The Audi 4.2L 40V Chain Driven BHF / BBK Engine
The Audi 4.2L 40V BHF / BBK Engine
A summary of the 4.2 40V chain driven engine to help the community. This was written by Alex Musskopf @ AMTuned on 3/6/2023.
This particular engine is a five valve per cylinder (3 intake, 2 exhaust), 8 cylinder power plant. The most common bore size on this engine is 84.51mm which is found in over 99% of these engines. In the nearly 50 BHF 40v engines AMTuned has successfully rebuilt for customers as of 3/6/23 (not including our inhouse projects), we have only seen one 84.61mm factory bore size. Audi claims that some were also built with a 84.71mm factory bore. The larger, rarer bore sizes were re-machined during assembly when technicians found inconsistencies during the build process. They corrected this by machining a larger bore and using larger bore pistons appropriately allocated for that size. An interesting thing that was not changed when bores were enlarged were the ring packs, and this is why the Audi allows such a large acceptable ring gap (0.25-0.40mm top / 0.20 – 0.40mm second) with a wear limit of 0.80mm. We at AMTuned sell aftermarket ring packs at an attractive price that drop in within Audi’s specifications. We have accepted the fact that these engines can be rebuilt once with new ring packs utilizing the stock pistons while staying within a reasonable piston to wall clearance. Once an engine surpasses a decent life cycle (about 150-200k miles), rebuilding on the stock bores is not recommended. After this amount of run-time, we see cylinder bores too far out of spec to justify a rebuild as seating new rings may be prolonged or never actually fully happen due to an oval bore. Customers wanting a rebuild on out of spec bores will be recommended to go with our sleeving option which can be added to any spec stage rebuild we offer. As run time gets deeper into a engine’s life cycle, the thrust and anti-thrust (The Y-Axis when standing parallel to the crankshaft) will wear more than the X-Axis. Our rebuilds all get measured as part of our engine inspections. AMTuned will fail an engine inspection once a bore surpasses 0.0010” variance within a single bore. We take 6 measurements on each bore (3 on the Y axis, and 3 on the X axis) which include the top, middle, and bottom piston stroke placement. Our rebuilds utilize a final surface preparation for proper bore oiling in order for the new rings to seat. This is explained further in the next paragraph.
The cylinder block, Guide frame, and Cylinder heads are made of a Hypereutectic material called Alusil. This is a combination of roughly 17% Silicon with the remainder made up of mostly Aluminum. The bores themselves are linerless and utilize the same block material. Instead of a conventional crosshatch hone which iron based liners use, this Alusil bore has a different finishing process. During production, the final bore preparation involves a combination of polishing stones to achieve the proper surface roughness. Traditional honing stones cannot be used as they will produce a jagged cut on the embedded silicon. The silicon needs to have a smooth rounded exposure for proper ring seating. After the polishing stones, a final hone utilizing a felt based hone and special paste with suspended silicon is used. This paste and felt based hone is used to cut back the aluminum within a bore and further expose the silicon. This void between the silicon and aluminum is now the oiling channels of the bore to lubricate the piston sides and piston rings. On a traditional iron block, the crosshatch finish is what is utilized as the oiling channels. When re-ringing an engine and re-using the factory pistons, we do not nor do we recommend to re-do the polishing steps of the bores as too much material will be removed resulting in piston to wall clearances being too far out of spec and excessive oil blow by. Only the final felt process should be used to re-expose the silicon for proper ring seating. If aftermarket pistons are used, the entire Alusil bore finishing process can be used with appropriately sized pistons. Seating new piston rings on linerless Alusil bores requires much longer run time than a traditional iron bore. Because the silicon is much harder than the piston rings, it is mostly only the piston rings that will wear to create the optimal seal as opposed to a traditional iron bore where the rings and bore both wear. Our rebuilds have a 1k mile break in period with 3 oil change and driving habit milestones to be reached before changing to full synthetic oil. Even after switching to full synthetic, the engine will take thousands of miles to fully bed in the piston rings and achieve optimal compression.
The AMTuned break in schedule that goes out with all our rebuilt Alusil stock piston engines is as follows:
- Use Motul Mineral Break In 10w-40 Oil. First fill be over 11qts after filling oil galleys when priming engine. 0-25 Miles with keeping rev's below 4krpm, 50% engine load or less, city driving, utilize engine breaking.
- Use Motul Mineral Break In 10w-40 Oil (9.5qts) & new filter. 26-100 miles with keeping rev's below 6krpm, 75% engine load or less, mostly city driving, utilize engine breaking.
- Use semi-synthetic oil within the 5w-30 and 10w-40 range (9.5qts) & new filter. 101-1000 miles. sporadically rev vehicle to full rpm range while driving. 100% engine load allowed sporadically (don't attempt a 1/4 run). Normal road use. Utilize engine breaking when possible.
- Use full synthetic oil 5w-40 (9.5qts) & new filter. 1001+ miles. Drive vehicle as you wish. perform the first two oil changes at less than 3k miles. If forced induction we recommend a 2500 mile or less interval.
This engine is pretty stout from the factory as it can nearly achieve a 100% power gain with forced induction options before needing some upgrades. We have monitored over time that a safe maximum operating range on a stock longblock should be around 425whp/400wtq~ on a supercharged engine, and 475whp/450wtq~ on a turbocharged engine. The difference in numbers is due to the belt drive nature of superchargers and the slight strain addition to create the same amount of power when comparing to a exhaust gas driven turbocharger. We try to stay away from giving exact numbers as these are based on dynamometers that can have a rather large discrepancy depending on the style of the machine and how it is calibrated. For those looking to surpass these power figures with forced induction we have found three areas in which the engine should be upgraded to safely handle the additional stress. The connecting rods are the first area needing attention. Factory connecting rods for this engine are made through a cast powdered process with a sintered finish as opposed to some other forged variants in the Audi 4.2L lineup. Because of this alloy, almost all power related stock engine failures involve the connecting rods.
The second area addressed should be the piston rings. The stock piston ring material is perfectly acceptable for heightened power levels, but the ring gap’s do need to be enlarged in order to accept the additional heat that they will endure. Piston ring gap’s should stay to the smallest allowable gap for the expected heat that the engine will be producing. Many people will confuse ring gap’s heat requirement with maximum desired “boost” or “cylinder pressure” which is not the case. Piston rings have two main goals in an engine. The primary goal is to seal as much combustion pressure within the combustion chamber as possible. Their second goal is to distribute heat from the piston back into the water cooled engine block as a heat bridge. As the piston rings accept heat to transfer into the cylinder block, they expand like most other alloys. The gap on a piston ring is exactly there for this reason, it is there to accept the expansion of the ring due to heat. The primary (combustion sealing) and secondary (heat transfer) goals of the piston rings poses a fine dance of calculations to achieve a successful combination. If the ring gap’s are too large, it will affect engine compression and blow by for the engine. If ring gap’s are too small, the piston rings will expand until the ends touch. When this happens, a runaway effect is achieved to where the piston rings keep expanding outwards when there is no more gap, thus creating more heat from heightened wall pressure, thus creating more expansion in the piston rings until something gives way. In the case of this engine, it usually involves a cracked ringland and catastrophic engine failure.
The third area addressed should be the cylinder head valves. The factory valves are a thin-walled sodium filled valve. They are actually great at heat dissipation for their intended use on this engine. The problem with the factory valves is once valve guide clearances fall too far out of spec, they succumb to heat similarly to how piston rings were described in the paragraph above. Valve guides have a main goal of removing heat from a valve and distributing that heat into the liquid cooled cylinder head apart from keeping a valve aligned to its seat. Once the clearances get too far out of range, the valve guide ceases to properly remove the correct amount of heat away from the valve. Due to the thin walled nature of the valve, when they are superheated due to excessive heat, the stem becomes more pliable and failure rates increase drastically even in factory power level vehicles. The amount of time for this to happen on this platform is not consistent. We have monitored valve failures in the 70k mile range, and also see factory engines with over 250k miles on the stock valves. Because of this inconsistency, we have developed our in-house solid core valves which are utilized on our stage 0-1+ rebuilds. We also offer our stage 2 engine rebuilds which are a fully sleeved, forged, studded, and Supertech cylinder head component build for those looking for something spicy.The chain driven timing system! This is the area which gave this platform a sour taste. The timing system on this engine is a bit complex when compared to different systems. It consists of four timing chains, four tensioners, eight separate guides, mechanical adjusting sprockets with internal cams driven by oil pressure, electronic solenoids to drive those sprocket mechanisms and various sprockets and bearings. There are three main failure points on these timing systems with the most common by far being the left main chain guide. The factory plastic main chain guide was construction of injection molded plastic. This material has been used successfully in many timing systems throughout many different manufacturers. Unfortunately, on this platform that is not the case. Some blame it on multi-injection site molding, others on too much chain pressure. When this guide fails, it adds additional stress onto the upper main chain guide which at that point also becomes failure prone. The second main failure point is the mechanical sprocket assemblies. This assembly utilizes a pin which locks into a pin bore on the sprocket upon vehicle startup when the rotating assembly starts moving. There is a small period of time on startup where the ECU will allow the system to build appropriate oil pressure before activating the electronic solenoids which allow oil pressure to unlock the pins from the sprockets. This period of time is also crucial for the engine to confirm a camshaft position base figure so it can adjust accordingly. The issue here is that the bores on the sprockets wallow out over time and eventually allow the locking pin to escape its locked position resulting In the engine having a bad timing correlation and being unable to adjust variable intake cam timing. We remedy this by re-machining a new pin bore into the factory sprocket and allowing the sprocket to achieve a new life cycle. We recommend that this service be performed every 100k miles in order to ensure reliability. It can be done without removing the engine from the vehicle. The third main area of failure is the electronic solenoids which control the mechanical sprocket mechanisms. Over time and as this platform ages, we are seeing more and more failures of all three main areas. Our AMTuned Ultimate Timing Kit addresses all of these failure prone areas as well as all the other less common failures that are witnessed