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Old 06-24-2007, 06:26 PM   #1
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Question muffler/exhaust and gas mileage

I have a 91 Cutlass Supreme 4dr SL 3.1L. I have managed to get a one time high of 607 km over a period of 9 days. I go to the last quarter then fill up.

My question is this. Is there a difference in gas mileage when the stock exhaust system is replaced with an after market one, dual etc?

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Old 06-24-2007, 06:53 PM   #2
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I missed your intro thread ...so Welcome!
I think I'd start with a good basic tune up myself . Then I'd go ahead and really concentrate on how I drive and see if there are any bad habits I could change . Check your tire pressures and add some to achieve close to max sidewall pressure . As far as an aftermarket *anything* ...research is key . In my case an aftermarket catback hasn't really affected FE at all . I'd really focus on the driver first - you could surprise yourself .
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Old 06-24-2007, 07:03 PM   #3
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I replaced my muffler with a flow through and noticed no difference. Doesn't mean it didnt help.

Better intake and better exhaust will help, but you may need to do both to see a difference. In contrast, driving style can give you a 25% increase without too much effort (and a good intake/exhaust replacement can be a $1K or more)
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Old 06-24-2007, 07:05 PM   #4
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Ditto re. high-flow exhaust. Those are supposed to give a few percent more hp at WOT (wide open throttle) for anyone looking for max hp and acceleration.

When reaching for max fuel economy you're rarely if ever at WOT so I'd expect no gain from that mod. An exhaust system that's slightly restrictive at WOT is going to be basically not stressed at all at 3/4 throttle and less. Which is where you'll be anyway.
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Old 06-24-2007, 07:07 PM   #5
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Well not many people worry about exhaust flow. The issue being that when getting the maximum FE, the engine will be turning slowly. Thus added flow offers no real advantage.
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Old 06-24-2007, 07:09 PM   #6
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MnFocus,

Next week hopefully, I will be getting a tune up done actually. I have made one main improvement in my driving. At one point I got 499km using the gas pedal. After using cruise control, I increased the distance greatly. I am just as happy getting 600 km per 3/4 tank. Hopefully a switch from 10w30 - 5w30 will help. I am very surprised that I am doing this good for mileage for the car, considering the engine size as well as weight of the car.

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Old 06-24-2007, 07:16 PM   #7
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Quote:
Originally Posted by brucepick View Post
Ditto re. high-flow exhaust. Those are supposed to give a few percent more hp at WOT (wide open throttle) for anyone looking for max hp and acceleration.

When reaching for max fuel economy you're rarely if ever at WOT so I'd expect no gain from that mod. An exhaust system that's slightly restrictive at WOT is going to be basically not stressed at all at 3/4 throttle and less. Which is where you'll be anyway.
...good points on the exhaust and why I said research is key . In my case the catback I chose was dyno proven to give increases at the low end and top end . 6 whp /8 wtq . Not a big difference and a bit on the pricey side for the 'improvement' .

Antares , Keep working on technique ! Next to airing up the tires(which is probably the easiest ) ,it's the best and least expensive mod you can do .
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Old 06-24-2007, 08:52 PM   #8
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to large of an exaust can give negative affects for fuel economy, as the exaust looses velosity, and stops flowing as smoothly.
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Old 06-24-2007, 09:36 PM   #9
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I suspect if you do it right, you'll get a good 5% FE improvement. But right would mean a tuned exhaust header and pipes to match/optimize to your target RPM's. Same with the intake... AEM has a well respected tuned intake.

The tuning is to get a waveform match to the incoming air and outgoing exhaust. When done just right, these factors assist airflow. Exhaust for example will be sucked out of the cylinder at the end of the piston stroke due to the timed flow of the overall exhaust through the system.

Of these, the exhuast header and piping probably is the biggest gainer, especially if you have a bad header to start with, such the the exhaust stroke has a fair amount of back pressure to push against. As mentioned earlier, most FE driving is at closed throttle positions, so the positive effects of a change in pressure in front of the throttle will be much less than at WOT.
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Old 06-25-2007, 06:55 AM   #10
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11 mpg out of a 6? That is not good; that thing has some fundamental problems that need to be addressed.
No kidding. My 89 Buick Regal has the same engine, weighs more, and is getting 28mpg without me even trying really hard yet
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