Intake manifold construction, intercoolers

[Bruce]

From: "Kevin _" <kiggly at hotmail.com>
Subject: Re: Intake manifold construction, intercoolers
> > > Hmm... so why high perf turbo engines have multi-valves heads and high
> >lift cams just as their NA counterparts? If tomorrow the atmosphere
pressure
> > > changes from 101kPa to 202kPa, will this change something to what
> >happens inside an engine intake manifold?

> >Many engines use multi-valve arrangements to increase flow area.  The
same
> >benefits go for a turbo'd car as an NA car in this area...more volume
into
> >the cylinder at a given pressure loss, or the same volume into the
cylinder
> >at a lower pressure loss...at the price of less turbulence.  With more
> >valves and/or more valve lift you increase the flow area, reducing
pressure
> >loss at a given flow rate.  With more valve duration, the intake valve is
> >open for a longer percentage of the compression stroke.  If boost
pressure
> >is high enough, more air can be packed into the cylinder even as it
starts
> >the compression stroke, further improving volumetric efficiency.
> And when you're making a high rpm engine you keep the valves unseated for
> over 300 cranshaft degrees.  Usually about 250 to 260 degrees for 0.050"
> valve lift or more.  The more/less air being packed at a certain time
during
> the intake stroke is no different at all for a turbo or non-turbo engine.
> The only thing you design slightly differently for a turbo is the speed of
> sound is higher because the intake air temp is higher in a turbo
> application.  Thus, you need slightly longer runners for the same
Helmholtz
> rpm, but its nearly inconsequential.

Higher rpm, OK, how high are you talking about.  The thread started with
street cars, are you still working the street or moving into the race field
I'd hardly call 250d at .050 as a street engine.

> >If atmospheric pressure doubled, I don't think it will make an
appreciable
> >difference in the way an intake manifold worked.  The whole question
still
> >seems inconsequential to me, since by tuning an intake you are trying to
> >increase intake air pressure by a small number of pascals.
> Have you ever datalogged and seen pressure pulses at the back of a valve?
> When its all tuned right with the proper sized runners it can be over 20%
> beyond atmospheric pressure during valve closing.  We're certainly not
> talking single Pascals here.

At the back of the valve, OK, and how fast does that decay as you move back
from there?

> >This helps the
> >engine because the intake pressure becomes slightly higher than exhaust
> >pressure due to acoustic pressure, helping flow into the cylinder during
> >the overlap period.  If atmospheric pressure doubled you'd still need to
do
> >this, as exhaust pressure will double accordingly, but with turbocharging
> >the exhaust pressure increase (it's all absolute pressure here of course)
> >is small compared to the intake pressure increase.  Tuning the manifold
to
> >gain a small number of pascals pressure will have an insignificant impact
when
> >combined with the already large pressure differential between boost
> >pressure and exhaust pressure.
> Proper flow during overlap is very important.  Have you ever seen an
exhaust
> pulse's pressure trace?  AVL boost's website has an example.  There is a
> good 2:1 or better variance in exhaust pressure at the valve during a
single
> engine cycle.  Time exhaust valve closing/overlap during the low side and
it
> scavenges the cylinder very well.

> I have to completely disagree on the turbo issue here.  Normally with a
> turbo setup you have a huge pressure swing from before spoolup, during
> spoolup, then maintain a good pressure differential for ~3000 rpm, then
HUGE
> PRESSURE INCREASES IN THE EXHAUST.  You will see drastic increases in
> exhaust pressure, I've measured as much as 60psi for 23psi in the intake.

That has to be about the worlds worst matching of turbo to engine.

> That means I've taken the turbo WAY out of its good operating range.  In
> this situation, you have TONS of exhaust pressure during overlap and toss
> all kinds of exhaust back up the intake and run crazy natural EGR, can't
> spark it, and make very little power.  This is also where its by far the
> most important to have a properly tuned exhuast on the low side of its
pulse
> during overlap, you can then extend your rpm range (granted with some
pretty
> bad pumping losses) by keeping some VE at rpm's when the turbo is making a
> lot of backpressure.

Well, ya, but your talking at the extreme.

> In an SAE car there is probably a very real balance of exhaust to intake
> pressure.  If you have too low of exhaust pressure I bet you lose power
> since you're limited on intake flow by the restrictor.  If you are
> scavenging really, really well, then you'll have a considerable amount of
> wasted air/fuel exiting the exhaust.  A properly tuned intake is also
vital
> or else you'll have dead pressure times at the restrictor.  Have you ever
> measured and logged pressure at the orifice?  You'd probably have to log
at
> 5KHz to get a good trace at high rpm's.
> Just my $0.02,
> Kevin
> BTW - Bruce said he was looking for 'new' stuff in engines.  Check out F1
> stuff before you call everything old and pushrods good.  Approximately
> 300hp/liter naturally aspirated on gasoline (yes, I do use that term
> loosely) and revving to near 20k rpm isn't trivial.

To whom?
I haven't seen any of that wizz bang technology filtering to anything on the
street yet.  They still have reached any level of performance that can't be
done with engines that would have transferable technology to the street.
The rules in F1 are just a game anymore to see who can spend the most for
something that can win.
Bruce

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