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Discussion Starter · #1 ·
I read a bunch of differing ways to approach the IMRC in a MN12 problem.

Most common is a rpm window switch that opens the IMRC at 3000rpm for about 150 bucks. That keeps the IMRC function, providing more intake velocity at low rpms for torque availability, throttle response.

Some people delete them with a supercharger installation.
There are delete plates available for 179-?? Bucks.
You can delete them yourself with jb weld
Others delete the IMRCs because they have steeper gears, compensating for torque loss.
Others delete them and run a high stall converter (over 3000rpm) because they never see 3000rpm.

We kind of fit the last two, we have 3.73:1gears right now and a 4200rpm converter.

It seems to me that running that converter and gear would pretty much nullify the IMRC function altogether.

If I was running the stock Mark VIII converter I would probably hook the IMRCs up even with the gear change.

I can definitely see the benefit of a less radical converter driving around. The car has the distinctive loose feel when off the throttle of a race converter.
It's awesome for getting power down, but takes quite a lot of throttle input and is pretty loud around town. It's funny to watch it go from 4200 to 5300 to 4200 to 5300 and into 3rd gear. The thing never drops below 4200 when on it but that's the point.

Anyway, I understand the B motors are dogs down low and that is why they have IMRCs. With my gear set and converter, is it at all beneficial to use the IMRCs?

If in the future, we ant to gear the car for high speeds, 2.73:1 and ultimately 2.47:1 For this we will still use the race converter. Should this be a consideration in the IMRC question?

I can definitely see running the stock Mark Converter in the car at some later date.
We selected 3.73 for our street gear and will keep that setup around always.
The real high speed gears will go in the aluminum housing, street gears in the cast Iron housing.

I don't want to delete the IMRCs and figure out later I should not have done that.
 

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With a 4200rpm stall on the converter, the IMRCs are never going to be closed, so I would absolutely delete them. If you do wind up wanting to put them back at a later date, it’s not like it’s hard to find another set.
 

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Will you be street driving much? I have a 4200 rpm converter, but with my current tune it’s normal to have the converter locked in forth gear at 35 mph and the engine is around 1,300 rpm. This is the situation where having a working imrc system would keep the car more happy.
 
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If you want to delete them, pull them from the car and remove the blades ; two screws for each blade.
Then if you change your mind you can go back to them.
 

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The Parts Guy
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Will you be street driving much? I have a 4200 rpm converter, but with my current tune it’s normal to have the converter locked in forth gear at 35 mph and the engine is around 1,300 rpm. This is the situation where having a working imrc system would keep the car more happy.
Bingo! If you're going to drive the car at throttle positions less than WOT, you'll see lower revs. If you're intending to cruise on the highway with the converter locked for instance, you'll see/feel less torque at rpms below 3200 than you normally would with active IMRC's.

It really comes down to how you intend to use the car.
 

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Discussion Starter · #6 ·
Will you be street driving much? I have a 4200 rpm converter, but with my current tune it’s normal to have the converter locked in forth gear at 35 mph and the engine is around 1,300 rpm. This is the situation where having a working imrc system would keep the car more happy.
We do not have an application where we would be in 3rd below 50, and will program the PCM so it does not happen.
Bingo! If you're going to drive the car at throttle positions less than WOT, you'll see lower revs. If you're intending to cruise on the highway with the converter locked for instance, you'll see/feel less torque at rpms below 3200 than you normally would with active IMRC's.

It really comes down to how you intend to use the car.
Sure, I can see your point well. It is always a compromise.

And since nothing is permanent, I won't do anything that can't be changed back.

I think for now we can get away with the delete, but later on it might be different.
 

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Anyway, I understand the B motors are dogs down low and that is why they have IMRCs. With my gear set and converter, is it at all beneficial to use the IMRCs?
The IMRC system is not much more than a crutch for cylinder heads that are the DOHC equivalent of the old 351C 4-barrel heads (both are victims of Ford half-assing the heads, the lower half of both the intake and exhaust ports of the 351C 4V are dead spots where flow actually reverses...unless you don't mind revving the engine to 9K all the time), both the B head and 351C 4V don't wake up until similar RPM's.

If at all possible, I'd update to a C-head or something better unless there are class limitations, or you're doing a supercharger, and then the B isn't a bad head at all.

Beyond that, I drove my '96 Mark VIII LSC around for two years with the IMRC's wired all the way open, and if anything, it actually drove better that way: It was still a turd down low, but it began picking up at a lower rpm versus the plates when they were operating properly (felt like the plates were choking something off), if you're running that type of gear and convertor, I'd personally get rid of the IMRC's.
 

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It's pretty easy to wire in an rpm switch to flip them at 3200; that way the only thing you notice is the boost of power. :)
My contour with IMRC's is a beast when they open.
 
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