camshaft overlap effect on afr at idle.

[kaeman]

I just installed a larger camshaft with more valve overlap, and have run into several differences in engine operation.

I have found that I get the best vacuum (about 50 kpa) with an idle of about 1200 rpm, lower than that and the idle kpa moves up into the low to mid 70's at about 900 rpm, and below 900 rpm the idle kpa is in the 80's.
I have been having the afr readings while idling rise up to 22.4 even though its running rich, after checking the innovate website they describe a condition where large cam overlap at low idle speeds will create false lean afr readings, and that when you move into the higher rpm and load ranges the sensors will read correctly again because of the increased airflow in the exhaust. The sensors are not giving error codes, and if I hold the throttle open and let the engine run 1600 to 1800 rpm the readings will drop back down to 13 to 15 afr's which is what I would expect to see in that part of the ve table.
I have been trying to get the engine to idle about 900 rpm, but after going to the compcams forums, one of their techs replied that this camshaft needed the 1200 rpm idle because efi systems using speed density couldn't cope with the large valve overlap at idle speeds below 1100 to 1200 rpm due to the erratic kpa readings.
Anyone else run into this kind of issue?

[krisr]

What's your idle timing?

I have a 242/248 @ 0.050 on 110LSA hydraulic roller in a 400 Pontiac with a 3200 converter/turbo 400 behind it. To get it to idle properly, I setup the shifters inhibitor switch so I could table switch spark tables and have 1 spark map for in gear and 1 for park/neutral. In P/N I command about 20°BTDC at idle but in gear I command about 38°. My idle vacuum is about 80kpa and is pretty much rock solid. Having 2 tables keeps the engine alive when dropping it into gear and not having to wait for the IAC to open up. At idle it runs at about 30 steps both in and out of gear when up to temperature. Commanded (and met) AFR is about 13.5 with an idle speed of 1000rpm.

Also what sort of cam are you running and what valve springs? That may restrict how low you can idle it too (alot of seat pressure etc...).

[Peter Florance]

Very common. When I have exhaust with lots of reversion (cam) or leaks at joints, I just tune idle for lowest MAP and stable RPM.

[Yves]

Strange, my cam has 247/249° at .050 with .600 lift and 106° LSA. Pretty agressive cam. I was able to get it to idle decently at 14° with a 2KPa penalty versus the lowest map I could get...

There is more to it than meets the eye.

[Peter Florance]

Strange, my cam has 247/249° at .050 with .600 lift and 106° LSA. Pretty agressive cam. I was able to get it to idle decently at 14° with a 2KPa penalty versus the lowest map I could get...

There is more to it than meets the eye.

Probably timing stability and AFR.

[kaeman]

my cam is 254/260 @.050 and 600 lift. solid roller. my advance at idle speed is about 16.5 at 1200 rpm. I am having problems with idle stability from one start to the next, I think my iac is giving me issues, I think I will just turn that off and use the butterfly set screw to control idle speed. the forum for compcams told me that under 1200 the idle with this cam would real hard to control. because of the overlap and intake reversion... I am going to make a couple changes to make tuning simpler.. remove the iac and go back to throttle blade setting....

[Peter Florance]

my cam is 254/260 @.050 and 600 lift. solid roller. my advance at idle speed is about 16.5 at 1200 rpm. I am having problems with idle stability from one start to the next, I think my iac is giving me issues, I think I will just turn that off and use the butterfly set screw to control idle speed. the forum for compcams told me that under 1200 the idle with this cam would real hard to control. because of the overlap and intake reversion... I am going to make a couple changes to make tuning simpler.. remove the iac and go back to throttle blade setting....

Post a log when you are able to.

[kaeman]

I guess my biggest frustration is that I get different idle speeds with the same idle step numbers.... I will load 2 short logs trimmed from a long one. and the current tune. the ve table isn't close to being tuned yet, I am still trying to get idle working then tune the rest of the ve table. I think I have something simple in the iac settings that must be messing me up.

2016-01-31_1st edit.msl

2016-01-31_3rd edit.msl

2016-01-31_20.07.59.msq

[kaeman]

Should I turn off the iac, and work on the idle using the throttle blades and get a good idle, and figure out the ve and timing values needed to keep a smooth idle.
I read in the manuals that full sequential idle afr targets should be set about 14.7, should I try for this or should I run richer (13.6 or 13. like I was in semi-sequential.
I am just trying to get a handle on where to start tuning.

Thanks for any ideas.

[Peter Florance]

Should I turn off the iac, and work on the idle using the throttle blades and get a good idle, and figure out the ve and timing values needed to keep a smooth idle.
I read in the manuals that full sequential idle afr targets should be set about 14.7, should I try for this or should I run richer (13.6 or 13. like I was in semi-sequential.
I am just trying to get a handle on where to start tuning.

Thanks for any ideas.

I'll check logs when I get to work later. Yes, you'll need much richer targets than 14.7 at low RPM. I'd start around 12.9; if better, try leaner by .2 or so until you find where it won't idle.

[2swe]

just out of curiosity, is it wort trying to retard injection timing to avoid injecting fuel to open exhaust valves ? I mean at idle and low load/low rpm..

[kaeman]

I wonder if changing the end of squirt to somewhere in the 300 degree area might help with the afr readings. I am going to have a friend come help figure out the iac steps from wide open to fully closed. I would like to use the iac, but if I cant figure this out I can use the butterflies and tune for best idle and then try to use iac.

[Yves]

I wonder if changing the end of squirt to somewhere in the 300 degree area might help with the afr readings. I am going to have a friend come help figure out the iac steps from wide open to fully closed. I would like to use the iac, but if I cant figure this out I can use the butterflies and tune for best idle and then try to use iac.

Gasoline needs time to go into vapor. You need to allow for that. 300 ° is not enough as you are injecting during the open intake valve and descending piston. I have been told it takes approximately 8 ms for the fuel to vaporize...

[Peter Florance]

I wonder if changing the end of squirt to somewhere in the 300 degree area might help with the afr readings. I am going to have a friend come help figure out the iac steps from wide open to fully closed. I would like to use the iac, but if I cant figure this out I can use the butterflies and tune for best idle and then try to use iac.

Try around 600 degrees

[2swe]

at 1000rpm, 1 engine revolution takes 60ms, so when the injection ends 300°BTDC and spark comes 20°BTDC you still have 280° or 46ms time to vaporization..

[kaeman]

well after much experimenting, I was unable to get my idle to reach 1250 rpm, so I opened the throttle blades until I was able to get the engine to idle at 1250 rpm. then I went to the iac test modes and ran the valve fully closed, the engine didn't noticeably (its pretty lopey)change idle rpm, then I opened it all the way using the run test and it made a huge vacuum sound, but didn't noticeably increase the rpm, then running it fully closed didn't decrease the idle rpm, but the suction sound stopped, I checked the idle valve and it was moving from full open to fully closed and made no noticeable change in engine rpm. The rpm gauge was bouncing around a lot, I am thinking that adding more smoothing to the inputs might help a lot....any idea's?
at idle my afr's are showing 22.4 and 17.5 afr ( I am assuming rich misfires causing the lean readings), but the plugs are reading dead rich. the sensors read correctly when I bring the engine up in rpm, I had to change my injection timing to 310 degrees to miss the overlap of the camshaft, which smoothed out the idle. the engine idles steady at about 1250 rpm, but the iac doesn't make any difference in idle speed, although it does richen or lean the afr readings a small amount between fully open and fully closed.
I tried idle timing from 22 to 31 degrees with little change in idle quality or rpm.
I am thinking that I might try adding more smoothing to the rpm and map sensors inputs.......they are currently 90 but I am thinking maybe 80 or 70 will give better results.
My next test/tuning session is going to include removing the iac and putting the block off plate into the throttle body and then get the engine warmed up and bring the engine up in rpm until the afr readings come back to normal readings, then I will turn off o2 correction until the engine is above that rpm.
here is the msq and the datalog.

2016-02-07_14.08.40.msq

2016-02-07_90 smoothing factor map rpm.msl

2016-02-07_16_input smoothing 95.msl

[kaeman]

by the way the camshaft at .015 opens the intake 40 degrees before tdc and the exhaust valve closes 34 degrees after tdc... 74 degrees of overlap at .015. its 110 lobe separation, comp cams recommends this cam for pro-street with 11 to 1 or better compression with good intake and exhaust and 3500 stall.......if this is useful info.

[slow_hemi6]

Hmmm my cams got 70 degrees overlap and idles 1000 out of gear and about 800 in gear at about 70kpa. Fairly slow hydraulic flat tappet ramps though. IO (.006) 33 BTDC, EC (.006) 37 ATDC.
Afr around 13.8 idling in gear.
I do have 50 to 60 in nearly all my lags.

[suberimakuri]

Few thoughts.
1) Give the engine what it likes. Don't worry too much about AFR reading if it drops out, whatever the best kpa it idles at will be the best mixture to idle at.
2) Box the idle part of the VE table. If you command the same VE (pw) for the varying kpa's it's loping in, the engine will run smooth as the actual fuel demands during loping are pretty stable. This should help with the <1200rpm area.
The fuel won't need to be increased until close or above the 80kpa @900rpm and maybe ~85 or 90kpa at less than this (~700rpm?).
If you try to increase VE with kpa in the loping idle part of the map you will go round in circles and introduce more lope.
SD has poor resolution at low rpm with big cams or ITB's.
3) I normally have a low rpm (~500rpm) row that is richer than the idling area (900-1100) so if the rpm drops (traffic crawling/easing off clutch with minimal throttle) the fuel mixture is rich enough to make the engine run strong.
4) Look at idle VE settings in MS. Pretty nifty, saves using up VE table resolution boxing idle like #2 above.

[kaeman]

I am trying to give the engine what it wants, not any specific target afr, but I am just surprised by how much different the engines setup is at this point, the afr readings are lean higher that 17 up to 22.4 at idle right now, but when reading the plugs they are still saying the engine is getting to much fuel. I am just going to keep working on the idle until I get the smooth idle I want.
I am using the idle ve and the idle advance in this tune, I have been working at smoothing the fuel section in the idle ve and then making the same section of the regular ve table close to the same.
I need to remove more fuel yet, I think the lean readings must just be misfires from the over rich fueling, when I rev the engine up and increase the airflow in the exhaust the 02 sensors will read in the 13 to 14.9 range. Is it possible that going to full sequential would require ve values 10 to 15 points lower than in semi-sequential mode at idle?