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MG MGA - Muffler..how close to fuel tank?

On my present restoration, a Coupe, I am installing a Moss S/S sports exhaust system, which basically comes with three lengths of pipe and the muffler. As I am also using telescopic shockers, I need to get sufficient clearance around the LH one, and in doing so, the muffler ends up very close to the edges of the fuel tank...about 1/4 inch. Is this considered too close?
Gary Lock

Gary, My silencer box is between 3/4" and 1" from the end face of the tank but it's a bit closer to the flange on the perimeter.
At 1/4" I would be worried about the exhaust banging against the tank not so much about the heat transfer....................Mike
m.j. moore

Personally I would be concerned about both. 1/4 is much too close and is a risk. I recently renovated a jeep and the exhaust was not as close as that and I was told by the 'qualified' engineer certifying it for the road in NSW that it should be 50 mm away or if not possible then not less than 25mm with insulation on the pipe locally.
In addition I would say that at 1/4in you would be locally boiling fuel.
Neil Ferguson

I sympathise with the safety minded people, but to suggest you would be boiling fuel locally is completely over the top. Even if there was enough heat radiating from the exhaust to the tank over that distance, the tank and the fuel inside it will soak it away much faster than heat will be transferred, and airflow from driving along will be more than enough to keep everything well under a dangerous heat level.

As a comparison, using a big candle to boil water in a paper bag takes forever, even with only very little water in the bag. You can hold your hand near a hot exhaust pipe for much longer than the same distance over a candle flame.

You may (and even here I have strong doubts) get fuel to boil locally using a very hot flame applied directly to the tank, but that would worthy of a nomination for a Darwin Award. The real danger comes when the tank is completely empty and the gases are potentially explosive. But even then an exhaust pipe is not hot enough to produce sufficient heat transfer over any air gap.

Or is Gary making up a dragster with nitrous injection and flame-throwing exhausts.

And unless the engine mounts are really weak, there shouldn't be that much flex in the exhaust system at the back of the car for physical contact to take place between tank and silencer. But I know Gary does a fair bit of gravel road driving, so on the rougher roads he may need more clearance than we need on Swiss roads.
dominic clancy

dominc..Disagree totally..and so did the vehicle regulator down under approving my 1975 restored jeep. The realistic situation..(rather than the rather irrelevant paper bag theory using a good insulator ( paper..compared to steel ) and a high boing point single component liquid ( water compared to a low boiling point volatile mixture )is that safety is paramount and there is a risk ..You may get away with it but why do it ??
6mm ( 1/4 inch) clearance is sub minimal and I noted from hot to cold stationary that my Jeep exhaust moved about 9mm by expansion laterally. I am sure I could juggle the mountings to reduce the thermal movement . I also had it wrapped with local protection and it had about 15 mm clearance before it was inspected and when I took it for such the local insulation was worn ..so movement vibration combined with thermal expansion was working the tank and exhaust.
Here in Oz when the anmbient is 42c and the fuel is the same temp.and you are slow moving in city traffic without airflow ( does the latter happen in Europe)I prefer caution. Up there in the frozen heights of Mont Blanc etc ..well.....
In conclusion I prefer caution and would ensure that there was ALWAYS an airgap in any condition of driving to prevent heat transfer and /or metal rubbing.
Interesting footnote.. I imported the 74 jeep CJ6 from Queensland . It was amess but on the road and the regulator in NSW found 50 items of basic safety deficiency ( some very big ones eg safety belts had no reinforcement at attachments through body work..I pulled one through the sheet.) but it had a 2 month old Queensland roadworthy cert.. must have been a mate..a dangerous one!
Neil Ferguson

Dominic..sorry about the ref. to Il Bianco.... should have said 'when you are driving up the face of the Eiger"
Neil Ferguson

Niel, iso-octane bp is 99.5C, n-octane 125C,
so - petrol boils about the same as water.
But it has less latent heat so it will boil easier.
But, boiling it doesn't ignite it.
But......
Art Pearse

Common sense would dictate that you get a hot exhaust as far away from a fuel tank as possible.Shift it and ,if possible ,lag it.
M Blencowe

Let me add a few comments to clarify my thoughts and background...
..I worked as an engineer and in my time have designed onshore and offshore oil, gas and LNG plants.
..We never put engine exhausts net to any hydrocarbon storage plant. ..and any one who suggested such would soon be out of work.
.. as I noted there was mechanical interaction on the one exhaust to which I referred...as well as heat.
...the closeness of hot areas and fuel, lightweight hoses , fuel overflows, filling points etc etc on a car make it a relatively risky configuration. There are no real options ..not like an industrial process plant.
...Given the last point it is best not to increase the risk if you are sensible.

Art....I am amazed at your simplified view of gasolene ..it can contain anywhere between 200 to 500 hydrocarbon components and refiners would admire the clean two component cut you seem to think can be achieved . It can have an initial boiling point of 35c at atmos. pressure for the lightest component. Heat it up to 50c..smell it if you can get close....don't put an ignition source nearby. It is not like water and the comparison is dangerous !! I have always treated all hydrocarbon fuel mixtures as volatile dangerous liquids at all temperatures...even ambient.

However ..enough is enough. The discussion for me is ended and I am surprised ( and a little concerned )that such an obvious comment solicited such division of approach.If someone considers out there thinks it is ok to have a bare hot metal exhaust close to and therefore at times up against a bare petrol tank then it is their risk. I live in Oz and note the two adverse contributors live far away from me .
I am wary of heat, sparks , hydrocarbons, explosions and fires... spent most of my life trying to design , then build and commission plants to make sure they don't result in deth and destruction .. I follow the same basic standards as far as possible and with difficulty in the restricted confines of a car.
The current issue for me is a no brainer!!
Neil Ferguson

Neil, I don't think anyone was advocating having an exhaust actually rubbing against the tank. That could obviously lead to damage and leakage, so lets set that aside. The gap on my MGA is about 3/4" and does not shift around. I am pretty confident that the heat transfer to the tank is negligible. You are right of course that the initial bp of gasoline is lower than octane, but that doesn't change my belief that my tank contents won't boil. The tank may vent from time to time due to normal aspiration, but that is remote from the exhaust gas, which is not hot enough to ignite it anyway.
Art Pearse

19mm (3/4in ) without movement sounds sensible and safely clear ..but please do remember the discussion was about 6mm (1/4in )...
Neil Ferguson

Surely there can't be any significant movement as long as the rear exhaust fixing is in good condition. A supplementary question is which direction should the end of the point, up or down, left or right?? Of course the 2 points are directly related as the gap is all about the the orientation of the silencer. My Falcon system points in and slightly down with a gap of 3/4 inch.

Paul
Paul Dean

This thread was discussed between 30/08/2014 and 31/08/2014

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