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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
I wonder:

A turbo is always on boost. Even at idle.

There is vacuum on one side of the throttle plate and boost on the other at idle and cruse.

On a turbo the exhaust pressure before the turbo can run to about 3 times the boost level.

Thus, at one bar we might see up to 45psi in the headers!

If header pressure is at say 3psi at idle and will end up at about 45 psi at 1 bar boost, that is a lot of pressure that has to be built up in the exhaust before ones boost target is achieved.

Knowing this, it seem that it makes sense that header volume might have a notable impact on lag?

As such on a turbo car it would not seem that we should expect equal length or any headers to do anything at, for, or against low rpm (before 2000rpm) with the back pressure created by the turbo at idle.

Such back pressure might be more restrictive than any muffler a normally aspirated car might ever see.

On top of that "nearly zero overlap cams" do not seem to care very much about the header design except if it is overly restrictive. Especially early in the rpm range.


If we are dumping exhaust out the WG it seems we are making much more energy than we need to run the turbo at the desired boost.

If this is true are we concerned with a little heat loss due to how much surface area there might be from one header design to another?

On a turbo optimized for ultra HP operation with a large non restrictive turbine wheel, retaning as much exhaust energy as possible at the headers and turbine due to heat loss might be more important.

On a normally aspirated motor with a long tail pipe I can see how having exhaust gasses cool and increase back pressure a PSI or so might hurt peak HP.


Lastly, why can an equal length of proper design make more HP?

I suspect headers make more HP on a NA motor for two reasons. Less back pressure and savaging.

Can we expect any scavenging from turbo headers?

If we are operating at say 45psi at one bar boost I would think it might be minimal a best.

However, equal length should allow the exhaust events to exit in a more orderly manner. If so, this should help reduce back pressure just as it dose on a normally aspirated car.

If we can reduce the pressure in the headers from say 45psi to 43 psi, that is about a 4% reduction in back pressure.

That should allow for 4% less exhaust pressure remaining in the cylinder when the exhaust valve closes. That should intern allow some level of additional intake fill during the intake cycle...

I am not an exhaust expert, just trying to better understand.

Dose any of this make any sense?
Old 01-09-2010, 02:34 PM
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