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exhaust system vs. timing

Discussion in 'The Garage' started by onlychevy6, Mar 7, 2007.

  1. onlychevy6

    onlychevy6 1/2 ton status

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    ok i changed my exhaust yesterday finally. I went from a 3" exhaust with flowmasters that dumped out just undernieth the front seats to a 2 1/2" exhaust with magnaflow's that exits out the back of the blazer. Yes it is still dual exhaust. No cross over pipe. However it is running wierd. Almost like the timing is off. When you change your exhaust like i did how much do you add or subtract in your timing? To get it to run better?

    I never had this problem before and now after the new exaust the idle is real weak and wants to kinda stall out.
     
    Last edited: Mar 7, 2007
  2. vortec

    vortec 1/2 ton status

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    did it have a crossover pipe with the old setup? duals with no x-pipe can make the engine run funky.
     
  3. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    you may have to make a minor jet change. Have you tuned the idle circuit since the change? bump up the idle a little, and adjust the idle mixture screws. How does it run at cruise or WOT
     
  4. lochenjons

    lochenjons 1/2 ton status

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    What kinda funky? I have duals with no x pipe
     
  5. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    Ya X2...
     
  6. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Time for a new "tune". Changing the exhasut system from what you had to what you have is a pretty good change and the engine will need to be retuned because of that change alone.
     
  7. onlychevy6

    onlychevy6 1/2 ton status

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    Kinda what i was thinking.

    Vortec - Never had x-over pipes on new or old.

    76Zimmer - When i drive on the freeway it seems fine down on power a little but have noticed the torque has picked up. Very quiet inside the cab now.:D
     
  8. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    Well, I would suggest mapping out a timing curve, and tuning the carb more for cruising speed. Do you have a q-jet?
    The smaller pipes I think will produce a little more torque than the 3" pipes you had before. It looks like you had more of a midrange system on there before, like 3000rpm on up. Get your carb tuned in at idle, check your timing, maybe start around 10 static or base timing, and try not to exceed 35 or so total, by 3000rpm. You can really fine tune the carb, ignition, and exhaust to work in harmony, and really wake up your engine. And then you can get into replacing bigger parts for even more power. Just realize that your engine can only produce the best numbers when everything is in sync.
     
  9. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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  10. onlychevy6

    onlychevy6 1/2 ton status

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    The carb is a Q-Jet. It was actually completely gone through already by someone who worked for JET. The motor was built more for torque not speed. However yes i know some adjustments are needed. thanks for all your input.
     
  11. vortec

    vortec 1/2 ton status

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    c'mon guys, "funky" is a technical term. ha. my friend's 69 mach1 had duals with and x-pipe and ran fine. once that rusted, she had duals put on without a crossover, same diameter pipe. idle and accelleration both suffered, even with tuning. once she put a crossover on, idle smoothed and accelleration improved. another friend had a crossover-injection 1982 camaro that had similar driveability issues with an aftermarket exhaust. crossover helped it. i don't claim to know enough about scavenging, etc. to know exactly why they acted like this.
     
  12. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    but you did stay at a Holiday Inn express last night right:haha: :haha:
     

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