twinsinside

iTrader: (34)

There is no advantage and many disadvantages to running the pre-cat, as many have already covered.

The pre-cat was added to USDM FD's purely as a nod to environmental regulations. I'm not sure how you're equating a precat with reliability either, since it causes the exact opposite effect. A hotter engine bay means a hotter engine, hotter turbo intake temps (in most setups), worse heat soak, breakdown of rubber/plastic components, etc etc.

Adding more restriction to the exhaust path, as well as ridiculous amounts of heat, is the opposite direction you want to go.

If you want to cool your engine down and have it last, ditch the precat, do the reliability mods, install good ducting, and put in a low end AI system using meth/water. Problem solved.

No matter how much you polish the pre-turd it will still smell.

arghx

iTrader: (3)

It kind of depends when that's happening and how the pulsation effects are working out. It's a pretty big topic and there is a lot to read about it for both piston and rotary engines. Here's the stuff about the Skyactive-G 4 cylinder and its long time header system http://www.mazda.com/mazdaspirit/sky...kyactiv-g.html

I don't think there's enough information to prove or disprove that on an engine with a healthy precat. The rotary engine port timing and exhaust manifold design have the biggest effects on this. The problem is that the precat clogs and causes excessive restriction. Lots of engines do have precats and those precats last a long time without causing problems. I wouldn't put a USDM FD into that category.

Precats are typically used to pass the cold start phase of the EPA FTP 75 emissions test. Mazda eliminated the precat and smog pump (except during cold starts) on the Rx-8 btw.

sc_frontier

iTrader: (3)

http://www.mazda.com/mazdaspirit/sky...kyactiv-g.html
Arghx, thank you for this article! This shows how hot exhaust moves from one header tube to the next, and hot gasses enter the other cylinder during it's intake exhaust overlap period. This hot gas caused detonation in high compression engines. Egr gasses have time to cool before they are reintroduced, and wouldn't contribute to detonation.

dgeesaman

iTrader: (7)

Would you say that the rich tuning that kills the FD precats, or just exceedingly high (turbo + rotary) EGTs?

David

arghx

iTrader: (3)

Cliff's notes: it's the high EGT's, not the rich mixture, and in fact under normal operation it is rich mixtures that actually help preserve converters.


More detailed answer: The things to consider here are the design of the converter itself, the on-board diagnostic systems to make sure it keeps working, and the basic control strategy for fuel, timing, and secondary air injection (smog pump).

We know that the USDM FD precat has a metallic substrate and I think it may only be only an oxidation-type catalyst (HC and CO conversion but no NOx). It also has a sort of filtration function.



I'm not sure if there is anything particularly unique about this design compared to the other metallic pre-cats out there. Here's some basic info on ceramic vs metallic substrate designs:




Emissions regulations were a joke 20 years ago compared to what they are today, and unlike today manufacturers did not have to do follow-up testing on used cars to make sure emissions equipment still after x miles. The FD didn't have to meet strict standards for useful emissions equipment life, and it did not have a bunch of control systems to monitor the effectiveness of the cat, like an EGT sensor and a rear O2 sensor. The Rx-8's have a stock EGT sensor and so do the Subarus that come equipped with a precat mounted before the turbo (yes before the turbo). Both of those have rear o2 sensors to monitor the efficiency of the cat.


Your question centered around basic control strategy. Based on everything I've read, heat is far more important for determining life expectancy than the actual mixture of the converter feed-gas. My 8th edition (July 2011) Bosch handbook remarks here on page 588 that accelerated converter aging begins at 800C/1472F exhaust temperatures, which a rotary can easily achieve coming right out of the exhaust port.

arghx

iTrader: (3)

I think people are making the assumption that these cats are clogging up with unburnt hydrocarbons due to a rich mixture. An oxidation converter is not a particulate filter on a diesel; it is not an NOx accumulator or HC storage cat--it's not really meant to capture large quantities of anything. For the most part, excess hydocarbons and other substances are just going to pass right through a healthy precat without being converted. I have reason to believe that the root issue is the cat is actually clogging itself as the material inside breaks down due to heat. Remember that the richer the mixture, the more heat absorbed--it is a somewhat exponential relationship. Here is a chart showing the relationship between brake specific heat consumption, ethanol concentration, and mixture (lambda) on a single cylinder research engine:



Notice that as the mixture gets richer, more heat is consumed at a nearly exponential rate. Now this data isn't from a rotary, but the basic idea remains. Under normal operating conditions as we richen the mixture, exhaust temperatures go down and thermal deterioration reduces.

Now let's look at an actual engine with a very long-lasting precat. Any vehicle certified as a Partial Zero Emissions Vehicle (PZEV) in California must have the emissions equipment last for 150k miles. The 2003+ Prius is surely the most well-known PZEV. The Prius has a pre-cat and a main cat. In a published paper Toyota engineers explain that they did extensive testing to figure out the main causes of catalyst deterioration. Part of the improvements they made had to do with the internal arrangement of the precat. The other main strategy was keeping temperatures down to minimize deterioration:



The chart above shows the effect of temperature on Prius catalyst conversion efficiency over time. You can see in the chart that the 700C Catalyst temperature worked much more efficiently over time than at the higher temperatures, meaning that the converter was not breaking down nearly as quickly. Toyota engineers also came up with a system that disables the [presumably] deceleration fuel cut during high temperature operation because it was found to accelerate converter deterioration.


So if you look at how an actual OEM precat with a 150k mile warranty survives, it comes down to optimized converter design and a control strategy designed to keep exhaust temperatures down. Rich mixtures are not normally associated with high exhaust temperatures.

j9fd3s

iTrader: (3)

that's it. hot exhaust gasses are actually bad in a rotary too, for the same reason too. under boost its a big cause for detonation.

Ryan95

iTrader: (10)

This used to be relatively common knowledge back in the big list days.
When the cars were fairly new, a couple people with bad precats cut them up and found the material had basically come apart and was rattling around in there creating a restriction.

arghx

iTrader: (3)

Doesn't surprise me. But the idea that a rich mixture would clog a cat up with hydrocarbons certainly seems like it would make sense. As I alluded to earlier, certain types of converters function as traps and they can get clogged.

It's one of the reasons why Mazda focused a lot on exhaust pulsation effects on turbo rotaries during their research of the late 80s. Using a twin scroll turbo reduces the heat and backpressure that could affect the intake charge due to overlap between intake and exhaust stroke. The same principles apply to a piston engine, especially inline 4 and inline 6 engines. It has to do with the runner lengths, firing order, and engine speed.

j9fd3s

iTrader: (3)

back in the day, 97-2000, we used to stock the precat, they got changed under warranty here and there. the dealership probably replaced more turbos than precats though.

in the 2004-2007 time frame we used to do a LOT of Rx8 cats. there was a recall where we tested the engine (40206?) and the cat, and the engines always passed, but the cats were like 50/50. somehow the ECU could generate some kind of back pressure number.

of course these were all warranty, the bad one is the 99-2000 miata precat, those fail OUT of warranty, we sold more of those than the rotary cats combined...

sc_frontier

iTrader: (3)

Wow, great posts guys. Thank you thank you.

dgeesaman

iTrader: (7)

My mention of richness causing precat failure was not due to clogging with hydrocarbons, it was related to afterburn taking place within the precat and the degradation due to the high temperatures.

Dave

arghx

iTrader: (3)

I suspect actual afterburn on a healthy stock car is pretty limited; the FD had an anti-afterburn system just like its predecessors. On the older Rx-7 engines the ACV had an anti-afterburn valve that would deliver smog pump air directly into the intake manifold on deceleration. On the FD the anti-afterburn valve was eliminated and instead the ISC valve took over its function. The dashpot also played a role. See page F-58 of the FD Service Highlights.

It's certainly possible that the anti-afterburn system loses its effectiveness over time, meaning increased afterburn and accelerated catalyst deterioration. The Series 6 stock ECU did not have any kind of long term self-learning (fuel trims etc) to directly account for changes in the engine and driving conditions.

j9fd3s

iTrader: (3)

this is a good thread! so way back in the mists of time, maybe 95? 96? when these things were still under warranty, my friend an i compared 2 FD's with 2 different mods.

the first one was an HKS car, it had the Fcon, the boost controller, intake and the cat back, and an EGT gauge. it had a couple different boost settings but i think we tried it @15psi (maybe higher?) STOCK precat, stock main cat.

car 2 was stone stock R1 except the DP/MP. he even ran it on 87 gas! with no boost gauge, we didn't know to be paranoid about these cars until the forum in like 2001... this car survived a serious beating with no boost control and no fuel. the engine died later from the same coolant seal failure than most FD's have.

anyways the HKS car ran like 500C EGT's AT IDLE, i only remember it because it singed my leg hair off!

so anyways the HKS car @15psi went ok, but the R1 was much quicker. the R1 felt better too, the HKS car sorta felt like you were trying to force it too much.

the HKS car died of an accident, supposedly it was fixed, but IDK.

helghast7

iTrader: (6)

as much fun as everyone had giving a very definitive and technical explanation on this

im surprised nobodys asked the basic question to the OP

OP, why are you so intrigued about the precat when its already understood that these cars can pass emmisions with one cat? are you that paranoid about getting pulled over? or do you live in an area that constantly gets hit with emmission checkpoints?

sc_frontier

iTrader: (3)

We don't have emissions testing in Grayson County. The reason I posted this request for information was because I could not find any hard evidence why not use the precat. Anecdotal evidence has been proven wrong time and time again concerning various beliefs. On a side note, I hate getting behind someone with polluting exhaust in this area.

RotaryEvolution

iTrader: (30)

i would say the precat doesn't plug up so much as it heats and cools too rapidly. the material is quite fragile.

take a torch to a piece of glass and then toss some cold water on it or hot water on cold glass and see what happens.

some cars that weren't thoroughly abused the precats actually survived for a surprising amount of time.

1QWIK7

iTrader: (33)

This.

Implying switching out the stock precat is all hearsay and trying to conduct further tests to find real proof of its effectiveness or lack of is a waste of time and dumb Imo.

There is no downside to switching to a downpipe so why not just do that? Besides a small cost and labor, I can't see why people would keep the precat, even though it "could" last longer that people assume.