Sizing a turbo from the compressor map

A compressor map has mass flow along the bottom, pressure ratio up the side, and closed loops of efficiency in the middle. Sizing a turbocharger is finding the point your engine will actually run at and checking where that point lands on the map of the one you are considering.

The calculator below works out the point. It does not name a turbo, and that is deliberate: matching a wheel needs its real compressor map, and there is no honest way to have those without the manufacturer’s data. A calculator that hands you a part number it cannot substantiate is worse than one that hands you a coordinate.

The two axes

Mass flow, in pounds per minute, is what the engine swallows:

air mass flow = VE × displacement × (rpm ÷ 2) × charge density

The same expression as on the horsepower page, because it is the same question. Two adjustments the bare figure needs: a few percent for blow-by and any recirculated blow-off, and nothing for the intercooler — a cooler changes density, not mass.

Pressure ratio is absolute manifold pressure over ambient. Note absolute and note ambient:

PR = (ambient + boost) ÷ ambient

15 psi of boost at sea level is a ratio of about 2.0. The same 15 psi at 5,000 feet is about 2.2, because the compressor is starting from thinner air and has further to push. Altitude does not reduce a turbo engine’s power much, but it does make the compressor work harder for the same number on the gauge.

The pipework counts

The compressor does not make manifold pressure — it makes manifold pressure plus whatever the intercooler and the charge pipes take out on the way. Two or three psi is ordinary, more on a restrictive core. The compressor is therefore running at a higher ratio than the boost gauge suggests, which is worth knowing before you decide a wheel is comfortably inside its map.

Reading where the point lands

Inside a 70% island is what you want. Efficiency is not an abstraction here: everything the compressor wastes turns up as heat in the charge, and hot air is thin. A compressor running at 60% instead of 75% hands you air that needs a much better intercooler to be worth the same.

Left edge is surge. Too much pressure ratio for too little flow and the air stalls off the blades and reverses — the hunting, hammering noise at part throttle, and it destroys thrust bearings. A wheel that is too big for the engine lives here at low rpm.

Right edge is choke. The wheel is at its flow limit; more rpm buys nothing and efficiency has already collapsed. A wheel that is too small runs out here at the top end.

The rule of thumb, and what it is worth

Roughly 9.5 to 10 horsepower per pound of air per minute on petrol. It falls straight out of the air/fuel ratio and a typical BSFC, and it is genuinely useful for checking a compressor’s headline flow figure against a power target.

It knows nothing about your engine, though. It cannot tell you whether the flow arrives at 4,000 rpm or 7,000, which is the entire question when choosing between two wheels that flow the same.

The calculator shows both numbers, and they will not match exactly — on the default 2.0 at 15 psi the rule of thumb reads about 15% high. That is not an error in either. The rule assumes a mixture and an efficiency near 12.5:1 and 0.50 BSFC; the airflow model is set up for a boosted engine at 11.8:1 and 0.58, which is richer and less efficient on purpose, so it makes less power from the same air. Where the two disagree is exactly the amount your fuelling choices cost you, which is worth seeing rather than averaging away.