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Normal Looking AFR for NA 1.8?

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#1
Keith Novak

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I added adjustable FP and went to the dyno to get it tuned.  The dyno shop had done Miatas before but not SMs with a restrictor plate.  The tuning was done on a Mustang dyno.

 

We got pretty decent power out of it if it converts the way I think it does to Dynojet numbers but couldn't make much of the AFR.  The last trace was where we settled after monkeying a bit with timing and FP.   The numbers I'm seeing others toss about don't seem to fit with what I got.  He thought it looked too lean under 4k rpm and we never did get the high rpm close to where we thought they should be.  He thought maybe it had a rising rate FP regulator.  There is also a possibility the prior owner put in a different fuel pump.  The filter was awfully clean when I looked at it a few months ago.

 

Any thoughts on this?

Thanks,

Keith

 

(edit:  Middle AFR trace is where I started.  Top is where I finished)

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#2
Charlie Hayes

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How were engine temps for the examples above? Not very consistent looking, looks like a different map on the top line as it goes rich at 4800rpm. I usually start the runs at 3500rpm on the NA1.8s because the are a little crazy under there. It starts out pretty lean and I target 13.5:1 +/-.2 depending on the day at 5500rpm which usually leads to 12.7:1ish up top if I recall. That's out of the tail pipe and our AFR gauge on our dyno. Usually try to do 5+ runs in a batch or how ever many you can get from 180-210 and see how it looks.
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#3
Tom Sager

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I added adjustable FP and went to the dyno to get it tuned.  The dyno shop had done Miatas before but not SMs with a restrictor plate.  The tuning was done on a Mustang dyno.

 

We got pretty decent power out of it if it converts the way I think it does to Dynojet numbers but couldn't make much of the AFR.  The last trace was where we settled after monkeying a bit with timing and FP.   The numbers I'm seeing others toss about don't seem to fit with what I got.  He thought it looked too lean under 4k rpm and we never did get the high rpm close to where we thought they should be.  He thought maybe it had a rising rate FP regulator.  There is also a possibility the prior owner put in a different fuel pump.  The filter was awfully clean when I looked at it a few months ago.

 

Any thoughts on this?

Thanks,

Keith

 

(edit:  Middle AFR trace is where I started.  Top is where I finished)

I have a '95 also and have spent a lot of time on dyno and tuning AFR.  These cars do not have a very favorable curve to begin with.  My car when tuned best will read near 15:1 (high 14's) at 4000 RPM and about 12.6 at 6000 and it's fairly flat from there to redline.  Might even lean out just a touch from 6500 - 6900.  Yours looks similar to mine at the lower RPM range but yours reads very very rich above 5300.  I've seen other '94/'95 dyno AFR reports that look very much like mine. The accuracy of the dyno sensor could be a question but if those are accurate numbers, I think you have a problem that may not be solved by lower fuel pressure.  Lower fuel pressure to fix the top end is going to make you even leaner at lower RPM.  Does the car have an adjustable fuel pressure regulator?  If not you should probably install one and then consider swapping the airflow meter and possibly the ECU.  The airflow meters can get a little wacky if the wire sensor inside gets really dirty.  I doubt the fuel pump is causing the excessively rich.  Usually they go lean if they go bad .   Also the odd spike near the end of the top run coincidentally results in what I would consider normal AFR at that RPM.  Could be a sign that the dyno sensor is flaky or that you have something wrong in the car.   

 

The curve isn't going to get dramatically better IMO.  Normal for a '95 is a 2.25 point (or slightly greater) spread from 4000 - redline.  


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#4
Keith Novak

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Thanks guys.  You confirmed what I was suspecting.  It's not what a typical trace would look like.

 

The car was at consistent temps.  I didn't know what the old delivered fuel pressure was with the stock regulator so I set the new adjustable one to the middle of the stock range for a baseline.  Result at 14* of timing was very rich (mid trace) which was expected since my plugs looked rich before with the stock regulator.  We tried advancing the timing and that made it worse so we reduced the fuel pressure and it started getting too lean at lower rpm.  At that point we ran out of ideas, parts, and time.

 

The last guy before me on the dyno was a long time autoxer/national champ and I think he'd know his didn't look right on his heavily modified car unless he just plugged it up somehow.  I suspect there is a car issue.  The airflow meter and swapping out ECUs are both good ideas and I suspect there may be a coil issue creeping up at least.  His timing light had a hard time picking up anything on my 1 and 3 cylinders but no problems on 2 or 4.  Mine works fine, but his made me wonder.

 

Confirmation that it doesn't look normal, some good ideas to go forward, and some mileposts to know if it looks better.  Sounds like a plan.  Much appreciated!  :)


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