Greetings all,

I have heard that exhaust back pressure is required (to a degree) by naturally aspirated engines, but have no idea why, and have been puzzling over this for a while. Can anyone shed some light on this?

Ignoring noise output and any catalytic converter, what would be wrong with using no exhaust at all, or a really fat one to reduce flow resistance? Or is a short pipe tuned to resonate at the same frequency of the exhaust valves the optimum?

Also, how can the optimum-for-desired-rev-range back pressure or exhaust diameter be found?

My current logic says that any pressure needed to force gasses out of the exhaust, is pressure against the piston on the exhaust stroke, and will therefore reduce available power. What am I missing?

Thanks,

Julian

In general the previous response is quite true. There is another ramification
though.

If you have a cam with a lot of overlap (both valves open for some time)
it will get the best power at speeds where the exhaust valve closes just
before any intake blows out with it. Below that speed some of your intake
charge is blown out the exhaust, resulting is less torque at lower speeds,
poorer fuel economy (not usually very significant in the overall scheme of
things) and perhaps driveability problems around town. It is a lot quicker
and easier to add a little back pressure to your system than to change cams,
and it will result in better low-speed performance and poorer high-speed
performance. Back when I was a teenager (when dinosaurs roamed the earth)
they had "Lakes Pipes" which were branches off the forward part of the exhaust
coming out under the sides of the car with blind flanges bolted over them, and
when you wanted high speed performance you just unbolted the flanges and
let 'er blast. In my dad's day they had exhaust cutouts that you could install
but they became illegal in a lot of places (not long after they became
popular). They would accomplish the same thing.
--
George Downs, Bartlesville, Oklahoma, Central US

So does this mean that with two little backpressure, some of the intake air/fuel mix would be drawn into the exhaust? That doesn't sound too environmentally friendly, and might explain why exhuast bypassing is frowned apon by the powers that be.
--
Drive it like you hate it

I suppose that by some standards, anything done by humans is environmentally
unfriendly. Probably the most environmentally unfriendly thing is the
destruction of forests and grasslands (here read "urban development", "Paving"
and most buildings other than treehouses).
On the other hand all those hydrocarbons came out of the environment in the
first place and if we still lived in the forests, there would be plenty of
natural reactions to recycle them.

It is interesting to note that the most rabid environmentalists, and especially
those who push "Zero Population Growth" also describe themselves as "humanists".

But it should also be noted that the amount lost is slight and if you have a
cat it will turn it into CO2 (a greenhouse gas), H20 (steam) and SO3
(sulfuric acid anhydride) which they tell me has yet to be proved to be
hazardous, just like the carbon black particulates in diesel smoke, which
probably contribute almost as much to global warming as greenhouse gases.

Okay, I am getting down off my soapbox, which I plan to burn for firewood,
thus polluting the air even more (but not like a forest fire, especially
one started by lightning). Maybe the EPA will start filing charges against
God for such environmental disasters as forest fires, volcanic eruptions and
tidal waves....
--
George Downs, Bartlesville, Oklahoma, Central US

It sounds like you have a problem with EPA regulations and civic responsibilty. You aren't alone in the woods you know, what you do affects other people. As far as retuning the engine with lower backpressure goes, sure the change in emissions is slight, but depending on how much the change in the system is, it could end up failing SMOG. If it passes SMOG then great. I put on 2 1/4" exhaust on my B230F with a K&N to get out what little more power I could, and still passed Ca SMOG with flying colors. I didn't swap to the high flow cat though because of the increase in emissions and the possibility of not passing SMOG. Do what you want to your system, and if it still passes SMOG, then great don't worry about the environment becuase you are only one of hundreds of billions in the US alone and it can't be that bad because it does pass SMOG. One person messing with their car and increasing their emissions slightly doesn't have a large effect.

But when you have a beef with the EPA regulations being too stingent or the EPA having regulations at all, which ever it is you have a problem with, you have to realize that a slight increase in allowed emmisions muliplied by 300 billion people is not a small number. You would have to be a fool to think you should be able to do what ever you want regardless of how you affect your community or the world at large. Especially if your rational is because volcanoes do it too. We aren't in preschool anymore.

Try telling LA or Mexico city that air pollution is OK because car emissions are natural and come from the environment. By your logic we should also pour used oil down the drain along with antifreeze and whatever else we want. After all, "if we still lived in the forests, there would be plenty of natural reactions to recycle them." Do you really want to be exposed to more than you have to be?

Pb, As, and C6H6 are all natural but I still wouldn't want people do be able to put them where ever they want to, whether it be from ignorance or stupidity.

There is a reason why SMOG exists and I think it is a good one. Waste and emission regulation is what keeps people like you from destroying the environment in which we live. Please, do not try to rationalize your own irresponsibilty by pointing the finger to things we cannot control. There is a difference between environmental pollution by deliberate choice, ignorance, and acts of God. I want my family to be exposed as few carcinogens as possible, no matter how "natural" they are. If a volcano erupts, fine, but to watch my neighbor or the local industrial plant dispose of waste into our ground water due to recklessness is not OK.

If you look at everyone's tailpipes while you drive around here in California they are nice, new and shiny. Even on the worst 1970's jalopy. I could only imagine what it would be like without it; Mexico? The drawbacks are, I can't get my old wagon to go 0-60 in less than 20s without breaking the law, but I would rather have clean air to breathe. Besides, if I wanted to have better acceleration I should buy a different car and still not harm the environment.

It is interesting to note that I consider myself an environmentalist, but have never heard of a humanist until you stated it. I would consider myself to be resposible though. You should think of an environmentalist as one one makes an effort to minimize their destructive impact on the environment within reason. You have to realize that just by just existing in our society you will have an imact on everything around you including your community. So, you should do your best to be responsible and minimize the harm that you cause.

"you have to realize that a slight increase in allowed emmisions muliplied by 300 billion people is not a small number."
Umm, the number is 300 million.
--
1980 245 Canadian B21A with SU carb and M46 trans

You are right, 300million. My mistake.

We don't have SMOG in small towns in Oklahoma because the population density
is low enough that it is not a problem, and we have plenty of green fields and
forests to provide all the oxygen we need. And even here in the cradle of the
oil industry, where oil and brine used to run in the rivers from time to time,
you will be pretty hard put to find a place that has not naturally remediated in the 107 years that oil has been produced here.

No matter how you slice it, high population density is unhealthy. I don't
think people have it in them to live clean enough for it not to be a problem.
For that reason and lots of others I am happy to stay here in Green Country.
--
George Downs, Bartlesville, Oklahoma, Central US

Maybe we don't have it in us to live clean enough in a high enough population density. But I still do what I can. I live on the coast in a smaller town bordered by redwoods and the ocean, so I don't really feel the effects on air quality from the local industry, plants, or cars , unless it is a diesel that just drove by. The ocean breeze sweeps whatever we put out inland, unlike LA where they have a trapped pocket of air above them and so whatever they put out they have to breathe or rains back down on them.

Interestingly enough, in the SF Bay area most of our smog originates from across the Pacific and is blown over by weather patterns. The only time I notice the air quality is after coming back from a trip to the mountians. But it does make a point that what we do does affect other people, even if it is all the way across the Pacific. Canada has a similar problem with our polution that is created in the North East.

I can understand your perspective though. In a small town where the effects from everyone in it just disappear, except for the landfill of course, it would be hard not to think that environmentalists are overreacting. But from one who lives in an area that is affected by people from so far away, and to see other cities that have such horrible problems due to lack of regulation, I see a real and present need for it. And I would hope that you could see it through to not ridicule those who see a need for it as well.

Well, at least on the brickboard because believe it or not, I hate talking politics even though I can't help myself when exposed to it.

Problem we're seeing in spots around here is MTBE getting into the groundwater poisoning wells. Not sure but I think the EPA mandates this...

Must be tasty too, as the squirrels chewed right through our plastic 5-gal gas container - the one replacing the steel one rusted by the acid rain. Now all you can buy anywhere here (even in automotive departments) is a "spill proof" spouted container "not recommended for refueling vehicles". Since using that spring loaded gadget in the backyard my wife swears she has spilled more fuel in a few weeks than in all the years of using a normal spout.
--
Art Benstein near Baltimore

This is also why oodles of dollars are spent on research at header manufacturers to take advantage of this scavanging effect to find it's peak for each type of engine for racing applications at high rpms ( they really don't care too much about how it efects idle ) going so far as to thermal wrap the exhaust at different points to get the maximum effect.
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '82 Mercedes 300SD

Been to Speedy Muffler lately???

THERE IS NO SUCH THING AS BACKPRESSURE!!!!!!!!!!!!!!

It's a falacy created by muffler shops so that they can rip you off for whatever they feel like when you have to get your exhaust fixed at 3pm on a Saturday 150 miles from home. I've heard it all. Without it you valves will burn out. Your fuel economy will go in the toilet. You'll blow that rear muffler off without it. BAH!!!

All ranting aside, your camshaft releasing the exhaust valves after the expulsion of burnt gases to close them is all they need to shut. Muffler shops and grease monkeys who want to rip you off will try to explain that there is this magical reverse flow that seals them shut. It doesn't exist.

For the past 280,000 km between a pair of 240's I have run the same exhaust. High flow catalytic plus a single solitary OEM 240 Turbo muffler in the back. Straight 2 1/4 tubing all the way and in between. No resonators, no special flex tubes, just simple tubing bent to shape and welded in place. Gives a little bit of a snort on the high end as well...
--
Happy Bricking!!! - Richard - '87 245 DL , '82 242 GLT - Half a million km between them!!!

I don't know what you mean in saying that there is no such thing as backpressure. You can think of it as restriction, or limiting smooth UNRESTRICTED flow. Anything that limits flow can be considered backpressure. And I'm pretty sure the stock turbo muffler has baffles and creates backpressure, FWIW.

My experience thus far has been as such:
My car with a single cherry bomb installed right after the header was loud. I know that when i added a baffled "performance" muffler(probably one slightly similar in design to the stock turbo muffler) my car idled better and had more power and was smoother in the low rpms(under 2.5k). As far as my car having less top end, i'm not sure, possibly yes, but maybe not. I didn't really think about it or compare it when i switched, but there was a noticeable difference in the lower rpms. A difference for the better. Being that my car is carbed, i think possibly the lack of low end power might be less exagerated in a FI car, but that may be bogus.

From what i've read over the years, and gathered and what not, is that backpressure is pretty much bogus and an "old rodder's tale". I've always wondered what those "Lakes Pipes" were for and how they worked... Other than looking cool of course. From what i've personally experienced, it is not fully bogus, but i can't comment on high rpm performance, which is where it makes sense that it would limit the engines capabilities. I'm biased toward wanting my car to have a dual perforated core muffler system, where one is like my cherry bomb, just a straight barrel with a perforated pipe on the inside for the exhaust to just go through and the sound waves to be absorbed by the material outside of the core, and then a larger muffler of the same perforated core, straight through design but with a larger area for absorbing sound. That would be what i want in an exhaust for an NA car. For a turbo car, same thing if you want it quieter, otherwise just one resonator(cherrybomb pretty much...).

Recently, i've seen some people talking about how backpressure is necessary again, and all that other good crap. Who knows? It seems it's all myth these days. And then sure, you can think of it this way(these are not facts, well, not that i know of, but they make sense), straight through is good for higher rpms, and some backpressure is better for lower rpms/around town driving/traffic crap.

My bunch of two cents... :-\ Peace,
--
Kyle - attending Ore. State, while my lil '68 142 (256k, 74 b20, m40, iPd bars, other misc... =D) waits for its next outing... (My Cardomain site)

Richard,

That may be the solution I need for my '89 760 TI. PO had the exhaust replaced from the cat back. However it is smaller diameter and rattles in a few junctions. The knucklehead welded the smaller pipe inside the back end of the cat. I'll have to cut off an inch or so.

Where and how did you support the long run from behind the cat to the over-axel pipe?

Thanks

Dan W

Hello,

The pipe leading from the cat all the way to the rear muffler is one piece supported by hangers where the 2nd resonator used to be. I'd be worried if some of the material in that cat of yours has been melted by that knucklehead...

--
Happy Bricking!!! - Richard - '87 245 DL , '82 242 GLT - Half a million km between them!!!

The pipe out of the back of the cat is pretty long, so I don't think that is a worry. In addition, the car passed Emission testing with flying colors a few months ago.

Thanks

Dan