Does having four main bearings in a straight six severely impact bottom end strength?

Hi folks, I've got an old Rover SD1 with the 130bhp Leyland PE166 2600 straight six engine, and if it's not completely idiotic to do so, I'd like to try and get some more power out of it to build a competent highway cruiser. One of the many odd design decisions in this engine was to use four main bearings rather than seven to keep rotating friction down. To be fair to the designers, they did this with the goal of getting good fuel economy and it worked really well, but I'm having a hard time finding any concrete data on the impact this has on bottom end strength.

For most owners or tuners this is the kind of question they'd just take to the owner's club forum or a Facebook group, but I'm aware of only two tuned PE166s, one Zakspeed turbo kit from when the engine was new which was (politely) useless, and one NA engine in a not-quite-replica racing car, which makes somewhere around 200bhp. This is an engine with as near as I can tell, zero tuning scene and aftermarket so I'm just trying to get some general info about other four-bearing sixes to see if I can map it onto this.

It seems obvious to my untrained brain that 4 bearings is worse than 7, but I'm quite lost as to how severe the strength loss is. Intuitively, I feel like the mains will handle the tensile forces as they're (very scientific, I know) pretty beefy, but my gut says the bending moments from the longer unsupported spans between mains could get nasty, and it feels like the potential for distortion-induced imbalance and vibration at high RPM is quite high. Does anyone know if this is generally how things go, or where I might research to test this hypothesis?

Obviously there are a lot of confounding factors so I don't necessarily expect anyone to be able to say with confidence "that engine is doomed" or "that will be fine", but if it's universally true that four bearing sixes can hardly handle any power at all, or if there are examples of them surviving moderate power levels, I'll have a decent idea whether it's worth pursuing this project at all.

I guess given I'm on a forum where people talk about building 1000bhp engines and beyond, it might be useful to clarify that I'm only looking for somewhere broadly around 300bhp; the intention is to have a fairly boring reliable daily that just makes enough power to not be annoying when overtaking on the motorway. The NA 200bhp engine I mentioned earlier had only a bigger camshaft, headers, and a conversion from twin SUs to triple DCOE Webers, so I suspect I can reach my target with similarly mild changes, a conversion to EFI, and a spot of boost from a fairly small turbo, something along the lines of a Holset 221 or if needed, maybe something just a tad larger as I believe the Holset isn't going to be very far off its flow limit at that power level.

TIA for any thoughts. Would be nice to build something with one of these oddball engines if possible rather than dropping in a Rover V8 or something similarly humdrum, so hopefully someone can recall an example of a few-bearing engine making decent numbers!

Not sure about the direct relevence, as there will be a lot of differences other than the 4 bearing crankshafts, but the Chevrolet "stovebolt" in-line 6 has a 4 bearing crank and has been used well over 500hp, and the sixties' Chrysler "slant six" 225 type engine was also a 4 bearing crank design and has had some stout power levels out of them. Oh, there're also the GMC 'truck' sixes.

There are a bunch of tuning guides for both, and while not directly transferable, should give you food for thought.

I expect there are many more but at least those have some tuning information.

Ah, Googling "PE166 2600 straight six engine" brings up some articles you should find useful, with several mentions of the strength of the bottom end and the forged crankshaft. They would also suggest you should find it relatively easy to exceed the 200 mark, and maybe a fair bit more, with cam' and head porting to go with EFI/throttle bodies and a well designed 6>2>1 exhaust.

The problem will be actually getting components and you may have to do a lot of your own work.

Yeah I've read plenty quite a bit about the engine and its history and its supposed design strengths, but separating engineering fact from what might be optimistic visions of enthusiastic fans of a brand can be tricky, so I kind of ignored a lot of it!

I will say that from cursory examination of a dead PE166 I ordered as a mockup engine to build the EFI and turbo system on a while ago, the bottom end seemed very heavy duty, being an EN16 forged crank with what I seem to remember as being very large journals and some overlap between the journal and big end bearings, which seems to be a useful property. If nothing else, it weighed a bloody ton. My fear was that all of this beef was needed just to make up the overall system strength lost as a result of the bearing shortage, though by the sound of it the engineering at play here might actually mean a fairly stout bottom end.

The pistons are also forged, from Mahle, and are fairly robust. Sourcing a stronger connecting rod is one place I expect to have to bend over and spend some money, but there is some promising discussion on the SD1 forum where an enterprising member has noticed that Datsun L28 rods are remarkably similar in some dimensions, and have forged versions available off the shelf. Whether those are adaptable is a question I dare not think about yet though!

Knowing there are some four bearing sixes around that make decent power is encouraging enough that I'm at least happy to carry on with this idea. I have a habit of having ideas and not actually following through on them though (which I insist is less because I'm lazy and more because I just have too many ideas to actually realistically work on most of them) so I'm gonna try not to get too excited about this just yet. Lots of more boring things to do first, like getting rid of my rear drums and getting vented rotors in the front and finishing the cooling system upgrades and finishing the full-custom race car I'm building with an old housemate...

I'm sure your 300bhp goal is achievable quite easily... 2600 cc engine should give you about 200+ bhp in NA mode providing that efficient head porting is done and proper camshaft is chosen combined with high compression ratio - that should be piece of cake for you to get it around 5500-6000 RPM. It means that in order to achive required 300 bhp you'll need only 0.5 bar of boost. Installing turbocharger will not require increasing compression ratio so you can keep it as is.

As far as bottom end strength goes it also should be quite easy. ARP 2000 bolts will secure it good enough as long as you won't overrev the engine (much past 7000 rpm) and make sure the crankshaft is balanced ( it would be critical for high revs reliability). There is another way to secure it even more but it's an overkill - you can ask a mechanical workshop to install the secondary bolts to main caps if there is enough room for two bolts per side of the cap. I got the same thing for our 2uz engine and installing 3uz caps with 2 bolts on each side.

Getting correct bearing clearances will be also very critical...

Yeah I wouldnt expect too much drama with the crankshaft at moderate power levels and RPM, provided it hasnt been worked hard all its life. The main disadvantage of a 4 main crank in a 6cyl is the counterweighting. You have the front and rear pairs of cylinders that have to share a counterweight - yet the pins are separated by 120deg, so the counterbalance effect is far from ideal, then for the two centre cylinders you will have a massive shared counterweight hanging off one side that is swinging around in fresh air with no journals to support it. So compared to a 6 main crank there will be a lot more torsional vibration in general and a lot more stress through the most stressed part of the crankshaft (junction between rear BE pin and rear counterweight). I do remember seeing a few cracked Triumph 2500 cranks in my previous life which I think were based on the same engine? but they were also probably from engines that had been raced all their lives too.