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Dec 21, 2006 12:21:05 GMT
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OK heres a question for y'all...
backpressure is bad, right... having a low restriction exhaust frees up power and allows the motor to breath better.
However is no backpressure a good thing or not?
I have always been told and have read places that for a street car a small amount of backpressure int he exhaust is necessary for torque development, to improve idle quality (smoothing out tickover) and improving the warm up cycle to get the car off choke quicker.
My own "experiments" with this involve running a car with just the manifold and finding that it ran badly and hesitated. Simply reattaching the exhaust system cured all its problems.
Similarly when I have had an exhaust rot through at the down pipe I noticed perfomance was reduced and idle was rough and occasionally it would not tick over. Once the exhaust was relaced it ran like a swiss watch.
Also also a guy I knew had a Chevy with a mild 350 in it. He ran big pipes off the headers and capped them off with a striaght through silencer each side. The thing would not idle when cold. He had to keep blipping the throttle until it warmed then you could drive it. He replaced the straight throughs with baffle type silencers and with no other change it started better from cold, idled correctly and drove fine when on the choke.
So anecdotally I have proven that you need a proper exhaust and some back pressure which is resultant from that.
However a recent magazine test they had a Chevy on a rollig road and they dropped the exhaust off and found 12 BHP and more low end torque.
They stated that they had always believed the "you need a little back pressure" story to be complete bunkum and that this proved it.
Also I know a guy who drops his exhaust when he runs at Santa Pod and found over a tenth improvement from doing so. Dragsters run with open pipes.
I am as familiar as I am ever likely to be with the theory around exhaust gas velocity and the like but don;t see how this can be effected by swapping silencer types right at the back of the system...
Any thoughts anyone?
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1941 Wolseley Not Rod - 1956 Humber Hawk - 1957 Daimler Conquest - 1966 Buick LeSabre - 1968 Plymouth Sport Fury - 1968 Ford Galaxie - 1969 Ford Country Squire - 1969 Mercury Marquis - 1970 Morris Minor - 1970 Buick Skylark - 1970 Ford Galaxie - 1971 Ford Galaxie - 1976 Continental Mark IV - 1976 Ford Capri - 1994 Ford Fiesta
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stinkwheel
Posted a lot
Doctor Of Gonzo Journalism - One of gods own proptypes, never even considered for mass production.
Posts: 2,280
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Dec 21, 2006 12:23:30 GMT
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i have to say over the years have always found the same as you, a car will run and idle and perform just better with an exhaust system fitted. But again, drag racers run open straight throughs, so whats the answer? dunno.
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1973 Citroen Dyane 6 1980 Citroen Acadiane
1992 Citroen AX 1990 Citroen BX
1997 Citroen XM 1993 Citroen BX
1997 Citroen Xantia 1977 Citroen Ami 8
1996 Ford Escort 1989 Citroen BX
1997 Suzuki RF900 1988 Yamaha TDR250
1979 Honda CB400.
'I need less vehicles'
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Dec 21, 2006 13:00:41 GMT
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Having no back box on my 1 litre Polo killed it's "performance" - there's an entire science devoted to deciding how much to have on different engines. IIRC Will Holman had a restrictor on the exhaust of his Capri for a bit. Could be wrong though  |
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Dec 21, 2006 13:01:52 GMT
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Oh, and I ran my bug on open headers with a stinger at the Pod and it was great. I deliberately didn't go for the largest bore system available though as I knew it was a waste of time The wisdom is that too little back pressure on an engine with a long duration cam will cause scavenging of the burning mixture IIRC |
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Dec 21, 2006 13:20:26 GMT
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you don't want any 'backpressure' as that is a pumping loss, its energy wasted. However, exhausts contain a certain mass of gas with its own momentum and the pipes are of a distinct length such that the system as a whole is designed to take advantage of the pressure waves which occur as the exhaust valves open and close. the pressure waves can be used to encourage scavenging and cylinder filling at the engine end. Thus if you change the exhaust you change these two characteristics and you can end up with worse performance, even if the new pipe has a higher flow rate than the old one. 'do i want backpressure or not' is a gross over simplifiction which cannot be answered unfortunately.
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1972 Fiat 130 1985 Talbot Alpine 1974 Lancia Beta Saloon 1975 + 1986 Mazda 929 Koop + Wagon 1982 Fiat Argenta 2.0 iniezione elettronica 1977 Toyota Carina TA14 BEST CAR EVER!!!!!!!! 1979 Datsun B310 Sunny 4-dr 1984 Audi 200 Quattro Turbo 1983 Honda Accord 1.6 DX GONE1989 Alfa 75 2.0 TS Mr T says: TREAT YO MOTHER RIGHT! |
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Dec 21, 2006 13:29:22 GMT
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Yeah, I guess I'm mixing an old wives tale of exhaust fitters re: backpressure with what is effectively just a rapid deceleration of the exhaust gas as it leaves the restriction of the pipe/manifold and enters open air.
I still find it odd that dropping the exhaust off on one car makes more power and on another it makes it run badly...
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Last Edit: Dec 21, 2006 13:30:08 GMT by akku
1941 Wolseley Not Rod - 1956 Humber Hawk - 1957 Daimler Conquest - 1966 Buick LeSabre - 1968 Plymouth Sport Fury - 1968 Ford Galaxie - 1969 Ford Country Squire - 1969 Mercury Marquis - 1970 Morris Minor - 1970 Buick Skylark - 1970 Ford Galaxie - 1971 Ford Galaxie - 1976 Continental Mark IV - 1976 Ford Capri - 1994 Ford Fiesta
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Dec 21, 2006 14:36:03 GMT
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Basicaly you need an exhaust of the correct length with as little back pressure as possible. same principle as using longer/shorter intake trumpets.
If you just drop the exhaust off then you will need to adjust fueling to see what its relly doing tbh.
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Last Edit: Dec 21, 2006 14:41:39 GMT by slater
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Dec 21, 2006 15:04:11 GMT
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my backbox fell off last week and i noticed a lack of a deadspot when it was cold ;D
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Dec 21, 2006 15:18:40 GMT
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Basicaly you need an exhaust of the correct length with as little back pressure as possible. same principle as using longer/shorter intake trumpets. Aye, I read a very interesting article about exhaust system (rather than manifold) design by our Mr Vizard and it was very interesting. He was all on flow benching different silencers, explaining the effect on velocity and pulse tuning of different internal silencer architectuire and all that. Took me a few reads through to take it all in. Never really mentioned backpressure other than keep it low. quote author=slater board=general thread=1166703665 post=1166711763] If you just drop the exhaust off then you will need to adjust fueling to see what its relly doing tbh. [/quote] I hadn't really thought about it but if I drop the exhaust of a car its different to a magazine guy dropping one off on a rolling road where they will of course then retune the thing... Good point... I shall in future just ignore folks prattling on the subject of backpressure I think. The only way to prove the point would be to do something on a dyno using a variable restriction which increased back pressure without constricting the pipe... I think we know what the result would be anyway... |
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1941 Wolseley Not Rod - 1956 Humber Hawk - 1957 Daimler Conquest - 1966 Buick LeSabre - 1968 Plymouth Sport Fury - 1968 Ford Galaxie - 1969 Ford Country Squire - 1969 Mercury Marquis - 1970 Morris Minor - 1970 Buick Skylark - 1970 Ford Galaxie - 1971 Ford Galaxie - 1976 Continental Mark IV - 1976 Ford Capri - 1994 Ford Fiesta
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awoo
Posted a lot
Posts: 1,506
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Dec 21, 2006 18:58:25 GMT
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depends alot on the velocity of the gases. on a turbo car you will find it improves performance if you have a straight through exhaust as it kicks the fast moving gasses out and expells them rather than hinder it as it would with a normal system. then on a more regular smaller n/a type engine you would need a more typical system that would give it some back pressure to work effectively. a good basic way of thinking of it is this - take a straw of say 2mm diameter and blow through it - a straw of 5mm and blow through it - and then take a drain pipe which is few inches in diameter. now if you were to hold your hand at the end so you can feel your breath you would get a goodish flow from the smallest straw but your face would be red as its not expelling the air very well. then the next biggest straw would allow you to blow through it alot easier and the airflow would be good and strong. finally if you blow through a drain pipe and hold your hand at the end not alot will happen at the other end but you probably will go dizzy through essentially lack of back pressure, not allowing you to give a controled flow. that kind of shows in a basic way how it works, obviously using the 5mm straw would give you a good strong air flow which is essentially what an engine would want as its a breathing machine. remember it depends alot on the type and speed of the engine more than anything. opening up the airflow on a revvy engine will be beneficial but a slow low compression engine probably wouldnt benefit too much. on my motorbike i had a micron can which was just a straight through pipe. this opened up the lower end and gave more torque but the lack of back pressure took away all the torque in the top. i hope this is a good enough analogy, I'm no specialist btw, more a bodger |
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Dec 21, 2006 19:19:07 GMT
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how much back pressure you need is mainly down to the cams, as an example a BOA 24 valve granny cossie engine running standard cams needs a fairly narrow restrictive exhaust, fitting a bigger free flow one kills its respeonse and chops 15 -20bhp off it.
fit cams from the later BOB engine and it'll run fine on an open pipe, no other mods needed.
the BOA runs cams with the same profile as the 12 valve 2.9 cologne engine it was developed from, while the later BOB version runs cams with the same profiles as the cossie turbo YB lump.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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Indeed but it could be the exhaust gas velocity which is killing it not the lack of backpressure...
A little reading suggests that everyone PSI of back pressure is a 2 BHP loss at the road wheels.
But then a little reading is a dangerous thing...
Interesting to know about them cossie V6s... One day I will have a Cortina with one in.
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1941 Wolseley Not Rod - 1956 Humber Hawk - 1957 Daimler Conquest - 1966 Buick LeSabre - 1968 Plymouth Sport Fury - 1968 Ford Galaxie - 1969 Ford Country Squire - 1969 Mercury Marquis - 1970 Morris Minor - 1970 Buick Skylark - 1970 Ford Galaxie - 1971 Ford Galaxie - 1976 Continental Mark IV - 1976 Ford Capri - 1994 Ford Fiesta
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Dec 22, 2006 10:27:26 GMT
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Using smaller primarys etc. in the exhaust manifold is all about keeping the gas velocity up at lower revs. If the same amount of gas is forced through a smaller tube it will have to move quicker and gain more momentum. It also takes more energy to push it (which is the back pressure) but this is offsett as the gas from a cylinder keeps its momentum after the exhaust valve has closed. The gas moving out the exhaust creates a partial vacume behind it when all exhaust valves are closed so as the next exhaust valve opens the vacume sucks exhaust gas out of the cylinder reducing the back pressure it sees!
Back pressure is allways the enemy its just that you can sometimes reduce it but making the exhaust manifold smaller and letting the 'pulse tuning' help you out. As with all tuning its a case of getting it right, too big is as bad as too small to a point. The diameter of the exhaust is largely dictated by the engines capacity and the rpm you want it to work best at (hence the amount of gas it will need to flow at optimum efficiancy) smaller exhaust will move the effect down the rpm range as at lower rpm less gas is being flowed so for a street motor you will want a smaller exhaust then a racer. The back pressure at top end may kill peak horse power but its a swings and roundabouts thing like normal!
The length of the exhaust and mainifold sections tends to have more to do with the cam timing in my experiance. Longer cams needing longer primary pipes etc.
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