does BACK-PRESSURE increase MPG vs straight thru...

[GeneW]

Originally Posted by 07WYarisRS

The driver affects the mileage way more then any muffler will

Within reason, yes....

I've had cars with plugged catalytic converters..... once the exhaust is sufficiently restricted things get complicated.... Mileage drops off, power is awful and the motor runs hotter.

Originally Posted by 07WYarisRS

Its the ECu's job to control the fuel system , using the MAFS and O2 sensor readings it will comtrol the amount of fuel needed to maitain the same fuel air mixture...
If one did modify the exhaust so that the engine combustion scavenging is greatly effected and fuel is being waisted... the o2 sensor will pick up the overly rich combustion gasses leaving the cylinders and the ECU will alter the fuel and timing to help control it to maintain the factory set air fuel ratio. Sure this may cost you a MPG or two at the most but, I have found in most cases everything kept the same just freeer flowing muffler actually improved MPG. its not until you get into different intake designs, and complete exhaust systems that you see major mileage changes

There isn't a set mixture ratio with narrow band O2 sensors. Usually in systems that use a narrow band O2 sensor the ECU will "switch" mixture in a cycle that centers around stoichometric so that the catalytic converter works properly. This is in part because the mixture response curve of a narrow band sensor is highly non-linear beyond a very small deviation away from stoichometric, and then if you go way too far you cannot resolve the mixture. Hence the cycling of mixture gives you an average value close to stoichometric.

Wide band sensors don't have this problem but they are pricey. The electronics is also pricey. The calibration of the wide band system is EXTREMELY pricey.

As I said in my "explanation" - the devil and salvation are in the details. You have to measure the results of changing the exhaust system.

Originally Posted by 07WYarisRS

Changing the exhaust will slightly alter how efficiently the engine will combust the fuel mixture but on a car with an exhaust system like the yaris (these things have what 3 cat convertors?) VVT-i, the elaborate ECU system the effects from a free flowing exhaust will be very small.

You've actually seen this in the real world? I would want to see such data.

Originally Posted by 07WYarisRS

The biggest issue with an open exhaust is the driver...
As a motorcycle mechanic I'm asked this daily and deal with this everyday.
Why after changing to louder pipes and re-jetting does it use so much more fuel... The answer... the operator.

If you're "rejetting" you're using carbs and probably not using a computer controlled system. I've worked on computer controlled carbs..... never mind.

Opening the jets will make the mixture richer, and MAY improve fuel efficiency if you're sufficiently close to Stoichometric on average.

Originally Posted by 07WYarisRS

With a quiet exhaust people tend to ride it or drove it much easier.. why because there is nothing impressive there... however with a loud exhaust people like to HEAR that exhaust and 90% of the time accelerate much harder and to much higher RPMs...

I have explained to many customers that before thier exhaust they would putt putt through town, now you can hear them honkin on it 4 blocks away...
You don't hear the guys on the Stock Hondas and Harleys flogging on it WOT through the gears like the guys with open drag pipes or fart can exhaust....

I challange anyone to drive /ride the same way with both quiet and open exhaust and you will usually find a very small change in MPG.. I understand there is no fun in that though.

I rode my GSX750F for a year with no problems, got 2 speeding tickets within a week after my new Hindle race pipe.... My mileage on the hwy actually went up slightly if I maintain a steady cruise speed, however now that i find myself shifting at red line or higher rpms a lot more fuel mileage suffers a lot when i'm accelerating like a tool...but man o man the pipe sounds sweet.

Yeah, some people like that "wide open" noise. I liked my straight pipes on my Nova. Sounded pretty bad assed.

Gene