Re: Exhaust back pressure

From: David Gersic (info@zaccaria-pinball.com)
Date: Thu Aug 10 2006 - 22:59:54 EDT


On Thursday 10 August 2006 08:19 pm, Terrible Tom wrote:
> As some of you may know, my Ram has no muffler. Sounds frickin
> awesome... probably was the exhaust note and volume that made me buy it
> in the first place.... but I've been recently wondering with the high
> gas prices, if its hurting my fuel economy.

It could be. It's probably hurting your performance, at least a little bit.
The engine's designed to have some backpressure, and does like to have the
scavenging going on. Plus, most places make it illegal to run with no
muffler. Having the right exhaust setup is probably worth a few horsepower
and lb. ft. of torque. I can't think of any studies of it relating to fuel
economy, but it seems reasonable to expect the increased efficiency could
result in slightly improved fuel economy. Of course, the biggest fuel economy
gains may be found in your right foot's operation.

> So not knowing volumes on exhust dynamics and flow, I started
> researching. I read that the exhaust flow actually does create a
> negative pressure, as in a suction effect... helping to pull exhaust gas
> from the cylinder and decreasing the amount of effort the engine has to
> used to pump the exhaust out on the exhaust stroke. (thus increasing
> power by actually decreasing parasitic power loss through higher efficency)

Right. Adding a X or H pipe to true dual exhaust helps with this as well.
Running a cat-back dual exhaust is not quite as good as dual with an H pipe,
but it's better than a single all the way back.

> Have I grasped this concept correctly? If this is the case, I can
> clearly see the effect that increasing exhust pipe diameter too large,
> or reducing backpressure drastically, can have on this scavenging
> effect. It basically takes the suction effect away, forcing the engine
> to have to pump the exhaust out vs. having a helping hand pulling it
> out... that sound logical?

Yep. Sounds like you've got it. Depending on the cam, it also helps pull the
fuel / air charge in to the cylinders for the next stroke as well.



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