Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Terry Haines, bnoon, please read

Per your discussions in another thread relating to comparitive sizes between intake and exhaust valves, there was some discussion concerning rod ratios (rod length to stroke ratios)

I am of the opinion that increasing this ratio (longer rod, maintaining displacement) demonstrates increases in power/torque, especially in non-race tuned engines. I have several magazine articles that back this up. Just wanting to get your thoughts.

Sorry, not Terry, or Brad. I do agree with your thoughts on rod ratio though. I think that more
often than not, longer rods are used during a build-up of a "stroker" engine. Their main
purpose was to reduce stress on block, crank, and rods themselves by bringing the "ratio" back in line after a crank with a longer throw was added. The power increase was a welcome side benefit. Are you aware that using them almost always requires custom pistons? New pistons need a shorter compression height so they don't smack the heads at TDC. This often requires moving the ring stack up to make room for the new pin location. At that point you have to be careful about how much metal is left above the top ring land. The more combustion pressure you strive for [especially boosted or nitrous injected] the thicker the land should be to survive for any great length of time. Excessive heat and pressure can sometimes cause this area to fail. I don't know how much room there is to play with on our piston design. Worth consideration I think, if you were having pistons/rods made anyway.

Also one of the main benefits of increasing your rod to stroke ration is the fact that the piston will dwell longer at TDC and BDC, allowing for both more air to enter the combustion chamber and also for the mixture to mix better just before ignition. Some formula one teams are running 1.8:1 and higher.

Agreed,so look at the 3.0 Taurus bore/stroke Vs the LS 3.0 bore/stroke,the bottom end of the LS is the basis for the X Type Jag...do the math!

What I am talking about is a mod that will allow rod ratios of 2.25/1 or higher in nearly any production engine. I don't want to give out too many details here for obvious reasons, and it would seem that what I have just suggested would cause the connecting rod itself to contact the head, much less the piston. It is possible, however.

Terry, what kind of power gains do you think I will realize in a test engine, all other factors being equal? What if the engine were tuned to take advantage of the additional rod length and how it affects piston dwell, position, velocity, and acceleration.

Piston 'speed' is a crital factor,its not related to revs but to stroke as you know.Example small bore long stroke has more piston problems than 'oversquare' motors.

Tried and true way of increasing power. If you're going for stronger custom rods and pistons... why not? Of course we'd need to see two comparable motors tested side by side with the same mods to know for sure that it will be worth the money on the Duratec motors. :p

Longer rods do affect piston speed. Mean piston speed is conserved, as it must be, but piston speeds throughout the stroke are changed by longer rods. Max piston acceleration is reduced with longer rods.

Formula 1 teams run the under the 3 liter formula, with a compact stroke (42mm) and 95mm bore. *Jaguar Cosworth R2 Data*
The short stroke(and other factors) allow the engine to spin upwards of 18,000 rpm. You couldn't do that with a "stroker motor"

you would need a crank with a longer throw to get the piston down deeper in the cylinder wall to allow more air and fuel, increasing the overall displacement of the engine.
seems like it would be easier to just do the 3L swap IMO

I'm not talking about changing the stroke or bore at all. Just fitting longer rods. Longer rods do change piston position, speed, and acceleration.