A PROBLEM WITH JUNKYARD TURBOS

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft turbo blog)

"Don't send a child to do a man's job!" -- old Southern proverb

Someday turbos such as the tiny units used on the current Ford EcoBoost V6 engines will become widely available at the junkyard. And like an earler generation lured by JY T2s, T3s and T28s, some budget Car Crafters will undoubtedly buy the current generation of small JY turbos to use on big V8s with hopes of of dirt-cheap boost.

Years ago, more than a few Car Crafters grabbed up another small turbo, the once-ubiquitous Garrett AiResearch T3, and slapped pairs of them on all sorts of V8s, including Ford's 460.

Did it work?

It depends on what you wanted. If the goal is maximum pump gas power, the answer was no. Here's why:

Here's a turbo compressor map for one of the larger T3 variants. Note that the bottom axis is in lbs/min. of mass air flow. The highest flow mapped is ~ 35 lbs/min. Although the turbo doesn't stop flowing beyond that, the efficiency level becomes unacceptable (excessive air heating, potential overspeeding, and choking of the engine)

Recalling the general rule of thumb that one pound of mass air flow will support 9-10 horsepower, it should be abundantly clear -- even if you've got no idea how to read a turbo compresor map -- that a pair of these smallish turbos would be hard pressed to feed a 460 above 4,500 r.p.m. (If it's not, then stay tuned!)

Two of these turbos working together would struggle to supply enough "boost" at any pressure ratio for 630-700 h.p. And that's exactly what the Car Crafters who have tried to use pairs of JY T3s on 460s discovered.(See e.g. Trevor Cornwell's twin-turbo 460 Ford Fairmont)

And even that level would be adversely affected by excessive charge heating due to compressor inefficiency. Best efficiency would be at around a mere 200 to 225 h.p. per turbo -- a level that an "all motor" 460 should be able to easily achive on its own.

CONVERTING CFM TO MASS AIR, PART II

By 460-BBF-Turbo-In-CC (From the legendary Car Craft big-cube turbo thread)

STP isn't very "real world."

Standard temperature and Pressure (STP) provides a means to account for the fact that air density and air volume are not linked.

STP - commonly used in the Imperial and USA system of units - as air at 60 [degrees]F (520 [degrees]R) and 14.696 pounds per square inch absolute (psia) (15.6oC, 1 atm) See, Engineering Toolbox.

Of course, engines in the real world seldom if ever operate at STP. Underhood temperatures, heat radiation, convection, and conduction, humidity and altitude variations help all assure that. That's one of the reasons why the SAE and others have developed more realistic standard references for automotive use.

Recall that we learned turbo manufacturers use 85 degrees F as "standard" (A. Graham Bell).

Is there really much difference between the various methods of converting CFM to lbs/min? Let's compare them:

Assume we're trying to convert 500 CFM to lbs/hr:

Warner method: divide 500 CFM by 13.7 = 36.49 lbs/hr

A. Graham Bell method: multiply 500 CFM by 0.07 = 35 lbs/hr

Miller method: multiply 500 CFM by 0.069 = 34.5 lbs/hr

STP method: multiply 500 CFM by 0.076 = 38 lbs/hr

Note that Jeff Hartman in Turbocharging Performance Handbook (2007) also uses the STP method.

Although the spread among the various methods is not that large, it appears that Miller  and A. Graham Bell's conversion factors produce the most conservative results. Moreover, because they track with turbocharger industry standard practice, we'll use the 0.07 conversion factor.

But don't forget that these all of these factors are based on picking a somewhat arbitrary air temperature and pressure as a measuring point. Actual results may vary.