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VMP Tuning
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Discussion Starter · #1 ·
I worked on a paxton novi 2k blown non-intercooled 98 Cobra on Saturday. This was a pretty clean lower mileage car, about 32k.

The owner has not gotten real serious with it yet, he just wanted to slap on a blower and make a little more power.

The car showed a drop out in power on the dynograph, and this is also something you could feel during the pull. The drop out did not always occur in the same place.

The drop out in power was caused by a drop out in timing, the timing drop was way too large and way too quick for it to be the knock sensors.

Somehow, some way, the computer thought the IMRCs were closing up and then reopening, so it pulled back timing for a few hundred RPM and then put it back in. I saw this indicated in the dynograph, the spark curve, and the IMRC bank 1 and bank 2 position sensors that I was logging.

These issues go hand in hand with a number of weird issues I have seen with 96-98 cobras that have blowers and still have the IMRCs.

The IMRCs were never designed to work under boost, combined with weak motors, carbon buildup, sticky valves, and innacurate sensors, they can be a real mess.

I would delete them on any forced induction car, and strongly consider deleting them on an NA car that is raced often if you want to it run consistently at the track.

A few months ago I had a local car on the dyno that I tuned 2-3yrs ago, he brought it back to check a few things, and it was low on power. We eventually figured out that the IMRCs weren't opening on ONE SIDE.

Now one could pose the question, why not just command the IMRCs open off idle and force all the timing in through the BKT? (instead of using the IMRC adder tables). I believe this could lead to too much timing and the potential for knock. IMRCs were mainly put there by ford for emissions, but they also have the benefit of adding tq and the low end and at part throttle, however, you can get most of that tq back by adding in the proper amount of timing once you remove them. With IMRCs closed, the swirl created by air going through only one port on the head turns the chamber into the fast burn type, which requires much less timing to achieve max power. Adding in a bunch of timing with the IMRCs still closed and the chamber swirling is a bad thing and could result in knock, something you of course want to avoid at all costs with a supercharged car.

See attached dynograph showing inconsisent power curve:
 

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Hmm, that's the first I've heard of the IMRC causing issues at a higher rpm. Maybe once open the rod is causing a dead zone right behind the rod. But I agree, whether its n/a or fi.. delete them and add timing to make up for the torque loss. I always though ford did the IMRCs to make up for the torque loss cause the dual ports where just too much air for a lil 281ci motor. Never heard they did it mainly for emissions.

-Mark
 

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I thought the IMRCs being closed at low RPM gave the intake charge more velocity, hence the momentum of the gas molecules would help fill the combustion chamber more. It is sort of the invers of the scavanging effect on the exhaust side.

Not to hijack the thread Justin, but did any of the cars you tuned have a larger than stock MAF and tuned with SCT software? I have a theory about a bug in SCT's LLX3 and LLX4 code bases.
 

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I know this is a little of your guys conversation but, if some of you guys reading this are woundering where you can get some deletes now since they have been scarce for so long. MMR is making them now. I just bought a set and they look and fit great. My heads were ported so I had to do a little cutting to match them to the heads. Other than that no complaints here. You can see them on MMR's website. Hope this helps.
 

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VMP Tuning
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Discussion Starter · #5 ·
How much do the MMR deletes run? the other option is to just gut the stock valves and close up the holes with welding or JB Weld.

I have seen a few other 96-98 cars with IMRCs issues too, the above ones are not the first or last.

The IMRCs are definitely there to help the air get more momentum at low RPM.

I would agree that there are some funny things about 96-97 cars. I'm not sure that its a bug in the sct software, but maybe rather a bug in the ford strategy when we go and modify the code outside of what is normally done from the factory.
 

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How much do the MMR deletes run? the other option is to just gut the stock valves and close up the holes with welding or JB Weld.
MMR is selling them for $199.99 per their post over on Corral: http://forums.corral.net/forums/showthread.php?t=1084149&highlight=imrc+delete

I would agree that there are some funny things about 96-97 cars. I'm not sure that its a bug in the sct software, but maybe rather a bug in the ford strategy when we go and modify the code outside of what is normally done from the factory.
Justin, if you did any LLX3 or LLX4 tunes using SCT on a blown car with a larger than stock MAF, could you tell me what the max Load was that you logged during your WOT runs? Several of my friends and I have found that the max load comes in under 0.9 when it should be 1.3 or better in a blown car. I am looking for other examples where the load is screwed up in SCT tuned cars set up this way.

We have been able to fix it by scaling the engine displacement, BTW.

Thanks!
 

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issues

Currently have the IMRC issue with my 96 cobra along with misfire in cylinder 5 n a o2 sensor... those were the readings after i scanned it...

But wanted to know if i delete the IMRC would that affect it when i smog the car ?

Need alittle help since ford no longer sales IMRC ......

Thanks
 

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My car isn't blown but I figured I'd take this opportunity to post some feedback regarding the subject since Justin did my tune via mail order.

When doing my 2V to 4V swap I deleted the IMRC plates in the process, and told him I did. Because I still have my stock GT MAF but 24# injectors I didn't run it much until it was flashed because everything would have been a mess but once the tune was uploaded, the car has more low end power than the 2V motor ever did. Given that, I honestly see no reason to keep the IMRCs other than if you want to not modify the car more than you have to or are trying to avoid the work.

Keep in mind also I have a 4:10 rear, 5 speed, and a good 200#s of weight reduction by loose figures, so can afford to loose a little low end. That's not the case for everybody and I realize that.
 

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Just trying to make sure i can delet them with out messing with emissions only so that way i can smog the car already b/c it says there stuck close... if not i'll just delete them or see if i can just clean them out instead...
 

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Just trying to make sure i can delet them with out messing with emissions only so that way i can smog the car already b/c it says there stuck close... if not i'll just delete them or see if i can just clean them out instead...
Emissions wise they aren't part of the group of required equipment like EGR and converters are, so you should be fine. Most inspectors, I bet not even 1 in 100 would know to look for them. It'll all come down to the tune but I doubt you'll see any worthwhile difference in the test from not having them, but you might get a CEL if the computer doesn't get the feedback from the controller it was looking for. But, that's another tuning issue which can be taken care of.
 
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