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Running in
Most people think that running in an engine should be a gentle exercise that smooths down all the rough spots and beds everything in. Sounds reasonable, but evidence and logic say otherwise.
In fact there are no rough spots in a modern engine, and the only things needing to be bedded in (camshaft and bore/rings) need power and revs to do the job properly.
You don't have to go overboard and try and melt something - just good bursts of full power and good over-run, keep the revs above 2,000rpm to be kind to the cam, and you're looking at up to 10% power increase over stock, and cleaner oil.
New engine parts
These are the only parts in engines that move or contact moving parts:
- Ball races
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Seals
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Belts and chains
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Cylinder bores and piston rings
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Plain bearings
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Camshafts
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Brakes
Ball races
These are factory polished and do not need running in. They simply roll round a polished track. Look up some bearing manufacturers websites and bearing specifications to learn this for yourself.
New races are smooth and greased - they will never be in better shape.
Seals
New seals are smooth and greased - they will never be in better shape.
Belts and Chains
Rubber does not need running in, chains may bed in sometimes but as the loading is generally independent of engine rpm and power running in methods cannot affect these.
Cylinder bores and piston rings
A good ring to bore seal is essential for power, economy and reliability. Modern engines + modern running in techniques give a bad seal, which means less power and a shorter engine life. Think of the seal on a ground glass cork - or some ground steel - a ground surface is much more accurate than a polished or burnished finish. Valve seats are ground in because they have to seal perfectly, and piston rings are just the same - they need grinding in.
Effective piston ring grinding only occurs in the first 20 miles of a new ring/bore/honing set - so after a re-bore and when the engine is new. The new honing pattern allows the rings to be ground to perfectly fit the bore. If you run in gently, they merely smooth out the tight spots and that stops the loose spots from ever closing up properly, so you get a permanent bad seal. For the first 20 miles you need high cylinder pressure (i.e. hard acceleration below 3,000rpm) to push the rings out into the new honing pattern, to grind out the perfect cylinder bore fit. After you've done that, change the oil.
Plain bearings
Unlike rings and bores, these are non-contact parts. This simply means that the metal crank-journals and big-end bearings never cause any metal to metal contact. At All. When that oil film fails the bearings are instantly toast - you need roller bearings to allow that, like are used on two stroke outboard marine engines.
To see how little contact occurs yourself, try looking at some old bearing shells from an old, well used engine. This is a picture of one which is worn out - you can see the copper (The layers are steel on the outside, copper in the middle and a coating of babbit (soft alloy, .0005" - .0008" thick).
In fact new shell bearings are made to a fantastic accuracy and are the right size and finish when brand new. In fact - they will never be better than when new. Old bearing shells may look less 'matte' or less dull than brand new ones, but if you wipe a new bearing with tissue paper you'll see it suddenly gets shiny and matches the old one. That's just how soft the micro-thin babbit metal layer is on these bearings, if there was contact and the metal had to transfer power they would melt.
Plain bearings all ride on an oil film (hydrodynamic lubrication) and there is no metal to metal contact during operation. Without adequate oil flow to the bearing, the oil film breaks down (synthetic is a little stronger) and contact is made between the journal and the bearing.
In a typical 100BHP 4 cylinder engine there is 25HP trying to get through each big end bearing, so frictional forces quickly turn into heat and melt the bearing babbit until the bearing shell starts to extrude out the side of the connecting rod. Excessive clearance causes loss of oil pressure and bearing pounding on each other therefore quickly accelerating wear until the rod fails at which point it can exit stage left through your crankcase.
Plain bearings are never run in - they are exactly the right size and finish from the factory.
Camshafts
Camshafts need oil films too. They also need to work harden without being scraped away, and the cam heads are the things that suffer here. Idling is the most stressful operation for a camshaft. The oil supply pressure is low and the oil film is both stretched and under enormous strain to protect the slowly moving metal surfaces from each other.
To bed in a camshaft properly you need to keep the revs above 2,000 rpm for the first 20 minutes of operation. This is for the camshaft to prevent wearing the cam tops off. The slower the engine, the higher the camshaft load. Be gentle to your new camshafts by keeping the revs up while the lobe tips work harden.
Brakes
New brakes will need bedding in - however new brakes also have two major advantages over old brakes.
- Smooth flat surfaces
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New brake fluid
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No dust in any vent holes
This means that bedding in brand new brakes (new disc/pad sets) is much much quicker than when you put new pads into a used disc - because both surfaces start off flat. So don't be too hard on them - as they are your primary safety system, but I do not think you'll have to be too gentle either.
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Running it in properly - drive it like a rental car
Use mineral oil only for this (if your brand new car has synthetic, change it), and then after 20-30 miles change the oil, as it is now the most full of bits of metal that it may ever be - and the filter will NOT get it all. Keep your power - change the oil. Those little bits of aluminium will burnish the steel bearing surfaces and narrow the oil bearing gaps, creating more friction as the oil works harder (same shear speeds over a thinner film).
So the sequence is this:
- Ensure there is mineral oil in engine. Synthetic is too slippery, and prevents the bores from bedding in.
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Keep the revs above 2,000 rpm for the first 20 minutes. This is for the camshaft to prevent wearing the cam tops off. In fact you are work-hardening the tips now (at speed, with an oil film). The slower the engine, the higher the camshaft load and you just end up grinding the tops off because the oil film fails.
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Drive with hard acceleration and decelleration (engine braking) and varying revs between 2-4,000 rpm for 20 miles.
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Change the oil and filter to new mineral oil after 20 miles. Flush again if the oil contains too much metal.
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At 1500 miles of gentle running change oil (to synthetic).
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Enjoy a more powerful, reliable and long lasting engine than anyone else (2-10% measured)
The major caveat - and I believe the major reason for the gentle 'proper' run-in schedule is that you are more likely to smash up a new, unfamiliar car than an old one, especially when you'll have more power and efficiency (better ring seal) than anyone else too - so be careful out there!!
Engine wear - how and when it occurs
Dirt and contamination with anti-freeze and/or petrol is a fast way to wear an engine, belts and chains wear over time, chains and all bearings being mainly suseptible to dirt.
Bores and piston rings
Most engine bore wear however is not the result of mechanical wear at all however, but of nitric acid etching away the metal. The first 30seconds run time of a cold engine allows the cold bores to condensate combustion products onto the bores, and ideally a 30 second idle should be allowed the car. The more load, the more combustion products, the more acid, the more etching. So when you start up, let it idle to bring up the cylinder bore walls to a high enough temperature to prevent condensation and to allow the oil films to become established.
Bearing shells
The only wear you will ever see on these shells is caused by leaving the car switched off for long periods, so the crank's weight displaces the oil film and the you then get a scrape on start-up. Soft starts are the key here - no sudden revving, and use synthetic oil to have a higher shear strength in thin oil films.
For the evidence, photos, theory etc - look no further than Motoman's race engine tuning site
Basically (and please try it for yourself) you'll do more damage with tissue paper rubbing a plain bearing than almost any in-engine use - providing your oil pressure is there.
Ring/piston wear, oil contamination
So how does this affect running in? Well, the more ring blow-by you have in an engine, the more acid you end up with in the sump, which damages the oil, starts eating your engine, and encourages sludge. Once the oil additive pack is exhausted in it's battle with the oil you start to risk oil failure, or the 'black death', where the oil gels up and the engine therefore fails.
Useful references
MotoTune USA's Power Magazine
This is a useful resource for the engine builder - don't forget to read the bit about oiling the cap bolt threads.. it also explains how the oil film supports the load.
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