Exhaust corrosion using avgas

morrisman1, Aug 6, 10:03pm
Has anyone else experienced this, and can suggest how to reduce it? After a season of using it in the race car we got a lot of scale come out of the exhaust. The exhaust is mild steel
mrcat1, Aug 6, 10:15pm
Can you still get No1 Racing spirit? That might be a alternative to using avgas as from memory the racing spirit has a few more additives designed more for a car engine but still the same octane rating as avgas.
mark.52, Aug 6, 10:24pm
Lot of lead in avgas. Could that be what the scale is?
serf407, Aug 6, 10:26pm
bill-robinson, Aug 6, 10:45pm
no, e85 is used for taxi racing, real racing uses proper fuel
mrsdoobercoons, Aug 6, 11:34pm
When we had lead in our petrol there were exhaust repair shops everywhere. They were sort of like tyre shops. All gone now that the lead's gone.
mm12345, Aug 6, 11:59pm
Yes - that's it. The surviving exhaust shops have morphed into brake, suspension, and general service centres.
I had a Mazda 323 GTX 4WD turbo running on leaded fuel back in the day. I had to replace the exhaust on it twice in 80,000km, probably because the relatively wide bore exhaust seldom got hot enough to evaporate all the water out of it, exacerbated by turbo meaning that more of the exhaust heat gets dumped in the manifold (which would glow red hot), and the exhaust pressure drop post-turbo particularly with a free-flowing exhaust would cool the exhaust gases more quickly than in a non-turbo car.
mainlander05, Aug 7, 12:17am
Agree.since no lead,no exhaust rotting.so the conclusion must be that the lead caused it.however I can only suggest stainless exhaust.
kwkbrk, Aug 7, 12:43am
My understanding is that avgas attracts water, and I was always advised to use it all up for the very reason that things corrode if left sitting; and to avoid it run ordinary fuel for a wee while to get rid of any residue.
elect70, Aug 7, 12:48am
The exhaust tubing today is thin wall crap , my 89 rangie still has origonal exhaust system , heavy wall tube . & would have been running 95 leaded until the switch over to UL .
sr2, Aug 7, 12:56am
One of the by-products of combustion is water and when you're tuning for HP rather than economy you produce a huge amount!
Combine this with a race motor that is often sitting unused for days or even weeks, throw in some lead for good measure and you've got a rusty exhaust.
mm12345, Aug 7, 1:05am
One apparent miracle probably explains the other, I'd expect the exhaust to be almost ever-lasting when mated to the rest of a car which was designed by default to be almost never-running.
flossy64, Aug 7, 1:32am
Hey i know you
saki, Aug 7, 5:12am
As a former exhaust shop owner and racer I can tell you it is normal to get scale build up in the exhaust when using avgas only way to prevent it is stainless steel system or use E85.
kazbanz, Aug 7, 5:24am
saki-One suggestion made to me was to weld a drain plug to the lowest point in the zorst. One the engine is cool--remove the plug and water should drain out. Was a few years ago
morrisman1, Aug 7, 6:14am
Cool, thanks guys, I had the feeling it would be unavoidable. My preference would be to run unleaded but its hard to turn down free fuel. Might just consider it weight reduction for the exhaust!
reb53, Aug 7, 6:33am
Every car I've ever had has had a hole punched in the lowest part of the muffler to let the water drain out.
If the warrant guys complain, and they haven't yet, I'll stick a self tapper in there.
First time I've heard of lead rotting exhausts. Can't think of the chemical reaction that would cause that, anyone know ?
clark20, Aug 7, 11:56am
Yep, had a Supatrapp muffler rust out in 12 months on the race car, Cardwells didn't want to know
elect70, Aug 8, 2:22am
Wash your mouth out - brititsh quality engineering farked by unions
saki, Aug 9, 3:29am
Water is not the problem when burning avgas it just forms scale and eats at the exhaust, back in the early days of HQ racing someof the cars were running extreme compression and I am sure they doped (cheated) up the avgas, I used to leave there replaced flowmaster mufflers on the bench for a few weeks ond you could watch them corroding away, also had a different smell to burnt avgas. Most racing cars are shut down with a hot exhaust so dampness isnt a problem.
planespotterhvn, Aug 9, 10:29am
Pffft.
No. Its that leaded fuel has an acid additive to descale the lead build up from the cylinder head hemisphere and the piston crown, and the valve surface above and below the tulip and the plug electrodes. This acid compound combined with the water produced by the combustion of the hydrocarbon with Oxygen in the air. The Acid / water condensed in the mild steel exhaust pipes and mufflers and rusted them to bits. My 81Commodore needed a new rear muffler every year or two. My new 81 XR250 needed a new second muffler after a year. Nowdays with no acid lead scavenging compound in unleaded fuel mild steel exhausts last for decades. But the water from the combustion process is still there H-C-H + O2 = H2O + CO.
sr2, Aug 9, 1:25pm
Good post mate, makes a lot of sense.
topper, Aug 9, 8:21pm
I concur with plane spotter. A by-product of combustion in the leaded high sulphur fuels is sulphuric acid. Dilute sulphuric acid just eats ferrous metals.
clark20, Oct 23, 6:05pm
Yep, petrol makes about a 1L of water for every 1L burnt (from combustion and in the air), but most leaves the exhaust in vapour, like above says it was the acid that did most of the damage.