rod to stroke ratio

[rakkassan34]

Has anyone looked into the pros and cons of each setup (long connecting rod short stroke, short connecting rod long stroke) built an engine to take advantage of what each has to offer for the sake of mileage alone?

I know in the racing field each has been tried for max power. Some swear by higher rod ratio's others swear by the longer stroke shorter rods. Both can be made to put out excellent power but the cams, intake, exhaust, and rpm's at which power are made are totally different in each.

[dkjones96]

The lower your rod ratio the more you lose to side loads on the piston and the more piston acceleration you have while running, that is usually the limiting factor on rpm. The lower rod length lends itself to a good mechanical advantage to torque but doesn't rev well. It can make very efficient power down low.

Engines with high rod ratios rev well but don't create as much torque because of a loss of mechanical advantage. However, these engines have a high TDC and BDC dwell time that can be useful when cam timing is being adjusted for maximum scavenging and efficiency.

The Ford 460 Big block has a rod ratio of .96 and makes close to 275hp and 500 pounds of torque with a redline around 4500 rpm. About 95% of that torque is available below 1800 rpm by the way.

The new Civic Si has a rod ratio of 1.42 and makes 192hp and 139 pounds of torque with a redline of 8000 rpm.

So what do you want? An engine you have to rev up or one that pulls at low rpm. It's really deciding if you want horsepower or torque. (I vote torque)

[thisisntjared]

dkjones hit the nail right on the head. the higher the rod-stroke ratio, the safer the motor is revving high. it should also be noted that the piston speed is slowed down relative to the rpm with a higher rod-stroke ratio.

and almost like dkjones said, its a matter of where you want your torque.

with the variable valve timing (and variable everything now) most auto manufacturers are gravitating toward a higher rod stroke ratio to leave the low rpm for fuel economy with an economy set of cam lobes and then use the upper rpms to provide a completely second engine behavior. the 2nd isnt good for gas, but when you need to get moving, its there.

the lower rod-stroke motors have to work harder to optimize all throttle positions as its the same rpm that needs to save the fuel that needs to pull hard when more throttle is given. this can often lead to compromises, but if done right, the torque is always there when you need it without a downshift.

my current car has a rod stroke ration of 1.74:1 and i love it!

[GasSavers_RoadWarrior]

Marvin is at the high end, I'm thinking of tweaking it a tad lower in the new motor. Current plan for that is taller pistons, shorter rods and an offset crank grind. However, this is also aimed at getting compression a bit higher. Oh, will also add about 200cc of displacement.

[DRW]

Sometimes you can learn something by looking at the best designs. check this out people.bath.ac.uk/ccsshb/12cyl/
To improve efficiency, see if you can modify your engine to have a longer stroke with a smaller bore.

[dkjones96]

I love that engine. (take off the www)

The main reason that engine is so efficient is because it's so massive. The larger the volume in a cylinder the less percentage of heat gets lost to the chamber walls. Another reason why part-throttle is so inefficient.

[rakkassan34]

I know some of the pros and cons of each setup, especially when it comes to power. I was just wondering if anyone maximized the fuel efficiency of either setup or both to see which one produced more mpg. I myself would like to see a long stroke, short rod, small dia high static compression engine built with the purpose of higher mpg in mind.

[GasSavers_RoadWarrior]

That's what I'm gonna shoot for, +3mm on the stroke from offset grinding the crank and using undersize bearings, -5mm on the rod, and pistons 3.5mm taller that will get the dish ground out of them, tryna get compression up to 11.5:1 and gonna de-edge and groove the head, among other tricks. I'm projecting 5mpg from it all. Got a lot of figuring to do though, because it will be a frankenstein using nissan SR20D series rods and bearings, stock crank, G54B pistons... ricer math on what all the upgrades I'm planning add, says it's north of 300HP* but I'm expecting about 230, and 30+ mpg on the highway.

(* btw that's even using the low end of guesstimates.)

[GasSavers_Erik]

Road Warrior- With an 11.5 compression ratio, how will you prevent preignition/detonation? Atkinson cycle?

[dkjones96]

Depends on if that is his static or dynamic compression ratio. If it's static at 11.5:1 but the cam is either advanced enough or radical enough he could still run regular grade pump gas. With a stock cam and timing could run premium without issue. If that is his dynamic ratio it'll only be an issue under heavy load assuming he's using an aluminum head and premium fuel.