NEW GUY, FMU ISSUES, CHOOSING NEW EXHAUST SETUP. - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-12-2015, 04:39 AM   #1
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Default NEW GUY, FMU ISSUES, CHOOSING NEW EXHAUST SETUP.

Hi all,

I live in Athens, Ga and I own a 1990 Mariner Blue Turbo. I love the car and its a blast to drive but I have been dealing with a few issues lately. I will tackle them one at a time in this post and I hope to get a lot of responses

First is the FMU that I just had Bell Engineering rebuild. I am having problems getting it to hold the proper pressure according to the analog PSI gauge that it is hooked up too. I have followed all the proper steps that is on BEGI's website for getting it calibrated but it seems like the pressure is running too high. It is supposed to hold 30psi at idle and I am running probably 40. I adjust it using the hex screw, but it won't go below 40.

It seems to running very lean as well at idle according to the A/F. Idling somewhere around 16.8/17.0

Once the car is in gear and driving and even in boost it seems to be running strong. Car pulls hard and is blast to drive but I wish it was a little more consistent and the pressure was where it should be.


The second issue is the stock gearing in the car. I think it is the 430 gears which means the car drives just horrible at highways speeds because the rpms are so hight. ANYONE have any ideas about what diff I should switch too??


The last thing is trying to find what exhaust setup to go with. I don't want something SPEC MIATA loud but i do like the aggressive sound when i get to high RPMs and boost. I just don't know what exhaust to get that gets the most out of the car. I know im supposed to have backpressure (non straight pipe) to help the turbo. Someone help me out and set me straight!!!

Thanks again yall. I attend Caffeine and Octane at the Perimeter mall so look for the Smurf blue with white stripes!!
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Old 03-12-2015, 09:05 AM   #2
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if you have 40psi at idle but still 16AFR then you have major issues.
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Old 03-12-2015, 10:25 AM   #3
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you want zero back pressure on a turbo setup's exhaust. get those gases out as fast as possible. this is why most race setups have open downpipes or bypass cut-outs.
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Old 03-12-2015, 11:49 AM   #4
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I'm a fellow Georgian, I live in Lilburn. Are you the one who was recently selling the car on craigslist? Or did you purchase it recently? Maybe it is a completely different car.


16 is awfully lean at idle. I'm not familiar with the setup you are using so I can't comment anything helpful on that question.

I agree with impulsive - you want as little back pressure as possible. HOWEVER, I had a 3" catless exhaust on my MSM and it flowed far too well to the point of overboosting! I couldnt stay under 15 psi even on wastegate pressure! (overboost setting, so I don't even know how high it would have went) Ended up having to port the wastegate and add a cat. Stick to a 2.5 and you should be fine. MX5Atlanta is a GA based miata forum, I believe there is an exhaust for sale currently you may be interested in.
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Old 03-12-2015, 12:00 PM   #5
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What are your AFRs under boost?
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Old 03-12-2015, 12:00 PM   #6
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Quote:
Originally Posted by ASlickFinish View Post
Stick to a 2.5 and you should be fine.
3" or die in fire.
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Old 03-12-2015, 04:19 PM   #7
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@braineack -- I have tried adjusting the FMU but I cannot get the PSI to fall below 40. Is it possible that the analog PSI gauge is broken or inaccurate.

@impulsive.ca -- So basically your saying go straight pipe or close to it but keep 2.5 inch piping?

@ASlickFinish -- My AFRs during boost are 10.5 - 11.5/12. That may seem like a broad range but it normally starts at about 12 and slowly declines towards 10 as I stay hard on throttle. Doesn't stutter or hesitate at all when in full boost. Pulls hard, but that idling is driving me nuts and has me worried.

@concealer 404 ^^ See above.
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