Any time you change something on the car you have to reprogram the computer?

I don't buy it. Why then does the car have a hotwire MAS?

I gutted my cat and noticed a nice gain. Nothing HUGE but I wasn't expecting that. I agree a straight pipe is better than having an expansion chamber in the middle of the exhuast but the bottom line is turbos HATE backpressure. ANY backpressure and especially cats. I lost a PILE of hp in my Nissan by sticking on a 3" high flow cat. Granted this was over 275-300hp but still. The car just choked compared to a straight pipe.

By eliminating even a few PSI of backpressure post turbine you do several things.

1. Eliminate a LOT of backpressure in the manifold. Turbines have a pressure differential and the backpressure post turbine is magnified as a ratio in the manifold. ESPECIALLY when you turn up the boost and begin to work the turbo beyond what it's designed for. I have not done the calculations for the wheel in the Saab T3 but as an example a T25 (Which will support about the same hp as the 58trim wheel in the Saab T3) with a moderate exhaust with say 6psi of backpressure post turbine will see about 25-30psi in the manifold when pushed! I've seen it spike as high as 50psi when the turbine section chokes and can't flow anymore! That causes reversion which leads to bad things like detonation and burned exhaust valves.

This is also the reason external wastegates vented to atmosphere usually make a large increase in hp. They are venting manifold backpressure.

2. The turbo will spool quicker due to the pressure differential pre and post turbine. This can make a large difference in some cases.

3. This will also increase over efficiency of even the compressor wheel because the turbine doesn't not have to commit work to getting around backpressure. That work is now translated into the compressor side.

4. By your theory we can't even upgrade the exhaust system because we then end up screwing with the "flow charateristics" of the engine and the computer can't adjust. That's why it's a hot wire "Mass Air" system. It is supposed to make changes based on the engines VE...which will change with NO mods just from atmospheric changes. (I.E. Barometric pressure or elevation)

Just sharing some info from my experience turbocharging all kinds of cars and getting into late night talks with a Garrett Engineer who is a friend.

posted by 24.18.134...

• Gutting a SPG cat, BuHuSPG, Wed, 26 Dec 2001 23:37:14
  • So whats the verdict, BuHuSPG, Thu, 27 Dec 2001 23:35:00
    • Cat Guts, SteveW, Fri, 28 Dec 2001 10:43:49
    • the turbo is the backpressure!, dave n, Fri, 28 Dec 2001 08:59:52
    • Re: So whats the verdict, Jay Hassinger, Fri, 28 Dec 2001 07:45:10
  • Re: Gutting a SPG cat, SDP, Thu, 27 Dec 2001 12:36:29
    • Sorry but I have to disagree, Jay Hassinger, Fri, 28 Dec 2001 07:55:13
    • Great thread, now what kind of cat is the best...., lms, Thu, 27 Dec 2001 17:48:20
      • Search- high flow cat, but why 3" down? (long), SDP, Thu, 27 Dec 2001 20:11:21
  • remove it with the downpipe, dave n, Thu, 27 Dec 2001 06:43:01
    • Re: remove it with the downpipe, Johann F, Thu, 27 Dec 2001 08:01:00
      • So by your logic then, Jay Hassinger, Thu, 27 Dec 2001 08:47:30 <-- Viewing This Message
        • Re: So by your logic then, Johann F, Thu, 27 Dec 2001 09:50:11
          • Re: So by your logic then, Jay Hassinger, Thu, 27 Dec 2001 10:50:50
            • no theory here., dave n, Thu, 27 Dec 2001 16:10:07
            • Re: So by your logic then, Johann F, Thu, 27 Dec 2001 12:50:33
              • Well some final thoughts, Jay Hassinger, Fri, 28 Dec 2001 07:58:52
                • Re: Well some final thoughts, Johann F, Fri, 28 Dec 2001 08:27:46
                  • Re: Well some final thoughts, Justin VanAbrahams , Fri, 28 Dec 2001 11:57:46
              • Agree- and some, SDP, Thu, 27 Dec 2001 20:28:51
              • Dunno which FI system you're referring to... but,, bazil burns, Thu, 27 Dec 2001 15:26:46
                • Re: Dunno which FI system you're referring to... but,, Justin VanAbrahams , Fri, 28 Dec 2001 11:35:45
  • Re: Gutting a SPG cat, chrisx, Thu, 27 Dec 2001 04:09:34
  • yes but just a few considerations, Quinn S. Kurz, Thu, 27 Dec 2001 01:51:26

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