Around the shop we talked about this a bunch but never actually did it. Of course it sounds crazy to run your STI with no intercooler, but what if we added Meth injection?? Would this solve the air density issue? Would it cool temps enough to run the same ignition timing? Would this offset a necessary richer AFR do to the hotter charge temps? Would it be more responsive? Results were very interesting for sure.
The car we used was our 2011 STI sedan which has a typical Stage 2 setup but with a PERRIN equal length header, PERRIN CAI, and PERRIN Inlet hose installed. We flashed our Stage 2 map on and proceeded to do additional tuning by turning up the boost and adding some timing here and there. We left the car with a very high 327WHP and 387ft-lbs of torque (at this point the highest HP Stock turbo STI we had done). Our boost target was 19psi and like all STI's it tapered to 14psi at redline. Our results would be used as a baseline goal for the "No Intercooler" setup.
We installed the boost tube (no intercooler) with dual nozzles for the PERRIN water/meth injection system running a 50/50 mixture. Before doing any pulls I removed 8 degrees of timing everywhere on boost. We left the same boost target of 19psi like we did before. The first pulls we knew would be super safe with less timing, richer AFR (from the WI system) and we also knew it would be down on power. The biggest surprise was boost, check out how much more boost we were getting with no intercooler causing a restriction. The additional 3psi was very encouraging as this meant we had the potential for more HP!
Typically when a Water/Meth injection system is installed, you lose HP unless you tune around it. This is because you are replacing fuel and air with water (not combustible). In order to gain back the HP you need to lean out the AFR to a point that is just slightly leaner than before. From there you normally gain power because you can add more ignition timing, add more boost and lean out the AFR even further. Typically this is worth an additional 20-30WHP on cars were the turbo is large enough to support more air flow.
Well, we did all that and then some, but still couldn't match the power with an intercooler. First we slowly added back the 8 degrees of timing we removed to make sure it was safe. Then we leaned out the AFR and still we were not matching the HP and TQ of the stock intercooler. In this case we didn't add boost because we wanted to see if we could make the same HP with the same boost. Then I went crazy and added about 3 degrees more timing (3 degress above base line tune) across the board and still the "No-Intercooler" setup couldn't match the stock intercooler. And it wasn't like we were close to where a couple more degrees of timing would fix it. I was really hoping that the in-cylinder cooling from the Water/Meth system would overcome the cooler denser charge from an intercooler.
As you can see the TQ and HP down low was still off about 60Ft-lbs and 50WHP. Was there more to go? Yes, we could have upped the boost, or ran it really lean, but there was no way we were going to match the HP. So why didn't it work?
Typically an engine with a good intercooler sees temps that are 10-30F above ambient temp. In this setup the engine was seeing air temps in the 150-200 higher than ambient temps! The hotter air is less dense, hindering additional air and fuel from being crammed into the engine. Based on nothing but calculations, a 2.5L engine with a 75% efficient intercooler, at 4000RPM and 19psi will be making 50 more HP compared to the same car with NO intercooler. With the air being about 20% less dense you would have to come up with a way to get more power (water injection isn't enough). Using something VP C16 race fuel or other high quality fuels (do to their higher engery value) might be able to overcome the low end power losses, but I think you could still get more by installing an intercooler on top of race gas.
So in the end, was it worth it? It only took a few hours to make the pipe, and a few hours of tuning, so for sure it was. Now we really know you need an intercooler! (Like we didn't already know that)