HyperBaja

I read in a magazine about swapping out rods in a Miata for shorter ones, then another talked about going from 5.7"(?) to 6.0" Whats the different length do? can run morestroke? less of a compression ratio?

Thanks
Tommy

RLW

A longer length rod will allow you to smash your pistons into your combustion chambers, if that is all you are changing.

RumRunner

Changing your rod length changes the rod ratio effectively speeding up or slowing down your piston velocity. This will affect how complete your burn rate is in the combustion chamber.

jpclear

Well Russ, It's not really that simple. Longer rod length gives you a better push angle on the crank throw; ergo, better push; ergo, more power. And of course you have to change to a piston with a higher wrist pin to prevent the "smashing". Stroke remains the same unless you make a crank change. So unless you also change the piston dome shape, your compression remains the same. I can see NO reason to go to a shorter rod unless you want more displacement from a stroker crank change and have already maxed out the height you can get the wrist pin in the piston. Later! --- Jer

jpclear

And Rumrunner, I would have to disagree with you about rod length alone effecting piston velocity. That only happens when you increase the stroke dimension. --- Jer

FindMe

Not so JP... There is no way increasing the stroke will affect the piston velocity, unless you somehow change the rod angle at the same time, and you can figure it out with a little basic math, and simple physics. When any 2 crankshafts are at 0 degrees, 90 degrees, 180 degrees or 270 degrees, they will all be the same place in their total stoke by degrees, which does nothing to speed up the piston, as 1 revolution is still 1 revolution. BUT, when you change the rod length without changing the stroke you have to change piston pin location to compensate for the added length. Is it easier to pry a head off a block with a pocket screwdriver, or a 3' screwdriver or prybar? Step on the handle of the 2 1/2 " long pocket screwdriver, moving it 4 inches and the head will lift 1/8 of an inch. Step on the handle of a 3' screwdriver, and the head will lift 4"... Again, simple physics and math will confirm this every time, and thats why added rod length changes the point of peak piston velocity, not stroke. Some facts about the exact affect of rod length are listed below, and the information to back it up using a SBC as an example.

In most large cubic inch engines it is hard to get a very high rod length to stroke ratio. A Stock 350 Chevy has a 1.64:1 RL/S ratio, which is not very good. By increasing the rod length to 6" the ratio increases to 1.72:1, which is much better. You can squeeze a 6.1" rod in a 350 with little trouble, but longer than that requires plugging the piston pin bores after assembly to support the oil rings. It is not worth the extra expense for the little gains, so a 6" rod in a small-block Chevy has become common because everything fits right in. With endurance engines, longer rods are always better. Most endurance engines are using a RL/S ratio of at least 1.9:1 and some as high as 2.2:1.

Also, a longer rod reduces the maximum rod angle to the cylinder bore centerline. Less rod angle will reduce piston side loading; there will be less friction and less bore wear. Less rod angle also gives better average leverage on the crank for a longer period of time. A 5.7" rod with a 3.48" stroke (stock Chevy 350), will have a maximum of 17.774° rod angle. Switching to a 6" rod will reduce that to 16.858°, assuming that the wrist pin has no offset.

A longer rod reduces peak piston speeds slightly and delays peak piston velocity until the piston is further down the bore, which gives the intake valve more time to open more. Peak piston velocity is usually somewhere around 75° ATDC and since most cams cannot fully open the cam until at least 106° ATDC, it leaves the valve as a major obstacle when airflow demand is at its greatest. By delaying peak piston velocity, even if it's only 1 or 2 crankshaft degrees, it can allow the valve to open another 0.010-0.015", before peak airflow demand is reached. Not a huge help, but a step in the right direction. With a 350 Chevy, switching to 6" rods from 5.7" ones will delay peak piston velocity from 74.5° to 75.5°.

Reducing piston acceleration / deceleration from and toward TCD will reduce tensile loading of the rod, the number 1 cause of rod failure. A Chevy 350 with 5.7" rods will have a peak piston acceleration rate of 101699.636 ft/sec/sec at 7000 rpm. Swapping in 6" rods will reduce that to 100510.406 ft/sec/sec at that same rpm. That is a reduction of 1189.23 ft/sec/sec. A longer rod is moving slower at TDC, which reduces the speed of the exhaust gasses during the overlap period. This reduces the scavenging effect at low rpm and reduces low rpm power slightly (makes the engine run more cammie). A short rod on the other hand moves faster past TDC and increases the scavenging effect and help low rpm power.

Due to the fact that the longer rod moves past TDC slower, it gives the charge a longer time to burn. So you need less timing for peak power. Using less timing also reduces the chance of detonation; so higher compression ratios can be used. Switching from 5.7" to 6" rods on a 350 Chevy can allow as much as 1 full point increase in compression. In other words, if you could only run 9.5:1 with 5.7" rods, you could run 9.6:1 with 6" rods. So you are saying increasing the bore size will change the burn time, like increasing the stroke will change peak piston velocity? Goes against every bit of logic and simple physics I have ever seen if so.... but hey, maybe I'm wrong heh? ~smile~

And Rumrunner... you said the same thing a hell of a lot faster with less typing... you must have longer arms than I do... lol

jpclear

Find Me, I would certainly not dispute your facts in this discussion but I think that we are going the same place here while taking different routes. What I should have said to be more correct was that the AVERAGE piston speed at any given rpm doesn't change without changing stroke. Stroke alone determines how FAR a piston travels up and down in any given period of time and it will travel the same distance whether using a 6 inch or 6 foot connecting rod. So while velocities and thrust loads DURING a cycle will change with rod length alone, the AVERAGE velocity will not. I would also contend that the screwdriver prying off the head analogy applies to the C/L distance between main journal and rod journal (1/2 stroke dim.) and not rod length. This is good discussion that really makes you think. We need to talk more, Find Me. Later. --- Jer

HyperBaja

I see what it does. More than just Marketing Hype. As for the more displacemnet- I was thinking you could run a bigger crank (but you would also need more length in the combustion chamber)

ursus

This is simply false

changing from a 4.00 inch crank to say a 4.25 inch crank without changing rod length means at 0 degrees the piston will be 4.25 inches away from where it was at 180 degrees versus 4.00 inches for the 4.00 inch crank. if the engine is turning 5000 rpm, piston a will travel
5000 rpm* 4.25 inches * 2 /60sec /12inches= 59.03 Feet/sec

With the 4" crank the piston will travel
5000rpm * 4 in * 2 / 60 sec /12 in = 55.56 feet/sec

Rod length will not affect this wether it is 1 inch or 1 mile long , how far the piston travels is totally dependant on stroke, and since it travels a different distance in the same ammount of time the velocity is different, yes simple physics

blown1500

Find Me, you must have missed physics class too many times.
If you increase stroke (the distance the piston travels up and down) and you turn the same rpm, the piston must travel at a higher rate of speed to cover a greater distance in the same length of time.
Concerning the compression ratio increase for increasing rod length from 5.7 to 6 inch in a small block, the increase may be .1 but not 1 full point.