From:
doctorduke_REMOVE_898591_THIS_@yahoo.com
--- In <a href="/group/cosworthvegas/post?protectID=219233066105193209050199029077192253163098100009128121188190230091025019053061151110147">cosworthvegas@yahoogroups.com</a>, "twintonkas <<a href="/group/cosworthvegas/post?protectID=029181253180038031172158203219129208071">mbormann@s...</a>>"
<<a href="/group/cosworthvegas/post?protectID=029181253180038031172158203219129208071">mbormann@s...</a>> wrote:
> I will be replacing my exhaust system this spring and wanted some
> feedback on replacing the Cat. I will be replacing from the header
> back to the tailpipe. My question is, since I am emission exempt,
> should I even bother replacing the cat? Will it hurt performance
> without the added back pressure. Can I run the exhaust with just a
> muffler (probably Flowmaster), and not worry about back firing. I
had
> a '76 Capri with a 2.8 V6 that I removed the cat's on, and I lost a
> ton of low end power and every time I shifted, it backfired. Will
the
> cosworth also have this problem, or will it help the power with less
> restriction?
> Thanks,
> Mike #2874
For ever psi of exhaust backpressure you loose one psi of BMEP,
however, significant reductions of backpressure can upset fuel
metering and make a car run poorly, which is probably the source of
the myth "engines need backpressure". On the CV you can tweak the MAP
sensor adjustment to bias the mixture across the operating range if
poor operation from less backpressure occurs.
The CV suffers immensely from exhaust backpressure. For example, with
the OEM exhaust less converter and PAIR my engine made 109 RWHP at
6000. With my off road open exhaust and no fan it made 151 HP ad 6500
and was still climbing. (The MAP sensor was 1/4 turn rich) The fan
absorbs about 5 HP at 6500.
I recommend that you use a Walker Dynomax Ultraflow muffler. From the
data I have seen these seem to have the best combination of sound
attenuation and low backpressure, but it still might be loud without a
converter. If you decide to run a converter I would recommend a
modern monolithic converter as the OEM converter is much more
restrictive.
Use 2.5" pipe and mandrel bends are best, but expensive. A low
restriction exhaust system is the best way to gain useable power. The
next best way is probably reindexing the cams as has been discussed
here.
Duke
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