Well there you go 600WHP! It was limited by fueling, but even if we had fueling dialed, I don't think there was too much more power to be had. Running more boost means more air flow, more air flow means we are going to be way off the compressor map. Going off the compressor maps means turbo efficiency goes down the crapper, and in turn less potential power.
Here is the boost curve of the 600WHP run and the 22psi run. Couple things here to take note of: The actual boost needed at peak power to hit 600 WHP was just over 23psi! In contrast a 30R on an STI barely breaks 400WHP at that boost.
Air flow = HP and boost pressure = more air flow. How ever you want to look at it, this turbo isn't big enough to flow too much more air or make too much more HP. Below are some lbs/min ratings of the 3.0L at difference boosts and RPMs.
16psi 18psi 22psi 23psi
3000 24.8 lbs/min 26.5 lbs/min 29.7 lbs/min 30.5lbs/min
5000 41.4 lbs/min 44.1 lbs/min 49.5 lbs/min 50.9 lbs/min
7000 58.0 lbs/min 61.8 lbs/min 69.4 lbs/min 72.5 lbs/min
So at 7000RPM and 16psi of boost flows 58lbs/min of air. According to how Garrett rates their turbos HP, that is about 580 engine HP. Since we made 470Wheel HP, and if we use a 25% drive train loss, we get 580HP. Sounds about right. At 23psi and 7000RPM, the engine is flowing 73lbs/min, which is about 730 engine HP. With drive train loss, that puts our 600 Wheel HP at about 750. Again pretty close.
COMPRESSOR MAP OF GT4088R. You can see using a basic air flow calculator that not too much more power would be had. Esspecially on pump fuel. But 600WHP on pump fuel, what more could you ask for. Thought this wouldn't be possible without water injection.
Since the turbo is only rated for 700 engine HP, according to Garrett, it is no surprise that we started to see diminishing returns as we got closer to 23psi. The word diminishing is not the best word to use as we were still making huge power. You can see by the compressor map that we were getting into the areas where the turbo was less and less efficient. Again showing why at high boost levels the gains were not as big as the boost went up.
So was there tons more on tap? Nope! Can the H6 hold more power if there was?? I think so. With the progression of power, and how easy it was to get there, and with power coming so easy with ZERO knock, there is sure good signs that it can. Think of this build as WHP per cylinders. 600WHP on the H6 is 100WHP per cylinder. 100WHP per cylinder on a 4 cylinder is 400WHP, which many consider and have proven to be safe (with non-forged piston engine). To get 600WHP on a 4 Cylinder you will be at 150WHP per cylinder. That is 50% more stress, and pressure. If we use the example of a forged piston, stock sleeved STI motor, holding almost 600WHP (150WHP per cylinder), the H6 would be good for at least 900WHP. That is pretty scary! Add sleeves into this, and say 1200?? Thats only 50% more power per cylinder. But that is all speculation, and stepping into dream land.
So why didn't we go too much further? Fuel. As we got closer to 600WHP, we noticed that we were adding more fuel and nothing was happening. A PERRIN chassis fuel system is the easy answer to fueling and making it 100% safe to run at 600WHP. As many have found, 550WHP is kind of the limit of the Walbro, and we just proved that stat isn't too far off. As important as fuel is to this build, it is one of the simplest things to fix. This will allow, I believe, a safe 620-640WHP. But to go beyond that, it would be time for a bigger turbo.
SO WHAT'S NEXT???
Most likely nothing. At this point we have awesome power with 4000RPM of spool. Any bigger turbo and we will sacrifice spool and all the low end power we just gained with the bigger engine. If we did go bigger, the next best choice is the GT4094R, but the extra 100 possible HP would only be for showing off. At 500WHP, this car was pretty scary, add 100WP to that and it just became a dry weather only car! In the state of tune it is in, it is far faster than most all other supercars at 1/2 to 1/8 the cost depending on if you compare it to a Z06, or a Ferrari. This car isn't a trailer queen, or a dedicated drag car, or dedicated track car, or street car. It may not be the best at those things, but it sure will do well at all of them.
The one thing that will happen next is fuel. We need more fuel to get more power. The PERRIN fuel system and pump are installed and ready to do a retune at 600WHP! Above is just one part of the system. A fuel pressure regulator.
Alright, now the fun stuff. So how does this compare to your normal everyday STI's driving around out there? From all the info above, you can probably conclude it has more power. This is true, but it is kind of fun to see exactly how much and where it dominates the 4 cylinder.
The first comparo is our 07 STI limited. I think this is the best comparo as this is a very common setup. The 07 STI Limited uses a Full PERRIN package. Every single bolt on part we do is on this car. The rotated turbo we used is a GT3076R with a .82AR exhaust housing. Running a modest 21psi boost level this setup can hit 400WHP pretty easy. And this graph shows it. With similar spool to the Gt4088R on the H6, it does nothing but STOMP it everywhere!
The second comparo is highly tuned stage 2 STI. Again this is a good comparison as this is another common setup. This car only has a slight edge below 4000RPM. But when you are racing, auto crossing, dragging, you will not be in this range very long, so again the H6 wins! But on the road, below 4000, the Stage 2 STI will be much faster.
This comparo is a highly tuned GT3076R STI. Again this is a good comparison as this shows a similar spooling and similar low end power car with the H6. This GT3076R is really maxed out and on the 4 cylinder, a GT3582R will get close, but at the sacrifice of low end power.
The last comparo is a highly tuned GT3782 on an STI. This turbo flows about the same amount of air as a GT3582R, but with a freer flowing turbine wheel. Again this is a good comparison as this shows a 4 cylinder making about the same amount of power as the H6. But you can see what kind of sacrifices you make to get there. On the road, there is about 1000RPM different spool, and about 13psi more boost on the 4 cylinder. With the H6 running about 23psi to get this power, adding 13psi to the 4 cylinder to get close to this power, you are going to start having issues with head gaskets sealing, bearing taking a beating and quite a bit more stress.
The question is would I do this again? One could argue that the power levels we reached could be done with a 4 cylinder. One could argue that you would also save a ton of money doing the 4 cylinder. One could argue that the added weight will hurt handling. One could argue that it gets worse gas mileage. One could argue that it is untested waters. One could argue that 600 is way too much power for a street car.
I say so What! I think it is time to start a trend for a new Subaru Exhaust Note! Bye bye Boxer-rumble, here comes the boxer-smoothie............or boxer-Porsche-aru, or boxer-Sub-orsche, or something that represents that it sounds like a Porsche.