Power, economy, oil...

[johnl]

So the engine was overdue for an oil change. I hadn't been too concerned since not all the oil in there was past due date, due to topping up over time, and the car never does short journeys (where the oil doesn't reach full temerature for long enough or at all, the worst thing for oil longevity). A significant quantity, maybe half, was 'fresher' than it would be if the engine didn't use oil (like my old Accord, or our Saab, which will do 15,000km with virtually no lowering of the oil level). So some days ago guilt, and a vague feeling of impending doom prompted me to do something about this, and I've changed the oil.

Anyway, the old oil was 10w/40 (Penrite 'full synthetic'), with the top ups being 20w/60. Despite the topping up with thicker oil, when I drained the old oil (hot) it seemed very thin and runny. New oil is Penrite 20w/60 mineral oil, though I didn't have enough needing to top up with about another litre (the total amount needed was nearer 5 litres than the quoted 4.4 litres). The 1 litre needed to top up was what I had on hand, i.e. Penrite 10w/40 'full synthetic' (what I use in the Saab), so the oil fill will be a bit less than the full thickness of the 20w/60, but still fairly viscous as engine oil goes.

The immediate result was a total lack of variator rattle on start ups, and the engine is now generally quieter and feels smoother than it was (I suspect a lessening of 'piston slap'). The second thing I noticed was that the engine now feels significantly more powerful at lower rpm. However, it now seems somewhat less 'willing' to rev so freely as before, and feels to have a bit less power at higher rpm. This might possibly be due to feeling more powerful at lower rpm rather than an actual decrease in higher rpm power...

The third thing, fuel economy has improved markedly. Previously it was consistantly sitting on 8.4L/100km (displayed), but after a 'reset' (from cold) it fell to 7.9L/100km with my usual driving pattern. I did some highway driving yesterday (about 100km give or take), mixed in with some urban driving, and the readout is currently at 7.2L/100km. This is way better than it has ever been, i.e. previously it would take a great care and a featherweight foot (or a very long run on the highway) to achieve a readout near to or below 8L/100km (and then only barely below it). Now it's easy to get below 8L/100km.

My speculation re the apparent change in power characteristics is that the thicker oil has improved ring seal, and increased oil drag, simultaneously. This (increase in power) is more apparent at lower rpm because at lower rpm there is more time for compressed gasses to leak past the rings (on each compression and power stroke), so conversely if the rings are leaky there may be significantly less compression at lower rpm resulting in a power loss. At higher rpm there is less time for gasses to leak past the less than perfect rings, so with leaky rings there may be less power loss at higher rpm. However, with thicker oil there is more oil drag, so at higher rpm you may not gain much extra power from better better ring seal, but you may lose significant power from oil drag...?

Better ring seal giving better compression should have an affect on economy. Another thing that might occur with poorly sealing rings is a lessening of oil burning, resulting in reduced or eliminated detonation (in cylinder oil contamination of the inducted air / fuel can result in detonation). The ECU should detect this (if it is occurring with leaky rings), and pull back the ignition timing, which would result in power loss and worsened economy...

Regards,
John.

[V AR 164]

I'm not 100% sure on the science behind heavy and light weight engine oils and how it effects the running of an engine, but I too can speak from personal experience.

I used to run Castrol magnatec 10w-40 in my 164, to which I noticed it would burn a fair bit of oil every few thousand kms or so. The last oil change I switched to full synthetic penrite hpr-15 15w-60.

Immediately I noticed a drastic reduction in 'lifter tick' and a much more smooth running engine. I haven't noticed any power gains/losses but the oil consumption has pretty much halved.

Goes to show that oil really does matter and can play a big part in the engines reliability and durability.

Andrew.

[Citroënbender]

Mine feels slower when there are reds and blues behind it! 

Overall though, I also reckon it's the Placido Effect.

[Alfapride]

I had a similar experience too recently with my 147 twin spark, i've been using 5w-40 for about a year, i moved back to 10w-50 in the last oil change and consumption has dropped and the variator noise on start up is gone. Fuel consumption seems to be the same - i've travelled about 1000km since the change, next change i might go to 10-60 to see how that goes.

[poohbah]

A litre of oil for every tank full of fuel?

Crikey.

[johnl]

The placebo effect
My car always performs better after I polish it

My car also performs slightly better when it's clean...

But, this improved performance and economy with new and thicker oil is definitely not placebo, the effects are too substantial to be mistaken...

Regards,
John.

[johnl]

A litre of oil for every tank full of fuel?

Crikey.

My thought too...

Regards,
John.

[johnl]

I'm not 100% sure on the science behind heavy and light weight engine oils and how it effects the running of an engine, but I too can speak from personal experience.

Accepted wisdom is that the thinner the oil the better the fuel economy. This is due to reduced 'pumping losses', i.e. thinner oil requires less energy to pump it through the system. With pumped oil, and more so the thicker it is, this is energy parasitically lost and so not available at the crank. This means that with thicker oil a somewhat greater throttle opening is needed to make X power at the crank, whatever is needed to overcome rolling resistance and push the car through the air. Which of course means some degree of lessened performance and increased fuel consumption.

However, this assumes that the rings are in very good condition and need no thicker oil than is required to adequately lubricate them. If the rings aren't all that great and tend to leak a bit, then thicker oil helps improve the sealing performance, so improving compression, so improving power at Y throttle opening, i.e. a lesser throttle opening will be needed to produce the X crank power needed to propel the car. A smaller throttle opening equates to lessened fuel consumption, potentially despite the increased pumping losses associated with the thicker oil.

Regards,
John.

[Citroënbender]

"Powered by Penrite" 

[poohbah]

Isn't that what Ferrari got pinged for doing deliberately earlier this F1 season?

[Vne165]

Related question for the guru's here, appreciate any thoughts.

I'm just about to box up a twin spark CF3 engine which had a dubious history before it came into my hands. It's now clear that the cylinder head was repaired after a cam belt failure, however the previous owner neglected to inspect/replace the big end bearings. The engine had just begun to wipe the #2 BE when I inherited it. Now ready for reassembly, after a crank grind, rod re-sizing, new rings/hone etc, I'm torn as to whether I should drill some oil drain holes in the oil control ring grooves. The flimsy 2mm solid lower rings must surely be overwhelmed trying to scrape the bore clean of thrown oil. Not a hard mod to do, and I think is low on the risk scale for a road car, I'm just wondering if it will mitigate the chance of the dreadful oil consumption I hear of.

Has anyone else contemplated or done this mod?
Thanks in advance
Vne   

[Citroënbender]

How will you possibly make it worse? 

The evolution of CF3 pistons in Eper is interesting reading, the last iteration in V84 (change note D7995) refers to "cava", implied respective to the rings grooves...

[bazzbazz]

Related question for the guru's here, appreciate any thoughts.

I'm just about to box up a twin spark CF3 engine which had a dubious history before it came into my hands. It's now clear that the cylinder head was repaired after a cam belt failure, however the previous owner neglected to inspect/replace the big end bearings. The engine had just begun to wipe the #2 BE when I inherited it. Now ready for reassembly, after a crank grind, rod re-sizing, new rings/hone etc, I'm torn as to whether I should drill some oil drain holes in the oil control ring grooves. The flimsy 2mm solid lower rings must surely be overwhelmed trying to scrape the bore clean of thrown oil. Not a hard mod to do, and I think is low on the risk scale for a road car, I'm just wondering if it will mitigate the chance of the dreadful oil consumption I hear of.

Has anyone else contemplated or done this mod?
Thanks in advance
Vne

After the rebuild you have described if you use 10w60 full synthetic you should have no oil usage issues.

Sportiva - I must be lucky, my JTS just needs a minor top off every month or so.

[johnl]

I'm torn as to whether I should drill some oil drain holes in the oil control ring grooves. The flimsy 2mm solid lower rings must surely be overwhelmed trying to scrape the bore clean of thrown oil. Not a hard mod to do, and I think is low on the risk scale for a road car, I'm just wondering if it will mitigate the chance of the dreadful oil consumption I hear of.

Has anyone else contemplated or done this mod?
Thanks in advance
Vne

Never drilled pistons, but FWIW I recall David Vizard writing in his famous seminal work 'Tuning the BMC A Series Engine' that it is beneficial and doesn't appreciably weaken the piston.

Apparently some BMC pistons had no holes under the oil control rings, and some had long slots (the worst ones also had a long tangential slot that started under the oil ring and extended at an angle all the way down to the bottom edge of the skirt...). The slotted pistons were very weak and so not suited to high rpm work, since the top of the piston had a tendency to separate from the skirt. The 'solid' pistons were much more robust since there was no slot to act as a stress riser, but suffered from some oil related issue the details of which I can't recall (probably failure to 'scrape' enough oil from the bore, resulting in the compression rings lifting off the bore on a too thick oil film). Drilling several small holes behind the oil ring apparently eliminated or at least reduced this issue, without causing piston failure.

I would imagine that you don't want to drill such holes too close together, for fear of weakening the upper part of the skirt. From memory the holes were drilled in the ring groove through to the inside of the piston on the thrust faces, not on the sides of the piston. I would take care to ensure that the edges of the drilled holes were carefully chamfered (i.e. remove the sharp edge around each end of the drilled hole). When I built a semi competition A series engine (in another lifetime) I used forged Mahle racing pistons, which from memory had such holes from the factory.

Drilling such holes should have no significant affect on relative piston weights. The amount of metal removed is minimal, and will be similar from piston to piston. When 'balancing' a set of pistons it's amazing how much metal can be needed to be removed from the heavier ones to equal the weight of the lightest, one gram of aluminium equates to a surprising quantity of metal.

This (weight difference piston to piston) might apply more to cast pistons than forged ones, and modern production tolerances likely mean less difference between pistons than in the good ol days.

Regards,
John.

[Vne165]

Citroënbender - Very interesting. Is it possible to obtain a copy of the change note somewhere, or is it simply just a notation in ePer?

Baz - Yes have purchased Penrite 10W-60 Synth already.

JohnL - Thanks, some food for thought there. I cut my teeth on A Series engines as a kid many years ago, so do remember those pistons you describe.

So the decision I have then is to spend the time to mount the pistons in the milling machine, drill a total of twenty four holes (three per thrust side/piston, de-burr and clean...). Or just whack em in. Will let you know what I do.

Cheers
Vne