A harder working turbo produces a hotter air charge, as its higher in its efficiency range, right? This means less dense, which will produce less energy when ignited. Will that cause detonation, higher EGT’s, and screw up your timing?

 

Yes and no....The higher it gets in its compressor effiniency range the better. Compressing any air will heat it up even at 100% efficiency, that is the relative 0 point we are shooting for, above that is impossible. But turbo's aren't even that good, they run between ~60-80%. 1psi increase, will increase heat no matter what, but the higher efficnecy you are, the less heat per psi. So there is a point on the compressor map where you are at 60% efficiency, and as your pressire increases you get 80% efficient, even though you increased in psi, since you were so much less efficient, you can kind of level out and not have a temp increase. But the higher airflow you have, the more efficient your IC is, so generally the higher psi the more efficient your IC is. We really don't care about compressor efficiency (expect when chosing a turbo) while tuning, we care more about intake temps. And the density you lose to heat, is well more than made up by the pressure increase, or a turbo would be a waste if this wastn' true. Detonation could be caused by higher IATs, but it also has to do with timing and octane and CP (which you can't really change unless rebiulding your engind). EGT's are some what complicated and somewhat not important. Cylinder temps are important but there really is no way to measure them. But you can have sky rocket EGT's form retarded timing and your cylinder is completly safe from detonation and you will have no power. So advancing timing can lower EGT's, and there is a point where they start to go back up, that is bad if that happens. But you can't just look at the guage and know what's up, you have to make changes and observe.



Not really sure what you are getting at.... Are you looking to get a turbo and looking at compressor maps and efficiency or you looking to turn up your current boost?

 

To really understand what is going on in a turbocharger you need to understand more than a simple compressor map. The Ideal Gas Law comes into play as well as the efficiency of the whole system. The system being the volumetric efficiency of the engine as well as everything between the exhaust port and intake port. Think of a turbo's efficiency rating as a base number to go by, kind of like an EPA MPG rating. In a strictly controlled environment with ideal conditions you'll have a turbo generate the efficiency ratings listed on a compressor map. Just like MPG you will rarely have a set up that comes even close to the set up used by the EPA and your mileage just like efficiency will vary.



Take a big diameter straw and blow air throw it. Measure the temperature and volume of the air flowing through the unrestricted straw. Now take a much smaller diameter straw and connect that to the larger straw. Now measure the temp and volume again. You will see the volume decrease and pressure increase while the temp also increases. Same happens with a turbo system. So even though a turbo might be initially in its peak efficiency range the rest of your set up can cause it to run above or below that range. The best thing to do is measure the thermal efficiency of your system as a whole by taking air temp measurements before and after the intercooler.



Now as far as it jacking with timing most modern fuel injected engines adjust ignition timing based on a number of factors. These usually are TPS, Coolant Temp, Air flow (MAP, MAF, or both) and Intake Temp. In a OEM turbocharged car the ECU programming accounts for all the various factors and adjusts timing to match. In a car that was not turbocharged from the factory the OE programming still adjusts based on the above sensor inputs but to a much smaller scale. In most cases a Normally Aspirated ECU program cannot fully compensate for all the varying conditions a turbo brings to the table and detonation can result more readily. That's where using a good loaded dyno and monitoring the EGT, intake temps, AFR, and using a tuner that can adjust timing really helps keep your engine safe and happy. The reason why most tuners think EGT is worthless is because they don't know how to properly set up and read an EGT probe. To get the most benefit you need an EGT probe placed just outside of each exhaust port. Since most people don't have the money to do this then placing the probe outside your leanest cylinder will work fine. Since you cannot put a EGT probe into the actual combustion chamber having it just outside the exhaust port is the next best thing. Again you need to take into your set up as a whole when working with EGT. You may have a really efficient cylinder head but have a horrible exhaust manifold that will cause excess backpressure which will cause EGTs to rise abnormally. I won't get into the details of tuning with EGT here because people would probably fall asleep. Plus most younger so-called 'tuners' don't believe in the value of EGT and instead focus solely on AFR ratio. To those I say what happens if your AFR is fine, and your intake temp is good, but your ignition timing is too advanced? Most OBD readers that can display timing are too slow to register a detonation event, that's where watching a properly placed EGT probe can tell you what's happening before it happens..it will save you from a tow ride home.