There seems to be enduring folk law within some factions of the Austin Seven movement that a certain amount of exhaust back pressure is a "good thing" where A7s are concerned.
Ever since my motor cycle tuning days I have believed that back pressure is a "bad thing" for 4 stroke petrol engines. I tend to believe that Rolls Royce share that view as otherwise they would not have run Merlin engines on very short stub exhausts.
Anyone care to explain to me, please, why A 7 engines should benefit from exhaust back pressure??

Ian Mc.

Location: Shropshire

I don't have the answer but I know that smaller bore pipework on my aeroengine meant that it would scavenge exhaust gasses out of the engine due to the higher speed flow from the previous cylinder causing gasses to be 'sucked along' behind. Not easy to describe. Similar sort of thing happens with carbs in that you lose out if the carb is too big and the flow rate drops down. At least 150 more static rpm with the small bore collector exhaust manifold on it. Not so much about backpressure but the gas speed, both of which go hand-in-hand?

The standard A7 tailpipe does seem rather large bore for the given engine size, doesn't it?

I'm with you Ian.

I have heard it said that back pressure can help with smooth running, but I fail to see how this could work, unless the combustion volumes were drastically different between cylinders. Back pressure stops gas flow and therefore robs power, simple as that.

Location: Devon

Location: Devon

Location: Farnham

Ian,

you can't really compare to a Merlin engine. That engine, like the drag cars that run stubs is supercharged. With a supercharger you can use the overlap and boost to purge the combustion chamber of all the old exhaust gas, what you want from this exhaust is the minimum restriction. It's not the same with a normally aspirated engine, in that case you need the exhaust pulses to help draw the gasses out of the combustion chamber (not the ones that get squeezed out by the piston, but the ones that are left in the combustion chamber at TDC). We tried stub pipes on an F1 engine, all we gained was an enormous amount of noise, we lost a whole chunk of power.

I'm not sure what goes on with a standard 7, it has no overlap to speak of, so I doubt you get much from pulse tuning. As mentioned above, the standard exhaust is quite large in diameter for the power made. I suspect that no extra power was made from going bigger, and those tinkerers with longer duration cams that did something with pulse tuning found they lost power when they went bigger due to the reduction in gas speed.

Put simply, go too small on the exhaust and lose power from back pressure, go too large and lose power through gas speed, my feeling is that the standard exhaust is already too large. That said, I doubt it makes any difference with a standard cam fitted.

Andy

Location: Oxfordshire

I have pondered these issues over the years with no clear conclusion.

When you consider the cycle, valve overlap, the intermix of exhaust and explosive inlet mix, it seems a miracle that engines, and particularly 2 strokes, fire reliably on every stroke at all throttle openings, and do not ignite the carburettor or worse.

On my then stock RP my father used to meticulously check mpg and regularly achieved 50 mpg on trips. But he noted that with a home made silencer somewhat lacking in baffles mpg dropped to near 40 mpg with no obvious change in performance. Others have recently mention free flow exhausts on ordinary Sevens and I wonder if similar differences have been noted (Although economy is not now a consideration for most).

I gather it is common knowledge among tuning experts that free exhausts often require weaker jets.

Most later makes of cars and moderns have exhaust ports which seem to be deliberately quite small, particularly for the first few inches. Unlike the Seven.

I have also noticed the comparatively large diameter of the Seven manifold and front pipe. But it is hard to imagine that Austins would have made this larger than warranted.

The considerations for an immediately commoned manifold are somewhat different from separated pipes.

I also noticed on my car that any significant exhaust leak forward of the silence noticeably reduced performance. (Years ago I worked in Dunedin and the local traffic cops operated, of all things, Mini Coopers. They reckoned any leak in the exhaust totally killed performance.)

Bob Culver

Location: Auckland

Thanks for the responses gentlemen; it may be a case of my ignorance plus the danger of transporting experience into unlike situations - all my bike tuning experience was with each cylinder having its own pipe.
However, I have always believed that with side valve engines, breathing was the key to performance and, with no real knowledge of what appears to be the exhaust equivelant of "port robbing" on the inlet side, concluded that the more free flowing the exhaust system the better - especially at higher than normal road car revs.
Please, if you have experience, or just opinions, to add to the debate, keep it coming.

Ian Mc.

Location: Shropshire

You can get excessive back pressure from both too large and too small a diameter. The too small diameter is easy to visualise but if the pipe is too big, the gas speed is too low and can stall. I've found about 7/8 to an inch dia primary pipes works well.
Tuning pipe lengths is a different matter. You need to set the pipe length so that the standing wave in the primary pipe produces a negative pressure at the exhaust port when it opens. This helps sucking out the exhaust gases. This length can be calculated, in fact in the 1970s I wrote a computer programme to do just that. You have to calculate for temperature decay down the pipe and I meant to use 0.1inch steps. Unfortunately I got the decimal point in the wrong place and calculated every 0.0001inch. It ran for 13 hours. Our works computer department were very, shall we say, understanding.
Anyway the answer is that 27inch pipes should work well, and I've found that it does.
Then of course you have to consider what happens in the secondary pipes, and so on. The whole thing can get complicated.

Thanks, Alan. I should have realised that it was a more complex issue than I had at first thought.

Ian Mc.

Location: Shropshire

Getting off the basic topic of back pressure but...
Many texts cover design for speed with separate pipes, but the factors with common manifolds are less often canvassed. The penalty at modest revs cannot be enormous or so many cars would not have adopted.

The book Sports Car by Campbell suggests a resonant effect can be used even with a common manifold. The old text book by Smith? on Exhaust and Intakes described an experiment with a Reliant, same as a Seven. He observed a slight resonant effect at about 3000 rpm which he attributed to the distance to silencer. I could hear and detect the effect on my car; a tiny surge of "power". Presumably with a smaller pipe it would be enhanced.

The Seven front pipe is larger than my 1960s 1600cc 4 cyl car. Has anyone experimented with a smaller pipe but with the stock manifold?

I recall the Intake and Exhaust book also advocated a blind stub at the closed end of the common manifold!! As a radio technician in a past life I used to have enough trouble trying to explain these to the aerial riggers!

Bob Culver

Location: Auckland

Location: South Norfolk - Next to Suffolk

Hi Geoff

The topic was mentioned in passing and I hope not to see Sevens with curious protuberances on the manifolds!Those other than Geoff should perhaps skip the following!

The tuners will yawn but the theory is that when a pressure pulse, positive or negative, hits a blind end it reflects, but, being of wave form, is followed half a sound cycle later by the opposite. The exhaust pulse is a maximum soon after the valve opens. If the manifold has a blind end the pulse bounces off this and the following negative may arrive back at the valve about 1/3 of an engine rev after events began, in time to assist the last exhaust gas to exit approaching tdc. It is moderately speed sensitive, and with a common manifold the distance and so speed differs for each cylinder. I cannot remember the pipe added; probably about an inch dia by 6" to 9" long to the closed end of the exh manifold.

(Just as a pressure pulse reflects from blind end, a reflection also occurs from an open end, but the pressure wave starts on its journey back up the pipe immediately reversed; ie a positve pulse becomes a suck. The basis of tuned pipes etc. I am getting out of my depth here but I gather that with a sports engine with branched manifold the other branch can usefully provides a dead end. For tuned Minis and presumably some Seven specials, a blind stub was sometimes added to the common centre pipe serving siamesed ports,and which lacks a short branch.)

All of which has little to do with general back pressure, the original topic!

And if anyone wants to know how blind stubs on aerials work, I can't remember!

Bob Culver

Location: Auckland

An interesting subject, Ian. I don't pretend to fully understand all the clever stuff but I can't imagine a situation where I would ever need to make use of it.

In my limited experience of making up an exhaust system for my Swallow saloon, I found that by copying the original set up which had a large diameter tail pipe, there was a quite noticeable fall off in power. If there was any back pressure at all, presumably, it was caused by the fishtail? but there was no improvement with or without it. Could the problem have been due to the size of pipe which was at least as big as the down pipe? I don't know.

The Swallow layout is odd in having the pipe going across the car in a diagonal fashion exiting at the o/s rear.

When I made a tail pipe from 1" tubing there was a big improvement.
The pipe now goes straight down the near side passing under the axle with a turn to the off side at the back of the car and utilises a shortened silencer in the middle.

What can we deduce from this? Could the longer route create back pressure or does the more free flow eliminate it?

Incidentally, the car now goes better than it did with a conventional A7 system which I had used for a long time.

Location: Derby

As an interested bystander in this discussion, the fascinating 'bomb' shaped exhaust pipe and the Duck twin tail pipes with fishtails seem to show a different approach.

Tony.

Location: Malvern, Melbourne, Australia.

Thanks Bob,
That's just what I thought, and no I shall not be adding any protuberances inside the engine bay or elsewhere.
Racing a different story.
Talking of siamesed ports, David Vizard overcame that problem to a large degree when he came up with the scatter cam design. (Edit - For the BMC Mini engine)
Brilliant auto engineer is he, who has been doing great work in the US.
I did all my Ford head work following his designs.
There is still an ongoing debate whether to mirror polish ports or not. I firmly believe that airflow will stall at the surface of mirror finished ports, but not having a flow bench cannot test.
Any thoughts?

Ray,
Your big pipe probably made the exhaust gases stall me thinks.
Many young bloods think a nice big exhaust pipe will make a louder noise, but not so.
Smaller = louder.

Has anyone used copper tubing to make an exhaust system for ours or any other car?
I've heard it produces a rather nice mellow sound.
I just happen to have some 1" lying around

Geoff - This thread brings back memories of my time racing at Kyalami. Oh what wonderful days!

Location: South Norfolk - Next to Suffolk

Location: On the very Edge of Europe....West of Jelbert, Brian, Alastair, Bob and everybody else..

I discovered notes taken years ago from the book Scientific Design of Exhaust and Intake Systems, Philip H Smith 3rd edition.
For those with specials the accumulated knowledge of the fraternity will be better.

The intriguing thing about the Smith book is that he experimented with a Reliant engine and basic manifold. He found 3' 4" to the silencer and a 15" blind stub on the blind end of the manifold to be optimum.

As with so many books, competition type systems are discussed at length but very little reference to the goings on with the typical older car system with a common manifold.

Basic layouts and back pressure cannot be too detrimental. The Zephyr 6 of the 1950s and others had a manifold akin a Seven so had the full discharge pressure of a nearby cylinder to contend with.

Since the demise of lead in petrol exhaust systems are vastly longer lived.
My father was a plumber and in the 50s fitted a copper tail pipe to his/my car. Combined with an internally simple silencer it used to intrigue work colleagues when overtaken uphill in the bus at 3 times the speed and 4000 rpm.

Bob Culver

Location: Auckland

Hey Bob! You may have the answer there! Perhaps the reason why my car goes so much better with my home made exhaust system is that I used 28mm (1"i.d.)copper tubing. I only have one bend in it and by shortening the silencer by 4" and putting it at the back of the car I may have improved the flow a bit. Where back pressure comes into it I wouldn't know but it works fine!

I realised that copper pipe could have had too much heat dissipation so I wound it in thermal wrap. Reading the info it claimed that by keeping more heat in the pipe for as long as possible there would be an increase in power. Whether these claims are responsible or not, it's nice when something you do actually works!

It's all good fun. Nice thread Ian.

Location: Derby

This may be an urban myth, and I've never bothered to check it in the construction and use regulations, but I've heard it said that copper exhaust systems are illegal. It's something to do with acting as a catalyst in the production of something unpleasant. I'm not sure if the same applies if it's painted matt black so you can't tell.

Alan

This may be an urban myth, and I've never bothered to check it in the construction and use regulations, but I've heard it said that copper exhaust systems are illegal. It's something to do with acting as a catalyst in the production of something unpleasant. I'm not sure if the same applies if it's painted matt black so you can't tell.

Location: Derby

Alan

This may be an urban myth, and I've never bothered to check it in the construction and use regulations, but I've heard it said that copper exhaust systems are illegal. It's something to do with acting as a catalyst in the production of something unpleasant. I'm not sure if the same applies if it's painted matt black so you can't tell.

Location: Devon

I remember being told that the exhaust gasses, carbon dioxide and water,which emanate from a standard steel exhaust are catalysed by the copper to form carbon monoxide and copper sulphides will result in sulphur dioxide being produced.

Not being a chemist, I can't confirm that!

Oh, and going back to the original post, I ran the Chummystein with a straight through exhaust, while I was testing it and awaiting the arrival of a standard system from one of our cherished suppliers. Once that was fitted, the improvement over the straight through system was immediate and it ran noticeably smoother too...that was with a rock stock, low compression motor using the standard sidedraft Zenith.

Location: Near Bicester.

The Merlin was a supercharged engine and those short stub pipes were carefully designed nozzles which increased the aircraft speed the original arrangement was 6 pipes each one serving 2 cylinders. The naturally aspirated version of the Merlin was the Meteor which was used in tanks such as the Centurion had 2 cast iron manifolds one per bank each one made up of 3 separate sections (to cater for expansion) which looked a bit like a very large Seven manifold.

Using copper pipe does produce a good note. Not sure about the legalities in other places but it is not allowed to be used here. Can't recall why now.

Location: Queensland Australia

This an old thread but the issues have not gone away.
In Auckland (NZ) I recently saw some extractors done with a ceramic coating.
http://www.hpcoatings.co.nz/ExhaustHeatManagement.aspx
Can be many colours.
A brother of a friend tried copper pipes on his E Type but the Cops made him change them.
That was way back.
Bruce

Location: New Zealand, Whangarei

I've seen quite a few Sevens over the years using convulated pipes (like some rubber hoses), but suggest not a good idea (in case someone suggests them). When they first started testing the Repco Brabham engines here in Melbourne they piped the exhaust from the dyno out of the building via these ridged pipes until someone suggested a replacement with straight-sided tubing. Immediate increase of over 10 brake horse. Even a small reduction on a Seven is not desirable. Cheers, Bill

Location: Euroa, Australia

Location: Piddle Valley