Can anyone give me cold hard facts why not to use the precat? If it gets hot, tell me how much hotter the engine bay gets than with a downpipe, and under what conditions. If it get clogged, tell me what your a/f ratio is, or mods, etc. I have read a lot of "get that thing out of there because X", but can't find actual numbers or facts to back up the claim. Everyone that ask about the precat is wanting to increase the hp of the car, but I know less peak hp = less peak heat (a/f constant). What about steady throttle cruising temps? Any input would be appreciated.

 

The engine bay temp thing on a non-clogged precat... I don't know about all that. That's what the heatshields are for. It seems like one of those things that somebody said a long time ago and people keep repeating. Think about guys who ditch a non-clogged stock precat and then install a downpipe without a heatshield. I'm skeptical that this is going to reduce engine bay temps compared to a properly functioning (near showroom condition) exhaust system with all heat shields intact.

The real reason why the USDM precat get clogged is because emissions standards were much more lax back then and Mazda did not have to engineer a precat and associated control systems that would survive 80+ thousand miles. Modern cars have much stricter emissions standards as well as follow-up testing (In-use testing) required by the EPA to makes sure that used cars are still meeting emissions standards.

that's a statement that could lead to a lot of discussion...

 

So these precats will not flow with over 100k miles on them?

 

Probably not. I'm sure somebody somewhere on the internet has gotten one to last over 100k but it's only been a recent thing that cats have been legally required to last even 80k.

 

Who cares man. Getting rid of the precat is one less thing to worry about and in day to day, doesn't really contribute much.

 

Regarding to your question about higher HP engines making more heat you are correct to a point, let me explain (forgive me if I cover things you already know).

All engines have a thermal efficiency rating to them. For so much air and fuel consumed they will transform that into usable kinetic energy and waste heat. let's say for arguments sake that the engine is 33% efficient. That means that for every watt of power going to the eccentric shaft two watts of heat are produced. Some of this heat is absorbed by the oil and coolant, but most of it finds it's way out the tail pipe. By this example you can see that if you added more air and fuel to the combustion cycle, the power and thus the heat would increase.

If instead you made the engine more efficient by reducing its flow restrictions (removing a converter) the engine would in effect produce less heat for the same amount of power.

 

the real reason to ditch the precat is:

once it starts to break up and restrict exhaust flow all of that latent exhaust heat gets backed up into the engine causing EGT spikes and eventually broken apex seals.

if that's not a good reason i don't know what is.

main cats are much further downstream, causing less of an issue if they get constipated.

all the precat really does is help clean up emissions quicker on cold starts, a main cat on it's own will still pass even the strictest emissions standards with a well tuned engine.

 

like what ben said, only real reason the precat is there, is to begin the catalyst process faster during cold starts due to a small amount of substrate to heat up.
once the main cat warms up, the precat is pretty much useless, and like ben said, just the main cat by itself, on a well running engine, will pass smog just fine.
now, if the vehicle came with a precat, then you have to have it, due to it being an emissions device, it will show it on the emissions sticker: twc (2), indicating 2 three way cats, aka a warm up and main cat

Lloyd

 

So is it a sure thing that the precat will eventually kill an otherwise good, well tuned engine, or a possibility? I know that the EGR recycles exhaust gas back into the combustion chamber and this effectively cools down the combustion chamber when cruising on a warmed up engine. http://en.wikipedia.org/wiki/Exhaust_gas_recirculation
If the combustion chamber is filled with an inert gas (albeit hot) during combustion, wouldn't this cool down the EGT? Just trying to separate fact from hearsay.

 

hot day + moderately beating on the car and even a decent tune with a fairly broken/clogged precat = dead 7.

the EGR is useless on these engines so don't rely on it to aid in cooling the combustion with inert gas, if you remove the LIM and EGR take a look at the EGR passages, they are literally a pinhead hole drilled into the gaskets to restrict flow and often times are completely clogged up. you could bore out the holes but no one has ever tested the side effect to doing so, it may cause your idle to be extremely poor.

most of the time i omitted them even in california, it was a rarity that a smog tech even noticed it was missing.

 

The point I was trying to make is that inert gas lowers combustion chamber temperatures. Wouldn't inert gas accumulate in the combustion chamber if you had a clogged precat? The real question is how does a clogged precat kill an engine specifically? Does it have something to do with the raising temps incurred during pressurization?

 

EGR is still a free flowing gas, so it flows freely into and out of the engine.

exhaust restrictions have nowhere to push the exhaust out and down the exhaust system.

basically think of it like an eclosed explosion capped off, the backpressure and heat from the combustion cycles cause subsequent cycles to become hotter and hotter, to the point that it becomes unsafe for the engine.

the issue is that there is just nowhere for the flame to go yet the engine is still trying to cram more into that straw.

if someone put a cork in your *** eventually what would happen?

 

If exhaust backpressure and heat are high enough, there is a chance that this heat will essentially flow back into the engine and be recycled into the intake stroke. That ultimately heats up the combustion temperatures and could lead to knock.

Still, I am not convinced that a clogged cat will kill a 100% completely stock and otherwise healthy engine in the catastrophic fashion that Karack has described. I'm sure it's happened to somebody somewhere on the internet but realistically it will more likely just hamper performance and put additional thermal stress on components.

 

The capped off explosion analogy makes sense to me.

Once the fuel is ignited and squeezed in the combustion chamber a chain reaction has taken place. Even if there are few oxygen molecules in the chamber, the charges is very hot from compression/ignition . The precat prevented some inert gas from escaping during the previous combustion cycle, but the ecu believes the intake is full of fresh air. The ecu squirts fuel based on this assumption(a very rich condition). The precat tries to burn up this excessively rich charge, but can't do so fast enough, and continues to heat up all the while. Eventually the precat is overloaded, melts down, and the rich, burning mixture is forced back into the combustion chamber under extreme pressure/heat.

I have stated some assumptions above, correct me where needed. Does the precat slowly clog and then melt down, or is it fine one day, dead the next? Or is there anyway to know this?

 

it was a bit exaggerated but i have seen more than 1 FD with a blown motor with no other cause that i could find. the precat simply just doesn't need to be there so most people opt to not risk it and get that power back from the restriction they cause even when working fine.

the cats can both clog up slowly over time or fracture due to extreme thermal expansion/contraction rates.

generally i find the cores cracked and chunks turned sideways blocking most of the airflow, this is a common failure for catalysts in these engines.

 

So would you say that ultimately, pre-detonation kills the apex seals during the combustion stroke because of a superheated intake charge?

 

I have been around FD's for YEARS, including back when they were still pretty new cars ('96). I've been around SUPER low mileage, still new smellin' FD's with stock precats that were still good. Pop the hood on one after a good drive and the wave of heat coming from the engine is CRAZY. The precat puts out an INCREDIBLE amount of heat.

Back in the same timeframe, putting on a downpipe made for a massive reduction in underhood temps. Getting a ceramic coated downpipe even more so. I don't have hard numbers to go with this, but I can assure you on this.

The precat only has one good thing about it - it made the FD pass US cold start emissions so Mazda could legally sell the FD in the US. That's it. Everything else about it is a terrible idea - huge heat sink that bakes everything under the hood, restrictive, prone to clogging, hard to remove and in the way, etc.

Previous posts about EGR and the FD are correct. The FD's EGR system is worthless, I think Mazda knew that in designing it. The passage is literally pinhole size and will clog up in no time flat. Mazda got rid of EGR in the '95 US FD's as a matter of fact. Again, it's a system they had to put on to sell the car in the US. JDM FD's didn't have EGR at all. While the principle of EGR sounds nice (reduce combustion temps) you do so by introducing an inert gas that occupies some of the combustion volume. This ultimately reduces power since you have less fuel and oxygen in the combustion chamber.

For most people, what kills apex seals is not enough fuel or too much timing. Biggie is increasing boost with a stock ECU - it can't fuel the extra boost, the car runs lean, bye-bye engine. Or, general lack of maintenance - dirty fuel injectors, clogged fuel filter, etc. Many times it's a culmination of factors.

Reducing intake air temps is a good step to keeping a healthy motor, that's for sure. Higher intake air temps mean higher combustion temps, and high combustion temps mean detonation.

Dale

 

Here is a pic of a new FD EGR gasket. The tiny hole at left is the exhaust source, and the larger hole on the right is the EGR valve output to the intake manifold. ("Much ado about nothing.")

 

I do not have data. Long ago it was established that precats nuke the engine bay and end up clogged. With the precat removed the underhood temp definitely get better. It's so fundamental that people do not install EGT sensors/gauges before removing it, hence the lack of data.

The fact that they melt down and clog after 40k miles and cost over $1000 from Mazda probably doesn't help either.

If you're interested I have a nearly new (like under 1000mi) stock precat if you want to run proper comparative tests.

David

 

Can you drill and tap into a cast iron precat without any issues (cracking)? I have a precat, but it has sat outside for years, and could have up to 90k miles on it, so I don't think it would be a very good candidate for an experiment. I have a digital greddy egt gauge with a new sensor that could be used, and could easily mount up an under hood digital thermometer. Since I'm rebuilding the engine right now, it would be a good opportunity to do an exhaust test on a fresh engine. Wonder what shipping would be on that thing?

 

It is a complete waste to spend ANY time or money to test a precat. I can't understand why anyone would do that. Downpipes are not that expensive, and are superior in every way to a brand new precat. Put on a downpipe, and forget this nonsense.

 

I don't know and don't particularly care. Like Adam, I think this exercise is a waste of time. Just the reduction in heat off the hood was enough fo rme. If you're willing to pay me $30 for shipping you can have it and see how it turns out.

I understand the impetus behind asking the question "why" with regards to popular ideas. But IMHO the advantage of a downpipe over the stock precat is simple and true. The physics is simple - you have heated exhaust gas exiting the turbos. With a precat, you have additional backpressure (more heat) and additional frictional losses (more heat). Worst of all, the heat is up inside the engine bay where there is no active air flow. The best strategy found yet is to reduce restriction and insulate that gas to get it out and away from the engine. With a downpipe in place of the precat, there is ample heat for the catalytic converter to do it's job, there is enough restriction in the exhaust to maintain correct boost output, and the engine bay is cooler for it.

David

 

Mazda did some testing in the development of the Rx8 engine, and in part of their testing they put a CO meter on the top of the rotor housing and one in the exhaust to come up with an "internal EGR ratio" basically a measurement of exhaust gasses that stay in the engine on the intake stroke. at idle the rotary has a 3.0+ CO concentration in the combustion chamber.

the rotary has no valves, so restrictions in the exhaust are important. a piston engine, or even the Rx8 engine is under 1% CO.

the internal EGR thing is actually why the new Mazda SKYAKTIV engines have such long headers.

so there is already so much exhaust gas in the intake stroke that the puny little EGR valve is like a grain of sand on the beach...

if you want to know about the precat, the JDM cars has 265 hp, and no precat, the us cars are 255. the precat is the ONLY mechanical difference...

 

Has anyone mentioned the stock, pre-cat's weight compared to an DP? I'm guessing 15lbs more.

 

Thank you for this info, any chance you can post a link? Increasing exhaust gas in the combustion chamber would lower combustion chamber temps. I'm not talking about lowering combustion chamber temps through the use of an egr valve, but conceptually the same thing with a restrictive precat. There is something to be said about the Jdm engine with a downpipe, but then again, the precat cost more. If cost were not an issue, and reliability ruled the road, would Mazda use precats on all rx7s? What if we have a hot precat, hot engine bay, but a cooler engine?