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Bob uk
Unregistered guest
Posted From: 94.197.122.71
Posted on Monday, 01 September, 2014 - 09:32 am:   Edit PostDelete PostView Post/Check IP

A V8 is often thought of as 2 in line fours on a common crankshaft. At an angle of 90 degrees.

Early V8s were exactly that. But this will double the vibs.

So Chrysler Ford, GM and RR put the four crank big ends at 90 degrees to each other so that only one piston at a time is at tdc. Where as a 180 Deg crank would mean two pistons at tdc. Cross plane crankshaft can be balance by counter weights, which restrict its ability to rev.

The 180 or flat plane crank because it is lighter will rev quicker but be rough at low rpm. If the stroke is shortened then the piston speed will be less for a given rpm. Ferrari build engines like this. Ferraris sound different because they fire LRLRLRLR bank.

RR V8s fire RLLRRLLR bank. Notice the two adjacent pulses on the same banks. This creates exhaust bubbles and carb sucking problems which are solved by daul plane inlet manifold. Which are now unnecessary due to multipoint injection. The exhaust should have a cross over pipe to connect the two banks exhaust. This gives the burble and better scavenging.

Ferraris don't need a cross over.

If smoothness is required then cross plane and if power and revs, then flat plane.

How ever recent developments in motor sport (Audi) show it might be possible to build flat plane engines with long strokes and therefore torquey at low revs that can rev hard. Also modern Ferraris seem to be smoother. Some were convinced that Ferrari were using cross plane crankshafts. Definitely not.

(Message approved by david_gore)
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Bob uk
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Posted From: 94.197.122.91
Posted on Tuesday, 02 September, 2014 - 05:44 am:   Edit PostDelete PostView Post/Check IP

Correction

Whoops.

I meant to say that a cross plane crankshaft V8 has one piston in each bank at tdc. The flat plane has two pistons in one bank at tdc.

(Message approved by david_gore)
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keith pearson
Yet to post message
Username: dud_fivers

Post Number: 1
Registered: 11-2017
Posted on Tuesday, 28 November, 2017 - 09:34 am:   Edit PostDelete PostView Post/Check IP

In the W12 bentley 6 litre engine, the stroke is longer than the bore, very unusual for a modern engine. I suppose helps with the high torque at low speed.
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richard george yeaman
Grand Master
Username: richyrich

Post Number: 877
Registered: 4-2012
Posted on Tuesday, 28 November, 2017 - 09:48 am:   Edit PostDelete PostView Post/Check IP

Hi Keith welcome to the forum, I am sure you will enjoy it.

Richard.

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