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Matt Cramer

MegaSquirt idle tuning tips

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As there's often a lot of questions about how to get the perfect idle, I've put together a lot of tips to idle tuning in a new article.

 

http://www.diyautotune.com/tech_articles/megasquirt_idle_tuning.html

 

Let me know what you think of this one - we may end up expanding the article a bit if you need more coverage of specific issues.

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Matt I do a similar thing that you do with the timing map, only I do it with the fuel map. I create a little rich "ramp" area, below the idle speed so that if it does drop down, the extra fuel helps to catch and stabilize the idle. Same principle as your timing map.

Edited by cygnusx1

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Matt, great article - wish I had this when I started, would have sped me up the learning curve.

 

I'll add one more thing. Make sure that the injector opening time is accurate, especially for large injectors. If it's off, then even if you set the VE values the same for two different MAP levels (say 40 and 45 kPa at 750 rpm), the amount of fuel delivered will be changing more than what the changing amount of air requires and this will only make it harder to tune.

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Rough idle is the final gremlin I have to slay with my MS install. My car is NA N42/N42 with a 60mm TPS and no IAC. Nissan turbo injectors w/wideband. Although I have a mild cam, my idle surges 200 rpms with a base of about 1000 rpms and the car shakes like I have a V8 under the hood. It's pretty annoying.

 

Until yesterday, all of my .MSQs have used a ReqFuel of 11, and 2 squirts/alternating. If memory serves, I used 2/alternative based on info in Megamanual. When I initally wired my injector banks, I thought I was clever to put 1, 3, and 5 on one bank and 2, 4, and 6 on the other. The idea was to spread the alternating squirts as across all cylinders as evenly as possible. While it didn't hurt anything, I don't think this idea accomplished anything useful either. Might also be exacerbating my idle issues. ?

 

As noted in Matt's idle tuning tips, lowering my idle AFRs from 14.7 to 14.0 did help my idle a little.

 

What made a significant difference was changing from 2 squirts/alternating to 3 squirts/simultaneous. I referred to the permissible injector firing graph in thefuel injector section of Megamanual. The manual does state that increasing squirts per ignition event smooths idle. In addition to a smoother idle and less rmp surging, the car also doesn't seem to hit so hard when the accelerator pedal is pressed, i.e., the throttle is less twitchy. Just so you know, changing the number of squirts requires a new VE table, so you can't just change the number of squirts and expect the same operating AFRs.

 

As soon as I get my VE table adjusted to 3 squirts/simultaneous (shouldn't take long using Tunerstudio Live VE) I am going to work up a .MSQ for 6 squirts/simultaneous. Six squirts is the only greater option than three squirts simultaneous (assuming you are not full sequential using MS3).

 

The one potential isssue with going 6 squirts/simultaneous is that, according to the manual, the 'dynamic range' of the injectors is reduced. While I'm not positive what this means, I think it means that because your injectors are firing much more often, the total amount of fuel that the injectors can deliver at high rpms is reduced. Someone please correct me if this is way off.

 

What ameliorates my concern about reduction of injector dynamic range is that I'm using 265cc turbo injectors in lieu of the 185cc stockers. This could be an issue for other folks though. Another potential issue with 6 squirts is that the idle PW becomes so low that idle fuel consistency is at risk. According to my 'engine constants,' 3 squirts is a calculated PW of 3.9ms. Going to six squirts will halve that PW to a litle under 2ms. This could be too short a period of time to get consistent injector firing. Dunno.

 

I'll try to add to this after futher expirementation. There should be a single injector firing setting which works best - or is at least consensus best among L6 squirters - with the stock intake manifold. So far, I'm a fan of 3/simultaneous.

 

More to follow.

Edited by tennesseejed

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"I thought I was clever to put 1, 3, and 5 on one bank and 2, 4, and 6 on the other."

 

What you did was insure if you loose one injector driver, the engine will ABSOLUTELY not run at idle and if under high speed load doesn't run like ON off ON off ON. Engine goes 'brumm.....brummm...brummm...' Fires on the front three or the back three sounds like a rotary sorta....

 

If you wire 1,2,3 & 4,5,6 if you loose one injector driver the engine runs rough as hell, but it gets you to the roadside where you can affect the changes to get home on one driver.

 

:blink:

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"I thought I was clever to put 1, 3, and 5 on one bank and 2, 4, and 6 on the other."

 

What you did was insure if you loose one injector driver, the engine will ABSOLUTELY not run at idle and if under high speed load doesn't run like ON off ON off ON. Engine goes 'brumm.....brummm...brummm...' Fires on the front three or the back three sounds like a rotary sorta....

 

If you wire 1,2,3 & 4,5,6 if you loose one injector driver the engine runs rough as hell, but it gets you to the roadside where you can affect the changes to get home on one driver.

 

:blink:

 

Ha, you are absolutely correct. On the other hand, being an optomist, glass is half full kind of guy, if I lose an injector driver, my engine will be running normally at least half of the time. B)

 

I managed to get the 3 squirts/simultaneous VE map tuned pretty well. The 6 squirt/simultaneous is a work in progress. Having a very hard time getting consistent AFRs. I'll do some more testing before jumping to any conclusions.

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The 6 squirt/simultaneous is a work in progress. Having a very hard time getting consistent AFRs. I'll do some more testing before jumping to any conclusions.

Keep in mind that each squirt costs an injector opening time which is typically 1.25 ms. At 6 squirts that's an extra 5 opening times or 6.25 ms. At 6k rpm there is 20ms total per combustion cycle. Make sure there is enough injector duty cycle left at WOT.

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Good point, and it helps me understand the 'dynamic range' issue better.

 

With a ReqFuel of 11.8, three squirts adds 2.5ms (14.3ms total), and six squirts - as you point out - adds 6.25ms (18.5ms total).

 

The IDC with three squirts is 75% at 6K rpm and 90% with six squirts. Pretty much maxing out turbo injectors on a NA application at 6K.

 

I stil plan on fooling around with six squirts a little more, just to see if it will tune and idle better, but I'll certainly keep things a low rpms.

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I didn't intend to turn Matt's OP into my personal idle troubleshooting thread. So, apologies if it seems that way. Hopefully, some of this information might be helpful to current and future MS tuners. I wanted to add this information to one of my previous posts, but can't seem to figure out how to edit, so a new post seems necessary.

 

Without using IAC, there are not many factors that effect idle quality aside from MS settings. Basically, you have the condition of your (1) cap, rotor, wires, and plugs (2) valve lash (3) ignition timing and (4) any potential vaccuum leaks. My intake manifold is stock/non-egr and shaved.

 

Having decided that a MS setting of 3 squirts/simultaneous provides the best idle potential for my car, I set about to adjust the valvle lash and check the iginition timing. Surprisingly, adjusting the valve lash had had a significant effect on idle quality. All other things remaining the same, tightening my valve lash to spec caused the idle rmp to drop. Not exactly sure why that is the case.

 

My ignition advance also had moved to 22 degrees BTDC. The distributor bolt was tight and timing was previously set at around 15 degrees. Returning the ignition timing to 15 degrees made the idle more stable. And lowering the timing further - to around 12 or 10 degrees - would likely increase idle stability.

 

In addition to Matt's idle tips link above, the Fuel and Injector section of Megamanual suggests using alternating squirts for best idle. I did not find this to be the case after comparing similar maps using 2 squirts/alternating and 2 squirts/simultaneous. Essentially no difference between the two. Just my observation.

 

The Megamanual also suggest manually adjusting the VE table at the idle point to obtain the highest vaccuum. I found that manually adjusting VE values around the idle point had virtually no effect on the idle rpms and MAP reading. Not saying this is bad advice, just that in my experience it was not helpful.

 

As others have pointed out, I did find that a richer idle AFR smoothed the idle and reduced the lag or stumble when transitioning from idle to part throttle. 13.5 to 14.0 is in the ballpark for my application. Why adjusting the VE table did not produce a similar result is a bit of a mystery.

 

The last observation may be specific to my cam (Stage III - 270.280/.460). While a pretty puny cam compared to many of the other HybridZ cars around here, the cam does seems to need at least 900 or 1,000 rpms to draw enough vaccuum to idle stable. Below 800 or so rpms, the MAP reading drops dramatically (80-90 Kpa(?)) and the engine slowly dies. My mechanical cam timing was adjusted within a degree within the last couple of months and my head is recently rebuilt. So I have no reason to believe that this condition is related to an engine issue other than the normal operation of the camshaft. Idling at 900 or 1,000 rpms, the car now transitions smoothly between idle and part throttle and the idle quality is not nearly as rough as in the beginning.

 

I gave up trying to turn for 6 squirts/simultenous. While I am not sure why, probably inconsistent fueling across the rpm range due to very low pulsewidths, I could not get the VE table to tune at all using Tunerstudio/Live VE. Maybe someone else will develop a good 6 squirt tune down the road.

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Since it's okay to muddy this thread up I might as well throw in two plugs worth. On a completely stock 1976 280Z EFI engine I found that having plugs 3 - 6 all indexed between 10 and 2, and plugs 1 and 2 at 7 o'clock and 5 o'clock, respectively, gave a poor and rough idle. Getting 1 and 2 indexed between 10 and 2 also, smoothed the idle out dramatically. This is with NGK BPR6ES-11 plugs. I was surprised that indexing had a big effect, but it was real and noticeable. I only put the effort in because my engine's idle just doesn't seem as smooth as it should be, even with essentially identical cylinder pressure numbers and all new tuneup parts.

 

And a couple of side notes - projected tip plugs really do increase the tendency to ping/knock/detonate, requiring retarding the time a few degrees. And the batch firing seems to give a different idle depending on which sets of two cylinders each start the injection cycle. Stopping and restarting the engine can clear up a bumpy idle.

 

Just wanted get that out there while the details were fresh...

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I'm running basically a stock 280ZXT my only changes are a cone filter still running through the stock AFM (using the air temp sensor through the stock harness) and a crush bent 3" exhaust no cat stock downpipe. I have a MSD SS Blaster coil driven by MS.

 

I've got a solid idle using the set screw on the stock TB to adjust to about 800 RPM. Advance is set a 19 degrees in the idle portion of the map. AFR is around 14.0 Req Fuel is 11.6, 2 squirts per cycle and none of the plugs are indexed (manufactures recomended NGK turbo). I ran VE Analyze Live! and it has the VE around 44 or so and did help smooth idle a bit.

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