Do we add six different bigger, stronger, more trick parts or just three or none? As I have explained, the best way to think about turbo engines is that they are just a convenient hot gas generator for a turbine. In the final analysis we will conclude there are a surprising lack of extra do-dads that need added to make some power.
If you have read the other engine sections leading into this article, we are speaking now about running a turbo engine every day on 91-93 octane, some areas in the east get 94.
At a minimum, changing the rear section muffler to any proven straight through design will noticeably improve spool-up, midrange torque and top end power. This is a simple bolt-on operation. I use the high quality, all stainless Strömung exhaust mufflers with a choice of single, rally car style tip or standard dual tips. These are tasteful designs with a really cool, deep flat four sound enjoyed by many owners. I also usually leave the mid-pipe in place and change out the third cat for improved high rpm power. I would certainly encourage all owners to try these modifications. Better part throttle response at low rpm and noticeably quicker spool-up with more torque over a broad rpm range above 3000 rpm.
I suggest 2.5 inch exhaust pipe for road cars, while many others sell 3 inch. Why? While higher power 300+hp engines can benefit from larger exhaust piping, I have found that a fully tuned “pump gas” turbo engine benefits from keeping their standard 2.5 inch exhaust pipe diameter. We are not going to make so much power on the high rpm range that 2.5 inch is a restriction. If we go to 3 inch pipe size we create a very lazy spool-up in the lower rpm range with no gain on the top end on our ordinary super gasoline. This is very practical advice.
Since we live in the clean air era, I do not recommend removing the large down pipe cat on road car. This main down pipe cat is known to run a clear 300-320hp before is considered a power restriction. It is just not a significant restriction at the power levels we can achieve on pump gas. Yes, if we eliminate the main cat there is a mild improvement in spool-up of the turbo, but not huge gains that justify turning you car into a pollution machine. Just do not worry about it. For a mild tuning of the engine please keep your main cat. The power gains will be quite similar with or without, if other details are taken care of.
There is an up pipe cat ahead of turbocharger that is one of the two main power restrictions on the engine. (What is the other one?) Removing it will improve spool-up almost again as much as changing the rear muffler section. But it will also cause the engine ECU system check engine light to come on. When we remove this cat and replace it with a standard pipe we should reprogram the ecu as it alters temperature and pressure seen by the engine monitoring system. There is another reason we usually do not recommend changing this pipe. A replacement needs to be made with very accurately aligned flange surfaces and of high quality temperature stable alloy if it is going to last any length of time. This makes them relatively expensive, either in initial cost or in later repairs. Our pipe cost was about $420 for a heat shielded, Inconel metal unit that will not fail. Changing the up pipe cat does cause the check engine light to come on, but the third cat and rear muffler section replacements are quite harmless in this regard and we encourage their use. Again, better power, better throttle response, earlier spool-up are all plus effects.
Now, why not just turn up the boost? It is not widely understood just how the modern engine control unit computer system works. The new style (ODBC-2) engine control computer is fast and extremely powerful. All one would need to do is to gain control of the code and no other engine management computer would be needed. You have your 100 octane gas, you have your engine strengthened internally and a larger turbocharger and intercooler. You adjust your boost, timing and fuel of your ECU to make smooth reliable 300hp. What could be simpler?
Well, it has not worked that way yet. Since they have had no way of understanding how a modern ECU works, much less a way of modifying the ECU, the first wave of tuners just added a control valve to let the boost rise up to high levels. Their world was 300hp on tap every day. The motors would put it out, too. For brief periods of full throttle running things seemed to work, until many engines were lost to broken pistons or other effects of detonation. Just cranking the boost is a problem because the motor will often have periods were it will detonate above 15.5 psi boost, if no other modifications are made. This can be an expensive lesson. Use caution. Simple is not cool in the turbo engine world.
The second wave made a small piggyback computer to try to force the control signals for boost and timing to bring up the power. There are now piggy back ECU controls availible in the aftermarket. They work quite well within limits. These units are forcing the timing and boost signals coming out of the ECU to achieve more power. However, they are defeating the stock ECU’s ability to back off the timing and/or boost should conditions indicate the engine in trouble with detonation. We saw an owner pay big money to have this chip installed and shortly come to us with a ‘miss-fire’ around 4000 rpm. The engine was detonating with no means for the ECU to correct the condition due to the modified output signals forcing a boost and timing values back to their target values. An aftermarket intake had altered the acceptable target boost values and was causing detonation around 4000rpm.. When they inquired with the manufacture, they said, “I guess we need to pull the timing back a bit more in that area”. I guess so! I do not suggest these types of systems, although they are in wide spread use, because they do not have any fail-safe provisions. Within limited power targets, using stock intake, larger intercooler with modified exhaust they do work reasonably well. But they are not meant to run on the ragged edge, despite the temptation. They are nothing like what is going to follow.
Notice we are not making horsepower by the dollar. We are tuning the engine to a power level against possible detonation for the fuel it is running. This is the proper approach to turbo engine management. We will not tell you, “for $800 I can give you ‘X’ power and for $1500 I can give you ‘Y’ power”. It sounds nice, it is simple, but it is not the best way to approach making power. Instead, we are giving the engine ECU a target power level and letting it figure out how much power to make today. ECU systems have always worked this way. It is one of the great things about these engines.
If one elects to use a ECU replacement or even a piggy back unit, there is two important additional components that should be considered. First, we can now eliminate the up-pipe CAT for better spool-up and high rpm flow to the turbine. Second, the stock intercooler is by standards, quite large, but as we bring up the boost we are also bring more heat into the engine. Compressed air is hot air. By installing a larger and/or more efficient intercooler we can hope to get this heat out of the intake air before it goes into the engine.
Here is the my suggested engine tuning program for road cars running pump gas and no internal modifications. Our 260-280hp target power range is dependent on the weather as much as the quality of pump gas you run. We feel this power range is the safe limit for the engine’s cast pistons and quality, but standard valve materials. It is also the limit of the stock turbo charger’s efficiency range.
1) Better flowing exhaust.
2) Reprogrammed or modified ECU program.
3) Larger, properly sealed intercooler, up-pipe conversion. Refined ECU tuning.
4) After our 260-280hp we need access to higher octane fuel and add re-enforced internal engine parts for additional power gains. But that is not why we are here. Just street power, all day, every day.