Exhaust Conundrum

[LordWorm]

Originally Posted by TERRA Operative

Oh, and the reason cars come from the factory with small pipes, is actually cost. A smaller pipe is the most cost effective way of reducing emissions/power levels to within the relevant standards.

Cost - and emmissions.

Added restrictions in the pipe such as resonators and mufflers that are not straight through also keep noise to a minimum.....

Cosmo Dude: it wont keep power where it can be used.. It is true, adding too big a pipe can have ill effects, but thats because the velocity of the gas escaping the system is reduced (but, with aftermarket headers and such, you get that velocity back, with interest). If you never plan to mod passed a sports exhaust, put a smaller pipe on...it'll remove restrictions imposed by the manufacturers and get you some more power... but if you then go and do other things, that smaller pipe you put on will eventually become a restriction in itself. If you plan to go further, its better get something slightly larger and sacrafice a bit of velocity, knowing full well you get it back when you put new headers on.

The "location" for want of a better word of usable power is dictated by the primary lengths of the header. Since gas escapes the exhaust ports at a given speed (which can be determined by the engine speed, size of the cylinders, compression etc), you can calculate the point at which the gas will enter the collector and produce the most stuction (i.e, suck the next pulse through the moment the exhaust exits the port), this is your tuned RPM. Stock manifolds have very short runners, thus you will never achieve their tuned RPM (F1 cars have very short primaries, but have ludachristly high RPM, so they are tuning their peak power to come on that point).

So, if you work out where you want your power to come on, you can tune the length of your primaries to produce maximum power almost anywhere in your rev range (mines being built to come on at 3000rpm with peak RPM at 6000).


Gav: given your short primaries, your "ferrari power" advantage you speak of is likely to be near the bottom of your powercurve in effect. Longer primaries would then move this powerband down to where its usable (i.e. at an RPM your engine can actually reach)

[chicaboo]

Originally Posted by LordWorm

The "location" for want of a better word of usable power is dictated by the primary lengths of the header. Since gas escapes the exhaust ports at a given speed (which can be determined by the engine speed, size of the cylinders, compression etc), you can calculate the point at which the gas will enter the collector and produce the most stuction (i.e, suck the next pulse through the moment the exhaust exits the port), this is your tuned RPM. Stock manifolds have very short runners, thus you will never achieve their tuned RPM (F1 cars have very short primaries, but have ludachristly high RPM, so they are tuning their peak power to come on that point).

Refering to my useable power yes? I've only ever had the one set of headers on my car, but I've had 4 or 5 different muffler/catback configurations. Hence I can only comment on pipe size, and I did.

[LordWorm]

Originally Posted by chicaboo

Refering to my useable power yes? I've only ever had the one set of headers on my car, but I've had 4 or 5 different muffler/catback configurations. Hence I can only comment on pipe size, and I did.

your car is weird

You are probably just altering the restriction in the final pipe.. more or less restriction from one muffler to another.....its having an effect on the power curve no doubt....but never a positive one... one muffler is simply allowing the headers to work better than others...but at the end of the day, the headers are the master of your exhaust system, the headers are dictating how and where your powercurve occurs....

[chicaboo]

It is a phenomenom. Gay one at that though...

[AstinaBoiV6]

lucky for me there are no 90 degree bends in a v6 exhaust system, and i have the original in my boot to prove it. so therefore i have no restrictions to flow by using the bigger pipe in the bends even though it is press bent. personally i dont have the time to be running around all brisbane looking for a place that does mandrel bent systems. i had a spare hour so i decided to hook my car up. in my opinion why go full mandrell bent when u can get almost the exact same power if no the same by using press bent with a half inch bigger pipe on the bends.

Besides the power advantage is not very noticable and you dont get many benefits unless you are running a full system with high flow cat and extractors. And yes i do have a CAI and K&N filter but as i said the power increase is not very noticable but i have not been freeway driving yet to see if i got high end power increase.

for the $$ i payd i think its a great deal and i can spend what i saved on something else.

[Cincinnatus]

If back pressure was such a requirement for power flow and all the other things we've been discussing here, then why do top fuel dragsters operate without any extractors attached.

It's common sense, and if you've done any tertiary Physics courses you'll be able to figure it out. Power restrictions on cars are: amount of air and to a lesser extenet fuel we can combust (turbo and super chargers overcome the air, bigger fuel injectors with a more powerful fuel pump take care of the fuel probs). The second problem is getting rid of the exahust gasses, which clog up the system. Why do you think multi valve engines produce more power? More inlets and out lets.

Back pressure is a concept used by Exhaust specialists to sell you an affordable system, and explain away the obvious flaws as benefits. Who want to be told they should fork out $1500 for a real system and the $500 p.o.s. they can actually afford and are buying is almost next to useless.

Cosmo, dc your extractors, throw away your air filter and dyno your ride. Aside from killing the earth and farkin your engine with dust and dirt, you might get a pleasent surprise on your power and torque curve. Don't take anyones word for it. Find out for yourself, best kind of research there is.

[rodhog]

all I'm going to say is what I've known for years with small 4's

bigger is better.
IMO min is 2.25 inch under 2L and 2.5inch upt to 2.5L min

mandrel versus crush- depends on diameter, motor and path.

we always ran 2.25inch crush on the gemini - as well if you knwo gemini exhuast not too many bends. one day the motor was at it's peck basicly I suggested maybe bigger exhuast guys though nah man it's okay. But we had done a cam etc all required more air to get out at hi-revs. I saggested we remove the last section before the diff. The difference was amazing.



yes car's need back pressure if your after economy. You run your car when you need power on most engines in the top end range. This is why you need larger piping.
On some motors take ford 4L OHC note they never have fitted large pipes even on sport models. they are tiny but cats and mufflers flow well. This is because it does not rev it flows and yes a big improvement when you do but It's low end car so improvement is there but compared with small 4's it's nothing.

To make more power you need more air in and exhaust out.

the only thing stopping us from putting the right size pipe is noise polloution. even I would rarther fit a nice 4inch titainum setup so my baby turbo can breath.

[Exodus323]

Originally Posted by LordWorm

firstly, i'd say mandrel all the way. crush bends are nasty, and create restriction in the pipe.

Noone looks at the exhaust pipe, and with shiney muffler (i.e. stainless muff), it'll do the job fine. However it will be prone to rusting etc...so some sort of high temp coating should be looked at to protect it a little.

i find it funny that the guy is saying on one hand that 2in mandrel will shoot you in the foot, then on the other offering you a 2.25in crushbent system that'd probably be even worse....

mandrel 2.25in is the go reckon....

unless you live on a beach or a high salt area aslong as your car gets driven frequently your mild steel exhaust should last you the life of the car.
thay only get rusty when there not driven eg a grandmas car.
as long as the mild steel is of a good quality then there should be no problem

[LordWorm]

Originally Posted by AstinaBoiV6

lucky for me there are no 90 degree bends in a v6 exhaust system, and i have the original in my boot to prove it. so therefore i have no restrictions to flow by using the bigger pipe in the bends even though it is press bent. personally i dont have the time to be running around all brisbane looking for a place that does mandrel bent systems. i had a spare hour so i decided to hook my car up. in my opinion why go full mandrell bent when u can get almost the exact same power if no the same by using press bent with a half inch bigger pipe on the bends.

Besides the power advantage is not very noticable and you dont get many benefits unless you are running a full system with high flow cat and extractors. And yes i do have a CAI and K&N filter but as i said the power increase is not very noticable but i have not been freeway driving yet to see if i got high end power increase.

for the $$ i payd i think its a great deal and i can spend what i saved on something else.

I still maintain you wouldnt be getting nearly the same amount of flow. Shove it on a flowbench.when you crush the pipe, at any angle, the size of the pipe at the bend becomes..er... crushed =). The more severe the bend, the more restriction you get. Going to a bigger pipe doesnt help you any..

As an experiment, get a drinking straw, put say, a 60 degree bend in it, and try and drink some water through it... not as easy as it was without the bend is it? now get one of those coctail straws,, the ones with the bendy thing in the end. Bend the bendy thing to around 60 degrees. its as easy to drink through as if it were dead straight.

[Cosmo Dude]

Originally Posted by Cinncinatus

If back pressure was such a requirement for power flow and all the other things we've been discussing here, then why do top fuel dragsters operate without any extractors attached..

Top fuel exhausts don't feed into a collector. If it didn't matter then why do their exhausts have a short primary pipe before bending to a longer section? Hell, why do they use an exhaust at all? Heat shields between the heads and the tyres should provide enough protection.

Originally Posted by Cinncinatus

Cosmo, dc your extractors, throw away your air filter and dyno your ride. Aside from killing the earth and farkin your engine with dust and dirt, you might get a pleasent surprise on your power and torque curve. Don't take anyones word for it. Find out for yourself, best kind of research there is.

Post your results

[LordWorm]

Originally Posted by Cosmo Dude

Top fuel exhausts don't feed into a collector. If it didn't matter then why do their exhausts have a short primary pipe before bending to a longer section? Hell, why do they use an exhaust at all? Heat shields between the heads and the tyres should provide enough protection.

They are called zoom pipes. And quite frankly, i'd be wanting to direct exhaust like that away from me if i was the driver of a to fueler. the heat that comes off the exhaust on those things is enough to superheat the atmosphere and boil the water in the air around it INSTANTLY (the white stacks you see coming off the ends of them at night is superheated, supercompressed steam).

They dont need a collector, because the gas is expelled so voilently that they dont need to "suck" it out of the primaries... The speed that gas is expelled from most other cars is not fast enough to get out of the exhaust ports, through the manifold and out the exhaust pipe before the next pulse comes through, so scavanging is used to help suck the gas out as quickly as possible.

If we all drove around in nitromethane powered cars with 20:1 compression, we'd all have zoom pipes too... some hotrods have zoom pipes, but i think they are more about show than go.

[Cosmo Dude]

Originally Posted by LordWorm

They dont need a collector, because the gas is expelled so voilently that they dont need to "suck" it out of the primaries... The speed that gas is expelled from most other cars is not fast enough to get out of the exhaust ports, through the manifold and out the exhaust pipe before the next pulse comes through, so scavanging is used to help suck the gas out as quickly as possible.

So someone shouldn't have used this as an example in the first place, should they???

[AstinaBoiV6]

yer true LOrdworm But then again these systems are for different uses. if youre a full revhead and will be modding your car heaps and heaps then yer may aswell go for a mandrell bent as you will get most benefit. but most ppl r like me they want a affordable and good power increase and nice sound. The press bent system is a good addition for those who want an affordable modification to their car. Im happy with mine and i bet the other (well almost id guess heaps like over 70%) members are happy with theirs.

[LordWorm]

Originally Posted by Cosmo Dude

So someone shouldn't have used this as an example in the first place, should they???

Well, actually, as an example of "zero backpressure being good", its as good an example as any.

They dont need a collector because they get enough velocity without one. With a full system, you have so many other bits and pieces in the way that you need to take whatever advantage you can get. Hence you use tuned headers and high flow cats, straight through mufflers etc to limit any restriction you can.

Astinaboy: not saying yours was a bad choice..was simply pointing out that losing backpressure is good, not bad. For your application it may be perfect.



I havn't posted here to be a smartass, just to put to rest some myths about exhaust etc. At the end of the day the number one factor in chosing a system is going to be cost for most people - however arm yourself with the "secrets" of how its all spose to work, and you'll not be baffled by bs when you go in to get a quote. Alot of exhaust shops are very precious about these secrets and would do anything to stop people knowing about them, else more people would go out and make their own stuff.....

[FLiP]

It's final...

Lordworm, you've talked me into it...

2.25 mandrel in mild steel is the go...

I'm doing tertiary physics and never really applied any of it to my thoughts on exhaust... so thankyou for refreshing my memory...

Awesome... the argument is over.

back pressure is gay

[LordWorm]

Originally Posted by FLiP

It's final...

Lordworm, you've talked me into it...

2.25 mandrel in mild steel is the go...

I'm doing tertiary physics and never really applied any of it to my thoughts on exhaust... so thankyou for refreshing my memory...

Awesome... the argument is over.

back pressure is gay

Hahaha good stuff =)

find yourself a set of awesome headers for the BA as your next purchase...you may be slightly overaspirated with the 2.25in in the short term - but once some decent headers go on it'll bring big smiles =) (the 2.25in system will give you a theoretical maximum flywheel horsepower somwhere around 200hp which gives plenty of head room for other mods before the exhaust becomes a restriction again... a 200fwhp 1.8L BA would be quite the rocket i'd think)

[Cosmo Dude]

Well it isn't 'zero' back pressure. A 90+ degree elbow is a restriction that many exhaust systems do not have.

[LordWorm]

Originally Posted by Cosmo Dude

Well it isn't 'zero' back pressure. A 90+ degree elbow is a restriction that many exhaust systems do not have.

Large enough mandrel bent angle = no restriction.

[Cosmo Dude]

I said elbow not bend

[LordWorm]

Originally Posted by Cosmo Dude

I said elbow not bend

eh? every single set of zoom pipes i've ever seen are mandrel bent.

in the persuit of 8000hp, i dont think they are going to skimp on something as simple as exhaust.

show me a picture of these "elbows".

If you read that link i posted, it has something in there along the lines of, in general, every litre of air and fuel creates 13L of exhaust gas due to expansion. if even 1/13th of the gas is allowed to linger in the system, you cant get air and fuel into the cylinder. That can't be good.

Provide the argument for backpressure being a good thing - you havn't been able to convince me so far..