How restrictive is a catalytic converter?
Discussion
Hey guys,
Just had my car fail it's MOT on emissions due as the catalytic converter needed replacing. When I had a look at the old cat it was almost completely empty, which got me wondering:
- Exactly how much performance does a cat cause a turbo car to lose?
- Also how much of a difference there would be between a catalytic converter that was hollow and a decat pipe? Also if this would make a difference to sound?
May just be a physiological effect but feels slower with the new cat on, and sounds quite a bit quieter
Just had my car fail it's MOT on emissions due as the catalytic converter needed replacing. When I had a look at the old cat it was almost completely empty, which got me wondering:
- Exactly how much performance does a cat cause a turbo car to lose?
- Also how much of a difference there would be between a catalytic converter that was hollow and a decat pipe? Also if this would make a difference to sound?
May just be a physiological effect but feels slower with the new cat on, and sounds quite a bit quieter
Depends on the car and how restrictive the original cat was. Without other tuning it probably won't make much difference however de-catting will reduce exhaust back pressure which generally allows the turbo to spool up quicker - that might well be the difference you can feel.
Very car dependant though - what's the car?
Very car dependant though - what's the car?
It is hard to make an exact - 'will loose X bhp'
The only figure I know is from an old '92 ford book, at the point that some cars where cat equipped and others where not. The 2.0 DOHC N9A (non cat) engine was 125ps and N9C (cat) was 120ps if I remember right. The catalysed car was also a couple of MPG down.
There are different cell densities for cats. 'Sports' cats are usually a lower density. They go right down to 100cell, but a lot of vehicles may only scrape the emissions test (with plenty of heat). A lot of tuners consider a single 200 cell to be a pretty good balance unless you're going for every last hp.
As an example (although the remap also has to be considered here) it's generally considered that you'll struggle to reach 250bhp with the stock 'new age' Impreza WRX exhaust. The cars have a small cat before the turbo, a primary cat in the downpipe and a secondary cat further back in the system. The dealer fitted Prodrive kits did away with the secondary cat (and changed back box) and run a map with a conservative 260bhp.
The only figure I know is from an old '92 ford book, at the point that some cars where cat equipped and others where not. The 2.0 DOHC N9A (non cat) engine was 125ps and N9C (cat) was 120ps if I remember right. The catalysed car was also a couple of MPG down.
There are different cell densities for cats. 'Sports' cats are usually a lower density. They go right down to 100cell, but a lot of vehicles may only scrape the emissions test (with plenty of heat). A lot of tuners consider a single 200 cell to be a pretty good balance unless you're going for every last hp.
As an example (although the remap also has to be considered here) it's generally considered that you'll struggle to reach 250bhp with the stock 'new age' Impreza WRX exhaust. The cars have a small cat before the turbo, a primary cat in the downpipe and a secondary cat further back in the system. The dealer fitted Prodrive kits did away with the secondary cat (and changed back box) and run a map with a conservative 260bhp.
charltjr said:
Depends on the car and how restrictive the original cat was. Without other tuning it probably won't make much difference however de-catting will reduce exhaust back pressure which generally allows the turbo to spool up quicker - that might well be the difference you can feel.
This.Flexi pipe attached to the cat was shot on my BMW E39 530d so de-catted it and found only marginal increase in overall power (barely detectable), but the turbo did spool up a little faster. Slightly throatier exhaust note, too.
When they first appeared back in the early nineties a general reckoning was up to a 10% drop in bhp - though the non-cat versions (pre-1992) were still running 98 ron 4 Star.
No doubt cat design has improved over the decades, as have the engine management systems and most engines run 95 ron, or higher, unleaded anyway so probably not so much difference these days.
No doubt cat design has improved over the decades, as have the engine management systems and most engines run 95 ron, or higher, unleaded anyway so probably not so much difference these days.
A lot of non catted cars where running unleaded from around '89 I think. Older engines where also 'uprated' with hardened valve seats and revised timing.
You could still run 4* in these cars but not the other way around as it'd knacker the cat rather quickly. Unleaded pump nozzles are actually a smaller diameter than 4* ones were and size restrictors were added to the fillers on catalysed cars to prevent unleaded nozzles fitting. Imagine how much work the AA would've lost out on if there was a similar system to prevent confusing diesel and petrol?
You could still run 4* in these cars but not the other way around as it'd knacker the cat rather quickly. Unleaded pump nozzles are actually a smaller diameter than 4* ones were and size restrictors were added to the fillers on catalysed cars to prevent unleaded nozzles fitting. Imagine how much work the AA would've lost out on if there was a similar system to prevent confusing diesel and petrol?
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