944JM

If, in exhaust systems, low end torque is affected by how much back-pressure/restriction, and high end HP is all for free flow, where is the 'best' place for a 'low end torque' exhaust system to have some restriction? At the header collector, muffler, throughout the system?

Let's assume you have a high flow system and want to boost low end torque via exhaust. Would a restrictive tip do the same as a change further upstream for example? Or would the change need to be closer to the header?

orcfromthesouth

In my experience you want the restrictor either inside the muffler (say 1/2 way in) or at the exit. Stay clear of the headers. The BMW larger engined 3's have 2 tips, and one of them stays closed most of the time. At WOT it opens and lowers backpressure.

Orc

944JM

So BMW has made a high flow system with a variable restrictor in the exhaust tip. Is there any data on just how much this helps? Something like a before and after dyno run? Is there a reason that Porsche doesn't have something like this?

earlyapex

Porsche's now have continuously variable valve timing, which maximizes usable torque and hp.

1999Porsche911

You are correct that back pressure will give you more bottom end power, but only up to the point or choking the engine. I perfer power at the low end and that is why I stayed with the stock exhaust on my 996. As far as placement of the restriction device...it does not matter where in a properly installed exhaust. Flow for the entire exhaust will be limited by the smallest pont in the system wheather it be the tailpipe or restrictive header gaskets.

944JM

Speaking of the restriction placement, that's what I thought. Was curious if the added volume of the system as a whole (restrictive tip) would give it a sort of lag as pressure built up, but I guess the time involved is miniscule and the spring/flap affair keeps it fairly even. Lacking the ability of variable valve timing in the 944, is there a reason there is little to no one looking into this? (other than $$$) I have a Bursche(spell?) header on my N/A and was told it was a low end unit due to the 4-2-1 design ending with a 2.5" dia collector. Would a high rpm unit be one of a 4-1 with a 3" collector?

Skunk Workz

Backpressure is NOT needed in ANY exhaust system. Reason people think so is just because they've added an open exhaust and lost hp down low. Reason for this is that you need to have a richer AFR from idle to about 2400 rpm to counteract the more free-flowing exhaust. Nothing else.

NineMeister

Low end torque is affected by poor scavenging of the cylinder, usually caused by overlarge headers/port/valve or an excessively long cam. Big ports = low velocity = high back pressure, therefore in these cases restricting the flow in the port will help to bring up gas speed and reduce pressure.

Therefore any restrictor worth fitting to an exhaust should go in between the header and the head, usually large washers will do the trick if tack welded into place, you will have to experiment with the hole size .....once a suitable size is found you can make up a small bore headerif funds permit.

ebaker

The variable intake valve timing on the 996 turbo has 2 positions, normal and retarded. The retarded (low overlap) position is to provide cleaner running at idle and very light low rpm loads. It does not increase torque. At any engine situation with over ~20 ft. lbs. torque output and over 1200 rpm the cam is advanced.
I believe only Honda has continously variable valve timing.
Increasing back pressure does not inherently increase low rpm power. Smaller header pipes will increase the low end torque. This is because the air column in the smaller pipes has increased velocity and momentum. This momentum tends to pull more residual exhaust gases out of the cylinder near the end of the exhaust stroke.

911pcars

I agree with skunkworks. Restricting the exhaust system is not going to increase the torque output. Stuff a potato or other restrictive object in the exhaust tip and see.

However, adding a crossover (H) pipe to the exhaust system (given two banks) before the mufllers has been known to provide increased low/mid-range torque.

Sherwood

1999Porsche911

More back pressure equals more low end torque. (up to a point) I don't know why this fact is being argued. Energy cannot be distroyed. In a combustion engine, the amount of energy created by the combustion process is used in 3 ways. Heat, exhaust and pushing the piston down. The more energy that is allowed to escape through the exhaust, less energy is available to drive the pistons and therefore the crank.

Decreasing the exhaust back pressure results in the same low end torque as does increasing your valve lash. It allows more exhaust to escape which draws more of the recently injected air/fuel mixture out of the combustion chamber befre it can combust. That is why your engine runs richer with a more free flowing exhaust. (assuming no adjustments are made to compensate) At higher rpm's this waste of fuel is overcome because the intake system is sucking in at full speed and the combustion process is too fast to lose too much mixture out the exhaust before it combusts.



A simple example would bethat if the engine uses the energy equally between heat, exhaust and piston pushing, then obviously, if you increase the amount of energy used by the exhaust, you have less for pushing the car. That assume everything else remains constant.

In order to maintain the same low end toque you had before decreasing the back pressure, you would have to modify fuel/air mixture and flow as well as timing settings.

Skunk Workz

Strangely,every engine I've ever changed to a more open exhaust ran leaner...not richer. Had to increase,not decrease, the amount of fuel at low rpms to re-gain the low rpm torque,throttle response and idle quality. If you lower the backpressure,you need more fuel to compensate because the engine is now dumping the exhaust more efficiently,thereby increasing the amount of fresh charge in the cylinder to be ignited. More air and less residual exhaust to be re-ignited (except the exhaust doesn't burn or yield the combustion process in a positive way at all)= more fuel needed.

NineMeister

...or more probably, the reduced back pressure causes more intake charge to be lost into the header pipe at low rpm overlap, so you have to run richer to ensure that the same ammount is available in the cylinder to sustain combustion. Have you measured the hydrocarbon emisssion change on the same engine at low revs with small and large tube headers?

1999Porsche911

I re read what I wrote and, yes, I did say "rich", which is not correct. Obviously the engine will run leaner becuase of the scavenging effect. Brain fart? Everything else I stated is correct.

944CS

"The more energy that is allowed to escape through the exhaust, less energy is available to drive the pistons and therefore the crank."

This statement is actually false. It is proven that by leaving residual exhaust gases in the combustion chamber due to a restrictive exhaust, you are not allowing as much fresh charge into the engine. The fresh charge that could have entered would have produced a MUCH higher pressure on the piston if it was burned. Depending on your valve overlap and how restrictive your exhaust is, the backpressure of the exahust could push the exhaust gas back up through the intake valve as the piston arrives at TDC on the exhaust stroke.