Mike, please explain "all boost pressure is NOT equal". Other than the parasitic drag, what is the difference? It seems to me, that 14.2 psi at a set temp, in a 366CID engine, would always be the same. Can you please expound on this?
Turbos are interesting critters. Lets suppose you have two turbos. Your engine needs 600 cfm of air flow at 14.7 psi. One turbo has its sweet spot at 450 cfm at that boost pressure and the other has a sweet spot of 600 cfm at 14.7 psi.
The first might deliver 600 cfm of air at 14.7 psi but its discharge temp is 250 deg F, the second delivers the same 600 cfm at 14.7 psi but its discharge temp is 150 deg F. That difference in discharge temp is wasted power that came from the exhaust flow of the engine.
The turbo that has the 250 deg F discharge temp might have an exhaust back pressure of 1.8x boost pressure so it is fighting an exhaust pressure of 26.46 psi when the cam is in overlap as it is trying to stuff air in the cylinder at 14.7 psi. Needless to say the exhaust burps back into the cylinder until the exhaust valve is closed contaminating the intake charge. This is why most turbo engines have different cam timing than the same engine with no blower or a mechanical blower, often a rapid closing of the exhaust valves and little overlap.
The properly sized turbocharge might only have an exhaust back pressure of 1.2x boost pressure, putting exhaust pressure at 17.64 psi. This means less blow back into the cylinder as the exhaust valve closes and the engine has to do less work pushing the exhaust out of the cylinder against the turbo back pressure in the exhaust system.
Not only that, if your run both turbos through an ice over water intercooler tank, the very hot turbocharger will lose more pressure due to cooling of the hot air charge, so it will really have to do the work to make perhaps 17 psi at the discharge outlet to actually deliver 14.7 psi at the intake manifold, where the properly sized turbocharger might only need to make 15.2 psi to overcome intercooler losses and still deliver 14.7 psi at the intake manifold.
(these are all just wild guess numbers but illustrate the trade offs you must make as you size the turbocharger.)
Your real power gain is due to the total increase in useful mixture in the cylinder when the valves close, and how much power you can make burning it, minus the power you will use up getting the mixture in there.
In a belt or crank driven blower the energy cost is directly from the crank power output. In a turbo there are still power losses but they are hidden in things like exhaust gas back pressure and the work the engine needs to do pushing the exhaust gases out against that back pressure, and the need to use a short overlap cam to keep from getting excessive exhaust gas contamination in the cylinder due to high pressure exhaust gas back flowing into the cylinder before the exhaust valve closes.
In a mechanical blower you can throw away mixture with over lap and get great scavanging so all the mixture in the cylinder is uncontaminated. In the turbo not always true, although a turbocharger with a huge hotside might have very low backpressure but the trade off is it spools slowly and may be lazy to come on boost. A smaller hotside that spools fast might come into boost violently when the engine comes under load.
In the real world you need to find a middle ground between those extremes. Fast enough spool so you have good and predictable throttle response, but low enough exhaust gas back pressure that you don't throw away too much of your power overcoming that restriction.
That is where the art of turbocharger setup is. Picking the right turbo size and compressor trim to get the right airflow and boost pressure with good throttle response, and a hot side sized to give reasonable spool up and low back pressure.
Then you throw in a few tuning tricks to manage boost by juggling fuel mixtures and ignition timing to bring on boost or soften boost onset at critical rpm ranges then keep all those balls in the air so everything works together in harmony.
Larry
