Back pressure; was Re: got my 2.5" centre pipe!

[list@cemetery.homeunix.org (Andrew Basterfield)]

On Thu, 13 Mar 2003 11:48:58 -0500
"Rick Alexander" <ralexander03@snet.net> wrote:

> Might I interject that it also has to do with valve overlap. The cams
> (diesel vs. gas) have very little in common. A diesel needs no (or
> very little) overlap.

I would say overlap is more to do with scavenging in a tuned engine;
normally aspirated diesel motors aren't (as a rule) particularly highly
tuned. If you have a carefully designed exhaust manifold that reflects a
low-pressure pulse back to the exhaust port at TDC you would want some
overlap. This is how (tuned) normally aspirated motors can achieve
considerably over 100% volumetric efficiency (think bikes and F1).

The principle would be the same for diesel as the intake/exhaust strokes
are identical. Increasing the overlap gives you power higher up the rev
range whereas diesels rev slower, so have less overlap for more low-down
torque.

--Andrew