Big end to crank centre line offset....
09 June 2004, 10:13 PM
#1
Big end to crank centre line offset....
Does the amount off offset alter the torque versus power ratio,in theory big end closer to the ctr line and more revs less torque but more power,yes? wheras bottom end further away from crank along counterweight then more torque (more leverage) but less power(limit to crank spin up???) is this correct to any degree anyone please?
09 June 2004, 10:43 PM
#2
yes, pretty much that is correct.
The dimension you speak of is crank throw, and is the stroke/2.
Long stroke engines typically are lower revving, as the same revs means higher rod loads and pistons speeds. But if the capacity remains the same, the longer stroke means there must be less piston crown area so less force exerted by the piston. However smaller bore means less distance for flame to travel during combusiton, and generally better combustion all round, less likely to detonate etc. So overall there is usually a gain in torque low down. At higher speed increased turbulence improves combustion making the extra piston area count. Long stroke engines are often power limited due to the limits in max RPM. Race built EVO engines are pushing this to the max though, and making massive power, the big turbos required kind of make the whole stroke thing a moot point though, as they spool up so late.
Evo engines are long stroke, cossie mid stroke, subaru short stroke.
Paul
The dimension you speak of is crank throw, and is the stroke/2.
Long stroke engines typically are lower revving, as the same revs means higher rod loads and pistons speeds. But if the capacity remains the same, the longer stroke means there must be less piston crown area so less force exerted by the piston. However smaller bore means less distance for flame to travel during combusiton, and generally better combustion all round, less likely to detonate etc. So overall there is usually a gain in torque low down. At higher speed increased turbulence improves combustion making the extra piston area count. Long stroke engines are often power limited due to the limits in max RPM. Race built EVO engines are pushing this to the max though, and making massive power, the big turbos required kind of make the whole stroke thing a moot point though, as they spool up so late.
Evo engines are long stroke, cossie mid stroke, subaru short stroke.
Paul
10 June 2004, 12:52 AM
#4
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To put the different "strokes" into perspective,
Subaru stock: 75mm Stroked: 79mm (the norm) 83mm max.
Cossie stock: 77mm (IIRC) ?
EVO stock: 88mm Stroked: 92mm (norm) up to 102mm available
When put into this context, 79mm stroke seems pretty tame.
Mark.
Subaru stock: 75mm Stroked: 79mm (the norm) 83mm max.
Cossie stock: 77mm (IIRC) ?
EVO stock: 88mm Stroked: 92mm (norm) up to 102mm available
When put into this context, 79mm stroke seems pretty tame.
Mark.
10 June 2004, 12:00 PM
#6
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Andy has a good point. Rod length to stroke ratio will affect piston side loadings and thus the overall longevity and efficiency of the engine.
Rod length also affects piston acceleration/deceleration and dwell time at the ends of each stroke... a shorter rod will have to decelerate/accelerate the piston at higher rates and the piston will spend less time at TDC than a longer rod for the same crank throw. Less dwell time at TDC should help reduce the oppertunity for detonation to occur as the piston is around the TDC region. Shorter rod to stroke also leads to greater piston side loadings than a longer rod would, so, as always, its a case of choosing the best compromise for the job.
Rod length also affects piston acceleration/deceleration and dwell time at the ends of each stroke... a shorter rod will have to decelerate/accelerate the piston at higher rates and the piston will spend less time at TDC than a longer rod for the same crank throw. Less dwell time at TDC should help reduce the oppertunity for detonation to occur as the piston is around the TDC region. Shorter rod to stroke also leads to greater piston side loadings than a longer rod would, so, as always, its a case of choosing the best compromise for the job.
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11 June 2004, 12:20 AM
#10
Does the different crank throw of the evo and scooby show up on the road as a more torquey drive from the evo (esp. at low revs...?) (or maybe this advantage was used to offset the loss to bottom end power from the designers other tuning aspects, tailored to give even more top end, ) talk about a stab in the dark 
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