Originally Posted by
Karl
Rod length is predominantly about optimising piston dwell time at TDC. This is largely related to piston speed which is a direct function of crank stroke and rpm.
On a YB a long rodded engine on std stroke will give negative benefits below 7000rpm. This is due to a reduction in engine torque at lower rpm with no benefit at 7000rpm.
By 10,000rpm there is a benefit due to the increased TDC piston dwell time allowing more effective use of the available cylinder pressure by preventing excessive piston acceleration and descent ahead of the available cylinder pressure.
Edited to add these are my optimum rod ratio against maximum engine rpm for std stroke YB: (i.e these will give the best overall torque spread for that rpm of operation)
up to 6000rpm: 1.5:1
up to 7000rpm: 1.6:1
up to 8000rpm: 1.7:1
up to 10,000rpm 2.0:1
Why were they used in a wrc engine then when it was a massive torque engine that didnt need to rev high??
And my own engine that was 600lb ft of torque at 4.8k had peak power by 7.4k and it worked very well ??
The less bore wear i take it is down to rod angle and less piston movement in the bore??
cheers danny