I've never really understood this.

Why does an exhaust give you more horsepower (assuming it's a good exhaust for the engine)? The air is already out of the engine...what does it matter as long as the air in the exhaust doesn't "back-up"? Is a better flowing exhaust actually just letting the engine take in more air? example: if you upgrade throttle body, intake, plenum, and not the exhaust...that extra CFM the mods add will not do any good since the motor cannot get it out at the same CFM per second/minute, right? So what is it an exhaust is doing? It isn't letting the engine gain more power...is it letting the enging take in more CFM per second/minute and getting it out without any "back-up" of the exhaust therefore it can gain more power?

Food for thought...i've been wondering about this lately...

 

huh? this is confusing, but a good question.

 

Exhaust helps to reduce the backpressure on the engine.

 

I realize that...but I was looking for a deeper reply addressing back-pressure issues and etc.

 

supposivly on N/a motors back pressure is used to help the pistons on there down cycle at slower rpms, thats why when you release to much back pressure on n/a motors you loose low end torque but in return allow the motor to breathe better up top where you can the HP (becuse when the motor is moving that fast momentum takes over for the down cycle of the piston and backpressure is no longer needed).

 

Will the back pressure myth ever die?
If you have ever watched a leaf floating down a stream, you will remember that in the narrow sections of the stream, the leaf moves quickly and in the wider sections the water slows and so does the leaf. Sometimes the leaf gets into a turbulent spot and just spins in circles, or has difficulty going around obstacles. The same thing happens in your exhaust system.

A short time after your ignition systems lights on your fuel charge your piston is already near the end of it’s exhaust stroke. At this point the exhaust valves open and the pressurized gasses attempt to escape as best they can through a somewhat convoluted path. Keep in mind it is not leaving through an open round hole the diameter of a hole with the valve head under an inch from it’s seat. Not exactly what I would call streamlined but the hot gasses have great motivation to get out of it’s cramped confinement as the piston comes up to squeeze the remaining portion out of the cylinder. I will mention now that the intake valve will open before the piston gets to top dead center and the exhaust will hang open a short time after the piston begins it’s intake stroke. This time is called valve overlap and it’s reasons will become clear further along in this discussion.

So the exhaust is moving through the exhaust port like a cleared Drano clog, a big slug. This slug has a beginning and an end and the goal is to keep it moving as smoothly and as quickly as possible without introducing congestion or unneeded restrictions. If we were only running one cylinder at one speed, exhaust tuning would be easy, find the tugging size that aids in the best savaging at our target rpm and call it a day. What we need is a balance at both ends of the spectrum. Keeping low rpm slugs, and high rpm slugs moving without interruption. If we can time the slugs as they reach the collector so they don’t crash but instead zipper together, they will help scavenge each other through the pipe.

On a even fire V6, 3 pipes a side, headers with equal length tubing will help but attention needs to be paid to the distance the slug travels before the collector or you may have unnecessary collisions are maximum rpm. On single exit systems the Y pipe is also a concern. All of this is also dependant on tubing size. Like I said before the bigger pipe the slower the gasses move. First you need to find the optimum size for one cylinder, calculate how long it will take the slug to travel down the pipe, the collector should arrive in a position so the slug has

Equal length tubing will give your slugs the best chance of missing each other as they meet at the collector.

, a smooth transition to the header is what is most important here, you want the ports to maintain or increase their size from the valve at all times, you don’t want to


The reason bigger pipes make less torque is because the low rpm exhaust slug looses velocity once in the larger pipe. If velocity remains high during valve overlap the last remaining exhaust, and some fresh air, are drawn out of the cylinders by the vacuum created by the slug moving down the exhaust system. This is one of the reasons equal length headers are important, so the slug from the previous cylinder has a vacuum effect on the next one. This is also why single pipe systems tend to get better low end numbers, because all 6 cylinders are pulling on each other.

When top end HP is the goal, the pipe that was sized just right before is now getting a bit overfilled, it still has good velocity but it can only take so much at a time, this is back pressure. Going to freer flowing pipe will stall the slug out at lower rpm but optimize its travel at higher rpms, so you get more HP. Typically these systems claim higher torque numbers too, but usually higher in the rev band.

It all comes down to balance, street cars feel better with torque, having to rev the motor up just to pull away from a light is annoying. Race cars and track cars are better with top end because they spend their time with the pedal mashed. I guess what I am saying is "how are you going to use the car?" granted, an exhaust system alone is not going to free fall your low end grunt, but throw in some other high HP mods (F/I not included) and they whittle away at it.

Intake modifications to flow, oh yeah, you got to get it in there in the first place.



Chris

 

hypersprite...excellent response... very informative

 

Thanks hypersprite, exactly what I was looking for. So, what would you use for an exhaust if you plan on 550rwhp, 650rwhp, and 750rwhp? (3 boost settings) headers->test pipes->y-pipe->single tube piping->single outlet muffler? headers->test pipes->dual tube piping->two mufflers with single outlets? headers->test pipes->y-pipe->single tube piping->one muffler with two outlets?

Low-end torque and more torque than HP is more important to me. I plan on using the Borla exhaust while i'm N/A, but once I go F/I and tune for 650rwhp i'm going to switch out exhausts.

 

Bump...

 

The best flow on any FI setup would no exhaust at all.

If you really wanted the best exhaust setup for a turbocharged engine, it would include test pipes and a true dual exhaust with 3" or bigger piping straight to the rear with the least amount of bends possible. No muffler. Another option you might want to consider is an extra exhaust outlet for each wastegate.

Just my $.02.

 

Bump...

 

FI and N/A are completely different animals when it comes to exhaust system designs. Positive pressure induction gives you much more room for error than negative pressure induction. A turbo or SC will simply push air through. If the exhaust is "too big" to optimize low end torque, push the accelerator further and the FI engine will easily compensate by simply pushing more air and fuel into the clyinder.

Proper piping design on the intake and exhaust side become much more critical when you are running a negative pressure system that pulls air and fuel through the engine at submospheric pressures or atmospheric pressure at best.

While it is true that a narrower pipe maintains a higher exhaust gas velocity, there becomes a critical point at which this is no longer true. This is due to increased dynamic resistance to flow created by the friction and increased turbulence present in any given cross section of the exhaust pipe. Turbulent gas flow is greatest at the gas-wall interface. If the pipe is smaller, more of the cross sectional area will be taken up by the turbulent flow, leaving a smaller "core" of laminar flow in the center of the pipe.

The botom line is that the best guess in terms of exhaust design will maintain tubing lengths to minimize the collision of exhaust gas pulses, the way Hypersprite desscribed, but also one that maintains a the proper cross sectional area to accomodate the volume, and optimize the velocity of whatever combination of exhaust gas pulses is coming through.

That is the physics behind it. It takes a LOT of trail and error in design to get it right in practice. Being in a profession that involves a lot of physics, I can assure you that there is always a sizeable discrepancy between theory on paper and what really happens in nature. That is a subject that is FAR too long to go into here.

In the end, the best guess exhaust design that will give the most consistent performance over a wide range of driving and environmental conditions is one in which the headers are equal and appropriae length and one in which the total cross sectional area is maintained constant from the exhaust manifold outlet ports all the way through the entire exhaust system, and the one that has the fewest connections which are invariably points of increased turbulence. I *believe* each exhaust port is 1.3-1.4 sq. inches in cross sectional area. headers should maintain this diameter per pipe. The collection pipe at the end of the header should be 4.2 sq. inches and the exhaust pipe should be about 8.4 sq. inches. This translates to an exhaust pipe diameter of 3.25".

Of course, cats and mufflers screw all this up too!!!!