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Old 01-17-2000, 07:04 PM   #1
Scooby Specialist
Member#: 607
Join Date: Dec 1999
Location: SF Bay Area, CA
Post Making 300hp. Full disclosure (very long)

I'm currently in the middle of writing an SCC Project Impreza installment covering all the work I've recently done to the turbocharged 2.5RS. I've been busy this past month testing different upgrades and trying to establish a safe and conservative upgrade path. That said, I'm very happy with the car's current configuration. It's running safely with an estimated 300 engine horsepower. That's a significant step up from the dyno-proven ~255hp (183 wheel hp) the car generated not too long ago.

How does 183 wheel horsepower equal 255 engine hp? Good question. I'm sure most of you know this already but I'll address this again before moving on...

Tested on UPRD's dyno a stock 2.5RS dyno'd at 95 wheel hp. A few weeks later, when tested with the as-is 7psi Minnam set up (with a 2.25" Stromung cat-back), the project car dyno'd at 183 wheel hp. All on 92 pump gas. From the looks of the monitoring equipment, that is as far as the stock fuel system would take it. But still, the Stage II offered a remarkable 88hp gain to the wheels. So conservatively, engine output is *at least* 165 plus 88 = 253hp. Of course, this assumes that the stock car really does make 165hp. That's the only big assumption I'm going to make here.

Now, a more optimistic tuner may say (and I WON'T agree with him)... "Wow, wheel output increased by 93%. That must mean that engine output is 93% greater. So, 165*1.93=318hp! Holy *****!"

Obviously, there is a problem with that tuner's calculation. He would be incorrectly assuming that drivetrain loss is proportional to engine output. Evidence (and a little common sense) suggests that this is not true. While engine ouput does influence drivetrain loss, my guess is that it does so to a small degree (esp. when talking about these non-obscene hp levels).

So let's go back and be grossly conservative. Meaning that engine ouput does NOT effect drivetrain loss. So, with this in mind, it's VERY safe to say that the stock Minnam Stage II system makes (at the least) a fuel-limited 253hp. Not coincidentally, the calculated safe and efficient limit (80% duty cycle) of four 320cc injectors is approximately 256hp. Also not a coincidence are the air flow meter readings from the AFC. Stock was 44% peak. The standard Minnam Stage II was 68% peak. That's a 50% increase in air flow. So, theoretically, that's a 50% increase in engine ouput. 165*1.55=255hp. We're right back to that magic number! Of course, there are other hp-influencing factors involved but I think their cumulative effects are minor and would likely come close to cancelling themselves out if factored in. Also, the math and physics behind it are beyond me so I'm just going to play dumb (well... it's not really playing, is it?) until someone beats me over the head with them.

But wait, we are still a healthy 45hp away from the big 300hp I'm claiming.

WHERE DID THE EXTRA HP COME FROM? The rest of this message will document the mods I believe necessary to make additional safe and reliable horsepower without resorting to a fully programmable EFI system (that comes later).

DISCLAIMOR #1: Given the user-dependant nature of performance tuning, nothing is guaranteed. If you make tuning error (and anyone can make them!), something nasty may happen to your engine. In other words, follow my advice at your own risk and discretion. Please note that this upgrade advice does not necessarily reflect the views/opinions of Minnam Racing, or anyone else other than myself. As always, be safe. Don't drive irresponsibly on public roads. Don't drink and drive. Wear your seat belts. Two hands on the steerig wheel at all times. Wear clean underwear. Don't shout at your mother. Help grannies cross the road.

DISCLAIMOR #2: The following advice applies specifically to my Minnam Stage II turbocharged MY99 2.5RS which includes (among other things) a T3/T04E turbo, external wastegate, GReddy Type S blow off valve, free flow cat, J&S Safegaurd (Absolutely necessary for anything over 6psi of boost!) and Field/S-AFC fuel controller (also to be considered manditory). Other cars with other systems will require different approaches. All testing was done using CA92 octane gasoline. Any lower and you are asking for trouble. AND MOST IMPORTANTLY, IF AT ANY TIME, YOU HEAR DETONATION (a.k.a., knocking, pinging, rattling, etc,.), GET OFF THE THROTTLE! Detonation, not sheer power output, is the cause for almost all catastrophic engine failures. Knock is bad. BAD BAD BAD. If you develope a healthy fear of (and good ear for) knock, your engine will thank you in the long run.

DISCLAMOR #3: Before we begin this horsepower hunt, it is absolutely necessary to outfit the car will the following gauges: Exhaust gas temp (readable up to 1600F), fuel pressure (up to 90psi), boost/vacuum gauge (up to 15psi), and optional J&S dual monitor (a/f ratio and ignition retard). To make your life easier, go with an electric AutoMeter fuel pressure and EGT gauge. They are more expensive since they both require a sendor and an associated little electronic control module. However, both are very reliable, accurate, and most importantly, fast-reacting. I mounted the EGT sensor on the driver's side header (#2) just a few inches downstream of the exhaust port. At highway cruise, EGTs measure 1400 degrees. This should be a reference for all further temperature readings. In other words, if you mounted your sensor elsewhere (i.e. further downstream), your gauge will read lower under all conditions. Take this offset into consideration when looking at my temp readings.

DISCLAIMOR #4: Also, as with any turbo system, a non-restrictive exhaust is critical. Minnam Racing went to through great lengths to minimize hp-robbing and knock-inducing exhaust back-pressure by using a big turbine housing/wheel, an external wastegate, free flow cat, and fat exhaust tubing. It's completely counterproductive (and possibley unhealthy for the engine) to ruin this by using a small diameter cat-back designed for non-turbo cars (like the Stromung 2.25" cat-back I used during dyno testing). Currently, I'm using a Ralispec cat-back. It uses a wonderfully non-restrictive Apexi N1 canister with 2.5" inch stainless steel tubing throughout. It's loud and big-tipped, but it works better (for a turbo car, that is) than anything else I've tried so far.

Now that the disclaimors are out of the way, here goes...

PRELIMARY MOD #1: Blow off valve re-plumbing
--Vent blow off valve (BOV) to atmos and properly seal the discharge inlet. Don't use something filmsy to seal it off or it will eventually be sucked down by the turbo and cause a good deal of damage to the compressor wheel. This mod doesn't incease HP but it could very well prolong the life of the notoriously flimsy MY99 MAFs. Note that venting to atmos may require adjusting the BOV a little tighter so it won't leak under cruise and idle. Doing so will also insure a stable idle when cruising to a stop. Doing this, there should be no need to use the AFC's 'Decel' function. Besides, when the surge pressure is released into atmosphere, the car runs rich for a split second. This rich run condition can't hurt. In fact, it may even help and offset the temporary lean-run condition that can occur between full throttle, full boost shifts. It's also a good idea to mount a small K&N air filter on the BOV outlet since it does allow a small amount of unfiltered air to enter the system. Even more critical for those who go off roading.

PRELIMINARY MOD #2: Install a modified stock airbox.
-- This is yet another MY99 MAF reliability mod. This may require a little trimming of the blue Minnam intake tube. Once trimmed properly, use a 45 deg. 3" silicon elbow from Samco to conntect the box-mounted MAF to the blue minnam inlet pipe. This effectively isolates the MAF from engine-side vibrations. The stock panel air filter is restrictive. Dump it and install a K&N replacement. Now, the car will make full power/boost. Also, get rid of the inside-the-fender resonator system. Don't need it with a turbo. It's restrictive and unnecessary. Not to mentiont that the idea of the pastic snorkus/resonator ducting BACK INTO the hot engine bay is silly, to say the least. Instead, seal off that secondary hole with tape/pastic/whatever. This basically transform the intake box into a true cold air system that is fed air from the fender. The fender is fed air from the small vent next to the passenger side fog light. I can't imagine this setup could be improved upon. It's cold air and effectively non-restrictive. The mod only costs the price of your time and a K&N panel filter. Also, I'd bet that the MAF won't fail again.

Now, let's get down to business.

TO RUN 8 PSI (est. 270-280HP)===>

-- I installed a Mazda RX-7 twin turbo pump. Almost any high-flowing fuel pump will do. In fact, the pump flows so well that the tiny stock fuel pressure regulator proved to be a big restriction, making it impossible to get anything less than 40psi of fuel pressure. No big deal but it should be noted. This explains why the car would temporarily run rich (before the o2 feedback system would correct) during engine braking and coast down. With the pump, the ran richer and stronger at the top end. Appearantly, the stock pump was beginning to show its limitations at 8psi. How do I know? J&S monitor shows that maximum ignition retard went from 8-9 degrees to just 3-4 degrees. Less retard is more power. MY a/f ratiometer also shows that now the car can hold a nice rich mixture all the way to fuel cutoff instead of becoming ever-so-slighly lean (or actually, less rich) at max rpm. Also of interest is that peak EGTs dropped from 1650 to 1500. A nice change and no doubt a result of less ignition retard and more fuel. Double bonus. You may also want to take this opportunity to remove the tiny stock fuel pressure regulator and replace it with an aftermarket unit. I used the Weapon R regulator. It's pretty. I have it set to delivery stock pressure (43psi of fuel at atmos. pressure). Make sure to confirm fuel pressure with your autometer electic fuel gauge as I found the gauge that is mounted on Weapon R regulator to be inaccurate.

-- As far as I know, this is the only unit of its type of the market. It is desinged to intercept the cam and crank signal, modify it, and effectively retard or advance (+/-15 degrees) ignition timing in 5 different rpm zones. But why do I need it if I already have a J&S Safegaurd unit? Well, theoretically, I shouldn't need it. But in application, the J&S has to be set very sensative to catch ALL the knocks. In essence can over-retards when you ask it to do too much (i.e. when you run too much boost). It's safe, but not optimal for performance. Ideally, the J&S unit should be used as a back up or fail-safe (hence, it's "Safegaurd" nomeclature). Used as a primary form of ignition retard in a higher boost system, it can get overenthusiastic. This can lead to too much retard, low power and high EGTs. Enter the ITC. To dial it in, I watched the J&S monitor and followed ignition retard activity. I noticed that retard was only necessary between 5000rpm and redline. So, I dialed back some timing in that rpm zone using the ITC. With a little trial and error, I retarded ignition by only 6 degrees in that range. The result? No J&S activity and less overall retard. A lot more power and even lower EGTs (now they rarely go over 1450 deg). Appearantly, it's more effective to prevent knock with the ITC than to react to it with the J&S. When both are used together, it's a killer package. Wonderful.

So why do I need the J&S? For added safety, that's way! If you are using an ITC, all that is needed for some engine damage to occur is an unexpected flurry of knocks/pings. This can be caused by fuel slosh, hot weather, a bad tank of gas, heat soak, etc,. In other words, unless you are always playing around with the ITC, constantly compensating for conditions beyond my control, you're in trouble. Of course, you can set it so conservatively, that knock is unlikey. But this sacrifices power. So instead, I let the J&S take care of these unexpected situations. In fact, I have tuned the system so I that I just *barely* see a little bit of J&S activity (by looking at the monitor). This assures me that the J&S is set sensative enough to take car of any knocks-- even the minor ones that usually go unnoticed by the ear. My healthy fear of detonation could explain why the car's cylinder compression is just as good as stock. But still, I know ***** happens so I will try not to get cocky. Really.

TO RUN 8 TO 10 PSI (est. 280-300HP)===>

-- Now that I have both passive AND reactive ignition controls, half the battle is won. What still remains before I can run higher boost safely are the proper fuel controls. I need more fuel. Plain and simple. With fuel (and timing controls) I could raise boost levels without running lean, knocking, and blowing the engine. And now that the car has a real fuel pump capable of some serious flow, I can go to work. Since I'm using stock injectors (which max out at 8psi), I need to use some type of boost-referenced 5:1 rising rate fuel pressure regulator. Now, there is a problem with using these contraptions on a car that already seems to do quite well when it comes to compensating for boost (up to a point). Namely, at there will be a rich spot during transition or at least until 02 feedback dials back fuel-- and that's only if it stays closed loop. Either way, this is not good for driveability or power. Why increase fuel pressure under any amount of boost when the stock EFI system works just fine up to 8psi? Forget it.

Another possibility would be to use a less aggressive 2:1 fuel pressure regulator like the one Adam is using. It's conservative enough that it won't throw the stock EFI system into a rich-running fit at moderate boost levels. However, I think it's too conservative. 2:1 means that for every pound of extra manifold pressure (boost), fuel pressure is increased by 2psi. At 8psi, max fuel pressure is 43 (stock@0psi of boost)+(2*8)=59psi. That's only 8psi more than what the stock 1:1 fpr would provide. This is a negligable increase of actual fuel injector flow. Even if the unit offers adjustable offset/preload (y-intercept for those who think in algebra), the slope-- 2:1, rise over run -- is fixed and your still left with the problem of not getting enough fuel under boost levels greater than 8psi. Need something else. Something that will provide a healthy 5:1 fuel rise while not making the car run rich at low boost. So, in other words, we need a fuel pressure regulator that starts "kicking in" when the car starts to run lean (at 7-8psi).

Hmmm... the only unit I know of that does this is offered by Bell Engineering (San Antonio, TX). They make a number of FPRs-- all designed to work differently. The one that I'm using on my car has no spring pre-load and is designed for modified OEM turbo cars. Suprisingly, it's not a big seller because not many people know about it. I should have kept it to myself But the unit works wonderfully because it is designed to become active only at 8psi and above. Below that boost threshold, it is just sitting pretty, letting the very effective S-AFC do all the fuel-related work. In other words, it doesn't screw anything up. This is good. But as soon as the injectors begin to max out at 8psi, it "jumps in" and raises fuel pressure agressively (5:1). To make things easier to visualize, here's a little chart of turbo boost and its associated fuel pressure.

Boost(psi) Fuel Pressure (psi)
0 43
2 45
4 47
6 49
8 60 <--injectors are maxed out and S-AFC becomes useless]
10 70
12* 80 <--RX-7 fuel pump is maxed out]

*too much boost

Even if you are using a colder spark plug, it may be a good idea to gap them smaller than stock at this stage. My RS came with plugs gapped at 0.042". I reduced them to 0.035". This improved power by eliminating the subtle misfire I would notice at high rpm during full boost runs. However, the elimination of misfire also made the car run marginally leaner which I took care of with AFC/FPR tuning. As with any modification, care should be taken to make sure the car still runs safely. At the end, peak EGTs should still be in the 1400-1450F range with ITC retarding 5-7 degrees between 5000rpm and redline.

TO RUN OVER 10 PSI (est 300+HP)===>

-- Don't do it. You will need a programmable EFI system to do this safely. While running 10+psi on the stock-ecu'd system is possible, it's bad bad bad. I've done it briefly and I'm ashamed. I did not inhale. Even though the car didn't ping or exhibit any obvious sings of running lean, there are several reasons why these boost/power levels are dangerous. First of all, almost every Japanese fuel pump is unable to reliably support the fuel pressure necessary (80+psi) to run high levels of boost with stock fuel injectors. Second, fuel injectors loose some mechanical efficiency when forced to operate at such unusually high pressures. Third, the "LEGO method" of EFI modifications that we have been following (stock ECU with lots of add-on fuel regulators and electronic gizmos) has its limitations when it comes to tunability, built-in safety margin, and response time. So basically, I feel that running over 10psi will require FULL and COMPLETE programmable control of ignition and fuel maps-- That only comes with the use of a programmable ecu (Haltec, TEC-II, MoTeC, etc,.) While the J&S Safegaurd is a wonderful safety device and has saved my a$$ more than once, even it has its limitations. Keep in mind that sh&t can happen quickly when you are right at the hairy edge between safety and complete engine failure.


When in doubt, be conservative. Conservatism is the difference between a good tuner and a bad one. It's easy to make a car fast. It takes a lot more skill to make it driveable and reliable as well. Personally, a powerful car is only fun to drive when you aren't constantly worrying about detonation, overheating, lean-run, misfire, etc,. Be picky. Don't get accustomed to bad tuning. The ending to that story is always unhappy (and expensive). Also, when upgrading your car, take your time and do one upgrade at a time. Doing more than that will make the tuning and troubleshoorting process much more difficult. Also, going from 165hp to 300hp in just one week may not be a great idea if you are unfamiliar with peformance tuning and the associated inevitable glitches that comes along with it. Please note that this post does not go into the in-depth tuning issues. That's impossible given the fact that every car is different. Instead, it just offers a broad perspective, requiring that you apply your own discretion and knowledge every step of the way.

Lastly, don't get swayed by the claims of others (including myself, of course!). That is, if someone is claiming, say 500+hp streetable hp on a small displacment engine running pump gas, so what? Chances are, they are either grossly optimistic or budding supertuner destined for international fame. The nice advantage that come with learning about the tuning process is being able to seperate the fact from the fiction.

And remember, 300hp is plenty. And if done correctly, it's safe as well. Keep in mind that our cars are very unique. It's EXTREMELY rare that one can add a turbo to a non-turbo, high-compression car, nearly double engine ouput with simple bolt-ons, still run pump gas, sacrifice no driveability, and still (presumably) provide years of reliable trouble-free service. Feel lucky.

Shiv Pathak

[This message has been edited by shiv (edited January 17, 2000).]
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