Originally Posted by mudpack
That's an old wive's tale you are repeating.
Exhaust restriction will not result in more torque....at any revs. It will result in less.
Care to explain the "physics" of how restricting the airflow through an engine will result in more power?
This company explain why you need a bit of back pressure quite well.
A few of my posts from other threads......too lazy to type it again!
Very low back pressure will only improve performance it the top of the rev range. Hence your referral to drag cars where only power at the very top end of the rev range matters. Torque will decrease throughout the rest of the range...not what you want for everyday driving or off road lugging. This is because more fuel will escape unburnt during the valve overlap period (the time when inlet and exhaust valves are both open during the induction stroke - to help cylinder filling) this means less torque where you need it in the real world. And worse fuel mileage. If you want a gutless engine that needs revving hard (not something these engines like) then get a short straight through pipe. If you want a system that gives a good wide spread of power and drivability then a lot more thought and design is called for to allow maximum power WITHOUT sacrificing to much bottom end power. How you use your vehicle (and its state of tune) will help decide which way you want to go and which pipe design to choose.
The only real exception to this is turbo charged engines where a more open pipe will allow the turbo to spool up faster. This will not in itself give you more power but the power will come quicker (at lower revs) and allow more tuning potential.
Originally Posted by wjjeepman34 View Post
"A muffler and tailpipe swap would not cause a 2mpg drop on its own."
Well it could and often does if the new exhaust has much lower back pressure, especially at lowish revs. At low revs the back pressure helps keep the newly injected fuel in the bore rather than going straight out past the exhaust valve unburnt on the valve over lap phase. So this makes it run slightly lean without the back pressure and inject more fuel to compensate. Does not tend to happen at higher revs. So this is why a freer flowing exhaust can make a car a bit flat in the mid range without affecting the top end or maybe increasing it .
Every time I have put a straight though pipe on cars I have had in the past (in my youth!) the mileage has taken a hit. At low/medium revs there is not enough back pressure to keep the gas in the combustion chambers on the valve overlap phase. Thats why straight through pipes tend to give more power at the top of the revs at the expense of midrange power. Not saying this is def what is happening here....just passing on my experiences!
Agreed for the most part. At the end of if day (and with nothing else in the engine changing) if you want ultimate high rev power at the expense of everything else you want the shortest and biggest bore pipes you can with the widest, shortest intake manifold you can....look at a drag racer...all geared to max power at peak revs because nothing else matters. If you want maximum torque at low revs you want a smaller bore and longer pipe with longer thinner intake. There is rather more to it than that of course for instance if you have very low back pressure the length of the exhaust becomes much more critical to as the pressure waves ( what helps keep the fuel in the bore on the valve overlap phase) can bounce back from an open pipe end and how long this takes ...as in the bore size, the distance travelled (so length of exhaust) get much more important....and so it goes on! In reality we want somewhere in between those extremes and that is what the manufacturer aims for with a nod towards noise and emissions. Now if you want to nudge the exhaust/intake design one way or the other then you can but there will be a price to pay somewhere in the rev range. Ok you can mess with cam/injector timing and cam profile to get back any losses but in reality most average people don't do that and put a bigger pipe on and want gains everywhere but generally it does not work like that.....and I have played with a lot of pipes on quite a few cars and bikes over the years and come to the conclusion that the car makers spent millions developing that exhaust and its very difficult to for an exhaust company (or private individual) to better their designs in more than one area without a cost elsewhere. Just my two cents!
Not surprising really. If you go to a larger bore and/or freer flowing exhaust the motor will make more power.....but only at higher revs and usually at the expense of lower down power and torque. That is the opposite of what you want for normal driving and exactly what you don't want for off road use. Same with shorter bigger intake tubes too....and that's why lots of cars have variable intake geometry (to get the best of both worlds..like BMW etc) and why lots of bikes have variable exhaust geometry (eg Yamaha EXUP) that actually restricts the exhaust size at low revs to maximise low down power whilst opening up to allow max power at high revs. Its all to do with back pressure stopping the fresh fuel air charge going straight out of the exhaust. You want it at low revs....you don't want it at high revs!......And then you have the annoying noise on the highway with a free flowing exhaust....another story!
Nothing to do with you ecu. Its because the back pressure is what helps keep the unburnt fuel/air mixture in the cylinder in valve overlap.....the amount of time that your inlet and exhaust valve are open at the same time on the intake stroke. The purpose of overlap is that the exiting exhaust gases help ensure a full fill of fresh air charge for the next firing stroke otherwise known as scavenge effect. If you have low back pressure more of the unused air (and therefore oxygen) goes straight out the exhaust without ever being burnt. At high revs this is fine as the time for this to happen is minimal and getting the air moving through the engine as fast as poss helps with power output. At low revs it makes for a poor burn and torque loss. I built a straight through system (near enough...it actually had a small silencer but straight through) for my 20v Audi Coupe and it was flat below about 2500 revs....where it spent most of its time on the highway. Put up with it for about a month...then went back to standard. Kolaks system (as referred to above)has a muffler and no doubt is engineered to have the right amount of back pressure so as not much is lost at the bottom to gain power elsewhere. Any engine without a knock sensor could in theory damage exhaust valves and piston crowns without enough BP at low revs causing the motor to run lean at those revs....but that would be an extreme example and much more likely to happen on an old high tune carb engine with larger valve overlap. On a modernish fuel injected car....it will just feel slightly flatter as the o2 sensors and ecu will stop it going too lean....at least in closed loop operation.
Modern motorcycles have a electronic valve in the exhaust to restrict the pipe and INCREASE back pressure at low revs to increase low rev torque . It opens up as revs increase to gain max power. On a Yamaha its called an EXUP valve.
A lot of duplication but you get the gist. Open pipes with minimal bends and back pressure for maximum possible power......well engineered back pressure for a torquy flexible engine. Don't get torque and power confused.
If you don't agree that's fine..... and what forums are all about IMO.Put the Jeep on the dyno before and after and prove it to yourself! I'm out!