VVT Control Theory - Offsets and Cam Timing

[barge]

I've read through the manual and I don't see this laid out but maybe i'm missing it.

We have cam angle offsets to define the actual position of the cam trigger. This tells us where the cam is but what if this is incorrect.

If the cam is shifted a couple degrees due to poor quality control and we set the cam offset to this position couldn't that perpetuate the error across the control scheme?

The tables say that 0 in the cam tables is the fully relaxed position. What if I want one came to be -2 at "Zero" and the other to be +2? Can this be factored into the cam offset if you degree the cams manually?

I'm currently in process of doing a built subaru motor and plan on taking a lot of time with eccentric idlers and adjustable exhaust cam gears to degree the cam. However with stock ecus people say degreeing the intake cam is pointless because the ECU takes care of it. I'm not sure how true this really is... but i'm wondering if there is a set way to basically use the avcs to control individual bank cam timing. If so it may make more sense to go to a quad cam avcs head.

[Brad Burnett]

Ive tuned a large amount of AVCS subarus.  There is no point to degreeing the intake cam as it is variable and will move via the ecu, so its best to just adjust on the dyno as you tune the motor in.  Im pretty sure that the setup of doing the "Cam angle test" to find the correct offset value for each cam is what you are after.

I dont know what cams you are using to be concerned with "poor quality control" but I never ran into said issue with the majority of the aftermarket cams I ran in subarus.  IE Kelford, GSC, BC, etc.

You can degree the exhaust on the stand as you wish.

[barge]

4 hours ago, Brad Burnett said:

Ive tuned a large amount of AVCS subarus.  There is no point to degreeing the intake cam as it is variable and will move via the ecu, so its best to just adjust on the dyno as you tune the motor in.  Im pretty sure that the setup of doing the "Cam angle test" to find the correct offset value for each cam is what you are after.

I dont know what cams you are using to be concerned with "poor quality control" but I never ran into said issue with the majority of the aftermarket cams I ran in subarus.  IE Kelford, GSC, BC, etc.

You can degree the exhaust on the stand as you wish.

I guess my main question on the intake cams is how does "0" relate to the cam angle test value. If the cams are not both "0" at the same offsets (I know they won't be the same value but the same offset to TDC) does 0 put those cams at that offset value essentially or does it sync them to the motor at a "real" zero. Maybe the cam angle test is already taking care of this and I just am not understanding it.

I've read that even factory cams have been measured 2-5 degrees off spec.

[Adamw]

“Zero” is just the point where the cams are resting against the mechanical stop - ie all the way home.  The engine builder needs to check that this “zero” is relatively close to where the cam manufacturer intended it to be.  Subaru’s are especially difficult as they change so much between hot and cold, most builders that are familiar with them will factor this fudge factor in to the cam timing when building the cold engine.

[barge]

I guess that could make sense why people have measured cold motors stock and found significant error.

As a DIY engine builder for this project I don't suppose you could point me in the right direction as to this fudge factor.

I was planning on just ensuring the oil control gear/control inside the cam gear was pushed all the way to one edge and degreeing it from there.

Basically it looks like you're answering my question with... degreeing the cams would be beneficial to ensure the cams are in the correct position. If nothing else ensuring both sides are matched while cold seems like it could be beneficial.

[Brad Burnett]

@barge is this some wild engine build with high compression, high lift cams and large valves?

[Davidv]

3 hours ago, barge said:

stuff

Hey,

(Disclaimer - not a subaru engine just some general vvt stuff)

I ran a test where I would do a full throttle pull at "zero", advance the cam 5 degrees, then do this all the way through to 50deg. 

Using a MAF I logged the airflow readings of each cam angle to see which gave the highest amount of airflow through the rev range.

What I found was that there was nowhere at full throttle where absolute zero gave the best power. Most of the time through cruising conditions you're advancing the cam a bit too. 
Zero position of the cam (in my case at least) is essentially only used at idle. So both zero and the advance amount are fairly arbitrary, as in reality you're just setting it to wherever it wants to go.

On this graph, whatever produces the highest results up the page gives highest airflow into the engine. 
The colour scale is degrees cam advance. 


 

[Brad Burnett]

12 minutes ago, Davidv said:

All of DavidV stuff

Davidv is 100% correct in this.  

You will primarily only see zero advance at idle and the primary reason for this lack of oil pressure to move the cam.  

On subaru applications, you will also see zero advance at the top end of the rpm scale as it makes the most power on stock cams.  Once you go to aftermarket cams with more lift and duration, then you will find that you advance the cams to keep the power climbing with RPM.  

 

Aside:

Davidv, was this a motor that only had VVT on the inlet cam?  Was it a turbo motor?  

[Davidv]

Yeah only intake cam, and NA motor. 

[Brad Burnett]

cool .. Was trying to get ideas  for my turbo/rewire thats about to happen.  The idea of a maf sensor for this very purpose was on the table.  But id be concerned with maxing it out in the turbo application as im not going to be running any huge pipes.

[Davidv]

The Hitachi MAF sensor that's used for the R35 and similar can flow some pretty big numbers without topping out.

(Which makes sense as it makes some fairly decent HP out of the box) 

It's only really for part throttle and really low load that you need a smaller pipe for decent resolution, if you're just using it for observational purposes at full throttle then you can bang it in a big pipe with no issues.

But you can find pipe size that results in hp maximums easily from the R35 people.

EDIT: Although thinking about it, I think they might use two MAFs and two pipes on those? 

I'm just going through the motions of installing a dual VVT engine, will be interesting to have a play around with the exhaust side cam too.

[cj]

If you are only looking for relative changes in MAF, I'd assume it doesnt matter if the MAF is pretty incorrect or calibrated incorrectly (in absolute terms). In this case, just stick any old MAF that fits in the side of the intake pipe. The fact that its potentially only metering say ~25% of the air because its not in the middle of the flow wont matter because you can still see "better" or "worse" for the purposes of VVT tuning.

[Adamw]

15 hours ago, Brad Burnett said:

There is no point to degreeing the intake cam as it is variable and will move via the ecu, so its best to just adjust on the dyno as you tune the motor in.

 

The bit you are missing though is that a Subaru has cams on opposite sides of the engine, so making sure the "home position" is correct at build time is well worth while for the two two reasons below.  In contrast to say a cast iron block inline engine you can usually just rely on the factory home position being close enough (assuming the cam grinder got it right).

1. When the heads/block are decked (most subarus have had this done many times by now...), the cams move closer to the crank, the tensioner is only on one side of the engine and the cam belt is a fixed length, so the cams on one side both advance and the cams on the other side both retard.  Effectively doubling the normal error this type of machining causes.

2. The block is aluminium and the distance from one side to the other is much longer than a typical inline engine, this means thermal expansion causes even more of the same effect explained above.  

 

Its been a long time since I had this discussion with someone that knew but it was surprisingly large numbers from memory, something like the difference between cold and hot was the cams on one side advance 8degs and the cams on the other side retard 8 deg.  So your two intake cams even though they are showing the same "position" in the software are effectively opening/closing the valves 16degs differently to each other in relation to the crank. 

I believe the factory cams are indexed differently to account for the thermal expansion effect but I believe most aftermarket or reground cams arent and the factory obviously never considered the decking effect.

 

10 hours ago, barge said:

As a DIY engine builder for this project I don't suppose you could point me in the right direction as to this fudge factor.

You could probably do a quick test, turn on the cam angle test before a cold start, set RPM and ECT lockout low so it works right from start up.  Start the PC logger recording all parameters then start the engine and allow to warm up, log the whole warm up.  You can then look at "Cam Angle #1,2,3" to see how much the cam moves from hot to cold.  You will only be able to do one cam at a time but just do one on each side.  

[barge]

Thanks @Adamw that's the kind of information I'm looking for.

I'm now understanding what you're saying about shifting during warmup. Not do to oiling or anything but due to the actual expansion of the block.

I may be able to do some maths and get a good approximation of I could heat a block up and see how far the deck height moved.

@Brad Burnett No it's not a "crazy" build... 400hp, 9k rpm ej22t with D25 heads. Stock cams to start since my initial power levels aren't crazy. However since it's my first real engine build and I have the winter I'm trying to make it "perfect". I'll be balancing the rods, getting the Pistons all matched, doing all the little stuff that people don't normally do. Having my cams where they should be is big.

@Davidv Good idea on the MAF. I actually have a slot style MAF that I was going to use blowthrough before I went Link. Could be a useful tuning tool if nothing else if I slapped it in.... Or added some easily removable piece.