Does Exhaust Backpressure Increase Performance?

I know a lot of people say exhaust back pressure is needed and you will get better performance with a muffler rather than just a straight pipe (I don't mean a muffler delete on a clapped out civic kind of straight pipe), but on that same note if there is less restriction to flow the engine can breathe more and in turn more power. So what is better, I know high performance drag cars run a straight pipe and so do multiple other high performance cars do. Is there an advantage to having back pressure in turbocharged systems or better for NA systems? Is it something to do with temperature and cooling? I just want to know what everyone thinks and maybe figure out this personal myth of mine.

As far as horsepower goes you don't need any back pressure as it reduces Volumetric Efficiency thus reducing engine power output. It's a myth that engine needs some back pressure for power - something like the saying that "horsepower sells cars but torque wins races"...

Look at it this way, backpressure is pushing back against the piston that's trying to push the exhaust gases out - the greater the back-pressure, the more force that is required, and that force comes from the crankshaft and isn't available to actually produce power/torque.

With production engines, there are many compromises in the exhaust and the exhaust tuning, and backpressure can reduce the differences over the rpm range.

There are a couple of problems, often seen with modified engines, and that's when a "tuned" length exhaust is incompatable with the actual needs of the engine's breathing and/or the fuelling and timing isn't changed to match the changes in the engine's breathing.

It makes sense to me that little to no back pressure makes more power but I was curious because my dad talked to a guy about his exhaust and getting a tune for his Yamaha Eluder motorcycle (1900cc V twin). The guy said the stock pipes with little modification is better than straight pipe or the zombie pipes (aftermarket mufflers) and you’ll get best performance out of the stock with a hole drilled in a plate inside the muffler. And I’ve heard others say that you need some sort of back pressure and you’ll get better performance but it never made sense to me.

It's all about exhaust gases evacuating combustion chamber and geometry of pipes. If pipe diameter (starting from valve seat) is too small - it hurts the flow and you get back pressure. If pipe diameter (again, starting from valve seat) is too big - same thing, you loose velocity and get back pressure. Same with pipe geometry - every single little turn slows flow down and adds up to overall back pressure so it's always a compromise between exhaust gases velocity and particular exhaust system configuration that dictates the amount of back pressure present. At the same time, as Gord said, if proper pipe size is chosen it can increase flow velocity and reduce the back pressure by evacuating exhaust gases faster...

Its the same as in acoustics and loudspeaker design, its a tuned resonnance system. This sounds silly but air speed, resistance and flow will always give a resonnance, where things works by magic, the resonnance frequency gives the magic. This means that more resistance gives a lower resonnance, which helps scavenging and torque at lower rpm (trucks etc), the lesser resistance there is the higher the resonnance is, aka racing. Same as our throat, it's ressonance system. Nature always repeats itself. Some are sopranos and some are baritone......

Ok now the resistance to flow to help with low end torque makes sense, my Yamaha R6 has an exhaust valve called the EXUP and its supposed help with low end torque by partly closing it then opens up in the top end, cause believe it or not a 600cc inline 4 engine that revs to 16,500 rpm doesn't have a whole lot of low end. The big V-Twin cruiser my dad has doesn't rev past 4000 rpm so it is always in the lower rpm and doesn't make its power and torque in the top end like my R6.

Guys, you are getting confused between two different things- back pressure and exhaust pulse resonating frequency. Once again- it's total myth that back pressure helps torque, it just doesn't at all simply because ANY amount of back pressure reduces efficiency of cylinder filling with fresh air and fuel thus reducing amount of energy to be had and converted to torque and power. It's just that simple.

However, the geometry of exhaust and intake configuration can generate differential pressure as much as 0.7 Bar between intake and exhaust valves during overlap. On naturally aspirated engines it's a great help for scavenging exhaust gases and improving engine efficiency. Depending on exhaust header runners diameter and length this help can be configured for specific RPM range (bottom end, mid or high revs) but again - it absolutely has nothing to do with back pressure but high and low pressure waves of exhaust gases.

As far as frequency goes- indeed it can be used for chosing camshafts when special transducer is installed near by intake valve and frequency at top RPM is measured. If the measured value doesn't exceed specific number camshafts with bigger duration can be used.

I will have to side with georg1970 on this one. Back pressure is another of those myths that just wont die.

What you do get is that the exhaust runner diameter need to be narrow enough to give the exhaust speed but wide enough to *not* give it too much back pressure. That way, the speed of the gas creates a vacuum in the other exhaust runners, like a venturi effect. The momentum of the gas will also create a vacuum behind it when the valve closes.

Back-pressure would work against all this, but I can see where the confusion comes from. If the runners are too wide, there is no back-pressure but also no scavenging.