Rod Length?
 

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

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

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.

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

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

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

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

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)

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

stroke vs piston speed
 

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.
:)

JP... yes this is good debate/learning material, but I think we may have got off track. You are speaking of average piston speed, and ursus is measuring the amount of distance traveled by the piston in a given time. It may help if we are going to talk about piston Velocity, if everyone was on the same page and knew the definition of Velocity so we aren't comparing apples to pistons... lol

ve·loc·i·ty
(v�El¼s“¹-t¶) n., pl. ve·loc·i·ties. 1. Abbr. vel. Rapidity or speed of motion; swiftness. 2. Abbr. V Physics. A vector quantity whose magnitude is a body's speed and whose direction is the body's direction of motion. 3.a. The rate of speed of action or occurrence. b. The rate at which money changes hands in an economy.

We all have valid arguements of sorts, but if we are going to talk about the effects of rod length changing the rate of piston Velocity as it is defined, then those same simple physics still will confirm that the average speed, amount of distance travelled in a given time or how far it travels does not apply to anything to do with the Velocity of the piston. While stroking an engine will change the events of the rotating assembly, the reason for adding rod length is not to gain or lose piston speed for cubic inches, but to change the rate and timing of a pistons velocity on a given cubic inch motor. Bigger is always better, but increasing the rod length does nothing to give you more cubic inches or displacement. Simple physics.... it changes the timing of a fixed event in a given engine, taking stress off the rods, side load off the pistons, and adding to the power event BTDC. A 454 is a 454, a 502 is a 502, a 350 is a 350.... simple physics..... ~I think~

When I was talking about piston velocity I was not talking about it's up and down movement rather it's time at TDC, & BDC will change with rod length (given everything else is equal) Plus the longer rod will change your side load on the wrist pins and piston walls

RumRunner, your exactly right. My point is, velocity is synnomous with speed, not acceleration or change in velocity and my statement stands.

Confusing the issue will not change physics. Average (and over 90% of terminal) velocity of a piston at a given/fixed rpm is completely dependent on the distance of the center of the crank pin from the center of the crank (that would be 1/2 total stroke). We need to end this before we get stupid (which obiviously we are not) and mess up everyone else. Catch you all on another thread cause I don't think we're helping anyone out here anymore . Later --- Jer

BINGO RUMRUNNER At least we are on the same page...

THAT is exactly what changes by altering rod length, and we can pull different aspects of a relative event into it and argue it's correct, or this changes that, but nothing anyone but you and myself said can change those properties. And Blown 1500 Im not going to argue with you and what you say may be true, but it has nothing to do with the specifics of changing rod length without changing the stroke,...PERIOD It is a timing event relative to TDC and BDC, and my physics work fine when I don't try to compare things that are not related, and stay on topic..... still some good info came out in this thread, and thanks to all for sharing...

1st of all I said the piston will travel a greater distance in the bore because it is on a crank with a longer stroke, 4.25 inches vs 4.00" and that the length of the rod is does not matter to this. At 0 degrees the piston pin and hence the piston will be 4" or 4.25 inches away from the point where it was when the crank was at 180 depending on the cranks stroke, period, rod lenth does not affect this in any way shape or form.

2nd, Velocity = distance / time

if you travel , 4.25 inches versus 4" in the same ammount of time, because the rpm is the same, the velocity increases, it does not get any simpler

Re: Rod Length?
 

Now to answer the original question
1. no you can't run more stroke just because of rod length.

1/2 Stroke + rod lgth + comp height = deck height of block

so if you change stroke you must adjust either rod length or compression height ( the distance between the center of the piston pin and the top of the piston) or both.

2. Although it can no you don't change compression ratio with rod length, if you lengthen the rod say 6.135" to 6.385" and change nothing else the piston would crash 1/4 inch through the head, see part 1. If you change the rod from say 6.385 to 6.135 and change nothing else compresion ratio would indeed be lowered from say 9:1 to 5:1

3. So what does dif length do? a longer rod will be at less of an angle when the crank is at 90 degrees maybe 18 degrees instead of 17 degrees. This will mean less side loading on the piston in the bore which is desirable, less drag=more power and less wear. To use a longer rod the the compresson heght of the piston must be decreased the same ammount though which is not desireable, less land are between rings = less reliability. So there is an optimum compromise for a given deck height.

Ursus, you have the right idea. Dwell time at TDC increases as rod/stroke ratio increases also. This is a good thing for performance. Smokey Yunick said to max out everything then put in the longest rod you can use. A boat motor is a puller motor and if you check rod length in industrial applications, they are usually very long, indeed. Short rods do make better response at part throtle, low rpm-not usually a priorty in our world. The dyno and real world has shown me that the longer rod, even if you have to do tricks to the oil ring is worth the effort. Use your own judgement. We like to run 6.8" rods in a 4.25" stroke engine-better power, economy, and longevity-even though most builders seem to prefer 6.385" rods for this application. ;)

Yes

Using a standard 9.8" deck block and a 4.25 " stroke builders have basically 2 choices, 6.135" rods and 1.52" comp height pistons or 6.385 rods and 1.27" compresion height pistons the short rod is a little steep the geometry is pushed to 19.45 degrees, with 17 or less usually being considered optimal. The 6.385" rod however only leaves 1.27" deck height which is not much room for rings and they sometimes have sealing/reliability problems, this is why when building a 540cid it is preferable to use a tall block, 10.2 or more

A 10.2 inch block allows 6.535" rods a 1.52" comp height and 18.3 degree max rod angle geometry. Using 6.8 " rods in a tall block (10.2") you get a nice 17.4 degree max rod angle geometry but you are back to 1.27" pistons probably not the optimum but it is a compromise eiter way :)

Ok.. very interesting.. But if you had the choice of building a long rod motor or a stroker motor.. which would make more power or better suit the marine enviroment?
I took things to the extreme and ened up with a 1.88 rod ratio in my BBC... I have not been able to run the combination to it's limits to see if I made the right choice..

FindMe...... concerning the timing issue.. how much timing should I run wiht such a ratio.. are we talking a great deal of reduction? or maybe a few degrees??
That poses another question.. how does everyone determine max timing for a motor combination.. without a dyno.. my boat is my dyno.

Thanks..

Dick

Long rod motors reduce piston wear and offer measurable power increases (especially at high rpm).

They do not in any way increase displacement. For this reason, a stroker motor will make more power than a nonstroker that uses a longer rod - displacement rules (especially at under 6000 rpm).

Timing - assuming that you are fairly well matched with your engine components, start around 28 degrees and do a pull to get a plug reading. Adjust your fuel mix if necessary and do a baseline run with known trim angles, etc.. Do it in two directions. Bump timing 2 degrees and run again - if you pick up rpm bump 2 more and run again. Keep doing this until you see no additional rpm - then back off 2 degrees...

If you have a b@stard set of engine components (huge cam with low NA compression and a 2 barrel carb on a homemade intake) then nobody can help you.

NO perhaps the picture will help understand, The piston travels a in a straight line because it is in the cylinder a total distance equal to the stroke , its position higher or lower in the cylinder is determined by rod length and compression height but this does not affect the distance it travels,,note the dif compresion height of the piston 1.645 versus 1.52 for the 4.25 inch stroke crank, rod length is the same in this example

Well dang fellas! I've only got the 9.8"decks with the 6.385" rods in my 540's. :(

Hmm,
Looking thru some tech articles on this I am getting some conflicting information. Everyone here is stating that above the 350 SBC ratio of 1.64 is good (if I am reading this right?). However, look at the following numbers. All the motors that make good torque like what we all need in boats have in the 1.5 to 1.6 ratio range.

327 SBC = 1.75
350 SBC = 1.64
400 SBC = 1.45 With standard Rod
400 SBC = 1.52 With 5.7 (350) Rod
427 BBC = 1.63
454 BBC = 1.53
502 BBC = 1.53

That being said, the motor I am currently gathering parts to put together will be either a 421 CI SBC, 3.875 stroke - 4.155 bore - 5.85 Rod. For a ratio of 1.51. Or a 430 CI SBC, 4.00 stroke - 4.155 bore - 5.85 Rod. For a ratio 1.46. If I understand correctly, this should be a good low rpm torquey motor.

Your comments and suggestions please?

Tee

P.S. Here is a link that is pretty good. http://victorylibrary.com/mopar/rod-tech-c.htm

I'm convinced that long rods are better. When Mercury built their Class 1 engine a few years ago (max of 500 CID allowed), they didn't use a 502 combination, R/S = 1.53. They went to 4.6 bore and used a 427 crank, 3.75 stroke, for 498 CID. Assuming they used a 6.385 rod in a standard deck that's a R/S = 1.70. If they used a tall deck with 6.785 rods the R/S = 1.81. That was a high revving, powerful engine making almost 900 HP.

But as was said above, if you're going to the expense of tall decks and custom rods, you're going to talk yourself into 4.75 stroke. That's what makes a 632, and I am also convinced that at normal RPM the extra 134 CID will far outweigh the disadvantages of the lower R/S ratio.

Good thread!

Ursus makes a good point about not comprimising the ring pack...short skirts w/big bores...forged pistons w/loose clearances...hot combustion temps w/cold block ---Stability of the piston within the bore is critical. Longer rods are best with taller decks in boats.

mcollinstn
You mean i have to scrap the two barrel.. was hoping to get some milage out of it.. oh well.
Seriously.. I am running 565" with an 8" rod.. I have not had the opportunity to run tests to dial in the combination. I started with 32 degrees and it hasnt melted down yet.. But I am still dialing in the EFI and this year I got close until I found the prop would hardly turn due to a broken bearing race/cage on the lower vertical shaft.. Another Imco part that gave up. The intake is kinda homemade.. Guy by the name of Hogan built it.. he's on the left coast.
I do thank you for your suggestion on the timing .. I will run that test when the water gets soft again in the spring..
I love this thread!! I built this motor on the log rod theory, but I have not had much support. If all else fails I could shorten the rod by an inch and add and inch to the crank.. lets see 4.600 x 5.25..... someone do the math please. I am gettin excited here.

Dick

8" rod? That must be a really tall deck block!

Well gang, I guess my short deck 540's aren't any good then----Guess I'll just have to toss them in the trash tomorrow when the garbage truck shows up and start all over. :crazy:

KAAMA,

You can throw them in my front yard if you want!!!LOL

Merry Christmas!!!!

Pi x Radius Squared = Area of Circle

Pi is approx 3.1416

Bore Diam = 4.6"
Bore Rad = 2.3"

3.141 x 2.3" x 2.3" = 16.619 sq. inches

Area of Bore x Length of Stroke = volume of cylinder

16.619"" x 5.25" = 87.25 cubic inches

volume of cyl x #cyl = volume of motor

87.25""" x 8 = 698 cu. in.

mcollinstn,
Thanks for the math!
tomcat,
The block has a deck ht of 11.665". Merlin Super block. They claim I can go 4.625 x 5.600 = 750ci. If I want to change bore spacing and go to the max.. 4.680 x 5.850 = 805".
If the long rod thing doesnt pan out.. a 750ci with a couple of hair dryers or a 18-71 on it would make some torque.. Oh the possiblities..

My next question has anyone experienced less tendancy to detonate with a much longer rod? Dwell time at TDC is longer allowing to complete the burn. Would this make the combination octane insensitive? In other words can I get by with regular 89 octane fuel with 10 to 1 compression with alloy heads?

Next one? Has any one heard of using an 8" rod in a BBC for Marine use? I know Oliver sells quite a few 7.75" and 8" rods..

Thanks for any input on this.

Dick

So without all this math, basically there is less angle=less friction=less heat, more power, less wear?

HyperBaja,
That kinda sums it up. Although the dwell time at TDC is suppose to give you something also.. But some say with today's modern combustion chambers that advantage is negated. I figured I would give it a try and not sure as yet if I spent my money wisely. But then what are hobbies for??

Dick

Rod length and stroke-

The dwell time at TDC/BDC is increased. Overall average velocity stays the same at a given RPM and stroke, but there is more...

Another item of consideration is the point at which the piston reaches maximum velocity- this does not occur at 90 degrees down from TDC.

The piston reaches maximum velocity when the rod and the moment arm of the crankshaft are at a 90 degree angle (Not degrees from TDC). This could only occur with an infinitely long rod and an infinitely short stroke (remember this is a family web site). I am too lazy to calculate it, but 80 or so degrees sounds right in a stock BB chevy.

As the piston travels from TDC, it pulls vacuum and initiates airflow. This increases until the piston starts to slow down. The piston slows down after that 90 degree angle point is passed. Increasing the rod/stroke ratio moves that point further down the bore.

This then results in longer cylinder filling time which can be used to gain horsepower.

Bulldog aka Ronnie :frog:

So I could make more power with a +500 rod over a +400 rod but my CH would be too small?;)