Yeah, the oil cooler is my first performance mod, just for that reason. I did bolt-ons and a tune in my S2000 (previous car), and yes it helped. I was still making ~85% of the power I would make at sea level, but with the bolt-ons and tune, I just would have made noticeably more power at sea level, so I enjoyed the improvement.
Sorry if all or some of this is old news to you, but just in case it's not, here goes. And maybe somebody doing a google search down the road will benefit from it.
The reason you're only making 80-85% of the power is because that's about how much air pressure/density you have compared to sea level. Any NA engine (NA = naturally aspirated = no turbo/supercharger) just takes air at atmospheric pressure and density and adds enough fuel to hit its target AFR, say 12:1 (12 parts air mixed with one part fuel) or 14.7:1 for gas. If you have less air, you add less fuel. The more fuel and air, the more power you get. At altitude, atmospheric pressure, which is what pushes air into your NA motor, is something like 11.5 psi compared to 14.5 psi in most areas close to sea level. You therefore have about 11.5/14.5 = ~80% as much air going through your motor, and you make about ~80% of rated power.There are a lot of higher order effects like efficiency ranges, but that's the driving force behind the problem.
CAI and exhaust improve things, but they do it by reducing restriction on the airflow exiting the engine. You have reduced exhaust pressure, which essentially pulls air through the motor faster. That increases the flow rate of air through the engine, so in the end more air moves through your motor, matched by fuel, and you get more power. I did I/E to my S2000 because I liked the sound better, and after also adding a HFC and tune to fix AFRs and change cam engagement/redline I felt a significant performance improvement over stock. I haven't done anything to the Z because we just bought a house (I feel broke) and I'm kind of enjoying having a quiet car for a while.
Turbos were actually invented to fix this altitude issue for airplanes flying high up in the sky. They can be set up to make up for altitude or at least to let you lose less of your power if you choose. If you set the turbo to boost up to a predetermined wastegate pressure, it will essentially add XX psi to your intake pressure, so at altitude you'll have atmospheric plus that boost pressure to match with fuel and make power. Say 11.5 psi atmospheric + 10 psi boost, or 21.5 psi driving air into your motor at whatever density and flow rate. At low altitude, you'll instead have about 14.5 psi atmospheric + 10 psi boost. So at high altitude vs low with a 10 psi turbo, you end up with 21.5/24.5 = 88% of the power you have close to sea level--better than the 80% for an NA car. If you boost higher, you lose a smaller percentage of your power.
You can just set the boost +3psi higher than you would at sea level and hit a given power target that way. Sometimes you're already pushing the turbo to its rated RPM limits, so in that case you can't spin it any faster and you won't be able to compensate at all.
Centrifugal superchargers are geared to an RPM and will give you what they give you. Like dal said, you can lose the same percentage of power on your boost pressure and end up hitting that same 80% number, but it's 80% of a far a bigger number. Like the turbo, you could compensate by increasing the boost (smaller pulley) until you hit your original goal, so long as you don't approach the RPM limits of the charger.
And of course with boost, you'll burn more fuel at a time and will have that much extra heat to shed. Like we're finding, it's harder for the car do deal with it up here. So at the end of the day you'll get back to square one, cooling, except the car will be a lot more powerful.
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