Jump to content
Click here if you are having website access problems ×

Why You Should Use 10W40 Semi Synthetic in a VVC


revilla

Recommended Posts

I've often weighed in on conversations about the correct oil grade for a VVC in the past.

I have always asserted that it was important to use 10W40 Semi Synthetic on a VVC (and not 5W50 as other Ks seem to like) becasue the ECU held a calibration for the response speed of the VVC mechanisms vs. temperature (effectively driven by the profile of oil viscosity vs. temperature, which is exactly what the oil grade tells you) and that the use of an oil grade which did not match this calibration lead to increased wear.

I didn't have the evidence to back this assertion; now I do...

Mech.thumb.png.2f36a7f4d3b784bed02ddb0f3bebbf9a.png

This is the table correlating VVC mechanism response time to temperature.

I set about proving this by making changes to the VVC advance curve and to this table and watching the results on an oscilloscope.

Firstly I modified the VVC advance curve as shown below:

VVCAdvanceStep_0.thumb.png.a67cb6d04f7ae74aba8fa05f4496d825.png

This just sets one VVC position below 2000rpm and one above; the idea being that as I blipped the revs from idle, the VVC mechanism would try to make one single fixed-size movement on the way up, and one in the other direction on the way down.

Then I watched the VVC mechanism increase and decrease solenoids on a dual channel oscilloscope as I revved the engine.

VVCTableNormal.thumb.png.02879e246bba91aefa18664dfa83e26f.png

The ECU seems to work this way; one long pulse to get the mechanisms moving followed by smaller top-up pulses to perfect the position. After each pulse it monitors the cam sensor position and compares the cam duration with the one set in the map, applying additional pulses as required. In this case, the initial pulse was about right, just one small additional pulse afterwards in the same direction.

Green line is the "increase" solenoid which is active as the revs increase through 2000rpm, yellow line is the "decrease" solenoid which is active as the recs decrease through 2000rpm and the VVC mechanisms return to their idle position.

Then I reduced the response times in the table and tried again. Below you can see what happened:

VVCTableReduced.thumb.png.2d0ec07f5f48659198e90c8c5904e436.png

The mechanisms were consistently undershooting on the increase and the ECU was providing additional pulses to try to correct them. It appears they hadn't reached their correct position by the time I let the revs fall again, so the first "decrease" pulse still got them back to the idle position.

Then I increased the response times in the table and tried again. Below you can see what happened:

VVCTableIncreased.thumb.png.01c58450005de0cdd489d031d721be60.png

This time the ECU applied the intial pulse and a puch wider "top-up" pulse, which overshot the mark so it applied a decrease pulse, which overshot the mark again so it applied an increase pulse etc. The mechanisms were hunting for the programmed positions, constantly shunting backwards and forwards.

This was the case when the actual response speed was faster than the ECU was led to expect from the calibration curve, which is the situation which would exist once warmed up if you ran it on 5W50 instead of 10W40, and this is why doing so wears the VVC mechanisms prematurely.

The actual response of the VVC mechanism look quite complex; it seems that fairly large changes in the programmed values only made a small difference to the pulse timing applied by the ECU, but the overall effects of those changes are in line with what I expected and backs up my insistence on the point that 10W40 is correct for a VVC (unless you have the time and resources to do the research to remap this table - which you could probably make a good stab at by cross-comparing the viscosity curves for different oil grades - but I'm sticking with standard on this).

Link to comment
Share on other sites

Well done, Andrew!  Yet another impeccable example of the quality of your work.  And it also demonstrates what clever people Rover must have been employed in their K team.

Just one thing:

When you say at the beginning:

...and that the use of an oil grade which did match this calibration...

Should that not read:

and that the use of an oil grade which did not match this calibration...  ?

(Unless I've misunderstood something -- which is entirely possible!)

JV

 

Link to comment
Share on other sites

I have been using 5W50 Comma Motorsport Fully Synthetic (recommended by Caterham also stamped on the engine) 

I asked the garage to replace it with 10W - 40 and was told 'I need to stop reading forums' & 'they have been using 5W50 in all race elises with no issues.

No matter where I look the information is always different. 

How much of a difference between the two will this really make?

Good post and info though Andy.

Link to comment
Share on other sites

I'm fairly sure that semi synthetic 10W40 was specified by MG Rover for the VVC but I can't seem to find a downloadable Owner's Manual to confirm that at the moment.

The story I heard was that fully synthetic oil was somehow "too slippery" for the sliding block mechanisms in the VVC drive units and tended to lead to excessive wear in them. I don't know if this is correct; what I do know is that when I once tried a fully synthetic the mechanisms sounded a lot more rattly and when I switched back to semi synthetic they sounded a lot happier. Tiny amounts of play here lead to appreciable backlash in driving the cams, which leads to rattling and more wear.

On track engine it may make less difference than on a road engine. The VVC mechanisms are fully extended by 6000rpm so if you spend most of your time above that they won't be moving as often as on a road car. With your foot planted, most of their movement happens in the 4000 to 6000rpm band, and under lower load  (75kPa MAP and below) it is spread out more across the range from 1300 to 6000rpm. They probably don't cover the same mileage either. So maybe it makes little difference and if you're tracking the car maybe a 5W50 will do you fine.

For a high mileage road car, which I think a lot of VVCs are, I would definitely stick with MG Rover's recommendation of 10W40 Semi Synthetic from what I've seen (as in this post) and heard (from my engine, not gossip).

Link to comment
Share on other sites

From what I gather, the increase in the number of pulses to correct the VVC mechs is due to you reducing the response time to replicate a more viscous oil used. This shows how the vvc mechs are calibrated to the oil temps related viscosity.

I think you would need to gauge if using a more viscous (50) oil has a similar effect to reducing the response time, the difference in the viscosity maybe marginal. 

The reduced response time may be the equivalent to filling your engine with treacle. 

It would be good to see the pulses generated using both 10w40 & 5w50 to show if it has any effect on the adjustments made by the ECU to show realistic effects.

 

 

Link to comment
Share on other sites

Well I'm due to do an oil change as part of the annual service any time now. If anyone wants to lend me a can of 5W50 I'll happily run it in my engine just for long enough to do the test and report the results. It may not be quite as clean looking when it comes back, but I can't imagine it would materially affected.

Link to comment
Share on other sites

  • Member

If anyone wants to lend me a can of 5W50 I'll happily run it in my engine just for long enough to do the test and report the results. It may not be quite as clean looking when it comes back, but I can't imagine it would materially affected.

You're thinking like an engineer! :-)

Buy some, run the test, drain it, then sell it with a signed certificate "Individually hand tested by Revilla". At a grossly inflated price.

Jonathan

PS: Please keep them coming.

Link to comment
Share on other sites

I've got a MG TF 160 with the same vvc engine. It ran lovely with Castrol magnatec semi 10w 40. I then serviced it last year but put in a different brand, still semi 10w 40, but the vvc was a lot chattier, especially from cold. I changed back to magnatec and went back to be a lot quieter, so I'd definitely recommend using that.
Link to comment
Share on other sites

That's what I use. Also tried the GM/Vauxhall 10W40 Semi Synth as it meets the specs, it's very reasonably priced and generally seemed to be well thought of on the interweb. My VVC mechs sound equally happy on both, but it's remarkable how sensitive they are to different oils. You can definitely hear them clattering on fully synthetic when you're used to how they sound on Magnatec.
Link to comment
Share on other sites

This makes interesting reading. I am using Millers 10-60 recommend by Dave Gemzoe based on his k Series research. Chris at Retro Sports cars races a VVC engined Rover and he uses the same. As I tend to cain mine quite hard I thought I'd play safe and go with their recommendations . But the VVC is chattery at low speed , well within normal levels I am told.

 

The downside of the oil it takes some warming and its bloody expensive especially as with the Apollo its 7ltrs

Link to comment
Share on other sites

  • 2 weeks later...

Thank you, Mr Revilla, sir, for this post.  I am about to do the oil change soon and I will follow your advice.

I've got some Fuchs Titan 10w40 Semi-synthetic and some Shell Helix HX7 10w40 Semi-synthetic on the shelf.  Will either of those do, or should I go and buy some Magnatec?

Incidentally, I was thinking about changing the gearbox oil.  The car has the 5-speed Ford T9 (?) gearbox.  I called Opie Oils today and they recommended using ATF.  This surprised me, so I thought I'd check before purchasing.  (The car's done 30k miles and it'll soon be 18 years old - so I thought I'd treat it to a gearbox oil change.)

Link to comment
Share on other sites

Had my type 9 gearbox rebuilt by road & race in Kent, they specified EP 75 or EP 80 as otherwise the it may be  difficult to smoothly select gears. It was some time ago, so may be worth contacting them to check,if you haven't already changed it. Apparently I had the wrong oil in mine when it was stripped down

Link to comment
Share on other sites

If wear is being investigated would it not be easier and more accurate to test used oil and possibly filters for contamination?

The overshooting is probably attenuated by 10/40 being slightly thicker.

My pseudo science brain says that 10/40 is very roughly speaking similar to a 7.5/50 grade oil and that 5/50 is roughly similar to 6/40. Not an accurate calculation but enough for me to think that the grades are at least closer than they first appear.

My guess on the semi vs full debate is that when the engines were first developed that full was a bit exotic for most people. There is not much point in specifying a product that is not easy to buy, Even now 10/40 full synthetic is hard to find. I believe the main difference is that full can take more abuse before it breaks down rather than having an significant lubrication benefit.

I have used both and I didn't notice an advantage in one over the other. I did think for a while that 5/50 made the engine more responsive but after switching between the two a few times I think it more likely that my imagination got the better of me.

Link to comment
Share on other sites

A 10/40 at 50C will be runnier than at 40C.

A 5/50 at 40C will be gloopier than at 50C.

I am not saying my numbers are accurate it just makes it a bit easier to get my head around what the numbers mean. It would be nice to know what the difference at normal running temperatures (e.g. 80-110C) as the difference could widen or narrow.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...