[Shotimes] Re: Dyno day for my project

[Chris Mayfield]

Going catless on a SHO reduces low end torque because they use 2.25"
pipes (or bigger...) which are too large to maintain proper exhaust
gas velocity as they exit the ports. You get reversion, which hurts
the engine's ability to scavenge exhaust out of the cylinder. This
dilutes the intake charge and reduces power. It also inhibits the
ability of the incoming air charge to be drawn into the cylinder
during the valve overlap period, which reduces the cylinder fill with
fresh air charge... Less clean air, smaller boom.

One might ensure proper exhaust gas velocity by using a header
designed with a large catback in mind. Unlike every set of SHO headers
out there, some people actually put thought into header design, and
can produce something that doesn't lose power, and gains it in certain
areas of teh powerband.

As for gaining power everywhere, that was from personal experience. A
friend installed a full 3" catback (along with modifying the collector
on the Y-pipe (w/ cats) to 3") and the before and after dyno showed
MORE torque across the entire rev range... Including a 25 whp increase
at 7000 rpm. This was a gain with *just* the catback. The stock setup
may have been a little plugged, but I'd say the SHO Shop's myth about
the stock cat-back not being restrictive is a little out there,
especially at higher RPM.

The myth about backpressure increasing low end torque is one that I
can't wait until the entire car community accepts as debunked. It is
100% BS. The reason people think 'backpressure' increases torque, is
that a smaller diameter pipe promotes better exhaust gas velocity at
lower RPM. However, this same size pipe causes a performance loss at
high rpm due to backpressure. It is the good exhaust velocity that
creates the low end torque in the first place... Not the backpressure
the exhaust creates at high RPMs.

Chris

On 4/16/05, Alan Fanning <Awfanning@earthlink.net> wrote:
> Please to explain how one might insure "proper exhaust velocity" in a 3"
> exhaust fed by a n/a 2 liter engine?
> 
> Also, what is the basis for your statement that "if you build it properly
> you will gain power everywhere"?  That seems to defy common knowledge that
> engines can't be tuned for maximum power over the whole rpm range. Example -
> going catless on a SHO will reduce low-end torque.
> 
>    Alan
> 
> Chris wrote:
> 
> > There's no reason not to run a very large exhaust, so long as it is
> > built to insure proper exhaust velocity. SHO or otherwise, if you
> > build it properly you will gain power everywhere. Backpressure does
> > *not* improve low end torque.
> >
> > Nice dyno numbers - 50 whp gain is very impressive. Whats next?
> >
> > Chris
> >
> > On 4/16/05, Alan Fanning <Awfanning@earthlink.net> wrote:
> >> 3" exhaust for a 2 liter engine? Shouldn't see any back pressure in that!
> >>
> >>    Alan
> >>
> >>
> >> Scott wrote:
> >>
> >>> Focus Sport UDP, Focus Sport CAI, Focus Central Cam Gears, 2000 ported and
> >>> matched intake, Custom 3" Exhaust, Focus Central 4-2-1 Race Header, Focus
> >>> Central Flex-Pipe, Focus Central 65 mm TB, Screamin' Demon Coil Pack,
> >>> FR/Denso
> >>> wires/plugs, Focus Sport custom tuned SuperChip, Focus Sport Cylinder Head
> >>> (10.6-1) and Kent stage 3 cams.