DIY_EFI Digest V4 #462

[Greg Hermann]

>
>Date: Tue, 10 Aug 1999 16:56:01 -0600
>From: "Kevin Yachimec" <keviny at cybertech.ca>
>Subject: Re: Turbo header design
>
>I missed the part of this post that stated the intended use of the vehicle
>in question, so the following information may not apply.
>If the header/manifold is for a street turbo application there is no
>measurable gain by using a tuned the length header unless you using a pulse
>type turbine and housing designed for receiving exhaust pulses. Most turbos
>you find on production vehicles have the best results if the manifold
>smoothes out all the pulses into a constant flow with heat lost being
>minimized. I have a promotional paper from Allied Signal outlining manifold
>design for their turbos. It covers both their new pulse type turbos (like
>the ones you see on all those 10 second Honda's with equal length headers)
>and their standard turbos, if I can find it I'll post it 'incoming'.
>
>>Greg Hermann wrote:
>>
>>
>> FOR THIS PARTICULAR V-8> (IT HAS A 90 degree crank)
>> >                Firing order
>> >          1  8  4  3  6  5  7  2
>>
Kevin--

I don't remember how far back in the archives you must have dredged to get
that post, but it applies to about any kind of engine service, if one cares
to spend the money, do the work and can fit the headers into the vehicle.
It applies to turbo headers just as well as to NA headers.

A "pulse" type turbine is a crutch to get better turbine performance at
lower speeds and pressure ratios as a trade off against top-end
performance. A header of the type I described will let the ENGINE perform
the best. A turbine will perform most efficiently if it has a steady flow
rate through it. The way to get from a to b is to empty the header
collector into what is known as a pressure recovery accumulator--basically
a device which evens out the gas pulses before feeding the gas into the
turbine.

This evening out is essential to turbine efficiency because the turbine
funtions most efficiently with an inlet velocity in the Mach 0.75
neighborhood. Too much pulsing with that sort of a flow rate, and the
nozzle begins to get choked (reach a flow velocity of Mach 1) at the peak
of the pulses, with the attendant backpressure and efficiency problems.

A variable vane turbine is a much better solution to the (poor efficiency
at below design flow rate) problem than a pulsed type turbine, because it
gives far better low flow response and efficiency than a pulsed flow unit
ever dreamed of having without compromising the top end flow situation.

Garrett has been working on VATN turbos for some time now, and apparently
does not have them quite right yet. I suspect that their published opinions
on the subject may change as their product development cycle progresses.

You might check out Obert's info, brief though it is, on pulse turbos ,
pressure recovery accumulators, etc. Very interesting stuff.

As to the headers--almost exactly what works best on an NA engine will also
work best on a turbo motor. Only small dimensional changes, so as to
accommodate the higher flow losses which happen with a denser fluid (bigger
tubes) are necessary. Otherwise, the intake/engine/header system operation
with a turbocharger is really no different from operating the same engine
at the bottom of a very deep mine with no turbo. A log type exhaust
manifold on a turbo motor will degrade performance, compared to headers,
just as much as it would on the same engine at the bottom of a mine. The
performance degradation is roughly the same, percentagewise, as it would be
on an NA engine at sea level.

Regards, Greg