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exhaust system vs. timing

onlychevy6

onlychevy6

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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.
 
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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.
 
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
 
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.
 
4X4HIGH said:
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.
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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
 
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.
 
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.
 
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.
 
vortec said:
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.
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but you did stay at a Holiday Inn express last night right:haha: :haha:
 
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