with nitrous, the exhaust side has to handle the additional airmass. A poor flowing exhaust combined with high cylinder pressures will cause your exhaust valves to glow in the dark. Do this for a few seconds and you will be buying another engine.

As far as kits, I like the NOS Sportsman Fogger. This is a complete port injection (not a spray bar) system. Use the 90 degree Soft Plume nozzles. They can easily be plumbed directly into the intake manifold plenum pointing directly at each port. This system, which costs less than $700 can easily produce over 150 extra horsepower. (I would NOT recommend this jet setting.) However, if you have done the gearing and exhaust, the lowest jetting will produce a reliable 65 HP. Combined with the gears, this should put an SVX in the mid 13 second range with 0-60 times well under 6 seconds. With all the money you saved over the Turbo or Supercharger system, you’ll be able to afford the inevitable driveline parts failures.

I have extensively researched putting a turbo on my own car and have definitely proven that you can mount both a single and a dual turbo setup in the car. The real problem is getting enough space for a decent intercooler. I would probably mount a single or dual setup the same way Mitsubishi does- under the bumper supports in the front fenders in front of the wheels. You’d have to cut holes into the front side of the fender covers to provide a vent to the low-pressure area in the wheel well. Since I can write extensively on the subject of turbos, I won’t.

I know every West Coast Honda racecar wannabe has had the throttle body bored out but forget it on the SVX. The twin throttle body flows over 900 CFM. That’s enough for about 675HP, so it doesn’t pose any restriction.

For modifications, the fuel pump is a monster (222lbs/hr) meaning it could support about 444 horsepower at .5 BSFC. Unfortunately, the injectors are a different story as they are 275cc jobs that are going to be maxed out at 250 HP. If you mount a turbo or do some other modification that would cause your injectors to be inadequate, you will need to upgrade the injectors. This also requires a re-tuning of the ECU. But, you can easily support nearly 500HP by using the following trick. Buy the Nissan Motorsports 555cc injectors. They are expensive, but they will bolt in. Replace the pipe from the air filter to the intake with two pipes; one for opening in the throttle body. Plumb the mass air sensor into one side so it only supplies one bank of the engine. You will have to supply a separate air filter to the other bank. You have now effectively halved the amount of air the ECU sees, while compensating by doubling the amount of fuel injected. This has the added benefit of reducing the intake pressure drop through both the meter and the (now) two airfilters.

I have done this trick on two other cars with success. However, I have not done this with the SVX. Some ECUs will compare the A/F readings against a known map and decide the air sensor is bad. Others (particularly the Ford EEC-IV) have no problem at all. I also did this with a Subaru RX Turbo with no problems. Best to disconnect the battery overnight before trying this trick to force the ECU to reset.

If this sounds like too much work, buy the Infiniti Q-45 370cc injectors. They are also a bolt in (and cheaper). Add the very slick Apex-i Super AFC fuel computer and decrease the air meter readings by about 30%. This will allow you to support 350HP. This also provides you with control over the entire fuel map.

Of course, unless you do something more than exhaust and intake, you’ll never need to do any of this.

Think about this before buying a cone filter that breathes hot engine air. Of course, they produce an extra 15hp. Why would the manufacturer lie?

Leading to the air filter cannister is a circuitous, plastic resonator housed in the right fender whose job is to quiet the induction system. Although I’m not convinced this causes any serious restriction to airflow, if you want a more aggressive intake sound, the resonator can be removed. If you do this, do NOT cut a hole in the bottom of the fender cover in the hopes that more air will fill the fender. A high-pressure area builds up in front of the radiator when the car is at speed. This air flows around the radiator into the fender cavities to create a source of high-pressure ambient air for induction. This is an old hot-rodders and is done on purpose by Subaru and other manufacturers. Air passing over a surface creates a low pressure area on the surface, so if you then put a hole in the bottom fender, you are in fact creating a low pressure area. This will actually pull air OUTof the fender, not into it. If you don’t believe the manufacturer knew what they were doing, go ahead and cut a hole, but fashion some sort of scoop facing the high-speed airstream.

If you’ve read the earlier Air Filter mod, you should know that horsepower is made by increasing air, not fuel. A chip simply cannot increase the amount of air that the engine ingests. A chip MAY be able to find some horsepower in cases where the A/F ratio or timing is not optimal.

If you put your particular car on a dyno, and worked hard enough you might extract a few more horsepower from your car at WOT by playing with a more aggressive timing map. Since every engine is a little different, if the map was conservative, there is some horsepower to be had. The manufacturer has to make some compromises to ensure that the engine doesn’t detonate and destroy itself. However, most late-model high performance engines use a knock sensor, so the maps are far more optimal now than they used to be.

Fuel is usually not a problem because most cars go quite rich under WOT, once again to help avoid detonation. Remember that extra fuel does NOT increase horsepower. Once you have a chemically stoichiometric ratio of 14.7:1, any more fuel remains unburned. Fuel is richened to help cool the intake charge, and to decrease any distribution problems that might result in mixtures leaner than stoichiometric.

Disreputable manufacturers will do nothing but put a nice sticker on your ECU or ROM chip and return it for $300. Even the reputable ones are only richening up the fuel mixture and adding a little timing. If the timing was a bit conservative for your vehicle, you might find a few HP with a chip. If not, you might find a little detonation. Paying $300 to spin a rod bearing or break a ring land is not my idea of a good investment. Subaru doesn’t want to replace engines under warranty. The chipmaker does not have to worry about this. Ask yourself, "What does the chipmaker know that the manufacturer doesn’t?" The way some people talk, you’d think the manufacturer went out of its way to detune its cars to create an aftermarket. Nevertheless, the chipmaker claims 15 HP, why would he lie?

On a final note, before you scream about the Supra Twin Turbo chip that adds 30 hp, remember that a chip for a turbo car is a different animal. In this case, by raising the boost level, they can affect the amount of air ingested. Unfortunately, the SVX isn’t turbocharged.