Originally Posted by
Joshy
am i reading this wrong?
so a longer rod gives an unrelated, but similar effect to increasing CR?
I.e. allows you to build power (torque)in high and low RPM ranges by increasing cylinder pressures earlier?
(ignition timing and detenation supression dependent of course..)
Hence with the fuels that ford were running in the above example meant they could increase cylinder pressures closer to det, at lower RPM's without having to increase the CR or advancing the timing?
The engine has to be specced to suit the requirements, its not just a case of throwing a longer rod in otherwise the piston clearance would be a problem
Originally Posted by
Chip
MarkK, You are missing my point mate about the fuel. I was saying they changed fuel at the same time.
The problem with having a rod ratio that leaves the piston hanging around for a long time at TDC is that it makes it easier to find detonation (or harder to avoid it) but when the rally cars are running very det resistant fuel it means that the rod ratio that is optimum for them is greatly different than on a normal road fuel at the same rpm.
Ie because of the fuel they were using (both before and after the rod ratio change) the rod ratio change worked well for them at those rpm, but on a more normal fuel it quite probably wouldnt have done so.
Fuel or fuelling ?
Also having the rod at TDC (or around the cylinder filling point) gives a greater opportunity and timing scale to fill the cylinder at any given revs.
Why would the YB have the shortest rod then ? because its the oldest engine design !
I cant think of any engine that uses rods that short anymore, the crank stoke on the YB is relatively short, hence why a decent specced YB will love to rev, add into that longer rods and you are heading into a territory that also elongates your engines life expectancy.