Exhaust: Monoliths and Main Cat questions
08-06-17, 08:57 PM
#1
Exhaust: Monoliths and Main Cat questions
Outside of patching holes in exhaust pipes, this is my first venture into catalytic converters.
BACKGROUND:
I suspected an exhaust leak and checked with engine at idle.
There was a leak at the gasket between the No. 1 and No. 2 monoliths, and at the gasket between the long pipe and the muffler. I did feel air movement behind the front flange of the main cat, but could not locate the source.
Got all 5 pipe gaskets, the split pipe gasket, and a new long pipe (behind the main cat). The bracket rusted off the original pipe long ago and I “fixed” it. I thought I should get the new pipe while it’s still available.
Once everything from the exhaust manifold to the muffler was out of the car and disassembled I did a pressure test on the main cat (pic 1). At 20, 40 & 60 PSI there was air coming out of the heat shield left-front corner. Next I took the heat shield off. There are 4 spot welds, 2 had rusted away and I cut through the last 2. It’s a clamshell design and bending back the lip on the top half allows the two pieces to separate (pic 2). The insulation on the top stayed mostly intact. The insulation on the bottom is in a baggie as it did not fare as well (pic 3). I’m assuming there’s asbestos in the insulation.
I did find two cracks in the seam of the main cat on each front corner (pic 4). Once I had the heat shield off I redid the pressure test to confirm that the two seam cracks were the only leaks. 10 years back the side air pipe rusted off. I had it welded back on and it is still holding. I want to try the same on the two seam cracks.
The No. 1 monolith still has some of its heat shield, No. 2 has none. I'm assuming since it runs this is not a major issue.
No. 1 has nothing inside (pic 5) and No. 2 is mostly intact from the rear except the screen is failing, but looks chunky from the front (pic 6). It is possible to see straight though No. 2 at the very top.
QUESTIONS:
1. Should there be something inside No. 1 monolith?
2. If there was something inside No. 1 monolith, did it burn up (vaporized) or did it get passed along to go inside No. 2?
3. If the guts from No. 1 are now in No. 2, would this have an adverse effect due to added obstruction?
4. Upon visual inspection, the main cat interior front and rear look intact. If the guts inside No 2 monolith fail, is it possible there could be damage to the main cat?
5. If there is the potential for damage to the main cat from crap coming from No 2, would it be advisable to manually clean out No. 2? Or, just leave it as it is?
Thanks in advance for any comments/suggestions.
The last image is the fix on the original cat-to-muffler pipe. I took a pipe with the ID close to the OD of the pipe to be fixed, and cut it lengthwise on opposite sides. Used a patching compound then clamp the cut pipe over the fix. Holds better than the kit solutions.
BACKGROUND:
I suspected an exhaust leak and checked with engine at idle.
There was a leak at the gasket between the No. 1 and No. 2 monoliths, and at the gasket between the long pipe and the muffler. I did feel air movement behind the front flange of the main cat, but could not locate the source.
Got all 5 pipe gaskets, the split pipe gasket, and a new long pipe (behind the main cat). The bracket rusted off the original pipe long ago and I “fixed” it. I thought I should get the new pipe while it’s still available.
Once everything from the exhaust manifold to the muffler was out of the car and disassembled I did a pressure test on the main cat (pic 1). At 20, 40 & 60 PSI there was air coming out of the heat shield left-front corner. Next I took the heat shield off. There are 4 spot welds, 2 had rusted away and I cut through the last 2. It’s a clamshell design and bending back the lip on the top half allows the two pieces to separate (pic 2). The insulation on the top stayed mostly intact. The insulation on the bottom is in a baggie as it did not fare as well (pic 3). I’m assuming there’s asbestos in the insulation.
I did find two cracks in the seam of the main cat on each front corner (pic 4). Once I had the heat shield off I redid the pressure test to confirm that the two seam cracks were the only leaks. 10 years back the side air pipe rusted off. I had it welded back on and it is still holding. I want to try the same on the two seam cracks.
The No. 1 monolith still has some of its heat shield, No. 2 has none. I'm assuming since it runs this is not a major issue.
No. 1 has nothing inside (pic 5) and No. 2 is mostly intact from the rear except the screen is failing, but looks chunky from the front (pic 6). It is possible to see straight though No. 2 at the very top.
QUESTIONS:
1. Should there be something inside No. 1 monolith?
2. If there was something inside No. 1 monolith, did it burn up (vaporized) or did it get passed along to go inside No. 2?
3. If the guts from No. 1 are now in No. 2, would this have an adverse effect due to added obstruction?
4. Upon visual inspection, the main cat interior front and rear look intact. If the guts inside No 2 monolith fail, is it possible there could be damage to the main cat?
5. If there is the potential for damage to the main cat from crap coming from No 2, would it be advisable to manually clean out No. 2? Or, just leave it as it is?
Thanks in advance for any comments/suggestions.
The last image is the fix on the original cat-to-muffler pipe. I took a pipe with the ID close to the OD of the pipe to be fixed, and cut it lengthwise on opposite sides. Used a patching compound then clamp the cut pipe over the fix. Holds better than the kit solutions.
08-07-17, 09:19 AM
#2
Waffles - hmmm good
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Hmm, do you have to run the cats?
The guts from the 1st is definitely in the second and will cause flow problems. I bet the main cat may have some gunk in it as well. You could hollow them all out, weld em up and put it all back together if you need to pass a visual inspection. If you get sniffed you will need to spend $$ getting new cat or cats I bet.
A full RB SP setup would be the way to go if a) you have the scratch needed and b) you don't have to run cats for inspections.
The guts from the 1st is definitely in the second and will cause flow problems. I bet the main cat may have some gunk in it as well. You could hollow them all out, weld em up and put it all back together if you need to pass a visual inspection. If you get sniffed you will need to spend $$ getting new cat or cats I bet.
A full RB SP setup would be the way to go if a) you have the scratch needed and b) you don't have to run cats for inspections.
Last edited by t_g_farrell; 08-07-17 at 09:22 AM.
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TimWilbers (08-07-17)
08-07-17, 11:51 AM
#3
Hmm, do you have to run the cats?
The guts from the 1st is definitely in the second and will cause flow problems. I bet the main cat may have some gunk in it as well. You could hollow them all out, weld em up and put it all back together if you need to pass a visual inspection. If you get sniffed you will need to spend $$ getting new cat or cats I bet.
A full RB SP setup would be the way to go if a) you have the scratch needed and b) you don't have to run cats for inspections.
The guts from the 1st is definitely in the second and will cause flow problems. I bet the main cat may have some gunk in it as well. You could hollow them all out, weld em up and put it all back together if you need to pass a visual inspection. If you get sniffed you will need to spend $$ getting new cat or cats I bet.
A full RB SP setup would be the way to go if a) you have the scratch needed and b) you don't have to run cats for inspections.
I've looked into the RB solution and that would be my fall-back solution. Currently I have about $50 invested in the project, less the new pipe ($179 plus S+H).
I'll clean out the No 2 to prolong the life of the main cat. Get the seams welded and put it back together. The go from there. I've attached 3 images of the front of the main cat. The foremost screen is intact. Monoliths 1 and 2 are basically nonfunctional at this point.
Thanks for the confirmation.
08-08-17, 12:26 PM
#5
I did clean out the No 2 monolith and the main cat is currently at the recommended muffler shop being welded.
I'll update when project is completed.
08-09-17, 10:56 AM
#6
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i kind of skimmed all this, so apologies if i missed something.
in CA we need to keep the cats, because we still actually need to smog these things.
its normal for the pre-cat to be hollow, they are more for warm up emissions than anything else, so we just remove the junk from the main cat, and call it good, emissions test results don't change.
for the main cat, they do split at the seams, and i've had a bunch welded up. the 30 year old Mazda cat is just better than the aftermarket so its worth fixing.
that perforated steel plate on the inlet is there to absorb the pounding from the engine. the exhaust port opens instantly, so the pulse from the exhaust opening is like a hammer.
so the cliff note? you're on the right track, sand all the flanges flat, use new gaskets, weld up the cracks, and you're good to go
top tip, make sure it uses all the hangers, the studs on the engine can't support the exhaust, and they will shear off
in CA we need to keep the cats, because we still actually need to smog these things.
its normal for the pre-cat to be hollow, they are more for warm up emissions than anything else, so we just remove the junk from the main cat, and call it good, emissions test results don't change.
for the main cat, they do split at the seams, and i've had a bunch welded up. the 30 year old Mazda cat is just better than the aftermarket so its worth fixing.
that perforated steel plate on the inlet is there to absorb the pounding from the engine. the exhaust port opens instantly, so the pulse from the exhaust opening is like a hammer.
so the cliff note? you're on the right track, sand all the flanges flat, use new gaskets, weld up the cracks, and you're good to go
top tip, make sure it uses all the hangers, the studs on the engine can't support the exhaust, and they will shear off
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08-13-17, 02:34 PM
#8
All back together.
Upon installation the rubber doughnut hanger on the right was floppy loose. I compared the original long pipe to the new one and the bracket hook on the right was longer on the new one. Checking the 85-85 Parts Catalog there are 2 versions for a 12a with manual transmission based upon built date. My Rx7 was built Oct. 83 and I should have used a 40-200B, but as of March 84 the new pipe is 40-200C. Only the 200C is currently available. I decided to use the new pipe over the old "patched" one. With a jig of steel bar, big bolt, vice grips and clamping grips, I manage to put an S-curve in the pipe hanger hook so that it was within 3/8" of the original position.
On a test drive I did notice more of a deeper hollow sound. Somewhat less of the family car sound it had before. Nice.
Upon installation the rubber doughnut hanger on the right was floppy loose. I compared the original long pipe to the new one and the bracket hook on the right was longer on the new one. Checking the 85-85 Parts Catalog there are 2 versions for a 12a with manual transmission based upon built date. My Rx7 was built Oct. 83 and I should have used a 40-200B, but as of March 84 the new pipe is 40-200C. Only the 200C is currently available. I decided to use the new pipe over the old "patched" one. With a jig of steel bar, big bolt, vice grips and clamping grips, I manage to put an S-curve in the pipe hanger hook so that it was within 3/8" of the original position.
On a test drive I did notice more of a deeper hollow sound. Somewhat less of the family car sound it had before. Nice.
04-23-19, 02:16 PM
#9
Full Member
i kind of skimmed all this, so apologies if i missed something.
in CA we need to keep the cats, because we still actually need to smog these things.
its normal for the pre-cat to be hollow, they are more for warm up emissions than anything else, so we just remove the junk from the main cat, and call it good, emissions test results don't change.
in CA we need to keep the cats, because we still actually need to smog these things.
its normal for the pre-cat to be hollow, they are more for warm up emissions than anything else, so we just remove the junk from the main cat, and call it good, emissions test results don't change.
04-23-19, 09:36 PM
#10
ancient wizard...
04-24-19, 12:55 PM
#11
Full Member
The explanation about the #1 pre-convertor's purpose makes sense to me. Now, using this same logic, could not a piece of pipe and two flanges be fabricated to replace the OEM unit? Would this affect tailpipe decibels?
Place in Ottawa, ON (premierperformance.parts) has offered to manufacture a 'clone' (#1) converter for me - if I ship them the original. But no idea of the cost. I have no objections to replacing that OEM unit with open pipe and pretty sure any competent shop in my own city can do this. No need to attempt to 'hollow-out' my original part. I suspect it would not be a 'slam-dunk' - due to hardness of material and confinement.
Ideas?
Last edited by ebodyboy; 04-24-19 at 12:59 PM. Reason: spelling error
04-24-19, 12:59 PM
#12
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Just curious as to how much louder at tailpipe with no #1 converter. I remember decades ago when enthusiasts would remove all 3 converters and replace with header and/or long pipe - all the way to muffler flange. Decibels were much higher but I have no idea of the volume. Anyone did this? It's no secret that the cat-cons significantly lowered the decibels - as the muffler alone was not very quiet.
The explanation about the #1 pre-convertor's purpose makes sense to me. Now, using this same logic, could not a piece of pipe and two flanges be fabricated to replace the OEM unit? Would this affect tailpipe decibels?
Place in Ottawa, ON has offered to manufacture a 'clone' (#1) converter for me - if I ship them the original. But no idea of the cost. I have no objections to replacing that OEM unit with open pipe and pretty sure any competent shop can do this. No need to attempt to 'hollow-out' original part. I suspect it will not be a 'slam-dunk' - due to hardness of material and confinement.
Ideas?
The explanation about the #1 pre-convertor's purpose makes sense to me. Now, using this same logic, could not a piece of pipe and two flanges be fabricated to replace the OEM unit? Would this affect tailpipe decibels?
Place in Ottawa, ON has offered to manufacture a 'clone' (#1) converter for me - if I ship them the original. But no idea of the cost. I have no objections to replacing that OEM unit with open pipe and pretty sure any competent shop can do this. No need to attempt to 'hollow-out' original part. I suspect it will not be a 'slam-dunk' - due to hardness of material and confinement.
Ideas?
04-25-19, 11:15 AM
#13
Full Member
Well, I think I mentioned somewhere that my original (no.1) monolith failed at about 11k.miles and was replaced under warranty. So not much more than 50k. on this one. All a 'moot point' anyway since I went ahead and removed all the inside guts - grid and webbing material. I trust those who stated that this piece is pretty much comparable to our body's appendix. (My word, not theirs).
It took me only about 15 minutes and a skinned finger. (d'oh!) Put a glove on after that.
I did get a better look at the pieces and can pretty much conclude that the (inlet) restriction was more like only 25%. The surface area not visible through inlet was actually in pretty good shape. As suggested, possibly excess of fuel (or oil) upon initial start-up. For anyone who still has the OEM (RE-EGI) setup, you are well aware of the common 'smoking' - especially if engine not run for a week or longer. It was posted in a thread somewhere. Injectors tend to 'slobber' and get dirty - when having to sit for several days or longer.
I did consider an aftermarket (Racing Beat, Mazdatrix etc). exhaust system long time ago but this is not a 'race car' by any category and I just don't drive it enough to recoupe the cost. Keeping originality and interior, etc. in decent condition more of a concern. Time to get everything back together and see how it runs and drives.
(thanks to USA friends)
It took me only about 15 minutes and a skinned finger. (d'oh!) Put a glove on after that.
I did get a better look at the pieces and can pretty much conclude that the (inlet) restriction was more like only 25%. The surface area not visible through inlet was actually in pretty good shape. As suggested, possibly excess of fuel (or oil) upon initial start-up. For anyone who still has the OEM (RE-EGI) setup, you are well aware of the common 'smoking' - especially if engine not run for a week or longer. It was posted in a thread somewhere. Injectors tend to 'slobber' and get dirty - when having to sit for several days or longer.
I did consider an aftermarket (Racing Beat, Mazdatrix etc). exhaust system long time ago but this is not a 'race car' by any category and I just don't drive it enough to recoupe the cost. Keeping originality and interior, etc. in decent condition more of a concern. Time to get everything back together and see how it runs and drives.
(thanks to USA friends)
04-27-19, 09:01 AM
#14
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04-28-19, 08:35 PM
#15
Awhile after I the exhaust system back together and installed, the volume increased as did a noticeable vibration.
The carb needs adjusting as it runs rough with the choke pulled until it warms up a some and I ease off the choke.
04-29-19, 02:30 AM
#16
ancient wizard...
The pounding being referred to is how violent rotary engine exhaust pulses are compared to piston engines. If you ever heard a rotary with no exhaust or one run thru an unmuffled megaphone system,versus even a big block high compression performance V8 with open headers,you would understand.
While the Piston engine is very loud,the rotary assaults your ears. 1st time i experienced this was at a hillclimb in Reading Pa. All kinds of performance cars going past a full throttle spot where we picnicked, big block cobras,big block vettes,both with sidepipes,musclecars,all with uncorked exhausts. The loudest car was a little yellow RX3 with a tuned megaphone system. Girlfriend couldn't stand the noise so she had my hearing protectors. I soon learned when that Mazda was coming to plug my ears. i could not believe the noise,felt like it rattled your bones sort of like sitting in stands at the drags when a funny car makes a pass but different.
A 4 stroke engine has an power cycle every 4 crankshaft rotations and the exhaust comes out of the cylinder head and the part where it exits the head is a little removed from the combustion chamber so the sound though unmuffled has the exhaust pulses damped a bit.
A rotary engine is essentially firing continuously,each rotor has 3 bathtubs or compression chambers and in one revolution of the e shaft there are 3 ignition events compared to the piston engine having one ignition event in 4 revolutions of the crankshaft,this is the big difference in noise but more than that unlike a pistons cylinder head exhaust port,the rotary exhausts directly out of the rotor housing so its combustion event has no damping and its exhaust pulses are akin to a rapid fire machine gun. There is no break from the power pulses and why these engines have massive cast iron exhaust manifolds to soak up all that noise,but the front part of exhaust system gets hammered with that machine gun power pulse. There is no break like in a piston engine,also the reason particularly early rotary engines consume spark plugs at a much faster rate the piston engines.
In answer to your question,no you can't feel the pounding. If you have gutted a cat there will be some increased volume of exhaust noise. Vibration could be caused by exhaust system contacting something it shouldn't,check all your system hangers to be certain they're intact an not allowing system to sag. Another real possibility that could contribute to the noise is the engine running rough as you say. Adjust the carb to get the engine to run smoother,may minimize vibration issue.
While the Piston engine is very loud,the rotary assaults your ears. 1st time i experienced this was at a hillclimb in Reading Pa. All kinds of performance cars going past a full throttle spot where we picnicked, big block cobras,big block vettes,both with sidepipes,musclecars,all with uncorked exhausts. The loudest car was a little yellow RX3 with a tuned megaphone system. Girlfriend couldn't stand the noise so she had my hearing protectors. I soon learned when that Mazda was coming to plug my ears. i could not believe the noise,felt like it rattled your bones sort of like sitting in stands at the drags when a funny car makes a pass but different.
A 4 stroke engine has an power cycle every 4 crankshaft rotations and the exhaust comes out of the cylinder head and the part where it exits the head is a little removed from the combustion chamber so the sound though unmuffled has the exhaust pulses damped a bit.
A rotary engine is essentially firing continuously,each rotor has 3 bathtubs or compression chambers and in one revolution of the e shaft there are 3 ignition events compared to the piston engine having one ignition event in 4 revolutions of the crankshaft,this is the big difference in noise but more than that unlike a pistons cylinder head exhaust port,the rotary exhausts directly out of the rotor housing so its combustion event has no damping and its exhaust pulses are akin to a rapid fire machine gun. There is no break from the power pulses and why these engines have massive cast iron exhaust manifolds to soak up all that noise,but the front part of exhaust system gets hammered with that machine gun power pulse. There is no break like in a piston engine,also the reason particularly early rotary engines consume spark plugs at a much faster rate the piston engines.
In answer to your question,no you can't feel the pounding. If you have gutted a cat there will be some increased volume of exhaust noise. Vibration could be caused by exhaust system contacting something it shouldn't,check all your system hangers to be certain they're intact an not allowing system to sag. Another real possibility that could contribute to the noise is the engine running rough as you say. Adjust the carb to get the engine to run smoother,may minimize vibration issue.
04-29-19, 10:22 AM
#17
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04-29-19, 11:06 AM
#18
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A conventional four-stroke piston engine completes all (4) events (intake, compression, power and exhaust) over two crankshaft revolutions - that is 720 degrees. Dividing into (4) allows 180 degrees for each 'event'.
A rotary engine sees the crankshaft turn once per each chamber firing. That would be 3 revolutions per all 3 chambers - and subsequently all (4) events. So, that means these (4) events are completed over 3 revolutions - that is 1080 degrees. Dividing into (4) allows 270 degrees (of rotation) per each event of a cycle. Result is more time and ultimately more 'efficiency and more power'. This is why, in earlier times, the comparison or similarities between rotaries and two-stroke engines came about.
Remember that a common two-stroke principal has all (4) events being completed over one crank revolution. Yes, this allows only 90 degrees per event and is reason why (two-stokes) basically make very little power at lower rpm's. Of course, like a rotary, no camshaft or valve train so the noise potential is very similar. I'm sure we've all used a chainsaw at one point in time.
I also remember comparisons which put a 1300 cc rotary equally against a 2600 cc piston engine.
A rotary engine sees the crankshaft turn once per each chamber firing. That would be 3 revolutions per all 3 chambers - and subsequently all (4) events. So, that means these (4) events are completed over 3 revolutions - that is 1080 degrees. Dividing into (4) allows 270 degrees (of rotation) per each event of a cycle. Result is more time and ultimately more 'efficiency and more power'. This is why, in earlier times, the comparison or similarities between rotaries and two-stroke engines came about.
Remember that a common two-stroke principal has all (4) events being completed over one crank revolution. Yes, this allows only 90 degrees per event and is reason why (two-stokes) basically make very little power at lower rpm's. Of course, like a rotary, no camshaft or valve train so the noise potential is very similar. I'm sure we've all used a chainsaw at one point in time.
I also remember comparisons which put a 1300 cc rotary equally against a 2600 cc piston engine.
05-18-19, 08:13 PM
#19
In answer to your question,no you can't feel the pounding. If you have gutted a cat there will be some increased volume of exhaust noise. Vibration could be caused by exhaust system contacting something it shouldn't,check all your system hangers to be certain they're intact an not allowing system to sag. Another real possibility that could contribute to the noise is the engine running rough as you say. Adjust the carb to get the engine to run smoother,may minimize vibration issue.
05-18-19, 08:21 PM
#20
Here is the text:
Suggest you revisit your idle adjustment procedure to make sure it's right. With your idle speed set where it is,adjust mixture screw in til rpm drops off,then go back the other way til no more rpm gain,then a little more. At this point begin turning down idle speed,if/when engine begins to stumble go back to mixture screw and richen it some more,go back to lowering idle speed,back to mixture screw to balance air/fuel as you lower idle speed to about 800 rpm. If everything else is ok with carb you should have a solid idle. If not you'll need to look closer at carb.
Suggest you revisit your idle adjustment procedure to make sure it's right. With your idle speed set where it is,adjust mixture screw in til rpm drops off,then go back the other way til no more rpm gain,then a little more. At this point begin turning down idle speed,if/when engine begins to stumble go back to mixture screw and richen it some more,go back to lowering idle speed,back to mixture screw to balance air/fuel as you lower idle speed to about 800 rpm. If everything else is ok with carb you should have a solid idle. If not you'll need to look closer at carb.