VVT activation with standalone

[Grant #2]

Hey guys,

I recently got my ae111 engine in my ae86 chassis running and driving last night. The fuel and spark tables are way off, but that is fine. Will be tuning it day by day.

I'm using VEMS, which uses megatune or Vemstune to tune the car. At this point in time, I can only turn VVT to come on at a given RPM, such as 4400, and it will stay on until redline.

I read that the factory ECU turns VVT off at higher rpm. Why is that? Is it harmful if I leave it on?

[dr1ft-pig]

shouldnt do it any harm mate.... my mate got more power out of his 20v by leaving vvti on all the time.....

my old 20v running microtech turned vvti on at 3500 and it stayed on till limiter also

[fantapants]

wait for an answer from ant :) i know he did a bit of work in this on the last site :)

[LittleRedSpirit]

If you turn it off shortly after 6000rpm it will make more power for the top end.

[Grant #2]

I guess you need more overlap at higher RPM? Why does advancing the cam increase power at all then if you are reducing the overlap?

[Frak]

I guess you need more overlap at higher RPM? Why does advancing the cam increase power at all then if you are reducing the overlap?

ok briefly(I'll use ST 20v cam timing)

at low rpm the camshaft is retarded, this is, the intake valve opens at TDC and closes 70deg ABDC, this reduces over lap(exhaust closes 16deg ATDC) means that not much is wasted to the exhaust during idle and light load, this means better idle, better emissions etc.

during heavier loads but at lower rpms, the cam is advanced, that is the intake valve opens early, the cam timing is now intake opening 30deg BTDC and closes 40deg ABDC, this increase in overlap allows the exhaust energy to scavenge the cylinder better, now this scavenging NOT only gets rid of residual exhaust gases in the cylinder BUT on overlap the intake valve is also open, so if a low pressure wave gets to the cylinder while both are open it will also pull fresh intake in :) at these lower rpm's the intake charge is travelling quite slow, so the negative exhaust pressure wave gives it a boost and helps to 'get it up to speed'.

Now at high rpm's due to the intake charge having mass, it now has some speed(hopefully 'up to speed') to it and it is not as important to use the exhaust to suck this intake in, due to the speed of the intake it will go into the cylinders now, so we retard the timing once again, the intake valve now closes 70deg ABDC allowing more mixture to be 'captured' and improving upper rpm performance.

The speed of the intake charge is very important and the VVT works around this, nice stable idle/ emissions and light load driving, increased mid rpm torque(by using pressure wave to get the charge in) and improved cylinder fill at higher rpm's to improve upper end torque.

remember that the overlap is like a 'window' this window is where the negative pressure wave from the exhaust can do it's magic, if you reduce overlap, you reduce this window and the effects of being able to use the exhaust pressure wave to your advantage. This pressure wave IS VERY strong.

Basically think of it like this, high overlap helps to deliver strong mid/upper-mid rpm torque/power and late intake valve closing delivers high rpm torque/power, high rpm camshafts WILL have very late intake valve opening. For those use to single cam engines, to improve performance the cam generally has lots of overlap and late closing together on the one stick, VVT is the way to go, have your cake and eat it :)

We always want the negative pressure wave to arrive at the cylinder when the valves are open to help get rid of exhaust/pull in new intake. If the positive wave arrives then this will hinder scavenging and may even force the intake back up the intake tract which will then cause a flatspot/reduced torque.

4-1's basically have 1 tuning pulse which is very strong, while 4-2-1's have 2 tuning pulses which aren't as strong but have a longer duration if that makes sense, also you don't need 100% of the negative wave to arrive during valve open time, any part of it will give some gain. I know this is a bit of exhaust stuff now, but it all ties in with how the intake is working.

Again, this is just a brief outline and I'm sure others will add to this.

[takai]

Also remember that most AM ecus can only do full advance or nothing, no inbetween. Makes it a bit of a pain sometimes.

[Grant #2]

Frak, thanks a lot for the post. I'm going to have to read it over a few times as you have brought up some points that I am pretty unfamiliar with.

Takai, VVT on all 20V is either fully on or off. No inbetween. It's not like VVTi.

Anyone have some baseline numbers where I should turn VVT on and turn it off?

[Frak]

Frak, thanks a lot for the post. I'm going to have to read it over a few times as you have brought up some points that I am pretty unfamiliar with.

Takai, VVT on all 20V is either fully on or off. No inbetween. It's not like VVTi.

Anyone have some baseline numbers where I should turn VVT on and turn it off?

No probs, that was just like a quick 101, I might write up a tech article on it.

[Oly AE86]

From what Ant has told me, On at around 2000/2500, off at 6000.

On my GTE its on at 2500, off at 6000.

Best would be back to back dyno runs with different switching points and A/F adjusted to suit. Pretty sure Ant has done this.

[Grant #2]

OK, thanks for the rpm info. Is this only under heavy throttle, or always set it to turn on at 2000 rpm? I guess the first gen ST that switched it on at 4400 was not ideal then?

I dunno if I can control it based on load..

[Grant #2]

I supposedly can switch it based on load actually. I have it set from 2500 to 6300 rpm now, but I'm pretty sure it's not activating right at the moment. Have a bit of work to do.

After how much TPS% should I have it turn on? I have it set at 60% throttle input.

[Sam-Q]

on my engine it was ideal to have it turn off at 6400RPM. I wrote a bit of an article on this here:

http://s-86.com/articles.html

its the third one down

[Frak]

AE111 VVt operation
http://www.ae86drivingclub.com.au/dcimages/2/1/9/23231.jpg (http://img183.imageshack.us/i/vvtswitch.gif/)

[Oly AE86]

Mine is on regardless of Load.

[Grant #2]

Thanks for the input folks. I have it now set to turn on at 2500 RPM and off at 6400 rpm.

Throttle input required is 35%. I don't want to wear out the VVT gear from constant activation if I have it enabled by RPM only. Any thoughts on this?

For motorsports use, I might have a special config that turns it on on RPM only, since I'll be at WOT or 0 throttle most of the time.

[Sam-Q]

just a note you should do a dyno run with and without to find the ideal cross-over points, because of the perks of my engine it is unusually late, 6000 is more common.

with the activation would you really go over 6000 revs with less than 35% throttle?

[Anthony]

when you back off you might. (dont forget the emmissions targets these manufacturers had to meet Sam ;) )

yeah those on and off numbers are pretty much right.

A couple of tricks for setups. On microtech's etc (and maybe your ECU) you can run your aux through the normally close contacts of a relay as the "turn on". then run the coil of the relay to another aux out (assuming you have more than 1 spare?!?) make this output turn off at 6000 or so. in this way you can trick the less sophisticated ECU's into doing proper vvt operation.

Grant # 2. advancing the inlet increase overlap, not decreases. so i guess you answered your own question :)

Another note. last few firmware updates for adaptronic have the means to use a spare fuel table (fuel 2 in the tuning screen for example) as a vvt angle lookup table. this lets you do load vs rpm vvt timing. unfortunately most adaps on 20V's are going to be running map x tps tuning which burns both maps, but it may help some ppl with single throttle and/or turbo.

The downside to these features is that tuning costs rise exponentially.

[Grant #2]

OK Anthony,

If duration increase when VVT is on, why wouldn't you want to have it on until redline? I need to re-read Frak's post I suppose.

Sam,
You think I should have it turn off earlier?

The 35% throttle is more important down low, I can't have it slope upwards like the factory did in that image Frak posted.

VEMS allows you to turn the VVT on at certain RPM, off at another one, min TPS, Max TPS, and Min MAP and Max MAP. But I'm using AlphaN so I can't have map input b/c of limitation of megatune.

[Anthony]

OK. I dont follow. Your first post says you're using VEMS and youc an only turn on at a certain point - then stay on.

Now you're using VEMS and you can turn it off again and add load interlock policies??

What are we trying to do here?? :)

As for timing question. The very simple answer as to why you don't leave it on until redline is that the engine makes more torque at those points if you dont. :)Therefore you could say that mmore overlap does not automaticlly mean more torque for all RPM and load combinations. Imagine the dyno time behind the settings in the blacktop ECU... Dont forget most cam timing settings with fixed adjustable cam gears are an "area under the curve" compromise determined by the intended use for the engine.

The whole idea of VVT and especially VVT-i is to have your cake and eat it too.

[dr1ft-pig]

i like cake :)

[Sam-Q]

I am totally with Anthony in his comments. I say just do a pure rpm switch without any load hookup though, but thats just me. You need to pick the change points on the dyno simple as that

[Grant #2]

I supposedly can switch it based on load actually. I have it set from 2500 to 6300 rpm now, but I'm pretty sure it's not activating right at the moment. Have a bit of work to do.

After how much TPS% should I have it turn on? I have it set at 60% throttle input.

No, I updated my findings the same day I think :DD

[Frak]

If duration increase when VVT is on, why wouldn't you want to have it on until redline? I need to re-read Frak's post I suppose.


Hi Grant2, re-read my post, at high rpm, intake velocity is high, not as much need for the exhaust pulse to pull in charge, retard intake cam, meaning that intake valve is now open longer after BDC, hopefully capturing more charge.Therefore capability to make more power, more air and fuel, bigger bang, more force pushing piston down, more torque, more power.

If you have the intake valve open too long after BDC at LOW rpm, then reverse pumping occurs and intake is pushed back up intake tract due to piston coming up on compression stroke. This is not a real problem at very light loads such as cruising as the engine is not really making much power at all. But I also don't have intake being sucked out of exhaust due to reduced overlap, therefore emissions will be better.

You get away with it(late intake valve closing) at HIGH rpm due to the speed/weight/inertia of the intake charge overcoming the piston coming up on the compression stroke OF COURSE this has to come to an end somewhere, the LATER the intake valve closing the higher up the rpm the engine will 'work'

VVT allows you to have lots of overlap in the mid range/high load point to draw intake in/reduce cylinder pressure at around TDC/scavenge exhaust and at high rpm allows you to capture more air/fuel where the intake charge has lots of speed and doesn't have to rely on exhaust scavenging as much.

Remember high overlap and late intake valve closing are improving torque/power IN TWO DIFFERENT RPM RANGES.

[Frak]

I am totally with Anthony in his comments. I say just do a pure rpm switch without any load hookup though, but thats just me. You need to pick the change points on the dyno simple as that

Yes rpm works well. On my old ST running Autronic I mapped it rpm vs tps position.