I did this in a couple of stages, as I got busy with other stuff, and didn't have time to finish it. I think it is all in order, but if I left off the end of a sentence here or there, let me know, I'll correct it and make it understandable
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very generally - read this - it's not a bad writeup
http://www.autozine.org/technical_school/engine/smooth1.htm- read it through a few times, it's more info than most people will absorb in one read through (myself included) .
you 'could' do it with 2 3 cyl cranks, if you could turn the crank in the rear engine around and mount it back to front. If you just rotate it, it'll still be a sequential firing order (of some sort). you need the rear to be a mirror image of the front crank, not just one spun around on it's natural rotational axis. of course that then means it's a lot of custom work to machine it so it will drive the camshaft etc, and have the right flanges to mate together.
And even then it'll still want to twist (sort of) becuase it's 2 separate blocks. The forces in an inline 6 only even out because the block is stiff enough for one, and that the crank pins are a mirror image from one to the other, so it always evens out end to end.
Ultimately you could do it if you really wanted to but the result would be heavier and bulkier than any existing inline 6 of the same capacity.
About the only semi-related real world engine of note would be the BRM v16 - a 1.5 litre v16 (which was actually a pair of 750cc v8s mated directly infront/behind the each other. It was made circa the early 50s. It made something approaching 600bhp (some have suggested more, I don't know for sure, but 600 is probably the max realistically) - it was supercharged and ran something over 60psi boost (using a 2 stage supercharger sourced from piston aircraft engines of the era).
It wasn't a great success (they were running it carburetted, and had significant issues with drivetrains, and also the very narrow all or nothing powerband it had. But it was devastatingly quick in a straight line. It should also be mentioned that it was made in a time where Britain was still heavily in debt, paying a massive amount out to cover wartime loans/expenditures (I think technically the last of it wasn't paid till somewhere around the year 2000 iirc) - and still with rationing in place for certain goods. The fact BRM managed to turn out any racer at all, let alone one with such power (albeit not without said issues) is actually fairly remarkable.
Anyhoo it worked, but it was far from optimal, and I personally wouldn't be surprised if it was designed as 2 v8s because that was all they could manage to make size wise with regard to casting heads and blocks etc. It would be (from that standpoint) easier with shorter heads and blocks, easier to cast, less thermal expansion, less length in the heads to help avoid cracking or warping etc.
During the 60s (possibly earlier) there have been cases of people running twin engined drag cars. Obviously things went well beyond the limits of tyres (even with modern technology, tyres are the 'true' limiting factor in top fuel at the moment, so much so that they shortened the top fuel races to what - 1000ft or something, as the tyres were coming apart at the higher terminal speeds if they ran the full 1/4 mile - even with new tyres for each and every 1/4 mile pass!) But anyway - back in the 60s they did try twin engined dragsters. Current top fuel motors would probably make 2-3 times more power total than the combined total of those 1960s era dragsters (and they were mostly experimental stuff, afaik not actually competing for nhra titles or anything, but I will happily stand corrected).
Similarly - in tractor pulling, the highest categories (overseas at least for sure, I must confess I haven't followed it in Australia) often feature multiple engines - multiple large capacity supercharged monsters of engines. but that's just because they are literally chasing power that it would require multiple existing/available engines to produce. When it comes to streeters (or even most circuit racers) we (*luckily) don't have to re-invent the wheel - there's just a truckload of options out there.
Whereas this theoretical buildup being discussed - we aren't hampered by rationing, and better yet, there's no shortage of existing engines in a v6 configuration that will do the job.
As mentioned inline 4s have inherent secondary issues with harmonics/balacing (just like flat cranked v8s) - which is usually the reason you don't see too many inline 4s with really big capacity. Yes there's a few out there going above 2.5 litres, but most tend to be 2.0 litres or less in capacity.
Whilst it has more than its fair share of issues, if we were having this discussion 20 years ago or more, I'd maybe make mention of the mitsubishi sigma 2.6 litre 4. Yep, lots of problems, but even a somewhat mild NA combination, you'd have more torque than practically any street turbo'd a series engine, and probably similar horsepower. We aren't limited to having to be NA on the street, and boosted combos certainly are teh answer for most of us. I'm reminded of the group B rally cars. Some were approx 2 litres, and boosted. The audi quattros were what? 2.5 litre 5 cylinder I think ? IIRC rover had a crack at it, but ran a non turbo 3 litre v6. And whilst it went ok for what it was, it wasn't even remotely as quick as the turbocharged opponents from peugeot, lancia, audi and whoever else.
Point of all this is that whilst interesting discussions can certainly be had on this topic, ultimately the 'tried and true' approaches - either more cubic inches (esp if it doesn't come with extra weight) or boost on well established reliable engine families/models will always get the job done, and without all the headaches of totally blank slate one off conversions.
I note that some people only want to run a nissan sourced engine (even if it was a modified one like a twin 3 cyl setup as being discussed in theory here) in their datsuns. To each their own, and I certainly respect anyone who has a passion for cars - if they are footing the bill or doing the work, then nobody else can knock that, and if they only want a nissan powerplant, nobody can criticise that either.
I guess I'm a little 'off to the fringe' - though I don't think I'm alone here. Having 'grown up' on a lot of US and Aus derived 'musclecars'. Every man and his dog has put the ford 9" diff into various cars (esp holdens, because the holden banjo is weaker than the current economy in Greece, and the so called 'stronger' holden salisbury unit is only about 5% stronger than that!. On toranas it was very common to run the toyota celica (and as they became available, later the supra) gearboxes. The aussie m21 4 speed isn't too strong, but the box it replaced was the 'opel' 4 speed, which would likely struggle behind a stock a12, and didn't last behind holden 6s well at all. Heck I even had a holden motor with a ford 221 crank adapted into the block (a major undertaking, and the irony is that - turbocharged at least - it won't make any more power than a 202, and for full race stuff, the 202 will make the same power and last longer. turns out the 'only' situation where the stroker crank setup will snot the 202 based one - on a streeter where you aren't revving the ring out of it. With a more 'reasonable' cam and so forth, you have your cake and eat it too, more mid range than a 202 by some margin, still good top end (just not full race top end power levels/rpm ranges). To be honest, it's definitely the experience I had building the stroker cranked motors (had a lot of trouble with early ones, there's some shonks in the industry that cater specifically to making these cranks - some of them are total professionals, but one (in particular) is a rat and his machining work is very ordinary). I won't go into too much detail there, but suffice to say, whilst I'm not against using parts from any other manufacturer (I guess if I was the chief designer at holden I might have a stake in that, but I'm not, so I don't see it as a personal thing whether I run this or that brand) - but anyway - using other parts or motors - fine. just don't end up spending years, and a small fortune on doing such things, esp if there are cheaper, 100% reliable and effective options out there.
Maybe a case in point is better gearboxes for the a15s or boosted a series motors. Obviously the toyota t50, with removable bellhousing, and adapter bellhousing from dellow is workable. It's a reasonably strong box, but they are getting harder to find in good condition, and no longer really cheap. IMO the 71b boxes are a fair bit stronger. Yes, it's certainly more work to mate one up - some lathe and milling machine time, and some tig welding. but it is a _reasonable_ amount of time/effort/money - it won't break the bank. And the 71b boxes (or a 71c if you come across one) are pretty affordably priced.
Another example might be the h165 diffs. They are stronger than the 145s and not too hard to find (though I do seem to be having no luck finding a 3.7:1 165 gearset!). The only thing I wish was different on them was the outer axle/wheel bearings. I wish they were tapered, but it's not so 'desperate' that I want to make custom axle tube end flanges, weld them on and adapt tapered bearings from some other source (or make custom axles and run bw 78 bearings, mated to the h165 housing/tubes), much less a full diff conversion.
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