Andyboy wrote:reggid wrote:
I have driven/been driven in a rebuilt C2 2.7, M20B28 and M20B31 so from a few examples most of the information being discussed in this thread is not all that accurate. The 2.8 with 284/272 was much much more torquey than the C2 2.7 but was less powerful as there was no headwork done and it didn't have a longtube 6 branch.
I have to say I've never driven a standard C2 2.7 that felt vastly quicker than a really good early 325i. My white 2.7 was quick though but was a very odd spec; 2.7 with early high comp 325i pistons, 325i head, 323i inlet manifold and small throttle body and LE Jetronic, plus a five speed overdrive box and a 3.45 diff. 100 mph was about 3300 rpm in top! You never needed to rev it over 4500 rpm because it was always in the torque band. It was so easy to drive fast I never used my M3.
Over 5000 rpm it tailed off because the injectors and throttle body weren't man enough.
89.6mm stroke eh? That must be an M54 3 litre crank? What pistons are you running? The gudgeon pins must be right up by the piston rings!
the C2 2.7 was quicker than the late m20b25 we get here by a noticeable amount but definately wasn't making any more than what Alpina claimed.
The 89.6mm comes from a S52 with forged 10.5 CR alusil pistons. The engine produces plently of power from 3500 its a hoot but its lacking a bit in torque below 3000rpm IMO but you get that with a 286/272 with 11.75mm lift and ported head and anything else designed to flow better at higher air flow rates which tend to kill air velocity at low rpms. On the same dyno compared to a stock m20b25 8.8:1 CR there was gains of:
18% @ 3000,
35% @ 3500,
40% @ 4000,
32% @ 4500,
47% @ 5000,
44% @ 5500,
42% @ 6000,
46% @ 6500
which is where my est ~240hp crank comes from
below 3000rpm i don't have data any dynos are not that accurate much below these values anyway but i guess its no more than 15-20%
The thing about SOHC m20 is you can't have the best of both worlds its always a comprimise for what you want i.e. you can have lots of torque and not a great deal of relative topend (any of the 81, 84, 86 or 89.6 cranks with standardish heads and manifolds) or lots of power with a reduction in bottom end (81, 84, 86 or 89.6 cranks with modded heads, cams, BBTB and 6 branch manifolds). Modern engines can have both as they have better heads, variable cam timing etc.
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