my electronic MAP filter circuit - Miata Turbo Forum -Boost cars, acquire cats.

Welcome to   Members

ECUs and Tuning Discuss Engine Management, Tuning, & Programming

LinkBack Thread Tools Search this Thread
Old 06-18-2010, 02:07 AM   #1
Elite Member
Thread Starter
Join Date: Jul 2005
Posts: 6,319
Total Cats: 68
Default my electronic MAP filter circuit

I got tired of trying different restrictor and hose schemes to get rid of the MAP sensor "noise" (which is actually the manifold pulses getting through). If I used a small restrictor, the response was slow enough to cause throttle tip-in hesitation. Without it, it was introducing a ~2% error in my MAP value. I couldn't find a good compromise.

I discovered this when I was trying to tune my VVT by examining AFR's. I noticed run-to-run inconsistencies.

So I built and tested an electronic filter. (For the geeks, it's a 2-pole Bessel). 2-pole filters have a better combo of noise rejection and rise time (or delay time).

See MAP results, before and after. Left is before, right is after. The red trace is the MAP sensor voltage output. Green trace is the AEM MAP signal after the software filtering. (Blue below is RPM) As you can see, the spikiness gets filtered, but the MAP signal sometimes rides the peaks of the MAP voltage (when <4000 RPM), sometimes it rides the troughs (when >4000 RPM). It's some artifact of the smoothing method, I guess. However it's not consistent.

See schematic attachment. The diodes are to bypass the filter circuit when the MAP changes very rapidly. Without it there was a subtle tip-in hesitation.

See attached MAP rise time when I jab the throttle. It's between 60-80 ms.

If the input impedance of the ECU is < 1 Mohm, the total of 3k resistance of my circuit will introduce an error in the voltage vs MAP curve. In the case of the AEM, it has 100 kohm input resistance, and with the Motorola 2.5 bar MAP sensor I had to adjust the parameters in the software (was offset=10, scalar = 2.6, it's now offset = 2.5, scalar=2.58).
Attached Thumbnails
my electronic MAP filter circuit-map-filter.gif   my electronic MAP filter circuit-map-filter-schem.gif   my electronic MAP filter circuit-map-filter-rise-time.gif  

Last edited by JasonC SBB; 06-18-2010 at 11:32 AM.
JasonC SBB is offline   Reply With Quote
Old 06-18-2010, 09:01 AM   #2
Supporting Vendor

iTrader: (33)
Ben's Avatar
Join Date: Jul 2006
Location: atlanta-ish
Posts: 12,662
Total Cats: 72

Nice. A wee bit of overkill I'm sure, but there's nothing wrong with that.

Here's a simple filter for lazy people.
Ben is offline   Reply With Quote
Old 10-30-2011, 01:09 PM   #3
Elite Member
Thread Starter
Join Date: Jul 2005
Posts: 6,319
Total Cats: 68

I'd like to point out that the simple filter above has a time constant of only 220 us and a -3dB cutoff of 280 Hz (8400 RPM on a 4-cyl). This is too fast. To get a rise time of 25 ms (which I empirically tested is the slowest that will not affect fueling), it needs a time constant of 10 ms, so the 0.22 uF capacitor needs to be increased to 10 uF.

Remember the purpose of the filter it to remove out of band noise (including the pressure pulses in the intake mani).

At 4000 RPM on a 4-cyl, the 2-pole filter (mine) attenuates the pressure pulses 2x as much as a 1-pole filter (Ben's schematic) using a 10 uF capacitor.
JasonC SBB is offline   Reply With Quote
Old 10-30-2011, 10:09 PM   #4
Elite Member
iTrader: (3)
Join Date: Mar 2008
Location: Granbury, TX
Posts: 5,765
Total Cats: 544

Cool stuff. I've always noticed that the MAP signal has more noise than almost anything else we track. I switched from MAP-based accel enrichment to TPS-based because of this. I was actually getting MAP-based accel enrichments while just cruising. That makes it pretty difficult to tune fuel.

Need to try these filters. Thanks guys.
hornetball is offline   Reply With Quote
Old 03-12-2012, 11:30 AM   #5
Elite Member
Thread Starter
Join Date: Jul 2005
Posts: 6,319
Total Cats: 68

Here is the effect of the same filter (without the diodes) on the TPS signal in the AEM. The time constant I used was 2x10 ms (2 pole 10 ms each).

I totally hacked up my AEM's PCB layout/grounding and fixed newbie engineer boo-boos.

The effect is much smoother throttle enrichment, most noticeably on very light throttle application at low RPM. The decel fuel cutoff also doesn't jerk in and out anymore on trailing throttle turns with very light throttle application.

Attached Thumbnails
my electronic MAP filter circuit-filtering-tps-hack.gif  
JasonC SBB is offline   Reply With Quote

Related Topics
Thread Thread Starter Forum Replies Last Post
Project Gemini - Turbo Civic on the Cheap Full_Tilt_Boogie Build Threads 52 08-28-2016 04:18 AM
My solution for Oiltemp and Oilpressure input into Megasuirt (MS3) Zaphod MEGAsquirt 41 01-24-2016 12:25 PM
"Chimney Ethanol" E100 MiataGarage Engine Performance 5 09-29-2015 11:04 PM
In need of base map for stock 1997 brandonbkd MEGAsquirt 0 09-28-2015 11:52 PM
K&N Intake Filter nbdooey Miata parts for sale/trade 0 09-28-2015 08:46 PM

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 10:43 PM.