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It has been standard practice to make the exhaust duration
longer than the intake duration on a normally aspirated engine.
This has been done to promote scavenging and increase cylinder
pressure at high RPM because overlap is longer and one can run
narrower LSA's (Lobe Separation Angles).
Applying the same methods to a turbo charged engine, however,
will not have the same desired effect because turbo charged
engines do not make use of scavenging and reversion caused by
larger overlaps, which can cause havoc.
In a turbo charged engine exhaust pressure is almost at all
times (99.95%) higher than intake pressure. Reversion, when air
flows the opposite direction than what it is suppose to flow,
must at all times be prevented in a turbo charged engine. If not
compressor surge will occur which can result in serious damage.
If we take a set of 264 degrees duration cams we will see that
the inlet opens at 4 BTDC and closes at 43 ABDC for LS of 110
degrees. The exhaust opens at 36 BBDC and closes at 10 ATDC for
a LS of 103. This gives us an overlap of 14 degrees and a total
LSA of 7 degrees. Not good for a smooth idle and even worse when
it comes to reversion
Now let's increase the exhaust cam to 272 degrees duration.
Intake still opens at 4 BTDC and closes at 43 ABDC for a LS of
110 degrees, but exhaust now opens at 45 BBDC and closes at 9
ATDC for a LS of 108. This now gives us an overlap of 13 degrees
and a LSA of 3 degrees.
Let's now go the opposite way, increase the inlet cam to 272
degrees duration but change the LS for the 264-exhaust cam to
112. Inlet opens at 8 BTDC and closes at 47 ABDC for LS of 110
degrees and the exhaust opens at 44 BBDC and closes at 0 ATDC
for a LS of 112. This gives us an overlap of 10 degrees and a
LSA of 4 degrees. Less change for reversion and great for
cylinder pressures.
The following are achieved:
1) longer fill period for a more powerful combustion which in
turn will lead to a higher volume of exhaust gases with a higher
initial pressure.
2)
smaller overlap which will prevent
reversion.
3) longer effective exhaust duration which will help spool
faster and stronger because of the higher cylinder pressures.
4) larger
volume of exhaust gases present when the exhaust valve starts to
open.
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