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Old 09-06-2004, 04:58 AM   #1
jzvette
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Default Free Flow exhaust, good or bad??

My floor is hot and I would like to up the performance of my 89. It was suggested that I get a new y-pipe with no cats and that I get a newer main cat or just hollow out what I have. I hear that a better flow isn't always good, supposedly I need some back pressure. I guess there's two schools of thought on this subject. I would like your feedback. Will removing my pre-cats cool the floor?? What are some of my options when it comes to getting a better flowing exhaust?? My engine is stock, although I wouldn't mind adding a chip. How much HP can I get from a free flow exhaust??? What do I do with sensor on pre-cats??
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Old 09-06-2004, 10:35 AM   #2
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cant speak for precats, because my vette doesn't have em... but if your floor is getting hot, you're precats (or cats) are probably clogged... just putting on new ones or high flow ones would free up some HP
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Old 09-06-2004, 11:08 AM   #3
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Back pressure is your enemy not your friend.

As David Vizzard puts it in his books on SBCs, you just need to tune the engine for the reduced back pressure.

Also install an AFPR so you can add a little fuel at WOT when you go to a cleaner exhaust system.
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Old 09-06-2004, 11:17 AM   #4
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the L98 will see about 15-25hp freed up by eliminating the chokehold from the factory exhaust. Long tube headers (re-locate the o2 sensor bung to the passenger side collector @ 11:00 position), front Y-pipe w/o pre-cats, gut your main cat (be sure to install a "test pipe" thru it), and a decent cat-back (or muffler elims) will get you on the high side of +25hp.....alos your vette will sound a helluva lot better. i agree with the previous post about the probability that your pre-cats are clogged (floor heat). be prepared for a nice SOTP kick.
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Old 09-06-2004, 11:23 AM   #5
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And would like to add header primary length and diameter also plays a roll. I can't find the dyno sheet I got with my car (88) but with free mods and headers it was putting down either 340 or 320 RW ft lbs at 3000 RPM with true duals and no cats.
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Old 09-06-2004, 12:50 PM   #6
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Upstate,

What else did you do to your car to get 340???
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Old 09-06-2004, 12:51 PM   #7
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Bradvette,
Can I put a completely free flow exhaust with a chip?? How much HP can I get with that??
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Old 09-06-2004, 12:54 PM   #8
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65Z01,
Any guess on how many horses I can get with a free flow exhaust and a chip from an 89 stock engine???
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Old 09-06-2004, 01:29 PM   #9
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Quote:
Originally Posted by 65Z01
Back pressure is your enemy not your friend.

As David Vizzard puts it in his books on SBCs, you just need to tune the engine for the reduced back pressure.

Also install an AFPR so you can add a little fuel at WOT when you go to a cleaner exhaust system.
Just need to tune for free flow.
I showed this pic yesterday on another thread of my handywork with my pre-cats..here it is again.
http://www.pixfiles.net/pix/336793284556831609.jpg
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Old 09-06-2004, 01:34 PM   #10
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All free mods and some extra's here
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Old 09-06-2004, 01:35 PM   #11
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"AFPR"... is that a fuel pressure regulator? How do you tune them to the right amount?
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Old 09-06-2004, 01:49 PM   #12
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I gained about 15-20chp from doing the front Y, RT cat and TPIS cat-back. Vic-89 had the same exhaust setup and picked up another 20chp when he installed 1 3/4" FL coated headers.

Since I have dyno data with A/F at WOT along with stock O2 sensor readings down the 1/4 mile I can tune my AFPR from the stock O2 sensor using my scanner.

If you don't have such data and can get consistent trap speed readings you can tune fuel flow for max trap speed, either at the strip or using a G-tech.

I prefer to use an AFPR for tuning as even a custom chip must be burned each time you make an air flow mod whereas you simply bump fuel pressure with the AFPR.
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Old 09-06-2004, 02:15 PM   #13
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Quote:
Originally Posted by jzvette
Upstate,

What else did you do to your car to get 340???
My old setup is nearly identical to cdnguyred88's setup, with the addition of an aftermarket converter, the AIR pump, and stock RR's. The dyno numbers are damn near identical to boot.

The previous owner of my car cut a best of 13.2xx with that combo running without an air cleaner.
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Old 10-08-2004, 04:06 AM   #14
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Quote:
Originally Posted by 65Z01
I gained about 15-20chp from doing the front Y, RT cat and TPIS cat-back. Vic-89 had the same exhaust setup and picked up another 20chp when he installed 1 3/4" FL coated headers.

Since I have dyno data with A/F at WOT along with stock O2 sensor readings down the 1/4 mile I can tune my AFPR from the stock O2 sensor using my scanner.

If you don't have such data and can get consistent trap speed readings you can tune fuel flow for max trap speed, either at the strip or using a G-tech.

I prefer to use an AFPR for tuning as even a custom chip must be burned each time you make an air flow mod whereas you simply bump fuel pressure with the AFPR.

Your stock O2 sensor data is useless at WOT. The narrow band O2 sensor simply defaults to "rich" at WOT, and the ECM goes into open loop and ignores the O2 signal & runs on preprogrammed look-up tables based on TP angle, coolant temp, and engine load or airflow.

OEM GM type O2 sensors can only detect air/fuel mixtures in a very narrow window that is centered at 14.7:1. Any stock GM car runs much closer to 12:1 a/f ratio at WOT. A wideband O2 sensor and accompanying aftermarket ECM are needed to run in closed loop at WOT.

Bumping fuel pressure richens the mixture across the RPM range, which isn't good for idle quality, emissions, catalyst life or low RPM cruise. It may or may not help your E.T., depending on your existing PROM settings.

Last edited by emissions_guru; 10-08-2004 at 04:11 AM.
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Old 10-08-2004, 04:06 AM
 
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