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Re: Back pressure...

Bill Sheehan
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


I think I might start polishing the inside of my pipes and silencer Bill,should get an extra bhp or tow!!!

Location: Piddle Valley

Re: Back pressure...

Ray White
........ 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.
Ray,
I lost the tail pipe on my bog-standard RP just behind the silencer when coming back on the Jogle (nr Exeter).
I would swear that gave me the wings to get from Exeter to Lands End in about 2hrs 40mins..
I then fitted a pipe to come out in front of the nearside rear wheel.
I'm still of the opinion the car is more responsive etc., with this arrangement.
It's basically the same set up as SWB cars.
.
Of course when I say responsive, this is a relative thing, nuances of subjective feelings.

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

Re: Back pressure...

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

Re: Back pressure...

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

Re: Back pressure...

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.

Re: Back pressure...

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.


If I am told to remove it by someone in authority I will but until then I will take my chances.

Location: Derby

Re: Back pressure...

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.


I have also heard this said, although I have no proof that it is either correct, or incorrect.

Location: Devon

Re: Back pressure...

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.

Re: Back pressure...

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.