Exhaust Question

Hey guys I have a question for you all. Do you guys think or have any data that a cat back exhaust hurts your mpg? I have a gibson cat back exhaust system with the 4in. deck plate mod and I just cannt get any better then 17.85 MPG. combind hwy/ city running the a/c. And 16.65 to 17.25 with winter blend gas not running the a/c. Iam just wondering if I had the stock exhaust system if I would get better MPG. I have cleanned the MAF sensor and the throttle body. It has been tuned up and everything is up to specs. OR is that average for a 99 prerunner with a 3.4L auto tranny, running 31x10.50r15 tires, and a fibber glass shell. Any input welcome.

 

thats pretty good for an auto

 

The tire size drops your milage. i was getting 18-20mpg stock 4.0 V6 auto. I put 2657516 and now i get 17-18 mpg. i have a flowmaster 40 and nothing changed until i changed thr tire size. I knew it would when i did it but i love the way it looks, so i'm happy.

 

i got 20 hwy with 31x10.5s. 31s is stock size man.


OP:The only way an after market exhaust system can negativly affect your motor is if they is a loss in Back preassure.

Now after all the reading i have done. I have read that full cat back exhaust systems aren't worth it and that just replaceing the stock muffle itself and leaving the stock pipe is best to let the exhaust breath a bit more, but not too much. The 3.4l motor is very picky about its back preassue and most ppl that opt for a large pipe size over stock tend to see negative gains.

I can tell you i put on a 14'' Magnaflow muffler and 2.5'' pipe out from it which is larger then stock. I saw a small decrease in bottom end and a tendency to stall out coming from high revs and sometimes starting.

I have no idea what the gibson system would do..but thats what i know about exhaust effecting the motor.

 

Yeah your right 31x10.50 15 are stock. The Gibson you cut the stock muffler off and install there muffler and a 2.5in tail pipe. I didnt notice any loss. It is doing what thay claim. Just wondering if I would be better off going back to stock?

 

I wouldn't think so no.

 

Ok thanks guys. I guess it is what it is!!!

 

A good post by Blackdawg - bigger isn't always better. With modern engines, the old school tricks of adding dual exhaust and radical aftermarket systems to improved performance may actually yield reduced power and fuel economy. Tuning the engine afterwords might be necessary to optimize both.

You are right to seek data to support your decision making. However, in doing research for an aftermarket exhaust solution, I found almost no data to support the claims being made by the manufacturers of exhaust systems themselves. And, you need to really understand the conditions of test for where there is published data on performance gains. Most claims made are from bench tests done under controlled conditions and while verification is essential, real world VALIDATION (with the scientific data to back it up) is also essential. Validation data is nonexistant in my searches.

What you get with aftermarket is a different sound for the most part. Unless you have a tuned exhaust system, the odds are that you'll experience reduced performance (power and fuel economy) throughout the range of operation. Some areas you might do better, but you'll need the data to support contentions. Most use the 'calibrated butt' to detect minor power increases and some nonscientific (but usable) data doing fuel studies (miles traveled/gallons at fillup).

Tire size and aggressiveness of the tread are also essential factors as are driving habits and geographical terrain. Some aim for a certain look for their vehicle, so the loss of performance is an acceptable tradeoff. Vehicle configuration impacts performance. But my suggestion is that you determine how you want to configure your vehicle, do the research like you are doing, understand and accept the tradeoffs, and drive with a smile on your face.

I'll sacrifice some MPGs to have a vehicle that sits up higher. That's a good tradeoff in my book.

Good luck!!

Kevin

 

Thanks Rackster. That gives me somthing to think about. But yeah I agree, being lifted and having a little more ponies when pulling a hill or trailer is a good trade off. I appreciate your write up!!!

 

Anytime!