Re: Turbos and tickets

[James Montebello <jamesm@domain.elided>]

On Fri, 2 Aug 2002, C M Smith wrote:
> Turbos are not completely free of course. They do take advantage of the
> remaining expansion potential of exhaust gas just as it leaves the exhaust
> port, but the resulting resistance provided by the turbine does increase
> exhaust back pressure somewhat. Turbos run well with low valve overlap
> partly because exhaust extraction and intake ramming do not work well if
> there's a turbine in the exhaust tract and a compressor in the intake.

This isn't strictly true.

Mr. Smith later describes the 'high pressure' and 'low pressure' routes
of turbo design.  In the high pressure route, not only are high pressures
used, but everything is geared to make the engine run well 'on the turbo'.
(i.e., while the turbine is spinning fast enough to provide useful
pressure).  The compression ratio is lowered to accomodate the high boost
pressures, and the cam timing is set to 'retain' the boost (low overlap).
The exhaust system is as short as possible, to get the turbine as close
as possible to the ports, so it sees the highest possible exhaust energy.
The intake side is largely ignored, as the boost pressure is providing
all of the power.  For good drivability, a small turbo is used to
spin up quickly and provide all of the power at the low to mid range.
For very high power, a big turbo is used, drivability suffers (sometimes
greatly), but the top end numbers are very high indeeed (700-800hp/liter
is possible if the engine will take it).  This was the common approach
in F1 during the turbo era, since the engines were small (1.5l), yet
the big boys made over 1000hp, running as much as 5 bar (73psi) of boost.

However, the other school of thought is to simply integrate a turbo as
an assistant to an otherwise conventially tuned engine, using cam timing
and the usual exhaust and intake wave effects to add power.  The intake
is normally unaffected by the presence of the compressor, since the
compressor is almost always quite far from the ports for plumbing reasons.
Wave effects work up to the first large change in port section, so as
soon as you're past the intake runners (say, to the plenum on a GTV6 or
a Motronic Spider), the wave effects are nearly finished on the intake
side (yes, some effects will continue all the way up to the air intake,
but at a weaker level).  Esp. if you have an intercooler plumbed in,
the compressor is typically nearly a meter away from the ports.  On the
exhaust side, one can design a good equal-length header and place the
turbine at the collector.  The exhaust side wave effects are also mostly
finished by this time, so the turbine only minimally effects them.
You lose some exhaust energy by placing the turbine so far from the
ports, so such a setup usually has a small turbo that will spin fast on
low energy.  High boost pressures are usually not possible, and this kind
of arrangement is usually better on larger engines.  What you end up with
is an engine will good drivability AND good top end power, but no more
low end power than the non-turbo'd engine.  Porsche used this approach
to good effect on the 917-30 to get a 5.0l engine to produce something
over 1100hp.  Volvo and Audi have also commonly followed this route.

james montebello
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