Back Pressure in NA, Is it really needed? Options

[FAQdaWorld]

Ok, i know everyone says that Back Pressure is needed in NA engines but i need to know whether this is true.

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If back pressure is actually REQUIRED, then why do you see dragsters w/ nothing but straight pipes coming directly off the engine? Isnt it more likely that a car simply loses power because its not tuned correctly? Granted there is going to be some back pressure caused by expanding gasses in teh dragsters exhaust, but minimal say compared to mufflers and such.

I have read in several places that new cams and such will solve problems associated w/ loss of backpressure, and basically cars just arent stock set up to run w/ much less back pressure.

So say you're allowed to do anything you want to an engine, besides overbore w/e, or FI, is almost no backpressure really bad? Say you had an exhaust like this.... CLICK
but minus the turbo of the wrx?


IONO maybe i'm rambling cuz i was just arguing w/ my friend about this....



-Ryan

[SpedToe169]

It would run like crap because it has no exhaust gas inerita to scavenge the cylinders(I never said a short exhaust was necessarily good, just that it should have as little backpressure as possible). Think of the moving exhaust as a continuous column. If that column is moving it has inertia. If there is something slowing down or blocking the front of the column, then the molecules directly behind the front will slam into the ones in front of them and so on down the line. Ovbiously this will slow things down, eventually it will slow things down at the beginning of the exhaust (in the head and the cylinder)(think of a traffic jam....you're stuck in traffic before you get to the accident). Any obstruction to the flow of the exhaust will slow it down and reduce its inertia. It relies on inertia (scavenging) to help pull the exhaust out of the cylinder each time the exhaust valve open. If an engine has no exhaust outside the head, then the velocity will slow rapidly (theoretically to 0) as soon as it exits the head. There will be no moving column of air, so there will be no inertia, so there will be no scavenging. The engine will still run because the exhaust gas in the cylinder will be above atmospheric pressure, but it will not make very good power at all because there is no scavenging.

There are however, exceptions. Top Fuel dragsters are the most notable. They have very short exhausts coming out of each individual cylinder and they make very good power. How you ask? They are supercharged. The intake air is pressurized to way above atmospheric pressure so the exhaust gas is also way above atmospheric. At first glance this would seem no different from the standard car except that there is a higher pressure gradient, but there is one other trick up the top fuelers sleve.....They run cams with large overlap. The intake valve and the exhaust valve are both open together for a split second during the exhaust stroke. This means that the positive pressure from the supercharger can 'blow' the exhaust out of the cylinders and scavenging is not necessary. The caveat to this is that they also blow a HUGE amount of unburned fuel and air out of the exhaust headers along with the exhaust. Its terribly inefficient but produces gobs of power. It also creates beautiful blue flames at night when the hot exhaust valve ignites the unburned fuel as it leaves the engine.

This might also work with turbo cars but not nearly as well and only at full throttle and on boost. The overlap in the cams would cause the engine to run really bad off boost and probably make it really laggy. It would likely also eat up the bearings in the turbo and possibly the turbine wheel from the raw unburned fuel (which would end up burning in the exhaust).

In short, its a fun discussion but its not practical for anything we'll likely ever do.

If you're really serious about power there is a LOT you can do with tuning the length of the exhaust and the placement and type of collector. Unfortunately this is very expensive and very time consuming (lots and lots of dyno time). We've done a lot of this with our race cars (read; hundreds of hours on the dyno) and found good gains, but theres still more to be had.