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In that case wouldn't you want a 60* total angle on the V12, if you're aiming to allow the exhaust from both banks to tied together...Otherwise the 120* angle with equal length headers would have two high pressure exhaust pulses trying to enter the intersection from both banks at the same time, which doesn't exactly seem desirable from an airflow management standpoint.

Not to nit-pick, which generally means that's exactly what I'm going to do, I'm also pretty sure the Slant Six was only a 30* offset angle from vertical...Not that it really matters, since either way a dry sump probably isn't a bad idea. Sounds like that's pretty well covered for now.

See, this is why I am tinkering around, to figure out the best compromise. Which is more important, combined exhaust or lower headroom? I feel the lower headroom would be more important, so may force a split exhaust. And you are right, the /6 was a 30 degree offset. But it still had a highly effective oil delivery system, as does the Subaru, so good examples to study regardless.

Ok, did a quick and dirty mockup of the V12 and realized it may be a hair too big for a Locost. The block came to 802mm long, although it only came to 420mm wide. With the head design, it should be easily narrow enough even for a book chassis, but I fear it may be too long for the job.

With a V12 you don't fit the engine to the car, you fit the car to the engine.

What diameter bore, and what bore spacing are you using for the basis of that mockup?

Same bore and stroke, each cylinder center is 129 mm off, so with an 80mm bore, you have 49mm for spacing. This is to allow for sufficient cooling, and to leave the possibility for increasing the bore in the future as well.

Of course you realize, with the same bore/stroke, the V12 would be running 3498cc, a 3.5L V12.

No wonder the engine is so long...I think most "big block" V8's have smaller bore spacing than that.

Which is totally understandable to provide a substantial safety factor on a 'proof of concept' engine, but even accounting for the homebuilt nature of the engine, on later developments for the 'final product' that is probably some pretty serious overkill.

That is close to the displacement range I was thinking of as well, if a bit on the large side.

The main reason why is due to the crankshaft, or to be specific, the connecting rods. I took the "industry standard" pulled from the engines I have available, and added 4mm to the high end of the numbers I tabulated, to buy margin. I would rather be a bit over, than severely under.

And that is also why the v-twin, to see how much margin I can shave safely. That is wgt I am not going hung ho on the larger engine, I know it is not ready.

Yes, it's definitely going to be interesting what the 'lessons learned' will be from the first engine. I'm certain there will be many expected ones, but even more curious will be the unexpected ones.

Very very true. One of the reasons for the larger rod journal is due to my home-casting limits. On a homemade engine forum, however, someone suggested a way around that by using a built-up, rather than single-cast design. By doing that, can make the rods with a solid crank bore, rather than a two piece, which would strengthen them and remove one of my concern-points. By doing that, can easily remove 16mm from each rod journal. Also would allow more experimenting, as could "swap out" the rod journal easily. For a full engine, the crankshaft would need to be oven brazed, of course, but for this test-stage, bolted would suffice.

FWIW, I think a 3.5L 12 would fit in the car I'm drawing up. There is something like 32" of length in the engine bay.

Two-stroke motorcycles ( at least some ) use built up cranks that are press fitted together.

Something struck me last night so I did some testing. The V4 is inherently unbalanced, while the boxer is more balanced. The I4 is also unbalanced. So, if the V4 will not be naturally balanced anyways, there is no inherent advantage for a 90 degree V4. So I took my throw-weight testing model and adjusted it to 120 degrees, and found it was more balanced than the inline, less than the 90 or 60 degree.

Also, Toby called the head design something I hadn't heard before, a "Fixed Nailhead". Curious, I looked up the Nailhead, and ran across the Buick Nailhead engine he was referring to. It was, similar to this, a very narrow engine, but with a hemispherical rather than flat head arrangement. Due to the hemispherical and side-by-side valve arrangement, the Nailhead had serious breathing issues, particularly on the exhaust which had to do a 180 turn to escape. By being flat, but using the nailhead side-by-side arrangement with low profile skinny head, you'd have the advantage of size without the breathing issues. He also asked again if I was sure I did not want a May style head, as he had one whipped up just in case.

Ok, so it looks like Toby and I had a mis-communication. Rather than putting the cam on the inbound, making the block a bit taller without adding to the width, he did it on the outbound, which widened the block without adding height. After some serious cursing, it struck me to take the design, and put the V12 at 60 degrees again, rather than 120, and viola, it now came to an a reasonably short size, with the whole block + heads + valve covers coming to around 440mm. Would this fit easily under the Locost hoodline? The new block looks to be coming to around 700mm long as well, which should fit pretty well lengthwise, and would be around 400mm wide.

That's a great number. Most engines are closer to 600 mm and the older Ford xflows are around 500.

I agree that those dimensions are sounding very reasonable to work with!

And from an auditory perspective, in V12 configuration you'd get to hear all 12 cylinders firing both evenly and independently again.