Fail to PLAN, plan to fail.

BY 460-BBF-Turbo-In-CC (from the legendary Car Craft Big-Cube Turbo Thread)

Corky Bell's celebrated turbo book Maximum Boost: Designing, Testing, and Installing Turbocharger Systems introduces readers to the acronym PLAN to explain power production:

P -- Pressure (as in cylinder pressure)

L -- Length (as in stroke length)

A -- Area (as in bore area)

N -- Number (as in number of combustion events per minute)

Bell uses PLAN to show why turbocharging is more practical and cost-effective than most other forms of Car Crafting at boosting the output of production engines.

PLAN is also useful to explain yesterday's low boost comparison. The Indy V8's greater ability to consume air (and thereby extract power from fuel) is the result of optimizing PLAN within restrictive rules. The Jetfire's weak numbers are mostly from ignoring PLAN.

First, let's look at the Jetfire:

The Jetfire V8 used an early, less-efficient, smallish turbo compressor drawing through a restrictive gasoline carburetor. It forced the hot supercharged air-fuel mix into a constrictive cast iron intake through small ports regulated by a mild camshaft.

Although the Jetfire's static compression was in excess of 10:1 (which in many driving conditions jacked up peak cylinder pressure beyond the detonation threshold), it exhausted the waste gasses into simple log manifolds that joined together before a single scroll turbine section. The turbine exhausted into a small, restrictive exhaust and muffler system. Maximum effective RPM was less than 5,000.

In terms of PLAN, average cylinder pressure (Factor P) was held down by inefficient induction. It was artificially spiked by high compression, but that did not increase the amount of "working fluid" (air and fuel) available for combustion and ultimately led to less available power than an equivalent peak cylinder pressure produced by other means. Heat and kinetic energy use was not optimized.

The number of compbustion events per minute (Factor N) was also limited by induction design, cam timing, valve size, port size, short-block design, materials and exhaust design. (Pumping up Factor N is usually the most expensive way to increase power because it requires either adding more cylinders or increasing R.P.M.)

Thus, although the Jetfire has a small advantage in displacement (Factors A & L) over the turbo Indy V8, it is far outweighed by other factors limiting mass air flow.

As Bell points out at some length, Factors A & L are generally hard to change and require huge increases to yield big results. Spending $2,500+ on boring and stroking most engines will typically produce only a faction of the power of the same investment in a turbo system.

On the other hand, starting with a big-cube V8 can reduce the need to hike the P and N factors to budget-crushing levels to obtain tire-melting power!