What's wrong with reversing cats and turbo?
What's wrong with reversing cats and turbo?
(OP)
Dear Gentlemen,
I rarely post to this forum but read what you fellows post... all the time. There is no other forum that quite compares to this one. And Pat Primer, you're the shizzle. Here is a question that's had me thinking for the past year...
If a turbocharger depends on high pressure gas, why wouldn't it make sense to put a catalytic converter before the turbo? Wouldn't a pre-turbo cat make the turbo run harder because of (a) combustion of the unburnt gas (more gas volume) and (b) the resultant higher temperatures of the exhaust? The only draw back might be temperatures too excessive for the turbine bearings but couldn't that be solved by adding a little distance between the cat and the turbo? In particular, I was thinking of running individual mini-cats on each exhaust tube (before a 4-in-1 collector). Any insights? Thanks.
Walter Carter
I rarely post to this forum but read what you fellows post... all the time. There is no other forum that quite compares to this one. And Pat Primer, you're the shizzle. Here is a question that's had me thinking for the past year...
If a turbocharger depends on high pressure gas, why wouldn't it make sense to put a catalytic converter before the turbo? Wouldn't a pre-turbo cat make the turbo run harder because of (a) combustion of the unburnt gas (more gas volume) and (b) the resultant higher temperatures of the exhaust? The only draw back might be temperatures too excessive for the turbine bearings but couldn't that be solved by adding a little distance between the cat and the turbo? In particular, I was thinking of running individual mini-cats on each exhaust tube (before a 4-in-1 collector). Any insights? Thanks.
Walter Carter





RE: What's wrong with reversing cats and turbo?
RE: What's wrong with reversing cats and turbo?
It might also adversely effect the life of the expensive cat.
Although heat might be picked up in the cat, some will also be lost in the extra length of pipe and there will be extra resistance to exhaust flow imposed by the cat.
I think keeping the exhaust as short and au non-restrictive as possible is the best approach.
Regards
eng-tips, by professional engineers for professional engineers
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RE: What's wrong with reversing cats and turbo?
In any case the earlier Subaru MY01 WRX had a small cat prior to the turbo mainly for emissions reasons. One of the modifications for this car was to replace the pre-turbo cat with a straight pipe which gave more power and faster spool up.
regards
Pete
RE: What's wrong with reversing cats and turbo?
Hey Pat- you da shizzle.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What's wrong with reversing cats and turbo?
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: What's wrong with reversing cats and turbo?
Yep on the ('02 in the US) WRX removing the preturbo cat provides full boost ~200-500 rpm sooner. Also if EGT's get too high, or just wear/tear, causing the cat to breakdown -- well what do you think an exhaust turbine will look like after processing solids?
RE: What's wrong with reversing cats and turbo?
Some years back, Merecedes made some engines with the catalyst before the turbo. The catalyst had a metal substrate that degraded, resulting in metal passing through the turbine.
The turbine didn't like metal mixed with the exhaust gases and Mercedes ended up with big warranty claims.
j2bprometheus
RE: What's wrong with reversing cats and turbo?