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Old 07-15-2018, 05:10 AM   #1
e30davie
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Batch vs semi sequential - duty cycle woes

Hi All,

Apologies for my long winded post,

my setup:

-m20b23 manual
-megasquirt 2 currently running fuel only - wired in semi sequential- 2 batches of 3 injectors (complete custom made loom)
-OEM vacuum advance distributor controlling ignition. - Plan is to setup some coils in wasted spark to replace the vacuum distributor eventually
-ngk powerdex wideband
-tunerstudio - paid version for auto tune.

Got the above going yesterday. I cant remember where i got it but i had an m20b25 tune file to work from. I did the required fuel thing for 2.3L and my injectors (which are oem 185cc units according to googling the part number on the injector).

Currently using 1ms injector dead time - no idea if this is correct - seemed like a reasonable number based on research.

I was stuffing about trying to get it to fire and i could get a cough or two. i switched it to untimed injection, which is batch fired and it immediately fired into life. ran really very nicely from this point on.

Got all excited, jumped in the car, drove around for an hour or so and got a fairly good tune on it using the auto tune thing. Drove it 200kims today and further refined the afr target got the accelerating enrich going reasonably well. Engine is very smooth in its power delivery, my brother had a drive and he coudln't make it stumble etc. quite impressed with the outcome so far.

side note: i'm getting like 110 odd VE as a maximum value at approximately peak torque area.

Got home, decided to open up the log file and notice that my injectors are at 98% duty cycle!

98% was at WOT at 6000ish rpm. I guess I didnt rev any higher on that log. but i know i have reved it higher flat out and the AFR looked ok from memory?

So now i start looking into the semi sequential, and trying to work out what to do. I understand the go is to set it to semi sequential and go for 2 squirts alternating.

questions:

- Is 98% duty cycle real and is it an issue? the engine is completely factory, i would have thought the oem injectors would be fine? WOT target afr is 13:1
- should i switch to semi sequential setup? is there any advantage? Internet researching doesn't give a definitive answer. of course full sequential is superiour but hey, need an ms3 for that.
-If i switch to semi sequential is my duty cycle issue going to be worse? I think it will be as you now have twice as many dead times to deal with. But its all a bit confusing.

Any input much appreciated.
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Old 07-17-2018, 08:52 AM   #2
Digitalwave
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My VE is that high around peak torque as well (built M20B25).

Is your motor stock? The duty cycle is a calculation based on what you inputted for injector size, the req_fuel inputs, and your VE. It may not be 100% accurate, but I would be upgrading to something larger if you are at 98%. I think the stock tune is much, much more lean than 13:1 -- anything reason why you are closer to maxing out stock injectors.

Sometimes people run over 100% duty cycle, as long as it's not leaning out past your target AFR. That is dumb to do, and leaves you with no head room. Very dangerous to do this, and could lead to a hole in your piston.

I can't help with the other questions, as I'm running batch fire on the stock wiring. I know semi and full sequential are desired for ease of tuning idle, but past that I don't think the benefits are that large.
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Old 07-18-2018, 04:45 PM   #3
e30davie
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Thankyou for the reply.

Engine is 100% stock and rebuilt ~30,000km ago. Only difference is it has a replacement exhaust. Just a 2 1/4" all the way through with straight though mufflers. I wouldn't think this would make enough difference to max out injectors...

I had the wideband attached to the engine when it was on OEM ecu and at WOT the AFRs were around 12.5 - 12.8. which is richer than what i have now (13.1).

On the weekend i am going to try and implement timed injection. I think two squirts per revolution alternating should help with my duty cycle woes as the injection times will be halved just twice as often. Just a bit odd that the oem injectors are so close to maxed out in standard form.

Still need to determine what degree i should be injecting at.

The oem distributor on the 323i auto (my car was originally auto) appears to be very conservative in the high rpms. really dies off after 5000rpm. I Reckon once i get the ignition running on the ecu I'm going to potentially need more fuel up high.

Might have to look around for some replacement injectors.
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Old 07-23-2018, 01:56 PM   #4
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2-alt (two batches of 3) is not semi-sequential - it doesn't really need timed to work either. In fact, the number of times each injector fires for 2/alt vs 1/simultaneous is the same (once per revolution), it's just that on 2/alt each batch is fired at a different time (180 degrees apart) where 1/simultaneous is all fired at the same time.

If you are seeing more than 100% DC on your injectors, that sounds like they aren't quite configured correctly. With stock injectors I'd bet that a 1ms delay is probably too low - newer injectors are about that fast, but ones from 30 years ago are slower. I'd bet it's at least 1.2ms in reality, if not 1.3 or 1.4.
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Old 08-01-2018, 08:21 PM   #5
e30davie
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Ah I didn't see your reply there.

Things have progressed in the last few weeks with many questions to work through. But generally the engine is running well and getting good fuel economy. The same if not better than OEM. So that's a food start.

I had not considered the effect of injector delay having an effect on duty cycle. Makes sense. I think I will leave it how it is for now. I plan to buy some modern injectors in the future (planned turbo) so I'll just deal with what I've got now. The 100% duty is not affecting anything so it's not a.big deal I dont think.

I'm having all sorts of dramas with the engine idling at 14.7 in the morning (no traffic and nice and cool). But then in the afternoon in stop start traffic it approaches 16:1 and starts to stumble. The air is hotter so less dense so I would think it should be richer not leaner. I've disabled the air temp correction and it still seems to do it a little bit. I've only noticed it in the last few days as I have been taking a different route home. I need to do some.more logging I think and look at the numbers.
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Old 08-02-2018, 07:52 AM   #6
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I've had all sorts of problems tuning a consistent and steady idle on my car. I've found that tuning your fuel, timing, and even the idle speed to produce the most possible vacuum (lowest MAP number) has helped A LOT. It's more consistent and stable, especially when it comes to differing conditions (cold start versus driving for an hour, etc.).

I ended up at around a 13.5:1 idle with 17* of timing and about 1,100 RPM on my car, it made a huge difference (but my engine is highly modified).

I had started at 14.5:1 idle, 11* timing, about 900 RPM. The idle was good sometimes, but very inconsistent. It also didn't like to catch well, it always wanted to dip down before settling into the idle.
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Old 08-02-2018, 06:19 PM   #7
e30davie
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Interesting insight. Thanks for that. I'm working towards ignition, that will open a whole new kettle of fish!

I will have a play with the idle and see how the map is. I think old school carby tuners aim for maximum vac at idle too. I must admit my current criteria for idle quality is basically what sounds the best which may not be correct.

Slight different direction in question. How much should I be relying on ego wideband correction? I enabled it rather aggressively from 800rpm up and only under 70% tps and appart from a few outliers in my tune I have a maximum of about +/- 4% over a range of temperatures driving to and from work. Lots of highway and constant driving so I can see it working well.

At the moment I am adding a fee percent of fuel in lower temperatures, pure guess at this stage. Needs a tad more I think.

Being a noob at tuning I would think it should be closer than this. But then again air temp has a fair effect on density and thus afr....so I'm.unsure what is good here.
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