TheSamba.com :: View topic

Sun Sep 13, 2009 3:57 pm

Ive read a lot of posts saying you NEED back pressure or you loose bottom end torque. If this is true you will gain torque at low rpm but choke the power at the top end.

On a header type system it seems running too large of a primary tube causes low end torque to suffer so it seems not to have enough back pressure.

I though that having the correct size tube (matching the head) allows the exhaust pulse to scavenge the cylinder. Is this what is considered needed "back pressure"?

Where is the balance of back pressure and free flowing exhaust?

I used to run a 1915 with 120 cam non ported 041 heads and a 1 5/8 header. I put a 1 1/2 header and noticed no difference other than sound.

A friend of mine is using a 1 5/8 header and fat boy on a 1600 with stock heads and 100 cam. Obviously too big for application, so what kind of tq did he loose and how much hp did he gain?

How bad is running too large of a header compared to running one thats too small? Ive been told an exhaust that is too restrictive causes higher head temps, is this true??

The whole reason I am wondering is I planned to put a 1 1/2 inch header on a 1775 with a flowmaster single chamber muffler. I do not think the "race" muffler will create any backpressure at all and am wondering what would happen to the torque curve.

Sun Sep 13, 2009 4:27 pm

You need a little back pressure at low speeds when you have low velocity thru your carbs. If you dont have any back pressure you will have scavaging which will tend to extract F/A mix from the cylinder.
It is the opposite at higher RPM where in incomming charge has enough velocity to aid in filling the cylinder and you want the scavaging.
But the reality is in a VW because of the reverse bends in the header you will never achieve a scavaging effect at low RPM with any exhaust system that will handle your high RPM flows.

So dont worry about it.

Sun Sep 13, 2009 5:11 pm

give this a read. this topic has been around for ages.

http://www.dsmtuners.com/forums/frequently-answere...ssure.html

Sun Sep 13, 2009 7:28 pm

Forget about scavenging, pressurize the intake instead. :twisted:

Sun Sep 13, 2009 11:14 pm

Quote: Exhaust: The straight scoop on backpressure
Since many people get confused about backpressure, scavenging, exhaust sizing, etc., I wrote this up for another board. I just did a quick cut and paste here, so enjoy...

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.

So is there some kind of formula (port cfm/displacement/rpm range??) to determine primary tube length and diameter?

How much influence does the cam make? does more overlap need larger diameter due to intended rpm range?

How small is too small?? 1915cc with stock port heads and 1 3/8 merged or stock port 2275cc and 1 1/2 merged? I would think you could go up a size with porting and bigger valves.

When does 1 3/8 header get "restrictive"?

How much better is the "merged" collector vs. the heater box non merged systems?

Equal length tubes?? anyone actually make them?

Ive been using my S&S 1 1/2 merged for 20 years and its finally rusting away. Out of all the systems Ive had its been the best, too bad they stopped making them. Anyone have an NOS one they want to sell?

Mon Sep 14, 2009 4:43 am

Its quite complicated to calculate but here is a calculator that will get you in the ballpark.

http://dairally.net/daihard/chas/MiscCalculators/DaiPipes.htm

Mon Sep 14, 2009 6:33 am

This is a great thread. I've wondered the theory behind it myself. That calculator is pretty intense! Love it!

Mon Sep 14, 2009 8:36 am

http://www.maxracesoftware.com/pipemax36xp2.htm

Basically an exhaust design program but it does a lot more.
Just bought it recently and I'm pretty pleased what it does.
It gives you detailed suggestions on your engine combo and
the end results match very closely to other higher end engine
simulation software. As long as you remember Garbage In, Garbage Out
when you make the inputs to the program. For 40$ I'd say it's a bargain.