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Discussion Starter #1
Is the flange on the output of the cat on the Mark VIII manifold the same as the flange on the MN12 and PS manifolds? The reason I'm asking is if it is, I would like to try and either get one off a bad cat, or possibly if my local junkyard gets one in and I can get the flange, make a jig (using a stock MN12 manifold as a template) to weld the flange onto the portion of manifold that I do have off the 98 motor I picked up (for $200 with 145k :)) so that I can use 'stock' manifolds with my stock MN12 exahust initially. Since I just got finished modifying a Mach 1 harness to fit the motor and my car, I want to confirm that I did it right by dropping the motor in as it sits now prior to actually doing my build.

The actual build is going to include headers of some description and true dual exahust, but I don't want to do all that now as the SOHC motor will likely end up back in before the built motor is finished.
 

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Is there a reason you don't want to go with Cobra or Mach 1 manifolds and just work them up to the stock cats??? Seems to me that'd be easier and better.... even with the steering in the way.
 

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If the build is going to include headers then use headers the first time, otherwise your doing more exhaust work for nothing.
 

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Discussion Starter #4
Is there a reason you don't want to go with Cobra or Mach 1 manifolds and just work them up to the stock cats??? Seems to me that'd be easier and better.... even with the steering in the way.
I have the stock manifolds from the Aviator. The problem is the drivers side one will not line up with the Y-pipe (I actually didn't think to check the PS manifold, I just assumed it would line up. I'll have to check that. :eek:)

If the build is going to include headers then use headers the first time, otherwise your doing more exhaust work for nothing.
I hear you.... but, it would really be more of a pain (IMO) to get it all set up with the headers initially since the SOHC motor will be going back in before the DOHC motor is fully built and perminantly installed. I want to keep the exahust stock for the 'test run' of the motor.
 

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I hear you.... but, it would really be more of a pain (IMO) to get it all set up with the headers initially since the SOHC motor will be going back in before the DOHC motor is fully built and perminantly installed. I want to keep the exahust stock for the 'test run' of the motor.
Not a bad idea. Its best to leave out variables when taking on a swap. Make sure the motor runs correctly, then have at it. At the same time, installing manifolds or headers on the engine while in the car is a royal PITA. Work smarter, not harder.
 

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Discussion Starter #6
Not a bad idea. Its best to leave out variables when taking on a swap. Make sure the motor runs correctly, then have at it. At the same time, installing manifolds or headers on the engine while in the car is a royal PITA. Work smarter, not harder.
The motor would be coming back out and the SOHC back in at which point the DOHC would get torn down and the work begun on building it up. Then once built right, it would go in with headers, exahust, etc.
 

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The motor would be coming back out and the SOHC back in at which point the DOHC would get torn down and the work begun on building it up. Then once built right, it would go in with headers, exahust, etc.
I'm missing something then. Why put it in in the first place then? Why not just build it?
 

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Discussion Starter #8
I'm missing something then. Why put it in in the first place then? Why not just build it?
Two reasons: I just got done heavily modifying a Mach 1 engine harness to work with my body harness and I want to be sure it will work right prior to adding a full engine build to the mix, and I also want to ensure that I have EVERY piece that I need in order for the motor to bolt in to the car and run properly. Sinc eI'm not coming from the 'normal' Mark VIII motor, but instead essentially a MAch1 motor, there are going to be differences that I don't beleive have been covered here. I'm sure I'm going to find SOMETHING that I'm going to need that I didn't account for. My plan is once I've confirmed it runs and I'm not missing anything, every modular motor part in my garage that's not off THIS motor will be stored elsewhere, and all the parts from this motor will stay in my garage for the buld so I know I have everything here and no extra or missing parts.
 

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Sounds like fun, but I think it would be hard to make that DOHC manifold work with the stock exhaust. Post some pics when you get it done! It might be easier to buy a Mark VIII Y or H pipe and make that fit yours at the rear.
 

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Can someone at least answer the question of welding the flange. I have heard it cracks after 1 to 2 years of use but maybe someone has done it right and it works. Id sure like to hear about it. Would be cool to cut out the pinch on one of the tbird manifolds.
 

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Discussion Starter #11
Can someone at least answer the question of welding the flange. I have heard it cracks after 1 to 2 years of use but maybe someone has done it right and it works. Id sure like to hear about it. Would be cool to cut out the pinch on one of the tbird manifolds.
The drivers side manifold isn't cast on the mark. I believe it's some grade of Stainless, so it would weld better than cast will, although cast can be welded and supposedly it'll hold just fine, as long as it's done right. Doing it right involves heating the parts so there isn't such a huge difference in temperature. when you do the weld.
 

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I think the hardest part when matching it to the factory SOHC manifold on the jig would be getting that kinked part where it clears the column right. IIRC the mark manifold/cat assembly tucks in towards the block to clear the shaft vs. the MN12 where the manifold kicks out and goes over the shaft. It would require more fab than simply welding the flange on if getting it to bolt to the factory exhaust is the goal.
 

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I still don't see the purpose of reworking the exhaust to fit modified ds manifold to stock exhaust if your doing headers later anyway. Your just doing the work twice for no real reason, but to each their own I suppose.
 

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I think it would be easier to get a Mark VIII h pipe and make it where it adapts to the stock single pipe thunderbird exhaust (the rear piece). The LSC had true duals all the way back though.

The LH mark manifold welds very nicely. I moved an O2 sensor bung into the very rear port of the manifold for clearance putting a 32v in a 93 f-150. I then installed a plug in the original bung.
 
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