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Rather than me choosing a stroke I've set some goals and asked Mike to pick whatever combination of stroke/cam/valve/etc would get us closest to them. They are, in no particular order:
8,200 rev limit
Peak power delivered ~7,800 rpm
Enough torque for a road engine in a (fairly) lightweight sports car
400 bhpThis seems like the most sensible way, you say what you want from the car, the engine builder builds a suitable engine.
Flippant remark: can't he build the equivalent of a 4.0 mezger but not using a mezger? That engine has everything you want, no?
NurseHolliday-
Flippant remark: can't he build the equivalent of a 4.0 mezger but not using a mezger? That engine has everything you want, no?
He routinely builds 4.0 litre Mezgers, they have a wider bore spacing, not having been based on the 2.5 litre Boxster engine, so they run 102 x 80.4 rather than the 100 x 85 that we'd have to run. That's a lot shorter stroke, which makes higher revs more straightforward.
The Mezger head carries a 41mm valve whereas the largest we could fit would be 38 (stock is 37).
However, if we get it right we'll have an engine specification that can be built out of almost any 3k scrap M96 engine - and it'll be powerful and reliable at maybe a quarter of the cost of a Mezger.
Dammit
I'd got into a bit of a fugue with the specification of the 911 engine - we have a range of options in terms of stroke, from short-stroke (72mm) 3.4 litre screamer to 4.0 litre long stroke (85mm) stump-puller.
The head design, valve size, cam profiles, compression ratio and so forth were all dependent on what we picked here.
The engine designer was asking me what to use - and I was getting somewhat frustrated because every answer I gave ended in further "well, this trades off with this, which trades off with this".
We weren't getting anywhere, basically, so I decided to change how we were working.
Rather than me choosing a stroke I've set some goals and asked Mike to pick whatever combination of stroke/cam/valve/etc would get us closest to them. They are, in no particular order:
Whether that's delivered from 3.7, 3.9 or 4.0 litres I don't care.
I'm very interested to see what specification comes back.