Ok, this is a little bit of a strange question for you guys. I know about trim levels, different turbos, etc... But my question is pretty simple probably, it's about different turbo sizes. Say a car can run 15 psi on a certain turbo maximum for that car's engine. If you change turbos, can you run perhaps more psi on a different turbo setup without it being harmful to the car's engine? Is a car's max PSI, the max PSI for every turbo for that car?

And, I lied, I guess I have two different questions. When people talk about pump gas, what range are they talking about? I assume something like 89-91 (but then again, who puts 89 in an FD). Is it possible to tune your car for 91-92 and then just be on the safe side by filling up with 94? I mean can you program the AIR/FUEL ratio to be as close as possible for the 91 and then fill up with 94 so it prevents detonation?

I guess in the end I don't even know if these questions make any sense because I'm a little tired, but they've been on my mind so I figured I'd ask. Thanks in advance, -Joe

 

No it's not the same. If your fuel setup and your engine can handle 15psi max on a small turbo, you cannot expect it to handle 15psi from a bigger one. It's not the pressure but the air flow that changes and therefore more power at 15psi from a bigger turbo.

As for the fuel, yes you can tune for 91 octane and use 94 and be safe but then again, if you are using 94 octane then why not tune correctly for 94 and gain more power and be safe too

 

Ok thanks a lot for answering those questions...the first one definitely makes sense about the turbos being different sizes. Also, would an N/A Rx-7 run at 0psi at or would the atmospheric pressure plus the engine/intake/exhaust change the pressures a little. Like, I would assume it would run at -17 or -18 like on our boost meters, but what accounts for that suction and what would 0psi actually represent in "eveness"?

And as for the fuel question, that also makes sense. I guess you would probably want to tune for the particular grade of fuel that you're getting to get more power out of the engine...I guess I just get worried that one day someone's going to screw-up with the gas station or something.

Thanks, -Joe

 

pump gas = 93 octane (or whatever you can get at the gas station, 91, whatever), as opposed to race gas, which is 100 octane or more.

 

And to add to that, a larger turbo may produce boost more efficiently, which means less heat at a given boost pressure, in which case the larger turbo may actually prolong engine life. Basically, every turbo is a little different, and the boost limit of an engine may depend on pressure limits, heat limits, or other factors.

Pump gas is what you buy at the gas station, but not labled as race gas. The normal octane range is 87-94 RON in the US. The important thing about "pump gas" is its composition and properties, not just its octane rating. Race gas, aviation gas, or other non-pump gas is not necessarily a certain octane.

You should tune the car for a particular fuel, and then stick with it. If you want to be "safe", then just have it tuned conservatively.

The NA runs at a vacuum due to the rotors (or pistons) sucking in air as they move through their cycle. At idle it would be somewhere around 15-18"Hg vacuum, and the vacuum decreases to near 0 as the throttle opens, but it never reaches 0 unless there is a ram air effect. A turbocharged engine also acts like a non-turbocharged engine until the turbocharger begins creating boost.

The changing atmospheric pressure does make a difference for both turbocharged and non-turbocharged engines. The boost pressure or vacuum pressure of an engine is based on a plus or minus value of the atmospheric pressure. For example, if the outside air pressure is 15psi, and your car is boosting to 15psi, the manifold absolute pressure is 30psi. Now if somebody lives at high altitude, and their outside air pressure is only 13psi, then when they boost to 15psi, their manifold absolute pressure is only 28psi. Yes, the outside air pressure does change with various weather conditions, as noted by the barometer reading in a weather report. Absolute pressure is sometimes written as psia (psi absolute), and boost pressure is sometimes called "gauge" pressure, and written as psig (psi gauge).

Edit: My absolute pressure calculations are not exactly correct because the do not take into account the pressure drop of the air filter, intercooler, inake, etc., but all that is beyond the scope of this subject IMO.

 

Wow, thanks for the great reply...that gives me enough info to actually learn something from It's very interesting to know this stuff since I wouldn't pick it up from anywhere else likely. Thanks guys for taking the time to post the great replies