VEry generally speaking, when you run them in series, you really have to look at what amounts to the combined inefficiencies of each stage (thermally speaking) - and unless you are talking about in excess of around 45-50psi (approx) you'll generally be able to find a single stage compressor of some type to do it more efficiently than 2 in series. Once you get above that, you get to a point of having little choice but to go to staged compressors.

FWIW - imo - you'll be breaking blocks before you saw the real potential of such high boost levels.

Alternately - on a streeter - where cost of the parts is often a big part of the overall choices, then compound charging certainly has its place. An 'average' roots type with a pulley ratio to give you (for example) about 10psi peak boost, and decent enough boost from a moderate rpm - well if you then put a 'big' turbo on that to feed it, and giving 7psi or so on top, you'd end up with about 20psi in the manifold (absolute rough guesstimate, and the specifics aren't important - if anyone really cares I will work out proper pressure ratios and likely actual boost at each stage).

Well on an a14/15 you could run something like a sc12 or sc14, producing a bit of boost (but not spinning so fast it will fail early. Then run a turbo off something like a stock SR20 - suddenly the engine will perform as if it _was_ a 2 litre engine with a turbo (thanks to the boost/gain from the supercharger) but it's not got quite as restrictive an exhaust side on the turbo (or alternately a big exhaust but real laggy). SO the comp discharge temp after the supercharger might be higher than a really efficient single stage, but with eough cooling, and for stuff all money, you have effectively the performance of a turbocharged 2 litre. - over 200bhp would be possible (engine internals willing)

So on a tight budget it's interesting. I'd have to wonder, when you add the weight of both the turbo and the supercharger, and the necessary plumbing, if it would be heavier than a ca18det swap. Dunnno on that one.

I understand the point about cumulative losses in the turbine/ compressor etc. It is true though that a super charger ahs a 0% effecient turbine in that it doesn't have one. And yeah I agree there's no point to series turbocharging unless you're shooting for 45-60psi boost. I still think it would be a cool project if anyone was brave enough to try it. 60psi boost would make one mean machine! Be verging on a turbo compound engine like was used in aviation and locamotion before gas turbines were enabled through better materials.

By breaking blocks do you mean the bore, block its self shearing it in torsion or something else? I'm interested to know more about the mechanical limits. I would think that lowering compression ratio would lower the peak pressure in the cylinder but this equally increases exhaust temperature so perhaps the exhaust valve will melt unless there's water injection (shown to reduce peak and exhaust temperatures).

The thing that gives me an idea it might work in a 4cyl is that the crank and everything else has to be strong enough for peak pressure and peak inertial forces (I think the inertial forces are generally larger once the revs get up) and that the power strokes don't really overlap. Maybe this is a project that needs to be done to find out? I'd gladly donate some time and labour to it. Or maybe I'm crazy, fun thinking about it in any case.

If you are chasing simply what the failure limits are of a std a-series, then its simple enough to hack up something like what you have mentioned.
Engine stand, two turbos, mig welder, safety cage around engine, jerry of fuel. boost guage.

Its a bit hard to really quantify what the approachable limits are unless you have specific details of what you start with.
eg A12, early model block. full beuild, including new bearings, line bored, new rods, pistons, rings etc.

Without starting from a clean slate, any results are difficult to apply to the real world.

You also need to decide on what the ultimate goal is. eg the maximum amount of boost that the engine can hold before failure. Determining what failure is, also needs to be decided. eg leg out of bed in less than 2 minutes or maybe just a blown head gasket.

If the idea is just to boost up a second hand original block till it goes bang just to see what it will hold, then I suspect that you may not get as much useful information as the effort is worth.

The supercharger and turbo idea is a more achievable and realistic project I think, in that the results that it will prduce are more usable to the datsun community in general.

For the record, If I had the cash to burn, and the time, I would do it this way.
A12 block. Full rebuild to std specs.everything new, but std spec parts. No performance or special gear. No custom parts for the turbos excpet the flanges to mount them.
Onlty modification is a carby that can feed enough fuel to allow increased combustion rates.
Failure is determined by the engine able to hold the boost, running for 5 minutes at max boost, without any parts, gaskets, or seal failing.

Once you have that baseline number, you can then begin to modify parts one at a time to find out what adds what.

Or you could grab that block out the back and add two turbos, and wait for the bang.

From general experiance of people from this forumn and around the place, approachable limits of usable boost would max out at about 20psi. But thats usable, and realible enough to be useful. Im sure you could wind it up to 30+psi, but its just not stable enough to be usable.

Quote:
I think that you would need to be quick to see it for the life expectancy of an A series at 400hp would be generally regarded as being fairly short.

While this thread is concerned with a compound turbo engine I was most interested with the Wright Cyclone Turbo Compound engine design.

This radial aero engine was already supercharged so they installed three radially displaced exhaust driven turbines around the crankcase &, via a fluid coupling, they coupled them directly to the crankshaft. [via a driveshaft & gears]

The big surprise was a 20% boost in power at any given fuel burn rate, or alternately, & 20% reduction in fuel burn at a given power output.
This was from exhaust gas that was otherwise just blowin' in the wind.
The engine [x4], incidentally, was used in the Lockheed Super Constilation.

Now I realise thet this is not practical in most automotive applications but in special vehicles, like a buggy, it might just be worth playing with the idea.

anything is possible but by the time you add the complexity, cost and weight of this kind of setup, a well-selected single turbocharger will provide as much grunt as an A-series can cope with, and also be lighter/easier/cheaper.

Unfortunately trendsetting is not only expensive, but difficult and often littered with plenty of well-meaning failed projects.

The turbo compound concept would be of benefit only on an engine that was already supercharged, otherwise, as you rightly suggest, it would be much better to turbocharge.

Not really practical in an automotive application but could be something fun to play with in other applications. [Pump, generator, wind machine etc.]