Robert Bowen
Robert Bowen
7/22/08 10:08 a.m.

It's not hard to see why racers and hotrodders are drawn to turbochargers like moths to a light: In theory, at least, getting more go from a turbocharged engine is as simple as turning up the boost. A turbocharger is simply a very efficient way of increasing airflow through an engine by pressurizing the intake manifold, and so it follows that the higher an engine's manifold pressure, the more power it will make, all else being equal, of course.

Without getting into all the messy details, this is true up to ridiculously high torque (and horsepower) numbers--or at least until the short block gives out. Turbocharged engines have the potential to make power at levels limited only by the builder's bank account.

That's the same reason why forced induction is banned in many forms of amateur-level competition outside of drag racing. In professional road racing series that allow turbos, such as the Grand-Am Cup and SCCA's Speed World Challenge, organizers long ago learned that restrictions are necessary to prevent a boost-driven arms race. Without some limits, the end result would look something like the NHRA, with engine builders assembling ever more exotic engines designed for astronomical manifold pressure and correspondingly high torque.

But despite the reputation of turbocharged engines as powerful time bombs--particularly those that didn't start out boosted--carefully designed and built turbo engines are no less reliable than other engines. In fact, home-built turbocharger systems assembled from collections of junkyard or aftermarket parts have the potential to produce a lot of reliable power for very little money. The keys are doing your homework and planning ahead.

Boost Theory

The biggest obstacle standing between a stock, naturally aspirated engine and high-boost power levels is proper control of fuel and ignition. Turbocharged engines have different fuel and ignition requirements than normally aspirated ones, and some way of modifying or getting around the stock engine controls is necessary. Without proper mixture and timing control, an engine will either not make power, or will run dangerously close to detonation and holed pistons from a lean mixture and advanced timing.

A turbocharger uses an engine's exhaust gases to drive a compressor that forces air into the engine's intake manifold. This compressed air is denser than the air in the surrounding atmosphere, so each time the intake valve opens, more air enters the engine than would otherwise be able to get in. So at a given rpm, a turbocharged engine is taking in more air than the same engine without a turbo, and will require more fuel than its non-turbo counterpart. The turbocharged engine will be producing more power as well.

Turbo engines also generally like much richer mixtures than do normally aspirated engines because a slightly rich mixture helps reduce the knock sensitivity due to the boost level and fuel octane combination used. Some engines are more sensitive to mixture strength than others, but while atmospheric engines might be happy at an air/fuel ratio near the chemically perfect (or stoichiometric) ratio of 14.7:1, forced induction engines like an air/fuel ratio that is much richer for both power and reliability.

According to Bill Cardell, noted Miata turbo tuner and developer of the Flyin' Miata turbo kits, fuel mixture is very strongly related to boost pressure. "Fuel mixture should be tied to MAP [manifold absolute pressure], so at cruise or light load you're running close to stoichiometric," he explains. "At boost, on the gas available in most of the country [91 octane premium], you'll need to be closer to 11:1 or 12:1."

Much leaner than that, and knock and high piston crown temperature start to become an issue, while overly rich mixtures can make an engine feel sluggish with poor off-boost performance. Many factory turbo cars dump more fuel than necessary into the air stream for other reasons, however. Cardell explains it as a strategy for warranty work avoidance: "Original manufacturer-installed turbos are all running 10:1 under boost to keep the catalytic converters alive, which is not particularly good for power."

The second important factor in turbo engine control is ignition advance. Thanks to the high cylinder pressures seen in turbocharged engines, they are more susceptible to pinging than non-boosted engines and must use less timing advance. As boost increases, the ignition timing needs to be retarded further and further, because the engine becomes more and more likely to ping. As Cardell says, "it's not that timing needs are reduced under boost; you'd like to run as much as you can, but if you do you'll get detonation."

The exact amount of boost-driven ignition retard depends on many factors, including combustion chamber design, temperature, fuel octane, rpm and cam timing, and has to be determined for each engine.

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Flattop New Reader
5/5/15 3:22 p.m.

Great read, Thanks for the article. I need more on this subject as I am not turbo knowledge blessed. Good thing for me there is a Nashco in my crowd.

Hungary Bill
Hungary Bill SuperDork
5/5/15 5:05 p.m.

Seriously a great read, thanks!

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