Intake manifold construction, intercoolers

[Kevin _]

> > 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.

>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.

>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 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.

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.

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