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Discussion Starter · #1 ·
Those who own a motorcycle with a catalytic converter will immediately read the title of this thread and understand what I am about to say. I promised this forum that I would get to the bottom of the precat problem that has hit the auto industry like a cancer. There are probably hundreds of thousands of damaged engines because of the precat problem. I didn't believe it at first after reading all the sorry reports of Altima engines that had died because of it but I am now a convert. I still haven't got the science of it exactly and maybe nobody has but I am not giving up until I know and understand everything about the problem. D0ugmac is right that the cause is the proximity of the precat to the exhaust manifold. Why this is so is still under investigation. However in the motorcycle industry where there is no choice but to have the catalytic converter close to the exhaust manifold, Yamaha came up with a solution. They invented what is called an Exhaust throttle valve. The following is from Performance and road car exhaust
"Some very smart people in motorcycle racing at Yamaha developed an ingenious device called an exhaust throttle valve (actually Yamaha called theirs an EXUP valve and were the first to use this idea; Honda called theirs the H-VIX valve). These valves are placed at the merge points of the header primaries. They are kept open and are continuous with the header. At cam overlap, THE VALVE PARTIALLY CLOSES. This prevents both the intake air-fuel mix from shooting into the header (called overscavenging) and blocks any reflected exhaust wave from arriving back to the combustion chamber. When cam overlap is over, the valve re-opens. So there is a brief increase in backpressure at cam overlap only with the exhaust throttle valve and nowhere else along the engine cycle. The valve is activated and then disabled by the ECU which measures ignition timing to determine when cam overlap occurs and the potentiometer to determine the position or angle of the throttle valve itself. It constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine speed. This ensures good low to mid-range performance. "


So the key words are THE VALVE PARTIALLY CLOSES. It it completely closed during cam overlap then the advantage of cam overlap would be zero. Why a partial opening prevents any reflected exhaust wave from arriving back to the combustion chamber and subsequent ceramic dust from the converter getting into the combustion chamber is still a mystery to me and maybe a mystery to exhaust mechanical engineers as well but the fact of the matter is that it does PREVENT the particles from reaching the chamber as any motorcyclist with a bad cat and an exhaust throttle valve will tell you. There is no problem if the cats are good because there is nothing to get sucked back into the engine. I personally spoke to Dr. Ron Heck Phd and the inventor of many closely coupled catalytic converters who worked for Englehard Corporation for 31 years and is coauthor of the 1996 bible on catalytic converters. He told me there is no problem if the cats are working properly and are not damaged. I then telephoned Martin Restoule the lead professor of auto mechanics at Algonquin college Ottawa Canada and who is a coauthor of the bible on auto mechanics 101, a 1700 page text book on auto mechanics called "Automotive technology A systems approach" He said if the cats are far from the exhaust port there is no problem. However for owners of vehicles where the precat is either part of the exhaust manifold as in Honda Civics or is bolted on to it in many Nissan vehicles, the paramount importance is to have undamaged cats. But how many owners check their cats?And even if you check your precat on the Altima you can only see the underside and not the upper side which leads to the exhaust port. So owners of Altimas are left with changing their cats every five years or so just to prevent engine damage. Note that by the time you get codes that show up in the computer it is too late. My cats were slowly being poisoned for 9 years by the bad camshaft position sensor that came off the factory floor and the ECM never spit out any codes. When I as the 3rd owner of my 2006 Altima took possession I immediately changed the 2 cats and the camshaft position sensor along with the 2 O2 sensors. Hopefully I saved my engine in time cause it has only 45000 miles on it.

There are at least 3 car manufacturers that closely couple their cats to the exhaust manifold on some of their models. Nissan, Honda, and GM. There may be more. My question is Why do the car manufacturers that do this, not provide exhaust throttle valves at the header. The motorcycle industry does it sometimes. Interestingly Kawasaki provides it on their larger engines but not the smaller ones. Do they consider the smaller bikes to be throwaways?


Meanwhile I haven't stopped my investigation. To be continued:
 

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GoDodgers!
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great report!

I'm gonna say due to cost, they put the cats close to the engine....? or emissions? I'm not quite sure but, glad you're on top of this. Would like to learn more, just as much as you would.

But thanks for the report. It's very insightful and beneficial to not just nissan owners, but the automotive world, in general.
 

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Discussion Starter · #3 · (Edited)
Well, more digging. It seems as if there is a cheap solution to catalytic exhaust gas getting pulled back into the combustion chambers. I will quote Sarah Forst in her book HOW TO BUILD PERFORMANCE NISSAN SPORT COMPACTS. "Anti-reversion headers are preferred by all drag racers. They have a stepped primary tube meaning that the primary tube increases in diameter by about a 1/4 inch within a few inches of the header flange. Sometimes a larger diameter reversion trap chamber is used in place of a stepped primary tube. The stepped diameter is thought to prevent reversion of the exhaust gases during the cam overlap period. If the reflected exhaust wave comes back up the primary pipe through the catalytic converter slightly early or slightly late, it wont arrive at the same time as the other cylinder exhaust gases are being pushed out. If this happens at cam overlap the cylinder will get contaminated. The step on the exhaust manifold dampens out the reflected wave at rpms other than what the pipe's length was tuned for and thus contributes to a wider powerband and thus more torque." If what Sarah says is true then it would seem that the distance of the cat from the exhaust manifold is not the critical factor. Let me quote another more universally recognized authority on the subject of exhaust reversion. Jason from Exhaust videos.com states " You always want to make sure that the header tubing is larger than the exhaust outlets on the head. There should be a short cliff or step between the head outlet and the header inlet. This acts as a barrier against exhaust gases flowing back into the head. " DO THE NISSAN ENGINEERS NOT KNOW THIS?

I will never let this topic go until I have all the math and science behind it. In the meantime I will change my header immediately on my 2006 Altima.

To be continued:
 

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Discussion Starter · #4 ·
Well to my chagrin after speaking to a couple of exhaust experts there is no legal way to change my exhaust header on my California emission compliant vehicle (PZEV) partial zero emission vehicle. I live in Ontario Canada that in the realm of car emissions falls under the California rules!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I guess I live in a world of the land of OZ. Because the car makers live in a legal environment of emission credits and quotas on emissions, the manufacturers can choose to limit emissions in different ways. Nissan and Honda among others have chosen to use closely coupled cat converters which actually are an integral part of the header to their exhaust headers. Nissan has admitted in their Recall Campaign bulletin of August 20, 2008 Nissan Reference NTB03-070d NHTSA reference 03V-084 that " Material from inside a damaged pre-catalyst could enter the engine and result in increased oil consumption." Ha Ha Ha Ha Ha Ha Ha This is a not a joke. They actually said this. I haven't yet got ahold of the science of why a closely coupled converter that breaks apart will get sucked back into the engine, but my previous post expressed dismay as to why a manufacturer would design cars that would let this happen. It may be a combination of the distance involved to the exhaust valve outlet and the fact that the piping should increase in diameter width just before the cat in the direction of the rear of the car so that any exhaust reversion would have a narrower path and thus be stopped from entering the combustion chamber. (see my previous post). In any case because the car manufacturers have not designed the piping properly and or the fact that they haven't put in an exhaust throttle valve like the motorcycle industry (See my previous post), the potential damage to the engine exists. Another factor may be that these 3rd generation Altimas do not have EGR systems because Nissan decided that variable valve timing would provide the method of exhaust scavenging. So anybody with closely coupled converters is left with constantly monitoring their cats and all their 20 or so sensors in their cars because you wont necessarily get any codes if one of the sensors goes bad. My cats were slowly poisoned for 9 years because of a faulty camshaft position sensor and at no time were any codes produced!!!!!!!!!!!!! To be continued:
 

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Discussion Starter · #5 ·
More digging. I found a 2013 study
Fuel impact on the aging of TWC?s under real driving conditions
where the impact of ethanol blended fuels compared to standard gasoline fuel on the three way catalytic converters (which are now standard on all cars and trucks) was studied.

The study concluded after 40000 km for 6 different vehicles. Even a blend of 5% ethanol did considerable damage to the cat. 0% ethanol did only minor damage. Since we can't get 5% ethanol in our gasoline (10% ethanol is sold in regular gas) there is even more damage. We are then left with paying 10 cents more per litre (38 cents a US gallon ) for high octane gasoline which doesn't have any ethanol in it. However high octane gas is even more harmful to your health than regular gasoline. Don't forget the cat doesn't work until it is heated up so you are always breathing in some pollution when you first start your car or idle it. So for those owners who use regular gas in their cars beware of cat damage.
 
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