| Author |
Message |
Timothy J. Dressel (Tjd)
| | Posted on Tuesday, November 13, 2001 - 2:54 pm: | |
I agree with Steve�s analysis, but would go a bit further and say that I believe this design quality is one of the fundamental �secrets� of the Ferrari V8 design that make it superior to other V8s.
You may have seen US race V8s with �cross-over� headers to produce this same effect.
As Steve indicated, the firing, and therefore exhaust pulses, are evenly spaced on each bank of four cylinders; and this provides a performance advantage. But I believe this also allows the opening of the intake valve to be closer to the closing of the exhaust valve in optimizing the cam design; that is, a street engine can use longer duration cams and with less compromise of low speed characteristics; and even more power can be squeezed out with race cams. One of the things that I first found puzzling about my 308 was the wide power range and good idling traits.
It looks to me that Ferrari handles the loads on the main bearings (and crank in general) by fully counter balancing each crankshaft throw. On the US V8s, there typically are not counterweights adjacent to the center main bearing, relying in part, I believe, on the two throws (and rod assemblies) to counterbalance each other. This is OK in modest use, puts stress on that center main bearing area at high revs.
--tim d |
Steve Magnusson (91tr)
| | Posted on Friday, November 09, 2001 - 1:13 pm: | |
To call it a "flat" crank vs "non-flat" crank issue is really a bit of a misnomer since that applies only to a subset of engine designs (e.g., 90 deg V8 blocks). A better description of the effect is "alternate bank firing" (ABF) vs "non-alternate bank firing" (NABF). For a 90 deg V8 block it works out that the flat crank shape provides ABF -- yet my TR, which also has ABF, does not have a flat crank shape.
The upside to ABF is that the exhaust pulses are all evenly-spaced time-wise for all cylinders in each bank so the (shared) exhaust system on each bank can be more highly optimized; whereas, for NABF the exhaust flow is irregularly spaced so the exhaust design/performance is compromised a bit.
The downside to ABF (at least in a 90 deg V8 block configuration) is that the peak combined load on the main bearings is higher (even though the average main bearing load is, of course, the same) -- for a 308, with its super-robust bottom end design (compared to it's modest displacement) this is not a problem, but for the typical (girdle-less) 2-bolt main cap "American" V8 design it could be a concern. |
Bob Campen (Bob308gts)
| | Posted on Friday, November 09, 2001 - 11:03 am: | |
Other than being easier to forge and balance, what are the advantages of a flat ( 180 ) crank vs a 90.
Yes they do sound great!!! |
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