back pressure myth

[Bushbasher]

It's my understanding , and I'm no expert, but ; efficient scavenging is achieved with tuned length headers where each pipe from the exhaust port is the same length so that the pulse of hot gas that is expelled from the cylinders reach the collector in sequence one after the other and then progress through the rest of the exhaust system with the minimal impedence possable. Cheap pipes and cast manifolds of unequal length cause the pulses to run into each other which slows down the flow and creates too much back pressure for maximum power BUT will give good economy when tuned properly. This is where gas velocity comes in. The less restriction means the faster those sequential pulses can get out and the more air passes through the motor at any given moment and as we all know - "more in = more out = more power". If you're after maximum power at all costs then go as large as possable with your exhaust BUT, (and this is where the backpressure argument should be focused IMO), if you want a street car that will return half decent fuel economy and efficiency then you do indeed NEED some back pressure. This is because of the valve timing overlap on most modern engines where the inlet is opening before the exhaust is fully closed. Without back pressure to regulate the flow of escaping gasses then a free flowing and well tuned open exhaust will suck or "scavenge" all the exhaust gasses out then take some of the incoming charge as well before the exhaust valve is fully shut. This in turn wastes fuel and increases emissions as well as heating your exhaust excessively when that unburnt fuel ignites. The most extreme example is any alcohol or top fuel drag car with huge flames blasting out the exhaust at WOT, you get the idea. Any exhaust is a compromise between power and economy/efficiency. The less flow means less power but generally, especially with stock engines, better economy and efficiency. The thing with factory fitted exhausts is they are even more of a compromise because they're looking to mass produce for the smallest cost possable and still meet minimum requirements, that's why when you fit a decent set of headers and exhaust system you will always get more power and usually better economy.

As I said, I don't claim to be an expert and I stand to be shot down in flames but that has always been my understanding.


Bushbasher

[frd906]

i only asked this question, just to see what you guys thought before i make my choice, thanks

[Thunder Time]

Lets just assume we are talking about an I6 road car that has to have an exhaust system to be registered.

The best performance is gained by having good exhaust velocity combined with less back pressure.

The bigger the exhaust, the less back pressure, and if the engine hasn't got enough power, the exhaust velocity drops and you lose torque - pulling power.

Yes, the car may make more power on the dyno with a bigger exhaust, and yes, top speed may be better with a bigger exhaust, but you don't want to wait 10 minutes before you get there.

If you go too big with your exhaust you will lose torque - pulling power, so instead of cruising up that hill like you used to, you will have to knock it back a gear. The burnout you used to do with ease will become a chirp if you are lucky. Acceleration out of corners will drop so you will have to change back an extra gear, etc.

If you look at race type extractors, a lot of them are stepped. The pipe out of the head is a bigger size and then it drops to a smaller size. The collectors narrow down to 2.5" and then expand up to 3". These type of things are designed to increase gas velocity to increase torque.

Torque is what allows you to do a burnout and accelerate hard, and on a street car it is all about 0 - 110km/h (unless it is also a 1/4 mile car) as that is where it spends most of its time (legally anyway). Being a NA I6, we are not talking about a serious 1/4 mile car anyway.

All cars go better - more power = better fuel economy (if you drive for economy) with a free flowing exhaust. Manufacturers have to comply with 90db (dead quiet), emissions, and fuel economy. Therefore the standard exhaust system is made smaller and restrictive. Sports Systems, if they sound good, are illegal.

On an I6, competition extractors (3 pairs of 2>1 collectors) give more torque, ballistic racing cat (100cpsi instead of 400cpsi) and a 2.5" system is plenty. If the car has heads & cam and a tune then 3" is a better size.

3" on a stock engine will produce excessive noise, less torque and chew more fuel than a 2.5" system.

The same applies on a "Stock" EA - AU V8. A dual 2.5" sounds better, but for torque, a dual 2.25" is the better size.

For a "Street Car" it is more about driveability and punch when you hit the accelerator.

I hope this helps.

Cheers
Greg

[PepeLePew]

Great stuff Greg....thank you ;)

[The Monty]

Thanks Greg, good lot of knowledge in there.

[Badcooky]

We obviously need some pipe length or we'll burn our exhaust valves out.
Simple actually.
Even top fuellers run pipes running as minimal as needed to stop deto.
A 2 stroke needs a pipe to work.
A good rule is to believe nothing you hear on the internet as it's usually wrong.

[The Monty]

I would love to see flames coming straight out of the side of the head though on a top fueler, that would look sweet

[castellan]

I have noticed loss of torque. right down low with a straight through short pipe.
I am with stav. on the re tune when you have put a high flow system on. if it's EFI or not jetted right you will loose power.

[frd906]

thanks gus this is really good infomation

[Stattic]

Originally Posted by Badcooky

A good rule is to believe nothing you hear on the internet as it's usually wrong.

smartest thing ive ever read ever... but how can i believe it?? :togo:

[Badcooky]

Originally Posted by Stattic

smartest thing ive ever read ever... but how can i believe it?? :togo:

Don't.

[Wally]

Originally Posted by frd906

does the i6 require back pressure from the exhuast or not, cheers matt

Your cam will be the biggest decider on that. No backpressure will result in a really fast blowdown, but will also result in blow through when the inlet opens during overlap. This will more than likely result in less fuel to burn in the lower revs where gas rarefaction is low, resulting in lower power production (which of course means lower torque). Higher revs won't mind it because even though there is less time opportunity compared to low revs for blowdown and blow through, there is less impedance to flow, but you will lose syphoning.

Larger exhausts generally make the torque/power curve peaky.

[sooty43]

FRD906, I have been constructing racing motors for many years and can honestly say that the only person explaneing correctly the answer to your problem is Pepelepew!

[frd906]

this thread is old news, who dug up this old fossil

[GS608]

Originally Posted by frd906

this thread is old news, who dug up this old fossil

don't be so rude, some good information in here.

[burnz]

Originally Posted by Thunder Time

Lets just assume we are talking about an I6 road car that has to have an exhaust system to be registered.

The best performance is gained by having good exhaust velocity combined with less back pressure.

The bigger the exhaust, the less back pressure, and if the engine hasn't got enough power, the exhaust velocity drops and you lose torque - pulling power.

Yes, the car may make more power on the dyno with a bigger exhaust, and yes, top speed may be better with a bigger exhaust, but you don't want to wait 10 minutes before you get there.

If you go too big with your exhaust you will lose torque - pulling power, so instead of cruising up that hill like you used to, you will have to knock it back a gear. The burnout you used to do with ease will become a chirp if you are lucky. Acceleration out of corners will drop so you will have to change back an extra gear, etc.

If you look at race type extractors, a lot of them are stepped. The pipe out of the head is a bigger size and then it drops to a smaller size. The collectors narrow down to 2.5" and then expand up to 3". These type of things are designed to increase gas velocity to increase torque.

Torque is what allows you to do a burnout and accelerate hard, and on a street car it is all about 0 - 110km/h (unless it is also a 1/4 mile car) as that is where it spends most of its time (legally anyway). Being a NA I6, we are not talking about a serious 1/4 mile car anyway.

All cars go better - more power = better fuel economy (if you drive for economy) with a free flowing exhaust. Manufacturers have to comply with 90db (dead quiet), emissions, and fuel economy. Therefore the standard exhaust system is made smaller and restrictive. Sports Systems, if they sound good, are illegal.

On an I6, competition extractors (3 pairs of 2>1 collectors) give more torque, ballistic racing cat (100cpsi instead of 400cpsi) and a 2.5" system is plenty. If the car has heads & cam and a tune then 3" is a better size.

3" on a stock engine will produce excessive noise, less torque and chew more fuel than a 2.5" system.

The same applies on a "Stock" EA - AU V8. A dual 2.5" sounds better, but for torque, a dual 2.25" is the better size.

For a "Street Car" it is more about driveability and punch when you hit the accelerator.

I hope this helps.

Cheers
Greg

just to add to that the exhaust header pipes (tuned length) is to size the pipe's length to match the fireing order, as each pulse passes it pulls the following pulse with it.

[pmpdau]

Originally Posted by XR-CHIEF

don't be so rude, some good information in here.

Second that, first bright topic you've posted and you go and ruin it for yourself.

Thanks everyone that contributed, some really good reading here.

[frd906]

lol sorry, it is a good topic,

[Stattic]

Originally Posted by frd906

this thread is old news, who dug up this old fossil

why do you even bother posting on this forum, fair enough youre dumb but when you act like that to people who try to help...

[NC1183]

I think we need to get a MOD to edit/delete the nonsense posts and leave the information and make it a sticky either here or in the tech doc's section

[superpursuit83]

I have a real life experience. I swapped my 2 1/2" system with paceys. To a 3" system with 1 7/8 difillipo's. Before a retune we had a little more of everything. After tune Picked up 20kw ALL through the rev range AND picked up torque. This was when the ute was NA and at its limits already.

I am no exhaust scientist, just personal experience.

[tones xr8 427]

u need some back pressure. put an exhaust on thats too big for the motor and u lose bottom end(tourque). u need velocity. if it was a drag car it would b a different story, but for the street u definently need back pressure.

[5.0 ED]

You do not need back pressure in a 4 stroke engine, but what is widely miss understood is that if you put a 3.5-4inch system on a 4litre, the exhaust flow is too slow, and therefore actually increases back pressure due to lack of air speed, and heat. the hotter the exhaust is the faster/better it will flow, go to big and it wont get hot, and wont have air/speed.

So in fact too big an exhaust creates increased back pressure. losing power.

[CPOCSM]

Originally Posted by T3man

No engine needs backpressure.

All normal engines dont need back pressure - except if you own an A series out of a mini/Morris. Due to the A series having a siamese port setup and a common exhaust for pistons 2 and 3, the back pressure helps with the scavenging of exhaust from the usually longer centre pipe (commonly referred to as a LCB or long centre branch)on aftermarket stage 1 kits, and relies on it to let the gas out the end.

A lot of the younger crowd like the 4 inch jam tin look on a mini...then wonder why they stall whenever the throttle gets opened up to take off and fuel consumption rivalling a space shuttle launch. Modern cars usually dont have an issue with backpressure but I will scan the dynamics of the shockwave principal from a popular mechanics book I have here. It is old school stuff but it is still pertinent if you run old school motors...A series is a motor designed in the late 1940's and still run great today but needs a lot of help t o work well

[frd906]

Stattic , becuase i can post and will keep posting,

no no. i thought this thread was finished and bieng that i havent been on for a long time is the reason i said this, this is old and did not realise it was still going

[Dave_Obsession]

Originally Posted by frd906

Stattic , becuase i can post and will keep posting,

no no. i thought this thread was finished and bieng that i havent been on for a long time is the reason i said this, this is old and did not realise it was still going

You don't need to post in a thread if you have nothing useful to contribute....even though you think the thread is 'old', don't waste a post by just saying that....like we've seen with other threads, it only serves to rub some people up the wrong way. It would appear that you have certainly created a negative impression of yourself with several AU forumites......you wouldn't have the 'Commodore' tag with the two sad faces for no reason!!!

Keep your non-important opinions to yourself, and only post stuff that will address the subject of the thread, and/or that will help to answer people's questions/concerns/thoughts/queries.

Just my two cents...apologies for hijacking the thread for this post.

[cj_aus]

An interesting question! Clearly shown by heated debate. It is clear that just chucking something on cause it looks goood or sounds good or boosts your ego (size does matter!) A balanced an informed aproach to designing your build rather than just chucking $$$$$$$$$ at a project wil give you the most joy from your passion and bang for bucks!

Informative article: http://www.burnsstainless.com/TechAr...ry/theory.html

[cj_aus]

http://en.wikipedia.org/wiki/Tuned_pipe

[MAD]

Nice articles there cj.
In a 4 stroke engine the tuned pipe works a little different to what is shown in that wiki artcile.
In a 4 stroke the return wave is timed to reach the exhaust valve just before it opens again so the pressure wave will be moving away from the valve which will effectively help to 'suck' the exhaust gases out. The initial pulse will also have a lower pressure behind it at the overlap period which will draw in more fresh air/fuel for the next cycle.

The reason tuned pipes only work in a particular RPM range is because the waves in the exhaust pipe, demonstrated in the wiki article, are sound waves and the speed of sound is a constant (relatively).

[Stattic]

Originally Posted by frd906

Stattic , becuase i can post and will keep posting,

no no. i thought this thread was finished and bieng that i havent been on for a long time is the reason i said this, this is old and did not realise it was still going

sure you can post and sure you will keep posting but dont be so rude... this thread is STILL going, yes, and there is a lot of useful information that a lot of people have found interesting, if you cant understand it thats your problem

cj, as others have said, nice find on wiki! i particularly liked that diagram
and you know what they say, if its on wiki it must be true!