VVT tuning 1

[pickler]

A 2013 Subaru Legacy 3.6 (H6) i'm tuning has dual VVT or Dual AVCS as subaru calls it. I can control both intake and exhaust timing (50*/40*) respectively. The cam specs seem to be very mild even though the car has no problem building torque up to 5200 RPM and power peaks at 6000 rpm at 260ps.

here is the cam specs:

the stock VVT maps can be accessed in the ecu and are as follows. However PLEASE NOTE that the exhaust retard has been mistakenly named Intake Advance and vice versa.

stock AVCS tables

I noticed the intake AVCS tables have 50* of advance stock at low-mid RPMs. The exhaust has been retarded at about 20-30* at ~2000rpm.

Now from my readings I have found that Advancing the intake whilst retarding the exhaust actually increases overlap which results in poor performance and reversion. Advancing both intake and exhaust however should create low end torque. So why has subaru retarding the exhaust cams at low RPM while intake is fully advanced? So opening the intake further after BDC (more advance) and opening the exhaust valve sooner before TDC (advancing the exhaust) will definitely give more torque no questions asked.

Using engine analyzer pro software, the theoretical best overall valve timing for low rpm performance is 45* advance on intake and 10* retard on exhaust:

so in the ECU this would translate to getting about 90% of advance possible on both the intake and exhaust side. This is very similar to factory AVCS on the intake side, however the exhaust side is still too retarded on the ECU.

31* of intake advance and 14* exhaust retard gave best theoratical mid-high range in the software.

only 15* of intake advance and 20* of exhaust retard gave best results for +5k rpm

results:

So theoretical conclusions from this suggest that:

A. Intake timing is well tuned from factory, however maybe a little too advance at low RPMs
B. There is no reason why the exhaust should be retarded at low RPM and fully advanced at midrange.

I did get a chance to take the car on the dyno and to test out the theory above. I only had time for a few pulls and here is what i observed for now:

- retarding the intake tables at higher RPM hurts performance throughout the revrange
- retarding exhaust by 10* at high rpm (from 0) made no difference in output
- advancing exhaust at low RPM degraded torque
- ignition/fuel was not changed

So i'm confused whether this car is already tuned at its best or perhaps I'm just not going about the right way in tuning VVT.

I have heard that retarding intake can allow one to run more ignition timing and hence make more power.

1. I'm just curious why the intake has been fully advanced at low rpm and exhaust retarded. Does intake advance and exhaust retard create more low end torque?
2. doesnt exhaust retard create more midrange torque? Why is at low RPMs exhaust retarded?
3. Why does retarding intake just by 5* hurt performance at high RPM? Should we perhaps even advance intake further?
4. overall what would be a better strategy to tune for power next time on the dyno?

 

[BrianPetersen]

1. The OEMs have put vast amounts of engineering into this.
2. The OEM's objective isn't always maximum possible performance - especially at low engine revs. In that operating region, the OEM's primary objective is emissions and the secondary one is fuel consumption.
3. The accelerator isn't always floored ... the driver does not always want maximum acceleration ... but having the throttle open enough so as to not have much intake vacuum even when the accelerator is not floored, artificially makes the engine have less output torque (the driver does not have the accelerator floored ...) but without much intake vacuum ... this results in a "wide open throttle but part load" setting that has (1) lower pumping losses, and (2) "internal EGR" for NOx control, than just shutting the throttle would achieve.

Retarded exhaust and advanced intake (= high overlap) but combined with a nearly-open throttle (so reversion isn't as much of an issue) sounds like a part-load-but-full-throttle optimization strategy for emissions and fuel consumption.

 

[pickler]

so overlap from advanced intake and retarded exhaust is good for wide open throttle power? I have seen some discussions where the tuners increase overlap through out the rev range and add timing and lean out the fuel.

 

[BrianPetersen]

No ... It's good for efficiency (lower pumping losses by allowing the throttle to be opened more) and lower NOx emissions (from "internal EGR") when the requested torque output is less than maximum.

You will note that the greatest exhaust retard and intake advance (maximum overlap) are near 2000 rpm and higher engine load. With an automatic transmission (which is the usual case nowadays) this is not the operating condition when the driver has the pedal to the metal (max acceleration). This is the operating condition when driving on cruise control up a moderate hill. It will want to keep the transmission in top gear with the torque converter locked at cruise RPM (2000ish) while not burning too much fuel in the process and certainly while not emitting too much NOx. If the driver requests more power than can be delivered in that situation, the transmission will downshift.

 

[pickler]

sorry i'm a bit confused, why would 2000 rpm be a target for such high overlap then if it's targeted for hill climbing? Are you saying it has max overlap because it is the most used engine speed and this will reduce overall emissions or is it because it improves lower RPM efficiency AND torque? I'm looking to improve torque only here, so I gather from your post I will need less overlap at higher RPMs. Should I advance the exhaust at 2000 rpm to reduce overlap and retard the exhaust a few degrees in the zero’ed out cells at midrange? Or should I leave exhaust alone and advance intake even further?

 

[BrianPetersen]

2000 rpm isn't a target for such high overlap; it's a target for minimizing emissions and fuel consumption in that RPM range and the cam timing that they've chosen is a means of achieving that. They're likely sacrificing a few lb.ft of output torque BUT they are maintaining closed-loop operation (happy catalyst) for emissions compliance. At the OEM level the calibration in the common driving speed and load ranges is ALL about emissions, emissions, emissions.

If you need more power than 2000 rpm will deliver ... downshift! And if you are climbing a hill on cruise control and it is not able to maintain speed, that's exactly what will happen.

You could play around with whatever cam timing you want but it's highly likely that you will not be able to do appreciably better overall than the OEM setup. A lot of engineering hours and testing by a lot of people went into that calibration.

 

[FeX32]

Minimizing emissions at low-mid rpm range coupled with partial loads is a huge focus these days. This is high in the duty cycle.
If forced induction was used a slightly different VVT strategy will may be used.

 

[pickler]

i understand that but i was wondering about the fundementals of VVT; valve overlap and cam timing vs. load and rpm to improve power. IE highly advancing intake timing and slightly retarding exhaust should improve low end power and do the opposite for top end. this means overlap is good for low end torque and bad for high end. this would make sense since the intake velocity is high enough at higher engine speeds and overlap is unecessary. also from testing on the dyno i found the stock intake valve timing to be best. however some minor tweaks to exhaust timing helped. for example retarding exhaust by just 5* added 3-5 horsepower at 4000-4600 rpm. however retarding the exhaust above 5k actually hurts performance. at low engine speed where factory exhaust timing is very retarded for maximum overlap, advancing the exhaust also led to some minor gains 2-3 hp.

obsevations
+45 intake and -5* exhaust retard gave best power at 2000 rpm vs +50 intake and -30* exhaust.
+35 intake and -5* exhaust best for peak torque area
+10 intake and -10* exhaust for best peak power (same as stock)

 

[BrianPetersen]

It isn't necessarily that "overlap is good for low end torque" but more that high overlap is an unavoidable consequence of using mechanical cam lobes with a fixed duration, and the bad side effects of that high overlap are less than the bad side effects of having a too-late intake valve closing and a too-early exhaust valve opening. Honda VTEC systems have VVT but they also switch over to a shorter-duration (and lower-lift) cam lobe at low revs in order to cut down on the excessive overlap in that speed and load range. BMW Valvetronic and Fiat MultiAir systems are capable of changing the intake duration period.

You will note that your VVT map moves towards less overlap at very low engine load ... because in that region, they have to start closing the throttle and having intake vacuum, which causes exhaust reversion, and then the bad side effect of high overlap is worse than late intake closure. All of this is why modern automotive engine designs have VVT!

 

[Lou Scannon]

Just curious, pickler, what is your measurement repeatability?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[FeX32]

So Valvetronic and Multiair do not need a VVT phaser?
Looks like toyota’s valvematic will always need a VVT phaser.

I wonder what the future will hold. Will we all go to something like Multiair or will we always be using VVT phasers?

 

[BrianPetersen]

Ohhh Valvetronic and Multiair (and VTEC) still need the VVT system. Valvetronic/MultiAir/VTEC are more or less used to play with the lift and duration (generally linked to each other - there is not independent control of these) and VVT plays with the timing.

Solenoid-actuated valve actuation - in order to be able to completely control the event timing without being stuck with a fixed duration - has always been the dream. It seems like Koenigsegg have built and tested an electropneumatic valve actuation system that might actually see practical use.

The other school of thought is that we're going to go entirely to electric propulsion, in which case a range-extender engine (if present) need not operate over such a wide range of speed and load conditions, in which case it has no need of varying anything, and can go back to plain ordinary camshafts.

 

[pickler]

@hemi i have been going to the dyno 2 times a week for the past month. So repeatability is high. So far I have come up with these tables, however it is still not fully done and yet to complete all combinations of cam and ignition timing and fuel ratios.

 

[FeX32]

Thanks Brian. VVT's are evolving quite fast. Many are not electric is seems.

I believe that by the time "free valve" type systems penetrate market in any substantial manner the all electrics will be there at the same time. It will be an interesting next 25 years in the automotive industry.

 

[utkuselamoglu]

you are retarding your intake cam too much and sudden. you should increase your exhaust timing a bit at higher rpm range.
I am sure you can achieve more power higher rpms!
you have advance your intake camshaft timing over oem parameters, and retarded the exhaust timings to achieve the similar torque curve of the engine and somehow get better results,



I think if the result is ok. keep it like that.