exhausts

[spoon]

Dont know if anybody has already brought up this topic by itself (couldnt find one in the search), but just wanted to clear up the whole "size" issue.

Alot of people are under the understanding that you can make exhausts too big... In actual fact exhausts pretty much have nothing to do with anything aside from transferring spent exhaust gasses between the manifold (ie extractors) to muffler/atmosphere

You are pretty much guarenteed the car engines standard size exhaust will work perfectly with little power to be gained by changing this for a larger diameter albeit going smaller will almost certainly reduce power.

The most power lost is between the Exhaust manifold itself and the actual design of the rest of the system - ie sharp bends in the pipe and restrictiveness of the muffler.

So when designing an exhaust system dont automatically come to the conclusion that changing the exhaust size is allways necessary.

Conversely, dont think that there is a maximum size either, as the exhaust doesnt really serve any purpose other than a medium. The extractors/headers is what is most important hence the terms such as tuned length etc.

Also with newer cars you might have noticed that the actual exhaust pipe seems quite small at the outlet compared to what you might think neccessary. Thats because in most cases... it is! Car manufacturers sometimes make their exhaust systems reduce in size from start to finish because as you would probably all know cool air is more dense than hot air, and as exhaust is further from the engine the cooler the air becomes. So using these two principals manufacturers can reduce material costs in making an exhaust without increasing backpressure.

People who tell you you need some backpressure in exhausts are also incorrect
Think about how many drag cars you've seen of late that have exhaust systems - the top fuellers dont even have collectors - only the bare minimum to promote positive flowing air.

So anyhow the moral of the story is... dont worry bout exhaust sizes but rather worry about the design of your headers - nothing really matters after that... but at the end of the day if you want more power, bigger is better.


Let me know if something I wrote is either incorrect/misleading too much writing to proof read!

[LeeRoy]

I believe you are totally missing the point of flow and air speed here. how much air do you think you can blow into a straw compared to a pipe of say 1" diameter. I would almost certainly guarantee that it is easier to blow air through the larger one. The link to engines here is that if an engine has to work to push exhaust gases through your exhaustand away from itself then that energy used is energy that isnt going to your wheels. A build up of exhaust at the end of the ports will provide force against other gas trying to move out and provide force against the piston which is trying to move up. A basic simile here would be blowing into a straw with the other end blocked up. Here is a basic relation to Newton who clearly states that no energy is lost but it merely transforms into another state ie an explosion exerts some force into blowing things apart, some force in the form of heat, and other parts of force in both noise and light.

With relation to turbochargers, the faster exhaust gasses move through the turbocharger the faster it comes on boost therefore making greater power.

Since we are talking about combustion engines of which heat is a by product and is linked with a decrease in performance, surely the faster heat can be taken away from the engine the better in both a performance view and another view looking at engine life. While on the topic of heat colder fuel burns better and materials become more conductive ie spark plugs and leads. Here the more heat in the engine bay, the less conductive electrical components will be and may result in such thing as poor burn and detonation.

To say that your point of view is incorrect is an understatement and it would be interesting to see where such a view could develop. Another thing to point out is that an exhaust if made smaller will reduce power, which is true, the obvious distinction to be made here i imagine would be that a larger exhaust increases power. You have also failed to mention the fact that there is a market that car manufacturers are aiming at capturing where the majority of which inside this market prefer quiet vehicles.

Maybe a dyno sheet of the same car with a larger exhaust will help clarify your theory on exhaust sizing

[oldcorollas]

The reason for having a tuned length is to avoid valve burning which is caused by cool air entering the engine through the exhausts after each firing, and rapidly cooling very hot valves among other things. This is why its a very bad idea to drive your car with a leaking exhaust gasket/manifold.

i think you will find that the tuning length of the exhaust is to time a pulse of negative pressure (or relatively low pressure.. whatever the positive pulse is reflected by the open end pipe as an opposite) to hit the open exhaust valve at the correct time to actually SUCK more exhaust out, and help pull the new intake charge into the cylinder.
i would recommend P.Smiths "scientific design of intake and exhaust systems" as a godo starter book.

it has nothing to do with burnt valves.. that is caused by leakage of hot gases past the valve and seat, esp when they cannot seal properly.

it's bad to have exhaust leak because it is smelly and not safe, and hot.. not because of valves burning

as for exhaust size.. size does matter still.. the extractors must be tuned length for this "sonic tuning of exhaust pulses" but that doesn't mean that you can just go bigger afterwards with no effect... it also depends on muffler design..
just pulsing hot exhaust into (effectively) a large chamber of cold exhaust gas, doesn't help it get out the other end... that said, if you have a tiny pipe, it will also restrict flow.. so there is a happy medium which allows the exhaust pulse to continue along without increasing pressure or decreasign velocity too much.

turbos work by the pressure differential across them, and it appears that the extra power a turbo can make by increasing the pressure differential (by having very large dump pipe) kinda offsets much oif the loss from going to a bigger exhaust system after the dump.. this is i suppose because the air gets chopped up so much that it is no longer pulsed flow, but is instead a much more consistent flow rate, instead of pulsing.

anyway...

[mullett]

+rep to LeeRoy for starting the comeback in what could get nasty.

Your headers ARE the most important thing in a highly tuned engine, but the rest of the exhaust sytem is also very important. The size does matter, as does the design. NA engines need some backpressure to operate at their most efficient in most circumstances, also tuned length headers (which are EXTREMELY rare and expensive, as they have to be made individually for every engine), need to be designed to suit your exhaust, to suit the pressure level.

OEM exhausts care very little for flow, the main thing is NVH. A quiet, well muffled and rattle-free exhaust are the main priorities, and of course meeting the relevant emmisions standards (which dictates alot of exhaust designs). So there is almost always power to be had in even just a catback, the more you do the more you get. This has been dyno proven time and again. Turbo cars get even more power gains. The exeption is cars like the Integra Type R, where the OEM exhaust (and even headers) are about as good as you're going to get.

Don't bother pulling comparos to drag cars, it's fruitless. They have nothing at all to do with anything you'll ever need on a road car. And are invariably boosted (which changes everything).

What the hell is that about valve burning? Where are you going to get cold air in an exhaust system (unless it leaks of course) Tuned length extractors work by scavenging the exhaust out of the port when the valve opens, this is done by tuning the length so that as the exhaust pulse comes out, it creates an area of low pressure behind it, this is resonance tuned to 'pull' the exhaust out of the next opening. (not the best explanation

There's heaps more to be said, but that's the main points.

RM.

Edit: OldRollas, you got in faster

[LeeRoy]

+rep to LeeRoy for starting the comeback in what could get nasty.

Cheers for Rep - LeeRoy

[oldcorollas]

Your headers ARE the most important thing in a highly tuned engine, but the rest of the exhaust sytem is also very important. The size does matter, as does the design. NA engines need some backpressure to operate at their most efficient in most circumstances, also tuned length headers (which are EXTREMELY rare and expensive, as they have to be made individually for every engine), need to be designed to suit your exhaust, to suit the pressure level.

lets get this straight

NA does NOT need backpressure. no engine NEEDS backpressure....

except of course if you have an old V8, with a 5000rpm redline and are running 300+ deg cams that is pretty much how the backpressure myth started. if your cam/s have too much overlap for the rpm, then some backpressure will reduce the amount of fresh charge going out the exhaust.

BUT... the time this is needed is low in the RPM range.... so if you have backpressure down low, you will have a shtload of backpressure up high, which is terrible... backpressure is terrible....
tuned length headers are not expensive or rare. i made them for my car and they cost about $100 in parts.
they need to be designed for your desired RPM range... not your exhaust pressure level... exhaust is meant to allow things to leave the engine, not try and keep them there

anyway....

backpressure = bad, unless you fecked up your cam choice

[andy]

OEM exhausts care very little for flow, the main thing is NVH. A quiet, well muffled and rattle-free exhaust are the main priorities, and of course meeting the relevant emmisions standards (which dictates alot of exhaust designs). So there is almost always power to be had in even just a catback, the more you do the more you get. This has been dyno proven time and again. Turbo cars get even more power gains. The exeption is cars like the Integra Type R, where the OEM exhaust (and even headers) are about as good as you're going to get.

what ive found is: by changing cat back slightly you make a little bit more power overall, but once you start going bigger you are getting more power in high revs, but loosing power and torque in the lower end.

[spoon]

im not saying anything oldcorrollas has said is incorrect and that I already didnt believe it, just that I may have not clarified myself very well!

If you were to change an exhaust, there is much to be gained by keeping the existing size exhaust and just removing all restrictions to flow such as large bends, press bends, restrictive mufflers, exposed welds etc.

Sometimes it's easier to just move to a larger pipe than reduce the drag caused by these systems...

I may have been a bit tired or something when I mentioned that valve burning is the sole reason for having tuned length runners, where in fact they are tuned length to operate most efficiently within a certain rev range. Such is the reason there are different lengths for different power delivery etc. ie for more torque at low revs you would have short length runners and also 4-2-1 layout as apposed to long runners with 4-1 runners in high reving/high power engines...

as stated by oldcorollas

"i think you will find that the tuning length of the exhaust is to time a pulse of negative pressure (or relatively low pressure.. whatever the positive pulse is reflected by the open end pipe as an opposite) to hit the open exhaust valve at the correct time to actually SUCK more exhaust out, and help pull the new intake charge into the cylinder."

-hence the principal behind expansion chambers on 2 strokes

All im saying is dont increase the exhaust size for the sake of increasing power... generally it will, but like I said if the design of the exhaust is poor ie sharp bends etc there is a chance you will lose power over the smaller design. Of course a properly developed system that is larger is going to work better in this case.

LeeRoy I think you have missed the point as I didnt ever say that going smaller is better.. or did i? Just think if an OEM decided to install a system in their cars dont you think they would have already figured what would be an ideal size for their cars exhaust flow. The problem arises when you have to meet costs and thats about it. You can make a high flowing, quiet and efficient exhaust but it all comes at a cost and this is usually put down to design. ie just about all OEM's use press bending which will automatically reduce flow. There are exceptions in alot of performance cars hence the use of the Honda Type R example where there is mandrel bends used (at a larger cost to the manufacturer) while still retaining a relatively small size.

so dont get your panties in a knot

[Lonewolf]

As mentioned earlier

exhaust is meant to allow things to leave the engine, not try and keep them there

anyway....

backpressure = bad, unless you fecked up your cam choice

The engine needs remove the waste product to make way for the next batch of fresh fuel/air so it can make more power. The exhaust system helps that, as A/F mix carries a mass and hence momentum. Usually (in cars that have valve overlap), this momentum will help pull through fresh intake charge. Exhaust pipe size and length with have an affect on how efficiently this occurs at a given RPM. This includes the exhaust port as much as it does the exhaust manifold, collector, muffler and pipe. So changing these will play with the powerband.

Spoon- Parts of your post do make sense and i agree with. Technically there is no maximum size for anything, but there is a Minimum and maximum practical size.

If you have an exhaust to small, it'll kill power as the exhaust will have trouble letting over X amount of gas through, on the other end having a pipe too big will slow down gas speed (and in doing so reduce scavenging). Hence why having a 3" pipe on a 2L NA engine pretty much kills the powerband, the engine simply does not pump enough exhaust gases to maintain a decent speed.

If you were to have 2 exhaust systems, one that is 1" and the other that is 3", both having been flowbenched to X CFM, I would likely go with the 1" setup, as the 1" is likely going to be of better design to be on par with a 3" setup, and also going to have better low and mid range power due to higher gas speeds (better scavenging) at these rpm and be no better or worse at the top end (though it'd probably be better). That said, its gotta be a pretty crap 3" system for a 1" system to be on par with it, but i hope u get the idea.

[LeeRoy]

If you were to change an exhaust, there is much to be gained by keeping the existing size exhaust and just removing all restrictions to flow such as large bends, press bends, restrictive mufflers, exposed welds etc.

This is true, however in many instances gas speed increases around bends and unfortunately many restrictions cannot be removed as in things like mufflers and cats.

All im saying is dont increase the exhaust size for the sake of increasing power... generally it will, but like I said if the design of the exhaust is poor ie sharp bends etc there is a chance you will lose power over the smaller design. Of course a properly developed system that is larger is going to work better in this case.

I'm unsure as to whether there is any other reason to increase exhaust size...is there?

Just think if an OEM decided to install a system in their cars dont you think they would have already figured what would be an ideal size for their cars exhaust flow. The problem arises when you have to meet costs and thats about it.

This is not so true, in instances such as either the ls1 or ls2 in the commodores adding such things as headers and proper intake setup has been proven to give huge gains. Adversely, holden has opted for poor quality cast manifolds and piss small intake setup as to reduce hp made as their target was 300kw. Here they have disregarded what flows well and opted for a worse flowing system. With reference to cost would it really matter when this stuff is being mass produced and the price difference between many things when you buy so much is almost nil??? Also im sure theres more metal in cast manifolds and exhaust parts than that of pipe.

so dont get your panties in a knot

I wear boxers

- LeeRoy

[oldcorollas]

With reference to cost would it really matter when this stuff is being mass produced and the price difference between many things when you buy so much is almost nil??? Also im sure theres more metal in cast manifolds and exhaust parts than that of pipe.

i think you would be shocked by how little it costs to manufacture a $30K car.
every little bit counts, and cast iron items are shit cheap to make, because they use scrap and have a high throughput.

making a manifold out of pipe however, involves man hours as well as machine, and thats always expensive...

[YelloRolla]

The engine needs remove the waste product to make way for the next batch of fresh fuel/air so it can make more power. The exhaust system helps that, as A/F mix carries a mass and hence momentum. Usually (in cars that have valve overlap), this momentum will help pull through fresh intake charge. Exhaust pipe size and length with have an affect on how efficiently this occurs at a given RPM. This includes the exhaust port as much as it does the exhaust manifold, collector, muffler and pipe. So changing these will play with the powerband.

Hmm, when the intake valve opens, the A/F it is at a standstill in the intake runner and needs to overcome its own weight (inertia). The pressure in the chamber is higher at than that of the intake manifold at this time and it may also be higher in the exhaust, this will result in the intake charge being diluted with the exhaust gas. This WILL result in a colder burn, which is a BAD thing - remember we are driving the piston by the heat produced from the air/fuel burnig process.
Exhaust extractor/header design ideally will have the negative pressure wave meet the exhaust valve at the overlap period and to help scavenge the last little bit of the spent gases.
Teh timing of this will generally only work in a portion(s) of the rev band.
As pointed out by oldcorollas - the timing of this "negative wave" to suit the rest of the engine characteristics (cam timing, intake manifolding etc) is where the secret to engines that run good (compared to the rest) comes from.

A lot of modern hi-tech racing headers are designed with a step to help arrest the "positive pressure waves" from arriving back at the port (therefore making some more torque across a broader rpm band.

Spoon - as the exhaust moves away from the engine the pressure is being reduced and it is therefore expanding thus requiring more room. Don't think that the designers of the old trumpet exhausts had no idea of what they were doing.

There is a lot more to write on this subject, and I must say that I am for conservative exhaust sizing.

[oldcorollas]

EEEEEEEEEXACTLY

guys, there have been many MANY books written on this subject and i urge you all to begin with the bible that pretty much started putting it all together..

Philip Smith
"THE SCIENTIFIC DESIGN OF INTAKE AND EXHAUST SYSTEMS"

he also has other books like the scientific design of racing engines.

there have been advances made after his books form the 1960's, but they are mostly just refinements and tweaking.

there is no point in waxing lyrical abotu your ideas for this and that because there is a shitload of information available if you search around.. there is also a lot of BAD information, particularly from the US, where massive engines run low rpm and stupidly long duration CAM (not cams ) so you need to sift thru the garbage also.

anyway, this thread is already full of opinions and half myths....

just read around abotu the scientific basis of exhaust design, NOT billy bob the mechanics $0.02 worth of dribble...

have fun

[jeffro ra28]

I think we have established that the extractors are probly the most important part of the exhaust to modify, without a descent set of extractors how we can expect to see gains from Cam tuning. There would be little gains from Cam tuning if you have shitting extractors as each exhaust stroke is pressurising the system and failing to draw fresh charge from the intakes.

Then we have to take into account the types of extractors, with regards to a four cylinders.

A four "into" one system will give best power gains but restricts the power band. Ideally for road cars with a broad power band and good mid range a 4-2-1 system is best altough it will probly reduce maximum power.

The art of exhaust tuning is to determine the length and size of the exhaust pipe for the negative wave to arrive at the cylinder during the valve overlap period.

The formula to work out the primary pipe length is:
P=850xED "divide by" RPM (-3)

RPM being the RPM which the exhaust is being tuned
ED being 180 degrees + the number of degrees the exhaust valve opens before BDC.


So i am tuning the exhaust for maximum Power at 5,500rpm.
My exhaust lobe on my wade camshaft opens at 75.2 degress before BDC.

My ideal length for my primary pipe is 36.4 Inches.

As stu said tho, there is alot of crap out there. For me its no problem to be able to practically try myths going around, which we do. But after seeing up to 4 different exhausts being used on the dyno on the same engine i can say that alot of this thread is BS to the particular application and specs of the engine. There are just to many variables to determine what is the wrong and right way.

[jeffro ra28]

For roadies we can have an exhaust pipe diameter the same as the exhaust valve.

For a more precise tune we can use this formula.
Where CC is equal to cylinder volume and P= Primary length in inches.

Inside diameter=