Stainless Steel in Classic Car Exhausts
An article by Jimmy Stewart of Classicfabs.
Jimmy has ran his own stainless pipework fabrication business for over twenty years before getting involved in Jaguar exhausts for the last ten
One of the main points that I can never understand when people ask us for a quote on a stainless exhaust system for their Classic Jaguar is that:
They very rarely ask what type of stainless we use.
How we weld it.
The wall thickness of tubing that we use.
How it is manufactured.
All of the above points should be taken in to consideration at the same time as comparing quotes from different manufacturers rather than the impression I am given that ‘it is stainless so it will last forever and won’t rust and they are all the same’.
I am not going in to the fine detail of all the different grades of stainless but trying to give the reader a basic understanding of what he should expect in quality from a stainless exhaust manifold and system. If he requires any further detailed information there are various detailed publications available.
When ‘Stain Less’ steel was discovered around one hundred years ago by an English metallurgist who found that when chromium was added to steel it gave it an element of corrosion resistance and prevented the knives he was creating from rusting. There were also German and French engineers working on the same process around the same time.
For stainless to behave as ‘non’ rusting it requires a chromium content of at least 12%, but as we will see later for the best corrosion resistance the stainless tubing to be used in a standard exhaust manifold and exhaust it needs to have at least 18% chromium plus other elements.
Some manufacturers use grade 409 stainless in automotive exhaust, not the most corrosive resistant (less expensive) as it has only around 10% chromium content. Confirm with the manufacturer which grade of stainless they use.
There are four types / families of stainless with many different grades in each of them that are in use today which are:
Austenitic: The most commonly used worldwide and with the 300 grades some of the most corrosion resistant. There are various grades of the 300s from 304 through 316L, 321 and 347.
For a quality classic car exhaust we should be looking at using 304 stainless. There is no need to go to 316L or the higher grades. 316L would mainly be used for marine exhaust applications and the others for very high temperature racing exhausts.
An easy test to find out if your system is austenitic is that it should be non-magnetic.
To be classed as 304 Stainless which is an alloy it must contain:
18% to 20% Chromium
8% to 10.5% Nickel
0.8% Carbon
2% Manganese
O.75% Silicon
0.045% Phosphorous
0.03% Sulphur
0.1% Nitrogen.
Molybdenum may also be present.
Ferritic Grade 409: These are of a lower grade corrosion resistant stainless due to having a lower Nickel content but are still used by some exhaust manufacturers as they are particularly cost effective (cheaper than the 300 grade Austenitic steels).
We would not recommend using this type of stainless due to the possibility of corrosion occurring long term if the tubing becomes pitted.
An easy test for Ferritic (409 grade) stainless is that they are mostly magnetic
Duplex: These steels were developed mainly for the oil and gas industries and are outwith the scope of this document.
Martenistic: These steels are mainly used because the can be tempered and are used for the manufacture of medical implements plus other hardened stainless steel products.
Material Thickness.
All our tubing depending on the system is either 1.2mm or 1.5mm wall thickness and silencers and resonators are all 1.5mm. If thinner material is used on silencers this can have a detrimental effect on sound at certain engine speed creating ‘resonation’ in the system.
Most companies use 1.2mm or 1.5mm on their tubing but it would pay to ask what wall thickness they use on their silencers and if they have sound recordings of their systems.
Welding of Stainless Exhaust Systems.
We would only ever use the manual T.I.G. (Tungsten Inert Gas) welding process for neatness and being able to control weld penetration.
Most companies also use the T.I.G. process but there are great variances in the finish achieved due to the ‘fit up’ of the assemblies and the shielding gas used.
High purity Argon gas should be used for the welding process to give the best finish possible (depending on the skill of the welder)
Always ask if the exhaust assemblies and manifolds are constantly purged internally with Argon during welding.
When you receive your system look up the inside using a bright light and see if you can see what looks like coking (Looks coking coal) on the inside, a sure sign that the tubing has not been purged while welding. This will lead to serious gas flow velocity restriction thereby lessening the performance of the exhaust. ‘Cheaper’ inert gasses are sometimes used for purging eg Nitrogen, but this should gas should never be used for the welding process.
The least expensive exhaust systems that are available use the M.I.G. (Manual Inert Gas and Ferritic (least corrosive resistant) stainless which enables the parts to be welded much quicker than the T.I.G. process but the components are not nearly as neat and well finished and in the case of any butt or face weld could lack in penetration whereby weakening the weld as there is not the same neat for an ‘accurate’ fit up compared to using the T.I.G. process. There is also the possibility of weld ‘spatter’ being left behind while welding and if the wrong amperage or gas set up is used while welding this could lead to weld ‘spatter’ corrosion due to ‘pitting’ later in the life of the system. A good anti- spatter spray will help to prevent this.
Another point to consider that some manufacturers are trending towards aluminised steel. This is fine but if the system gets scratched through to the bare metal then through time it will corrode at this point if not seen and treated right away.
Summary
As in any quote always make sure your getting a ‘like for like’ price.
Ask all the relevant points above regard material type, wall thickness, grade of material and what welding process is being used. This should then give you an idea what to expect in quality when you purchase your system.
Look for another article on the benefits of ‘structured’ manufacturing and the benefits of correct manifold and collector design on tubular manifolds that are available for the XK engine.