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Old 22 Dec 2006, 03:49 (Ref:1797322)   #1
chernaudi
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Crossplane vs flatplane.

I'm asking to see if there are major advantages/disadvantages to using a flatplane or crossplane crankshaft in a sportscar engine. Mercedes Benz used a crossplane crank in the CLK LM and CLR's 6.0 and 5.8 liter V8s, and Audi and Bentley used flatplane cranks in their engines. Does this really matter from a performance standpoint nowadays?
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Old 22 Dec 2006, 08:33 (Ref:1797416)   #2
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here < dont know if this helps but the whole article is a good article anyway !
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Old 22 Dec 2006, 09:46 (Ref:1797450)   #3
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The crossplane V8 has uneven firing on each bank whilst a flat plane V8 has even firing pulses. The odd firing produces the characteristic 'V8' noise whilst a flat plane sounds like a 4 cylinder.

The even firing of a flat plane gives much better exhaust tuning and is the only real configuration if you are after an ultra high performance / high rpm naturally aspirated race engine. The exhaust tuning potential also gives it a slight edge in turbocharged form as well. As explained in the link, a flat plane V8 has significant secondary inbalance ( like a 4 cylinder ) which cause the engine to vibrate which can start shaking the car to pieces. A crossplane V8 has good balance and the vibrations are much smaller, which may give it an advantage in long distance sports car racing.

Overall, a flat plane is probably the way to go. However, most sports car racing these days use inlet restrictors which will tend to limit the tuning potential of the flat plane crank. So it may be decided that the extra smoothness of the crossplane is a good trade-off against a small performance loss.
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Old 22 Dec 2006, 20:28 (Ref:1797924)   #4
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I wonder how Audi and Bentley kept from having problems with their flatplane engines? Did they use a balancer shaft? Or was it that their engines built like, as someone put it in the R8 engine topic(in the sportscar section), a brick outhouse?
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Old 22 Dec 2006, 20:30 (Ref:1797927)   #5
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For the noise. Crossplane.
For the ultimate power. Flatplane

I remember Honda's first Indycar engine (back when it was the CART PPG Indycar series) and Bobby Rahal ran it. It had a crossplane crank, and sounds much better than anything else out there!!

A friend of mine was (is) working on a straight 8 Bugatti engine. They tried a flat plane crank, but went for a odd config with the plane of the front four at right angles to the plane of the back four. It worked very well as the original config had terrible vibrations and shook everything apart.
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Old 4 Jan 2007, 13:12 (Ref:1806130)   #6
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I worked on the BMW-M5 Ascari 4.0 LMP900 twin turbo engine - we originally started with a cross plane crank and it gave reasonable power - about 580bhp if I remember correctly (audi had about 650!).........we had a flat plane crank made with new cams to suit and it did not make any more power.......but the race drivers liked the flat plane engine because the transient response (snappiness!) of the engine was much better - it revved with a much greater urgency. the engine was always going to be poor on power, as it ran a 5.5 litre BMW-M5, de stroked to 4.0, with a wide valve angle head, a 93mm bore which required a filthy great piston intruder to get a sensible compression ratio - which kills power in any engine - especially a turbo engine......if it had something like an 86mm bore it would have been a cracking engine - but not so.

I also heard Cosworth did a similar evaluation project for a possible Aston Martin LMP900 sports car project back in 2001 ish - they did the same flat/cross plane comparison and also found there was no power benefit of either.

BUT - believe me - a flat plane crank weighs about 25% less than a cross plane crank and due the the flat plane exhaust pulses being even.......the flat plane is the way to go........if I was designing an LMP1 normally aspirated V8 sports prototype long distance race engine, I would go for a low revving 6 litre, with flat plane crank, and with twin counter rotating balancer shafts in order to dampen out the horrible unbalanced 2nd order horizontal shaking movement, which rattles the drivers fillings out, and generally shakes the car to pieces as said above. hope that helps.

Last edited by knighty; 4 Jan 2007 at 13:19.
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