Exhaust affecting torque -- hunh?

OROutdoors · Post by **OROutdoors** » March 8th, 2007, 3:52 pm

Okay, so I can understand most of what I read about cars, and understand the reasons for the how and why. But I have been thinking on the generally accepted idea that if you put in too free-flowing of an exhaust, that it negatively impacts low-end torque. I have thought about this time and again -- I just can't come to terms with it. I just can't decipher a reason for why spent fuel and air having a low pressure would affect the power created by fresh air and fuel in the combustion chamber.

The only two affectors I can come up with are these: the oxygen sensor may not work correctly at low pressures; and, having a higher pressure exhaust (lower flow rate) would leave more exhaust in the combusiton chamber after the exhaust stroke. I can't believe the exhaust pressure would cause the O2 sensor readings to change unless the exhaust system was sucking in air -- but that's a problem with leaks, not pressure. The only thing I can think of is if more spent fuel and air remains in the combustion chamber, then the new mixture on the next intake will burn more slowly.

Does anyone here know the reason for the link between low exhaust pressures and reduced torque? Thanks...

xtreme velocity · Post by **xtreme velocity** » March 8th, 2007, 4:18 pm

i really dont know much about this..

but i can throw an idea out..

the O2 sensor tells the motor how much gas and air to use by detecting the fumes..with a free flow exhuast there is no back presure for the O2 sensors to read anything..the air will just blow by the sensor and it gets no reading..

just unscrew your sensor and let it hang and go driving..that would be the diffrence..i would think..

if im wrong pleaser dont hold it to me..im just throwing ideas out..

heck im still tryin to learn about this car stuff still..lol...

but i hope someone can give you a honest answer..

lowflyinmx3 · Post by **lowflyinmx3** » March 8th, 2007, 4:25 pm

People commonly mistake the words backpressure and scavaging. If you have too much of a free flowing exhuast the air coming thru the intake would just take its time and not really be forced into the engine in any way. This is what you would call scavaging, when the engine isn't bringing in enuff air to make power due to low back pressure. if you have a good amount of back pressure then, in theory, the air coming in to the engine would back up, kind of like a traffic jam, causing more air to be forced into the cylinders therefore creating power.

Thats how i understand it. hope it helped

Yoda · Post by **Yoda** » March 8th, 2007, 4:41 pm

This affect is the same an any engine where it is EFI or and old school carburated engine with no emission controls. What it comes down to is air flow dynamics. Air flow and hot gas dynamics don't necessarily follow what most people would see as logical. A very simplified explaintion is that as the exhaust gases cool they slow down and condense. If they slow down to soon in the exhaust system then the need additional energy to push it out the tail pipe.

On the other hand a straight through exhaust system will create back pressure at the top end due to air flow and hot gas dynamics

Do a search I pretty sure that I wrote several posts or people have reposted info from other groups explaining this in more detail in the past.

Tunes67 · Post by **Tunes67** » March 8th, 2007, 4:47 pm

First.. read this...

http://www.mx-3.com/phpbb2/viewtopic.php?t=41096

Wikipedia defines Scavenging as follows

Scavenging is the process of pushing exhausted gas-charge out of the cylinder and drawing in a fresh draught of air ready for the next cycle in automotive usage. This process is essential in having a smooth-running internal combustion engine. Modifying the exhaust system, (i.e. modifying the back pressure) can detract from the "ideal" scavenging effects, and reduce fuel efficiency and power.

Now.. in a nutshell..

Each pulse of exaust gas out of the engine creates a small vacuum in its wake as it travels down your exaust pipe. This vacuum helps "Pull" the next pulse of exaust out of the cylinder and into the exaust system. When your exaust pipe is too big in diameter.. this pulse slows down.. reducing the amount of vacuum and pulling less and less exaust gases out of the next cylinder in line. The result is.. there is more spent exaust gases left in the cylinder for the next cycle. Spent exaust gases dont burn anywhere near as well as fresh air & fuel... so you lose power.

Now.. the next step.. if your piping is too small in diameter OR you have sharp bends or curves in your pipes.. this will create backpressure in your exaust system.. which has a similar effect.. it slows down each exaust pulse which in turn doesnt have the beneficial effect of helping pull the spent exaust gases out of the cylinder.

Headers help reduce backpressure because they have a nice smooth pipe that moves the exaust pulse further away from the engine before entering the exaust system proper. This means that an exaust pulse would have to travel all the way back up each individual tube to the cylinder to create backpressure in the cylinder.. since each exaust pulse is split by the number of pipes per header.. it effectively helps divide the amount of backpressure each cylinder feels for any given exaust pulse. The idea is to increase the speed of the exaust pulse.. and eliminate backpressure.

When people talk about letting their engine "Breathe" better.. they are talking about both sides of the coin.. You have to increase the engines ability to Intake fresh air along with increasing the engines capability to Exaust the spent gases. Its all a balancing act really. You will notice minor gains from either side of the coin.. but you will notice maximum gains if you increase the engines ability to breathe efficiently overall.

This is the way I understand it more or less.. I may have some terms backwards or defined improperly. I am sure there will be someone to correct me if I dont have the gist of it correct

Hope this helps.

Tunes67

lowflyinmx3 · Post by **lowflyinmx3** » March 8th, 2007, 4:59 pm

^^^^ well written tunes

xtreme velocity · Post by **xtreme velocity** » March 9th, 2007, 12:04 am

made me look dumb..

but thanks for clearing that up..i understand it alot better...

basicly if u get more air in and more out at the same time you will notice your big gain in power..(i think)

but thanks..

good post..

wytbishop · Post by **wytbishop** » March 9th, 2007, 3:05 pm

The word backpressure is incorrect and should be stricken from the automotive encyclopedia. If we're talking fluid dynamics, there is technically no pressure in the exhaust system but astmospheric pressure as it is open to atmosphere at the end.

The hot gases exiting the cylinders have ENERGY. There are 3 ways a fluid can have energy...Thermal energy (Heat)...Potential energy (Pressure)...and Kinetic energy (velocity). The pressure in the exhaust is effectively constant so the forms of energy we can control to a degree are the heat and the velocity. Heat is effected by the size of the pipe because, as stated previously, as a gas expands it cools (Boyle's Law) and as it cools it loses energy and the ability to do work, ie. travel down the exhaust track. If the tub eis too small, the piston has to actually compress the exhaust gases out of the cylinder, wasting energy (HP).

The velocity of the gas is effected as much by the length of the runner as by the diameter of the tube. This is because of the hormonics mentioned earlier. The pulse from a cylinder travels down the pipe to the colector or some other obstruction like a sharp corner, bounces off it and returns up the pipe to run into the next pulse on its way down the pipe. This is called destructive longitudinal wave interference and is what kills the scavenging effect. By interfering with the flow of the individual pulses (witch are coming about 50 per second at 3000 RPM) the momentum of the general flow of gas in the pipe is retartded. Scavenging is all about momentum and maintaining that vacuum Tunes was talking about. The slower the flow the less trailing momentum is created. Harmonics is wiked complicated. Two strokes have to make extensive use of harmonics because the pulses are leaving twice as often. Harmonics are also very important with turbo's becausee the destructive interferance tends to slow the compressor.

Tunes is right on the money in a practical way. I just thought I'd drop some physics on y'all.

Wytbishop
Mobile Equipment Technical Advisor
Weatherford Canada

OROutdoors · Post by **OROutdoors** » March 9th, 2007, 3:55 pm

Tunes, that was a great description. I had not taken into account the momentum and inertia that the gas develops while leaving or entering the combustion chamber. You have helped open my eyes to an entirely new area of how the changes in pressure would affect and engine's performance.

It seems, this has numerous implications for tuning engines -- from length of individual intake tubes from the common intake, to the "bowl" just above the intake valves, to the lungth and design of exhaust headers. Further, when taking into consideration the amount of tme needed for air or exhaust to change velocity and vecor, one could easily imagine there would be a "sweet spot" range of RPM that an engine would turn which would yeild the greatest power.

On a normally aspirated engine, the pressure of air entering the cylinder during intake could be more than 1 atmosphere of pressure due to the "collision" of air's momentum (from the previous intake) with the closed intake valve. If the valve opens again before the slightly higher pressure equalizes, then presto, there is more than one atmosphere of pressure pushing the air into the cylinder.

Similarly, it seems the vacuum left behind the rapidly exiting exhaust could help the intake stroke. By closing the exhaust valve at just the right time, the combustion chamber is left with a lower pressure than a standardized atmosphere. The lower pressure (at the specific instant that the valve closes) would mean less exhaust remained in the chamber, so the intake air would be cleaner. And, that isn't even taking into consideration the possibility of having both I and E valves open for just a few moments while the pressure difference is greatest -- which could allow the fresh air to push out the remaining exhaust, yeilding an even more clean intake. And, with EFI, the fresh air wouldn't have as much change as igniting and backfiring (as carburated would have) if the fuel is triggered into the cylinder just after the exhaust valve closes. This would be a similar effect that 2-cycle engines rely on (2-cycle now makes more sense to me too now). AND, if a little fresh air makes it through the cylinder into the exhaust header while both valves are open, then even the cat conv would be more effective...

I've got to think on this some more... I'm sure there are more implications. Thank you Tunes, Yoda and Wytbishop for your great explanations.