confirm or reject: backpressure

[Kurt B]

Talking to a ricer IT guy on the floor and he suggested eliminating as much backpressure as possible results in loss of torque but a gain in high end HP. So ultimately, if you ran without any muffler at all and a huge ass header system, you might lose a little low end, but you'd gain a lot of high rev ponies.

In fact, someone can explain to me how backpressure helps increase HP at some point, because it seems to me, if the piston can return without any resistance (assuming some magical device instantly removed exhaust gasses) it would clearly operate much faster than it will when it is using the ignition phase of other cylinders to drive it to force those exhaust gasses out.
I mean, once again, the mathematical 'limit' idea. If enough backpressue would stop the motor comletely and work backwards, then none represents the ideal so you want to get as close to that as possible.
Help me out theoreticians.

[911nut]

During overlap (when both intake and exhaust valves are open for a few degrees at the same time) a very small amount of backpressure (which on a 911 would be headers and megaphones) in the exhaust header pipe holds the intake charge in the cylinder. Otherwise the charge, taking the path of least resistance, would flow on out of the exhaust port. This improves volumetric efficiency (like increasing displacement, a bigger throttle body and free-flowing exhaust improve volumetric efficiency; it's all about flowing the maximum air/fuel mixture through the engine) and thereby improves horsepower.

Of course too much backpressure inhibits volumetric efficiency and reduces horsepower. How much backpressure is too much? Porsche had to balance performance with exhaust loudness and cost and did a decent job. Headers and sport exhaust reduce backpressure and increase performance. Torque is more a function of displacement, camshaft timing and intake manifold length and diameter.

This is a simple explaination which doesn't take into account where the exhaust pulse is in the pipe and so on.

Some manufacturers (Fabspeed comes to mind) claim that a premuffler adds 7-10 HP by increasing backpressure. I have a friend with an M.E. PhD from MIT whose thesis was on engines who says that's BS.

[jluetjen]

The other thing to keep in mind is accoustic tuning. Exhaust gasses do not flow down the tail pipe like water down a hose. Rather think of the air moving through a trombone or a trumpet. Changing the diameter and chambers in the system can change the "tune" of the pipe which then affects the scavenging from the cylinder. Moving to a different engine speed changes the system all over again. Complex doesn't even begin to describe understanding the details of the system.
Anyway, a bigger pipe will have a lower "pitch". It might move the scavenging affect to a lower point in the rev range. The increase in the inertia from the larger volume of gas in the pipes might also increases the backpressure at lower speed and increase the exhaust scavenging at higher speeds.

This is definitely the type of thing that people get their PhD's in!


I'm getting a headache...

[Kurt B]

Excellent. Thanks guys.

[Chuck Moreland]

I think the idea that backpressure helps the engine make torque at any RPM is bunk.

I suspect that this notion got started because small tube headers can produce more torque at low RPM than large tube headers. That is true and it is not because of back pressure.

But since most people incorrectly think headers work by reducing backpressure by virtue of free flowing pipes, they also conclude that the reason big tubes kill the bottom end must be that some backpressure is needed.

As 911nut correctly points out, big tube headers kill the bottom end because exhuast velocity is too slow to give a scavenging effect.

This Elephant Racing tech topic explains scavenging and how headers work:

http://www.elephantracing.com/techtopic/headers.htm

Also your friends statement about: "loss of torque but a gain in high end HP" is very misleading.

HP = Torque*RPM/5252

So if you kill torque you also kill HP.

I think what he meant is it "Loses torque at low RPM but gains it at high RPM"

[Chuck Moreland]

As for this bit about the Euro pre-muffler, B. Anderson has said they do work and I believe Bruce.

The cars that are equiped with the pre-muffler don't have headers and therefor little to no scavenging effect. They have the lame exchangers from the Carreras, and I think the SCs too?

I'd believe that the pre-muffler somehow provides a bit of scavenging effect before I'd believe that backpressure is helping the engine.

[911nut]

Originally posted by Chuck Moreland
As for this bit about the Euro pre-muffler, B. Anderson has said they do work and I believe Bruce.

Granted, Bruce is very knowledgeable but I say in God We Trust and everyone else better bring their data.

[SeaDweller]

I can vouge for the fact that backpressure impacts engine performance. On my '72 LT-1 Corvette, I removed the stock dual exhaust and installed headers and side-pipes. At the time I installed the side-pipes, I did not have the slip-in mufflers.

The car had an automatic tranny, and with the stock exhaust, it would easily "chirp" second gear under brisk acceleration. With the headers without muffler inserts, it wouldn't "chirp" second at all, even under full acceleration. When I finally added the muffler inserts, the "chirp" in second returned.

[jluetjen]

don't have headers and therefor little to no scavenging effect.

I'm not sure that I agree with that statement. In many respects there is little if any difference between (tuned) "headers" and a normal exhaust manifold except for the accoustic tuning. Where as the length and diameter of tuned headers are chosen to create a resonance which scavenges the combustion chambers at a certain engine speed, a plain-jane exhaust doesn't do this. By virtue of being equal length, that tuning affect hits all of the cylinders at the same engine speed. As a result the exhaust will work most efficiently at that speed (think frequency) and it's harmonics.

A plain-jane exhaust system lacks this tuning so each cylinder may get it's scavenging affect at a different engine speed. As a result the affect will be far more defused if it even exists. If you are unlucky, the tuning will be such that it never comes into effect until frequencies and engine speeds far higher then the engine will ever be able to meet. This is often the shortcoming of manifolds with short primaries. In addition to the affects caused by the individual primaries and cylinders, there is also a tuning affect as a result of the collectors and secondary pipes. This is the result of exhaust pulses from one cylinder affecting other cylinders. It is this effect that allows "Flat Plane" V8's such as Ferrari's 3X8 and Cosworth DFV's as well as in-line 4 cylinders such as Toyota's twincams to get that extra little bit of horsepower. Just as the pulse from one of the cylinders is losing energy at the end of the primary, the pulse from one of the cylinders 180 degrees opposite hits the collector with enough force to pull even more air out of the first cylinder's pipe. V8's with 90 degree cranks (such as Detroit Iron) don't have this affect because it is very different to route the pipes while still having everything fit in the car. This is why they sound differently. Some NASCAR teams have on occasion run 180 degree headers (as did the Ford GT40) which get the same affect as the headers on a "Flat Plane" V8. If you have ever heard one of these cars you'll immediately notice tha they sound differently.

If you understand the tuning, it is possible to design a pretty good exhaust system by the numbers. Just like organ pipes can be manufactured to be pretty close to being in tune. The fine tuning will depend on manufacturing tolerances in the system temperature, humidity and things like that.

I believe that the pre-muffler that you described acts as a plenum chamber and performs two related tasks. One it most likely minimizes a certain resonance in the original European system which existed as a result of the cat being omitted. Secondly, it changed the tuning of the system so that it is in a "happier" zone for the engine.

In general, primary lengths (P) can be determined using the formala: P = ((850 * ED)/RPM) - 3. Note that ED is equal to 180 degrees plus the number of degrees the exhaust valve opens before BDC and RPM is the engine speed to which you want to tune the exhaust. (PS: The above formula is appropriate for a flat plane V8 or a inline 4. In the case of our boxer-6, the ED should equal 240 degrees plus the number of degrees the exhuast valve opens before BDC. This is because each bank of our 6 fires every 240 degrees. A different way of looking at it is that every cylinder will fire once each 720 degrees. 720 degrees divided by 3 cylinders = 240 degrees.)

The inside diameter can be calculated via: ID = sqrt(cc/((P+3)*25)) * 2.1 where cc = cylinder volume in cc's and P is the primary length from the previous equation. Note that decreasing the pipe diameter moves the torque peak down. A rule of thumb is that a reduction of .125 will move the torque peak down by 500-600 RPM or 650-800 RPM in engines smaller then 2 liters. If you move your torque peak down, then your HP will drop off as a result of the equation that Chuck mentioned. Moving it too far may result in an engine which generates it's peak torque at idle!

While these formulae are good starting points, variations in cams, inlet manifolds, porting etc. all have an effect also which might affect the tuning. Different arrangements of mufflers, pre-mufflers (AKA resonators), and cats will also have an impact. So the result will vary.

Finally, a creative application of cone design (think of a trumpet bell) in the collector and exhaust can also have an affect. Then there is the "reverse cone" like you can often see on 2 cycle mufflers which act similar to a trumpet's mute. An interesting side affect of this type of muffler is that they cause an echo inside the muffler which can also be tuned to scavenge the systems. A perfect example of this is Jerry Wood's "Phase-9" exhaust for the 911.

Notice that there is very little mention (if any) of back pressure. As long as you haven't gone "over the edge" on the low side or the large side, exhaust design is all about accoustics.

[island911]

John, you get a PhD for that. . .Pelicanhead Doctoate, that is

[DRACO A5OG]

Sorry Fellas for bring up this old thread.

But for those that use the Factory Euro Pre-muffler or their car was Factory installed ROW car. Do you have the notorious unburnt fuel smell coming to a stop. You know the plume of car fart at the end

Before two days ago, I was using the M&K pre-muffler. I love it but never been too happy with the smelly exhaust. Chalked it up to the Cat-less system ( her AFR is at proper levels )

First obvious thing I noticed was the lower sound, less resonance (expected) but what I did not expect was the lack of the annoying smell at stops, especially from heavy acceleration to sudden stop, the plume of smelly exhaust was just not there.

Prior to install, I looked into the factory pre-muffler. I noticed a perforated tube that protruded about 4-5 inches then the disk behind it with perimeter holes/ports. Could it be that PAG designed it to help burn off some of the used fuel, liken to a heating rod??? I am sure that part gets pretty hot in the chamber??? I can't image ROW owners accepting the smelly unburnt fuel smell from a new car, right???


original pic by MotoSook ( Thanks brother )
modified in red is what I observed

This is inside of the DANSK vs OEM where the disk is closer to the dual ports and the perforated tube sits in the center


Just curious if anyone had any thoughts on this?