Forced induction (was variable (non variable) whatever...)

[Nimpoc at aol.com]

There's been some good discussion, but clarification is in order.

[ramble on]

Different types of compressors:

- Roots style "positive displacement" blower.
These are units that move a given volume of air per rev. If you have a blower
that is geared to move 1.5x engine displacement, then you will always have a
(potential) 50% supercharge, instantly. Problems: generally poor compressor
efficiency (50%), very difficult to intercool, noisy, difficult to mount due
to size/shape, excessive drive requirements w/o by-pass valve, limited
realistic capacity. Using the by-pass to circumvent the compressor during
cruse, there is very little power loss due to the drive (>5 hp) = good mpg's.

- Centrifugal Supercharger.
Basically just turbo compressors attached to some sort of transmission driven
by the crankshaft. Nature of the beast is quasi-linear boost relative to RPM
(ie. 3k = 3psi, 6k = 6psi) or some multiple thereof. Units are capable of high
pressure ratios per stage (eg high boost). Benefit: superior compressor
efficiency (70%), easy to intercool, relatively small thus easy to install,
immune to backfire, simple to manufacture, boost curve often advantageous for
traction reasons.

- Turbocharger.
Centrifugal compressor driven by an exhaust turbine. Technically a speed
machine, meaning the compressor flow is based on its speed squared. This speed
relationship lends itself well to the use of an exhaust turbine to derive the
compressors power. With no direct connection between the engine and the
turbine, gearing/drive limitations (and complexities) are eliminated. Back
pressure is an issue, mostly just on factory applications though (remember,
almost all "factory" stuff sucks right...). A properly sized and tuned turbo
system will have a boost/back pressure ratio of 1:1. Indy cars do much better.
Benefits: superior efficiency, ease of intercooling, limitless
compressor/turbine selection, broad boost characteristics. Problems: exhaust
plumbing / heat isolation issues, poor converter light off, stigma.

Other issues:

Turbo motors benefit just as much as na motors from intake and exhaust tuning
(headers, etc.). The boost doesn't just "overcome" obsticles, it still has to
flow through them. Picture a forced induction motor as just operating in a
higher pressure differential regime than na motors and it makes sense. (fi
0-30 psi absolute, na 0-14.7 psi absolute)

Turbines derive their power from the mass flow of the exhaust, not just the
heat nor the pressure. Some also claim they can derive power from the acoustic
energy present... don't know about that. Sure a turbine won't make as much
power at the end of the exhaust than at the manifold, look at how much volume
is lost due to temp.

PV=nrT -> V=(nr) * T /  P  (gross assumption temp 1200 -> 200)

Properly tuned turbo car's don't have any lag. Audi 1.8T, newer SAAB's,
Volvo's, 911's. Just imagine if they used ball bearing center housings.

[ramble off]

watch those blue flames...

Mike Kent