Finding the con-rod length

[one.man.band]

I know that being off .5mm on your conrod length isn't going to drastically change anything in the port timing math.. it's just nice to know you're on the money!.. Calipers should do fine (mine go to .005mm), the trouble is finding reliable points to measure to/from

I guess you could also measure piston lift at certain intervals.. lets say 10*, and you'd see which one is the best fit mathematically.. again, probably only to .5mm!

can appreciate your quest for accuracy. the numbers do change quite quickly in a timing program (as you already have seen, i'd imagine).
after you add up all the many separate measurements with error on each, and see just how much patience you have taking things apart and reassembling, it goes kaput when the motor gets heat soaked and grow.

never read on here if folks do recheck clearance when hot?

*i should have wrote 1/2 stroke up there.

hope it helps. not a fan of digital calipers, imo should be thrown in the trash. hahaha

 

[Rx7man]

Good digital calipers are nice.. I have an old Mitutoyo that's been to hell and back and still works... the cheap ones are in fact junk. I like being able to zero them at any point to measure differences easily.. especially nice when you're turning something down on a lathe.. zero in at the size you need, then measure your part.

I have come to the conclusion the only way to really get good con-rod measurements is to split the crank, take the con rod out, and measure it directly.. If I had fancy tools I could do it without disassembly, but I'm not going to spend $500 to do it for as much as I need them.

 

[Chainsaw Jim]

The connecting rod length will not affect the piston stroke travel or dwell at tdc or bdc any longer if the crank shaft rotation diameter stays the same. All you'll do is tighten the squish and ease the side travel as you mentioned.

 

[Rx7man]

Did you read page 1? con rod length does affect dwell at TDC to an extent, and BDC significantly

 

[Chainsaw Jim]

Did you read page 1? con rod length does affect dwell at TDC to an extent, and BDC significantly

When the rotation of the crank is a larger diameter, the stroke distance increases and both ends will have more dwelling time.
Tdc and bdc of the crankshaft where the rod connects is what determines stroke distance. A longer rod will only leave the piston farther away from the crankcase with the same exact stroke. The only thing you gain is squish tightness and it slightly eases side play at bdc if the length increases enough.

 

[srcarr52]

When the rotation of the crank is a larger diameter, the stroke distance increases and both ends will have more dwelling time.
Tdc and bdc of the crankshaft where the rod connects is what determines stroke distance. A longer rod will only leave the piston farther away from the crankcase with the same exact stroke. The only thing you gain is squish tightness and it slightly eases side play at bdc if the length increases enough.

You forget that whenever the crank isn't at TDC or BDC the rod is forming a triangle with the crank and piston. So the piston height as a function of the crank angle is:
H = a * cos(theta) + SQRT( L^2 - a^2 * (sin(theta))^2)

Since at BDC the piston height is L-a we can calculate the piston height with respect to BDC.
H = a - L + a * cos(theta) + SQRT( L^2 - a^2 * (sin(theta))^2)

I'm lazy so I'll use a MBD software to plot the piston height vs. crank angle for my favorite saw and with 1.5 it's rod length.



As you can see, there is a slight difference. The longer rod will stay close to TDC longer and will be close to BDC for a shorter time/crank angle.

Also rod length greatly effects port timing, notice the large difference in piston height at mid stroke.

 

[Chainsaw Jim]

You forget that whenever the crank isn't at TDC or BDC the rod is forming a triangle with the crank and piston. So the piston height as a function of the crank angle is:
H = a * cos(theta) + SQRT( L^2 - a^2 * (sin(theta))^2)

Since at BDC the piston height is L-a we can calculate the piston height with respect to BDC.
H = a - L + a * cos(theta) + SQRT( L^2 - a^2 * (sin(theta))^2)

I'm lazy so I'll use a MBD software to plot the piston height vs. crank angle for my favorite saw and with 1.5 it's rod length.

View attachment 447496

As you can see, there is a slight difference. The longer rod will stay close to TDC longer and will be close to BDC for a shorter time/crank angle.

Now look at the green line. Does it change when the rod is longer....no. It only changes when the crank is bigger. That side of the triangle stays the same until you use a larger diameter crank. The triangle only gets taller when the rod is longer.
The dwelling time is affected by the size of the crank diameter. When you use a smaller crank the dwelling time will be shorter.

 

[Rx7man]

Of course the green line doesn't change... but look at the plotted graph... The two lines show the difference in relative piston position with different con rod lengths.
Sccarr that's the graphic I was thinking of, thank you

Chainsaw Jim... If you don't want to believe it.. that's fine by me.. but it's a fact that con rod length affects piston position

 

[Chainsaw Jim]

[/QUOTE]Chainsaw Jim... If you don't want to believe it.. that's fine by me.. but it's a fact that con rod length affects piston position[/QUOTE]

That's the ONLY thing it affects! Others you agreed with mention more dwelling time at tdc and bdc allowing longer combustion from stroke travel increasing with a longer rod as if the rod is collapsible on the down stroke.

What you are now arguing is the only aspect I initially agreed with.
And Marshy was correct.

 

[Rx7man]

What point was Marshy correct about? I never said that con-rod length affected stroke!
Con rod length is irrelevant AT TDC and BDC.. the short one and the long one will both be at the top/bottom of the stroke, and that's determined by the crank stroke.. but anywhere other than that, the con-rod length affects the relative position...
ChrisInPA understands what I'm saying and is on the same page

 

[srcarr52]

Now look at the green line. Does it change when the rod is longer....no. It only changes when the crank is bigger. That side of the triangle stays the same until you use a larger diameter crank. The triangle only gets taller when the rod is longer.
The dwelling time is affected by the size of the crank diameter. When you use a smaller crank the dwelling time will be shorter.

Jim, of course changing the stroke will effect dwell time, but that was not the original focus of this discussion; it was strictly on the effects of rod length.
As the triangle gets taller due to the larger rod length, the angle at the base of the triangle is decreased; which causes the difference in dwell time at TDC and BDC and most important the offset of piston height around mid stroke which effects port timing greatly.

 

[Chainsaw Jim]

Jim, of course changing the stroke will effect dwell time, but that was not the original focus of this discussion; it was strictly on the effects of rod length.
As the triangle gets taller due to the larger rod length, the angle at the base of the triangle is decreased; which causes the difference in dwell time at TDC and BDC and most important the offset of piston height around mid stroke which effects port timing greatly.

All I'm trying to say is the rod length cannot affect dwell time unless the crank size is also increased. Dwell time is only controlled by the size of the circle of the crank. A larger circle has a flatter line from one point to another. Like the earth compared to the epcot center. The comparison of dwell time would be how far you can see before you lose sight from curvature. With earth as the example you can see for many miles on a flat surface... That's a long dwelling time. But if you are laying on the epcot to peer across it then you would only see a few feet of it. That would be a very short dwelling time.

 

[Rx7man]

Look at the graph, it shows it clearly that you can have different piston positions (thus dwell times) with the same crank stroke but different rod lengths. If the rod were always vertical (infinitely long rod) you'd have a purely sinusoidal piston motion, but as you get shorter, the a small change in con-rod length is going to start to give you a significantly different dwell time... 1mm change in rod length on a 38mm stroke when you have an 80mm rod isn't going to affect the dwell as much as a 1mm change on the same stroke when your rod length is 60mm.

Here is an example from a Husky 65 (38mm stroke, 70mm rod length)

"Effective rod length" is how far above the center of the big end bearing the wrist pin is, it's a function of the angle the rod is at

The first picture is the 70mm (stock) rod, while the second is a 55mm theoretical rod that would probably be far too short to actually work. Look at the Piston Relative Height at 90* to see the effect of the shorter con-rod, as well as the difference in effective rod lengths... The effective rod length of the 70mm rod at 90* is 2.4mm less than it's real length... while for the 55mm rod it's 3mm shorter than it's real length at the same point.. Piston relative height follows suit being .6mm lower, affecting where your ports should be

......

 

[srcarr52]

All I'm trying to say is the rod length cannot affect dwell time unless the crank size is also increased. Dwell time is only controlled by the size of the circle of the crank. A larger circle has a flatter line from one point to another. Like the earth compared to the epcot center. The comparison of dwell time would be how far you can see before you lose sight from curvature. With earth as the example you can see for many miles on a flat surface... That's a long dwelling time. But if you are laying on the epcot to peer across it then you would only see a few feet of it. That would be a very short dwelling time.

That is only from the perspective of the crankshaft. The height of the connection rod pin to crank is a perfect sine wave, but the piston height is not due to the angle change of the rod.

Maybe you'll believe some publications. From "Stock Car Racing Engine Technology HP1506: Advanced Engine Theory and Design", written by the Editor of Stock Car Racing Magazine:

HOW ROD LENGTH AFFECTS PISTON MOTION

Since pistons momentarily stop at TDC and BDC, any change to the rod length affects acceleration and velocity between these two end point. Right here, it may be helpful to make a general statement about how changes in rod length affect piston motion.
As a rule, lengthening a rod tends to cause a piston to remain longer (increased crankshaft angle) around TDC and BDC than with rods of shorter length. Let's assume an example in which the stroke distance is the same for two different lengths of connecting rods. In this case, we know each piston will travel the same TDC-to-BDC distance, and the longer rod will cause increased piston dwell time at the two end points.
Lets examine what goes on in between TDC and BDC by cutting right to the hear of the issue. We know the piston for each rod length will achieve maximum velocity and acceleration somewhere during the stroke. What's important is that this point (crankshaft angle) will be different for each rod length. as a piston approaches or departs TDC or BDC, the longer the rod, the slower this motion will be. As you would expect, the corollary is true for rods of shorter length.

Or you can read MacDizzy article on it. http://www.macdizzy.com/update3.html
And here is another one by Hemmings. http://www.hemmings.com/hcc/stories/2009/06/01/hmn_feature11.html

 

[Rx7man]

srcarr52, both are good articles

So it seems that the Husky 65 with a 70mm rod and 38mm stroke has a 1.85:1 rod to stroke ratio which is pretty long by any standard, the Husky 61 with a 64mm rod and 36mm stroke is 1.78:1... My "manhattan project" will be 1.68:1

From the Hemmings page

The theoretical desired minimum ratio is 1.6; anything above that is considered exceptional. During the 1960s, Pontiac engines were known for one of the highest rod-to-stroke ratios in the industry. This has helped to establish their reputation for torque and longevity. The exceptionally high numerical ratio limits the internal friction, allows cylinder pressure to build and decreases bore wear.

I don't quite understand what he means by "The theoretical desired minimum ratio is 1.6; anything above that is considered exceptional." If it's a minimum, wouldn't it be anything LESS than 1.6 is exceptional?
It sounds like a case of the higher the numerical value of a rear end ratio, the lower geared it is..