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?

Thank You
Antares

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 .

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)

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.

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.

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.

Antares

...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 .

to large of an exaust can give negative affects for fuel economy, as the exaust looses velosity, and stops flowing as smoothly.

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.

No kidding. My 89 Buick Regal has the same engine, weighs more, and is getting 28mpg without me even trying really hard yet

Looks like he was entering KMs and Liters...I kinda doubt that Olds holds 50+ gallons. :D (just read his log and he wrote that it is getting 25 to 27 mpg)

Welcome.
HOW BIG IS THAT GAS TANK??

I barely do over 600km, and thats with the gauge below the last line and I'm worried I'll have to push soon!

Looking at your #'s, that must be a lot of fuel?

#1 Driving Technique
#2 Maintenance
#3 Up your tire pressure

The rest take a while to get your returns back.

The tank is 16.6gal (50 something L). Can be lots of fuel. Cruise control has gotten me very good economy.

Antares

[
I barely do over 600km, and thats with the gauge below the last line and I'm worried I'll have to push soon!

Looking at your #'s, that must be a lot of fuel?

#1 Driving Technique
#2 Maintenance
#3 Up your tire pressure

The rest take a while to get your returns back.[/QUOTE]

Given the subject of this thread, chew on this for awhile. Let's assume manual transmission. That means the engine rpms are directly related to vehicle speed--mechanical connection. Anything that increses horsepower means more fuel is processed for a given rpm/speed. Cold air intake--more fuel for a given rpm. Tuned exhaust--more fuel for a given rpm. If it gets more air, the computer will inject more fuel. Seems to me for FE the only effective modifications deal with how efficient the engine is. Lighter synthetic oil. Better engine blueprinting. More efficient accessories. Things like that.

Horsepower could make a difference if the drive ratios were changed to compensate. But, unless there are changes in tire diameter or differential ratio, it seems to me that more horsepower's not the way to go. This pains me 'cause I'm an old hot rodder.

Smaller exhaust?
 

I'm always late to the thread, but here goes anyway...

If the exhaust system needs to be replaced, go with stock if mileage is of concern not power. Nearly all vehicles have good exhaust systems when under light loads such as when driving for mileage, things change when you have high horsepower demands like when towing, as back pressure increases. Under high demand, energy is spent pushing the exhaust gasses out the pipe instead of propelling the car. Mileage improvements with exhaust systems are seen when the exhaust systems are heavily taxed with flow, this happens when you have high hp demands like in motorhomes, towing up hills, racing, etc. When driving for mileage in nearly all cases you will not have high hp demands - go with stock. If you are committed to a really light foot you can go smaller, just don't expect as much top end hp. Oh, and mandrel bent gives less turbulent flow reducing high flow system resistance.
For replacement systems I'm a fan of Walker DynoMax mufflers and stock pipe diameter. DynoMax has the flow directors for increased flow with reduced back pressure increase as flow increases, come in stock sizes and sound pretty good too, not loud and obnoxious. Alot of the auto-x ers use them, qualifies for stock in most if not all clubs, but flow much better than stock. I used stock pipe diameter to retain low end torque, but the most restrictive part, the muffler, replaced with one that doesn't constrict as much at high flow velocities.
For pure mileage, I think the ultimate system would be smaller than stock (size depends on engine hp demands), no muffler, smooth long radius bends and a 30" megaphone tuned to the most used RPMs, then more sharply tapered back down before the exit. For the street, replace the megaphone with a DynoMax.

Exhaust sytems have an increasing rate of resistance as hp output and flow increase. Bad systems have a rapidly increasing rate within the required flows, a large diameter pipe that reduces velocity, sharp bends that negate any tuning and increase turbulence. Good systems have a slowly increasing rate within the required flows, small diameter that keeps velocity high, smooth bends that keep turbulence to a minimum.
I read somewhere that he old benchmark Porsche 944 (naturally aspirated) actually performed better with SMALLER than stock size exhaust and a less restrictive muffler, the reason? Increased velocity at low RPM helping torque, and increased flow for a given pressure at the top end. Raw top end power was not as great as a larger system, but the system was designed for auto-x which values all-around performance, not the biggest advertisable splashy number.
Not to say all vehicles will respond this way, but my next system for the pickup will be smaller than stock.