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Old 04-27-2015, 12:36 AM   #1
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Default Tuning advice for big cams

I've recently done some mild DIY porting and bowl work on my N/A build, with the addition of some big cams, intake and exhaust differ but they are around 272 degrees duration, a bit over 9mm lift and some decent overlap installed "straight up" for now. I get a stable but lopey idle at 1,000 RPM at about 75 ish kpa, so I need a lot of resolution in the VE table. I've pressurized the intake and I'm 100% certain there are no leaks. Brake assist is not so great either so I may delete the booster, but thats getting off topic, but here's how I scaled the VE table. Please ignore the VE numbers, its just a start/idle tune at this point. I started with 8kpa just to control deep vacuum, and then from there went 40 on up.



As far as the timing goes, I have not touched the rescaling at all, Like I said just got a good, stable (as it can be) idle. I'm not even sure how to scale it here as to where I would need the resolution, similar to VE table I suppose? Its going to need to rise quickly from 75kpa up.



Since Its a "street" car, should I start tuning the cam gears now for best Vac at idle by retarding the intake and advancing the exhaust a bit?

here is the idle vid for grins. Sounds badass!


unfortunately no logs yet.

Any advice is appreciated!
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Old 04-27-2015, 11:09 PM   #2
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Did a little logging in the neighborhood and found I was hitting vac as low as 20 KPA. Rescaled a bit and ended up below. Its still rough under 2k RPM, but the car really wakes up around 4k RPM.

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Old 04-28-2015, 12:21 AM   #3
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Have you considered running alpha-n at low-ish throttle and switching to speed density at higher loads?
Others with aggressive cams seem to be more successful with that.
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Old 04-28-2015, 12:31 AM   #4
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I think I'd consider an alpha-n tune with a 75kpa idle as well.
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Old 04-28-2015, 02:10 AM   #5
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More timing at idle will give you more RPM, close throttle a bit to bring it back, and now you have more vacuuum.

Also, you can do a timing table by TPS that is additive, and have it throw timing at it as you touch the gas to give it more response off idle.

Have tuned bike engines with MS3.......
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Old 04-28-2015, 12:30 PM   #6
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I have considered Alpha N, but trying to avoid it, seems like smoke and mirrors at this point but will try it out If I can't get a good tune. Somewhere I read that was mostly for higher loads, just goes to show how much I know about it so far.

Pat, that makes sense at idle. For some reason I always try to keep timing at idle 12-14, but see no reason not to bump it up and use less IAC DC%

This car will be terrible to drive in traffic, but the way the engine wakes up in the midrange is just fantastic. It's going to be perfect for backroads hoonage.

Thanks
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Old 04-28-2015, 10:47 PM   #7
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Yeah with big cams you need more advance at idle. And I suspect once you get that idle vac down more you might find it to be bouncing between load cells. Try to get the cells its bouncing between the same or at least close to each other and it ought to take out some lumpyness.
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Old 05-17-2015, 10:32 PM   #8
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thanks for the advice on timing. I played with it a bit and it seems to like 19.5 degrees at idle. Got it down to 1000 ish rpm and 14.0 AFR, now it pulls 65 kpa at idle and a lot of the chop is gone.
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Old 05-18-2015, 12:39 AM   #9
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With cams there will be a lot of overlap, thus a lot of raw air/oxygen going out the exhaust pipe. The O2 sensor will see all the extra "oxygen" and report a leaner than indicated AFR. Thus AFRs at low speed are not going to be correct. So if it shows 14 now, you are richer than 14 at idle.

You may try tuning the AFRs for max vacuum also. With stock cams, my miata idled better at 13.0:1 than anything else.
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Old 06-07-2015, 10:04 PM   #10
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I've put 100 miles on the car now, and I've got it pretty close I think for an amatuer street "toon" Before the head/cam work, the car was starting out of breath at 6,000 RPM, now it just keeps on pulling. Wants to idle now around 1,100 at 64-68kpa, which is acceptable, anything lower less is just inconsistent and wants to die. When stopping after its been running awhile, it wanted to idle at 1500 rpm at the same IAC% due to running so much timing, so I created another row in the timing table at 1600 rpm and lowered the timing there to make it return quickly... If I ever decide I want the idle lower I'll use that technique. Using the higher idle advance even got the car to idle with acceptable stability with the A/C on One thing I've noticed though, a lot of cells are taking LESS fuel, than with stock cams. totally not what I expected.



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Old 06-07-2015, 10:24 PM   #11
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Quote:
Originally Posted by Stealth97 View Post
...One thing I've noticed though, a lot of cells are taking LESS fuel, than with stock cams. totally not what I expected....
Just so you know, at idle your wideband is going to read off. Your cams have a lot of overlap, so at low speeds, there is a lot of unburned air/fuel dumping across the valves and going out the exhaust ports. Your wideband will pickup the unburned O2 and think the motor is lean, giving you a leaner-than-actual reading.

EDIT: Haha I posted that right above this a while back.... Odd about it taking less fuel than before, I'm guessing that's in the low-mid range where the new cams are not tuned for. Right?
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Old 06-07-2015, 10:53 PM   #12
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Quote:
Originally Posted by patsmx5 View Post

EDIT: Haha I posted that right above this a while back.... Odd about it taking less fuel than before, I'm guessing that's in the low-mid range where the new cams are not tuned for. Right?
Yep. Looking at the VE, looks like power picks up around 3800, but making less power under that than stock cams. I'll probably get some back if I reduce overlap, but I'll tune that on the dyno. I havent spent much time yet over 5,000 RPM, but when I do, it gets into the high 13s... but does not buck or ping. It was originally tuned for 12.5 across the board at WOT.
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Old 06-07-2015, 10:55 PM   #13
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Keep it in the mid to high 12's. Don't wanna break it! I would recommend adding a bit of fuel up top so that it's not too lean, so when you tune it you're pulling a bit of fuel not having to add a bunch. Just to be safe!
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Old 06-07-2015, 11:12 PM   #14
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Quote:
Originally Posted by patsmx5 View Post
Keep it in the mid to high 12's. Don't wanna break it! I would recommend adding a bit of fuel up top so that it's not too lean, so when you tune it you're pulling a bit of fuel not having to add a bunch. Just to be safe!
Good call. will add some fuel up top.
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Old 06-08-2015, 04:38 PM   #15
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Remember that when you change valve timing, the relationship between MAP and engine torque output changes.

With a later intake close timing due to longer duration or centerline change (with cam gears or on a vvt engine), you have less trapped air mass when at lower engine speeds. That means the throttle needs to be opened more to make the torque. You also need more throttle to make torque due to residual gases.

Increased residual gases slow down the burn and increase instability, hence the need for earlier spark timing.

Assuming sequential, Have you swept injection timing? Set it to end of squirt, begin at about 300 btdc, and try moving 120 degrees in either direction in 20 degree increments. When you have good timing thr mixing will improve and you will have smoother combustion.

You mostly judge the injection timing subjectively as it really requires an emissions analyzer or combustion pressure indication.
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Old 06-15-2015, 11:27 PM   #16
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Quote:
Originally Posted by arghx7 View Post
Remember that when you change valve timing, the relationship between MAP and engine torque output changes.

With a later intake close timing due to longer duration or centerline change (with cam gears or on a vvt engine), you have less trapped air mass when at lower engine speeds. That means the throttle needs to be opened more to make the torque. You also need more throttle to make torque due to residual gases.

Increased residual gases slow down the burn and increase instability, hence the need for earlier spark timing.

Assuming sequential, Have you swept injection timing? Set it to end of squirt, begin at about 300 btdc, and try moving 120 degrees in either direction in 20 degree increments. When you have good timing thr mixing will improve and you will have smoother combustion.

You mostly judge the injection timing subjectively as it really requires an emissions analyzer or combustion pressure indication.
Have not played with injection timing yet, but yes running sequential. I will play with it once I get my VE table nailed, I'm getting closer. I see what you mean about tq and relationship with MAP changes. Light throttle can put me in the upper 80s, where my old map would get me into the 13s with near open throttle, now that is too rich. Once I get closer I'll add another bin before 100kpa and push the enrichment in the AFR table up there.

The timing "hole" I made to stabilize throttle made things a little jerky in parking lot speeds to I moved some bins around to tighten it up. I'll post it after I test the results.

Another thing I noticed now that I'm starting to give it more in the upper range.. about 6500 RPM it instantly goes from sounding like a Miata to a sportbike and it really starts pulling. I'll have to get a vid, it's not at all subtle.

Also ran though my first tank of fuel...23 mpg. and thats with overrun cut still off so my two big fears of running big cams - killing the smoothness of my ridiculously smooth engine (aside from lumpy idle which is cool) and killing my gas mileage, its just not bad. My old average was 27 highway, I can probably get close to that. All 'round its just more awesome.


EDIT - just checked my map. I'm NOT running sequential, not sure why.. I'll have to look into it.

Last edited by Stealth97; 06-15-2015 at 11:38 PM.
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Old 07-04-2015, 11:00 AM   #17
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I tightened up the timing hole and now without A/C, it drives almost perfect. Still adding/subtracting fuel manually here and there, and need to keep adding smoothness.



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Old 03-03-2016, 11:09 PM   #18
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Since this thread ranks up pretty high in google when searching for things like megasquirt and big cams, so I thought I'd update it. Might even be good for a few

So with a few tweaks, the posted spark table was almost perfect, then the temperature went down to 60 degrees, and the car wanted to idle at 2k rpm

The first step was to ignore my wideband at idle and reduce fuel BIG time. The gauge now reads 22-24 AFR at idle, but with a large overlap you are spitting lots of fresh air into the exhaust. This brought KPAs down quite a bit and increased stability.

Next was to dump timing at 1800-2k at 50kpa. This fixed the idle, but I would occasionally it this while driving and the car would start bucking at times like in traffic, making my commute a pain. I "tightened" this up as much as much as possible, but it still wasn't right. I knew the answer was Closed loop idle, but I just could not get it right, I kept getting high idle, so I locked the max DC right at my idle speed, which wasn't right, but it did help.

After that I got the idea of table switching the spark table at about 2% TPS. I kept the idle "feedback" loop I punched into the spark map in the idle table, and then started tuning the switched map for drivability and throttle response. This was epic. No more bucking, just smooth starts and drivability at low throttle. I can finally thank Atlanta traffic for teaching me something. Part of the problem with big cams, is that you are driving around in the part of the map where you would drive at low RPM, and it just wreaks havoc, but not anymore.

Finally I read my *** off on Closed loop Idle. This thread was especially helpful: Tuning Idle Valve - Ms2 - Guides: Forced induction - MX5Nutz Forum I studied up on what each setting does. Eventually I got closed loop to control my idle speed better than warmup PWM. The car actually idles BETTER now than it EVER did on MS, including on stock cams. This got my stable idle speed down to 1050, and my idle Vac down to 68-74 KPA. before CL idle, it was never lower than 70, ever. I can even let out my clutch slowly from a dead stop without touching the gas pedal, the RPMs will drop to 900 as the car moves forward, Idle valve will slowly open and bring me up to 1050, which is perfect in traffic.

TL;DR, you want good idle get your fuel and spark right. Then tune your idle valve, then repeat until you're happy.

VE table:


Idle Spark table <2% TPS


"Driving" Spark table


Closed loop idle settings


Closed loop PID table


And here is the idle log Notice the timing appears to be all over the place, but this is the "feedback loop" built into the "idle" spark table doing its job to stabilize idle RPM.



EDIT Actually, looking at that log, I could probably tweak the lower advance numbers to stabilize it a bit more.
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