Long rods vs short rods

[linuxman51]

This is a continuation of the chevy rod discussion below, but it doesnt deal specifically with that, so I'm starting a new thread.

There was some debate as to wether or not longer rods would be a good thing, so I've been researching the topic on the net (for my own benifit, but i'll share what i've found so far).

Long rod pros:
Less rod angularity
Higher wrist pin location
Helps resist detonation
A lighter reciprocating assembly
Reduced piston rock
Better leverage on the crank for a longer time
Less ignition timing is required
Allow slightly more compression to be used before detonation is a problem
Less average and peak piston velocity
Peak piston velocity is later in the down stroke
Less intake runner volume is needed

Cons:
Closer Piston-to-valve clearances
Makes the engine run a little more cammie at low rpm
Reduces scavenging at low rpm

Short rod pros:
Increased scavenging effect at low rpm
Helps flow at low valve lifts (a benefit if the heads are ported with this in mind)
Slower piston speeds near BDC
Allows the intake valve to be open longer with less reversion
More piston-to-valve clearance
Can allow for a shorter deck height

and cons:
More rod angularity
Lower piston pin height (if the deck is not shorter)
Taller and heavier pistons are required (again, if the deck height is not reduced)
More ignition timing is required for peak power

Things that I gleaned that are of direct interest for those of us that are boost-assisted:
Ignition timing.. less advance is needed with longer rods, which reduces the chances of detonation(well, detonation brought about by aggressive timing). Another thing of direct interest is longer rods don't move as fast and thus lower the reciprocating load placed on the rod, which is supposedly the leading cause of rod failure..

thoughts, comments, ideas?

 

[JohnLane]

Only one rule need apply.
Fit the longest damned connecting rod that you can.
Get that engine block with the higher deck so that you can fit a longer connecting rod.
Pluses in addition to what you mentioned: Wider powerband without changing anything else about the engine.
More 'dwell' time at top and bottom dead center.
At TDC this allows cylinder pressure to build much higher after the mixture is lit before the chamber opens up making for higher pressure during the 'work' portion of the power stroke. This means in many cases measurable increases in power just with changing connecting rods and pistons.
Minuses: Can't honestly think of any, unless hood clearance were keeping you from making a block taller.
What is your Rod/Stroke ratio as standard? If you are able to go from a ratio of 1.8 to 2.0 that will make a difference. Do it.
JohnLane.

 

[Hank Scorpio]

Exactly, iirc longer rods help make torque, but trade off for rpm (ussualy, unless going for forged rods) this was the big pontiac vs chevy thing.

Pontiac had long old, heavy rods, great for diesel like torque, but it was a 5500 rpm TOPS motor, vs a sb chevy that would wind to 6k or more without problems.

Doug

 

[linuxman51]

Hmmm... the articles i've been reading play them off more as a low rpm vs high rpm difference, in that shorter rods make? more power at the lower end and midrange, whereas longer rods come in the midrange and do better at the top end, and the reason was with higher rpms, the short-rods can out run the flame front...

 

[Captain Bondo]

well as rpm increases, piston speed increases, total timing requirement increases, piston dweel becomes a premium. Long rod=lower pistons speeds and longer dwell=more cylinder pressure.

 

[volvorsport]

Simple harmonic motion , the shorter the rod , the more vibration you will get . Thats why you have balancer shafts and such .

Keep the rods as long as possible

 

[Kjets On a Plane]

Wasn't that one of the things volvo was thinking with the B230? The early B230 is kinda weak, but I would imagine the light long rods and such made it balance pretty nice. What is the rod ratio on a B230 vs. B23 anyway? I always thought that teh B23 was better then than a late B230, but considering how much lighter everything is on the late B230 and the longer rods I am not so sure. Also dave barton has a stroker B23, now considering the B23 has a good but not great rod ratio, you would need shorter rods and a larger crankshaft for a stroker. This poses a problem since the rod ratio would be pretty bad with a stroker B23, just food fro thought. Volvos have a pretty decent rod ratio even on B21 B23 engines at stock stroke as is though. Do the volvo racing motors, like the Grp A B21ET and B230R use longer rods or have a higher deck height? The deck height on the B23 is about perfect with stock stroke, but no less could really be done since the pistons are already what .001 above deck...so do the volvo racing engines have a higher deck to allow bigger rods and such? It would be cool to have a B23 with a 96mm bore and 96mm stroke, but the deck height just doesn't seem to support that notion so well. Thoughts?

 

[Anonymous]

There is at least one big, brilliant name who advocates a shorter rod/stroke ratio. He builds turbo motors and small bore 4 cylinder motors as well as having a long reputation with American muscle. There are advantages to both styles of rod/stroke philosophies, and one must examine the engine being built and the uses it's being built for to determine which one is right for you.

Oh, and John: big rod/stroke ratios speed up the piston at BDC and slow it down at TDC, not slow it down at both ends. Granted, the last thing that a piston needs is to be slower at BDC, but there you have it.

 

[JohnLane]

There is at least one big, brilliant name who advocates a shorter rod/stroke ratio. He builds turbo motors and small bore 4 cylinder motors as well as having a long reputation with American muscle. There are advantages to both styles of rod/stroke philosophies, and one must examine the engine being built and the uses it's being built for to determine which one is right for you.

Oh, and John: big rod/stroke ratios speed up the piston at BDC and slow it down at TDC, not slow it down at both ends. Granted, the last thing that a piston needs is to be slower at BDC, but there you have it.

Guess again Tiger. Longer connecting rods make for longer 'dwell time' with the piston moving very slowly at top dead center and bottom dead center. Think about it.. ...What is happening differently at BDC then TDC? Exactly. The crankshaft is going round and round. You will have exactly the same number of degrees of the piston moving very little at BDC as TDC. Don't care to believe me? Get out your degree wheel and measuring calipers and check for yourself. (stifling a laugh).
Please feel free to point out no BS reasons to have shorter rods with the exception of doing so to have a longer stroke. Good luck. This is exactly why you will find slipper pistons in any manner of race motor with as long a connecting rod as they are able to fit. Hmmmm........
Thoughts? JohnLane.

 

[Anonymous]

Tiger???

Goemetry is what happens differently at TDC and BDC - that's all. When the crankshaft rotates, say 20 degrees, the crankpin goes sideways and up (or down). The sideways motion pulls the connecting rod over into an angle from vertical, which shortens the vertical length of the rod.

With the crank coming from TDC to ATDC, the crankpin pulls the piston down and the rod angularity pulls the piston down further. With the crank going from BDC to ABDC, the crankpin pushes the piston up, but the rod angularity pulls the piston back down a little bit. I could go into the math behind this, but go ahead and sketch it out for yourself. You may be surprised.

You ever thought about why peak piston speed is typically between 70 and 80 degrees from TDC, and not 90 degrees?

Think about this - a crank with an 80mm stroke and a rod with 40mm from wristpin to big end. When the crank's at 90 degrees ATDC, the rod will be at 90 deg. from vertical, and the piston will be stationary. For the next 180 degrees, the rod length is the same as the radius of the stroke, and the piston will not move. This is an extreme (and impossible example of the situation, but completely valid.

Don't be so condecending - there are other people out there who know what they're talking about. If you want to get out your own degree wheel and try it, be my guest.

 

[Kjets On a Plane]

Unless you are worried alot about piston skirt length, then longer is oftten better. The B230 has a rod ratio of about 1.905 which is pretty decent. IT seems to me that unless you are looking for tons of power, a well prepped B230 with late B230 pistons would do the trick. Does anyone know the skirt length of pre 93 pistons vs post 93 pistons? It may be pretty insignificant, but I would guess volvo did exactly what JL was saying fit the longest rod and longest skirted damned piston that will fit in there. I also found out the the B230R has a higher deck so you could make it a stroker with a good rod ratio and have a 3.0 to boot. Of course that is if you have >$3000 to blow on this thing. IT seems that unless you are after a displacement increase, you can rain tons more money into fancy turbos and head designs before this is a huge issue, atleast on a B230 and for most of us a B23 as well. Even though I have put some thought into this here and it is a very interesting subject, for 275 reliable to 300 reliable hp with plenty of torque and 2.3 liters this is about the last thing on my mind. So how many of you have broken a post 93-95 B230 anyway?

 

[JohnLane]

Tiger???

Goemetry is what happens differently at TDC and BDC - that's all. When the crankshaft rotates, say 20 degrees, the crankpin goes sideways and up (or down). The sideways motion pulls the connecting rod over into an angle from vertical, which shortens the vertical length of the rod.

With the crank coming from TDC to ATDC, the crankpin pulls the piston down and the rod angularity pulls the piston down further. With the crank going from BDC to ABDC, the crankpin pushes the piston up, but the rod angularity pulls the piston back down a little bit. I could go into the math behind this, but go ahead and sketch it out for yourself. You may be surprised.

You ever thought about why peak piston speed is typically between 70 and 80 degrees from TDC, and not 90 degrees?

Think about this - a crank with an 80mm stroke and a rod with 40mm from wristpin to big end. When the crank's at 90 degrees ATDC, the rod will be at 90 deg. from vertical, and the piston will be stationary. For the next 180 degrees, the rod length is the same as the radius of the stroke, and the piston will not move. This is an extreme (and impossible example of the situation, but completely valid.

Don't be so condecending - there are other people out there who know what they're talking about. If you want to get out your own degree wheel and try it, be my guest.

Me? Condecending? Never. Tiger? Oops. Sorry. Should have been "Ducky." Heehee. Relax dood.
I am curious as to just how you figure that the piston is to know the difference between the 60 degrees at the top of the stroke and the 60 degrees at the bottom of the stroke when the crankshaft is directly underneath the cylinders. Hmmmmm.........
Your 'impossible example' is just that. Not a valid example of just how things happen inside our engines. We are all running rod/stroke ratios of 1.7 to 2.0 or so. Please show me the benefit of running a shorter rod if you can honestly come up with one. That is about the long and short of where I am going with my tongue in cheek rant. Relax and have a nice day.
JohnLane.

 

[CNGBrick]

I have to agree with Matt on the longer dwell at BDC for short rods. His example is extreme but a useful illustration.

Now back to our regularly scheduled programming... :wink:

Richard Thomas

 

[Anonymous]

Okay, I guess you're just playing with me a little bit. I suppose that's alright...

Picture an engine in your head. Cut the view through the center of the cylinder, with the cylinder standing straight up, and the crank at TDC. Everything's in line, piston is at the top of the bore. The Y direction is parallel to the bore and the X direction is parallel to the floor. Zero X and Zero Y is at the center of the main bearing. Got it?

Turn the crank 30 degrees clockwise. The crankpin goes around in a circle, right? The distance it travels Y and the distance it travels X are determined by a triangle with the hypotenuse as 1/2 the stroke. The greater the amount you turn the crank, the further to the right the pin goes until you get to 90 degrees, then it starts coming back to the center at 180 degrees.

The big end of the connecting rod must also go to the right between 0 and 90 degrees, and then back to center from 90 to 180 degrees, agreed? Well, since the wristpin is held at X=0 by the bore/piston, the connecting rod lays over at an angle at anywhere between 1 and 179 degrees. This angle brings the piston down somewhat in the bore, because the length of the connecting rod is constant and again the hypotenuse of a triangle.

So, the greater the X distance of the crankpin, the more angular the rod is, and the shorter the Y distance is between crankpin and wristpin.

When you go from 0 to 30 degrees, the Y of the crankpin gets smaller and the X gets bigger. Since a larger X means shorter Y for the connecting rod, the piston comes down the combined distance of the two (two negatives, becoming a larger negative). When you go from 180 to 210 degrees, both the Y of and the X of the crankpin becomes greater. Since the X becoming greater means the Y of the connecting rod gets SMALLER, combining the two means the piston doesn't travel as far during the bottom half of the stroke as it does during the top half (positive and a smaller negative, becoming a smaller positive).

You don't have to believe me if you don't want. Go measure it. My impossible example was not for you to just scoff at and ignore - it's showing you the extreme case where the piston doesn't move AT ALL during the bottom half of the stroke. It is a valid example of the geometry inside an engine, but not an engine anyone's going to find in their car. The longer rod/stroke ratio you have means the less difference you have between piston speed at TDC and BDC. An infinately long rod would have the same piston speed and the same dwell at both ends.

Benefits of a shorter connecting rod? Well, since the piston moves faster in the top half of the stroke, it expands the combustion chamber the most when it's smallest, which generates higher vacuum, sooner. This pulls more air/fuel through the intake valve (or pushes exhaust through the exhaust valve as the combustion chamber is getting smaller), which gives a poor-flowing head an advantage. It also creates a stronger pull through a too-big port, which brings the velocity up higher at a lower RPM. Both of these generally bring the RPM band of an engine downwards, which is pretty much important in a street engine. As you know, max HP is fine but it doesn't necessarily accelerate the car. I outlined some reasons in the other thread (B23 Performance Ideas), some more important than others.

 

[JohnLane]

Okay, I guess you're just playing with me a little bit. I suppose that's alright...

Picture an engine in your head. Cut the view through the center of the cylinder, with the cylinder standing straight up, and the crank at TDC. Everything's in line, piston is at the top of the bore. The Y direction is parallel to the bore and the X direction is parallel to the floor. Zero X and Zero Y is at the center of the main bearing. Got it?

Turn the crank 30 degrees clockwise. The crankpin goes around in a circle, right? The distance it travels Y and the distance it travels X are determined by a triangle with the hypotenuse as 1/2 the stroke. The greater the amount you turn the crank, the further to the right the pin goes until you get to 90 degrees, then it starts coming back to the center at 180 degrees.

The big end of the connecting rod must also go to the right between 0 and 90 degrees, and then back to center from 90 to 180 degrees, agreed? Well, since the wristpin is held at X=0 by the bore/piston, the connecting rod lays over at an angle at anywhere between 1 and 179 degrees. This angle brings the piston down somewhat in the bore, because the length of the connecting rod is constant and again the hypotenuse of a triangle.

So, the greater the X distance of the crankpin, the more angular the rod is, and the shorter the Y distance is between crankpin and wristpin.

When you go from 0 to 30 degrees, the Y of the crankpin gets smaller and the X gets bigger. Since a larger X means shorter Y for the connecting rod, the piston comes down the combined distance of the two (two negatives, becoming a larger negative). When you go from 180 to 210 degrees, both the Y of and the X of the crankpin becomes greater. Since the X becoming greater means the Y of the connecting rod gets SMALLER, combining the two means the piston doesn't travel as far during the bottom half of the stroke as it does during the top half (positive and a smaller negative, becoming a smaller positive).

You don't have to believe me if you don't want. Go measure it. My impossible example was not for you to just scoff at and ignore - it's showing you the extreme case where the piston doesn't move AT ALL during the bottom half of the stroke. It is a valid example of the geometry inside an engine, but not an engine anyone's going to find in their car. The longer rod/stroke ratio you have means the less difference you have between piston speed at TDC and BDC. An infinately long rod would have the same piston speed and the same dwell at both ends.

Benefits of a shorter connecting rod? Well, since the piston moves faster in the top half of the stroke, it expands the combustion chamber the most when it's smallest, which generates higher vacuum, sooner. This pulls more air/fuel through the intake valve (or pushes exhaust through the exhaust valve as the combustion chamber is getting smaller), which gives a poor-flowing head an advantage. It also creates a stronger pull through a too-big port, which brings the velocity up higher at a lower RPM. Both of these generally bring the RPM band of an engine downwards, which is pretty much important in a street engine. As you know, max HP is fine but it doesn't necessarily accelerate the car. I outlined some reasons in the other thread (B23 Performance Ideas), some more important than others.

Interesting thinking here. I see your point about a short rod working for a really crummy head design (lets think 'Merikanski with pushrods from the 60's and 70's). Thankfully we are not working with that manner of animal in any of our cars that are Swedish or (in my case) Frenchie powered. Ponder this one: We are running turbo motors, all with a boost threshhold in the neighborhood of 3000rpm's and up. We are making our power in a fashion that it makes sense for our motors to have longer rods to widen our powerband up high with our milder (turbo) cams. At some point I would be curious to see the difference on a dyno chart in an engine that went from a 1.7 rod ratio to a 2.1 rod ratio. Bet that we would find a wider powerband that goes up higher in the rev range for zip for downside. Whattya think? JohnLane.

 

[Anonymous]

I'm not even sure about this - the highest revving street car motors right now (hondas) have a R/S ratio of about 1.55:1. I think the stronger pull through the ports would actually broaden the powerband rather than the other way 'round, but I've never seen any dyno pulls to verify this. Granted, Hondas often see improvements in top end power with longer rods, but again I've never seen back-to-back dyno pulls to show the point that the longer rod design takes over. The longer rods would also have less friction, from lower peak piston speed and also from less skirt loading, contributing to the higher RPM improvements.

One thing that sticks in my head is the fact that the short rod motor moves the piston faster near TDC - so what happens when the cylinder detonates? The piston is moving away from TDC faster, opening the combustion chamber quicker, reducing the time and pressure that the detonation exerts on the piston/rod/crank before the crank turns enough that the pressure can actually drive the piston downwards. The same could be said for the combustion pressure, but it seems to me that an engine that's more likely to be abused might survive longer under detonation if it's equipped with shorter rods.

On turbo motor design, because the shorter rod motor has less TDC dwell (that much we agree on, right?) it's less sensitive to valve overlap. Since the turbo motors are pretty valve-overlap sensitive to begin with, would it make sense to use a design that's less sensitive? Especially, given our limited cam selection (for cheap - for $$$ anything's available.) Also, a more thermally efficient motor (quicker burning of a long rod motor is more efficient, right?) would put less heat and therefore energy into the exhaust, making it less suitable for a turbo application.

We've still got heavy cars and relatively small motors, so I'd lean towards the better torque at lower RPM for my own car. Obviously different if I was in your shoes, with a race car.

I'm a believer in long-rod technology, by the way. I'm just tossing these design theories out for discussion and thought... I think a normally aspirated motor benefits more from a long rod design is all.

Matt

 

[iadr]

Matt and I discussed this at length :wink: offline almost a year ago. He has made up an Excel formula/spread sheet that does all kinds of factoring to do with geometry in a shortblock.
He's got it covered from an engineering perspective, John...
You accused me of "thinking to much" in another thread, so I'll admit that application of the figures may not be of immediate interest, but it's good to understand the motion.
The way I hear it, the general consensus from the higher levels of race are that there are ideal connecting rod ratios for each type of racing. Partly due to RPM range and partly due to the more difficult to describe quality of accelerative torque- the engine's abilty to accelerate under load, as opposed to the old steady state torque loading on a dyno converted to HP. The two will never be too far apart, but they do diverge. Inertia dyno's pick up things that water brake dyno's don't. Anyway rules of thumb might look like this:
pure street: 1.55-1.70
perf/hi-RPM street 1.65-1.75
short track, road race & most drag 1.75-1.85
other drag & superspeedway: 1.8-1.95
Like I say, my version of what I see/hear is believed best by different classes of racing. Nobody runs over 2.0.

On a lighter note, Matt you want to figure the geometry of this (or is it the same?: http://www.keveney.com/Atkinson.html

 

[Kjets On a Plane]

[quote:583b48a50a]pure street: 1.55-1.70
perf/hi-RPM street 1.65-1.75
short track, road race & most drag 1.75-1.85
other drag & superspeedway: 1.8-1.95[/quote:583b48a50a]
So how do you explain volvo going from a 1.77? rod ratio in the B23 to a 1.905 in the B230? The B230 is supposed to be a street torque motor no? Volvo by decreasing bearing area and adding longer rods must have been thinking efficancy and low end power. It is supposed to keep some of that exhaust heat in for greater thermal efficancy, though the exhaust side of your heads isn't exactly helping. I didn't see what Matt meant, so I didn't reply, but now I see what he's taliking about. Just didn't want to take sides too early when I didn't know. :wink: What do you get fro the rod ratio in your "french" beast?

 

[Anonymous]

Jim - I think you hit on the #1 reason for the longer rods earlier - better balance. If the pistons move the same speed at the top of the stroke as at the bottom of the stroke, the engine won't vibrate nearly as much as it would with a really short R/S ratio. (2 pistons decelerating at 200 Gs upwards and 2 pistons decelerating at 180 Gs downwards are more closely matched than 2 at 230 upwards and 2 at 150 downwards. These numbers are right out of my arse, here...) Longer rods within the same stroke allow lighter piston design, shorter skirt length, and slower peak speed, all of which should reduce friction, which is what the B230 was all about.

John's stroker B23 is a custom piece. Custom rods, pistons, crank... He doesn't have to adhere to B23 specs, so his B26 can have any R/S ratio he wants. In fact, he used 156mm rods and an 89mm stroke, so his R/S ratio is slightly less than stock. (1.75 vs. 1.81)

Ian - I never answered you about the Atkinson motor before, because frankly it scares the poop out of me. Some observations, though: The piston moves faster near the bottom of the intake stroke than it does at any other point - I'm not basing this on the actual HTML movement, but rather on the fact that the green connecting rod is at the "top of it's stroke" during that phase. The fact that the piston goes through four cycles within one crankshaft revolution makes it intriguing, but I think that idler link at the frame would suffer pretty badly in a modern, high speed application. Also the amount of piston deceleration at the bottom of the stroke is amazing - probably limiting this engine's top speed more than "toss mode" does in a conventional engine. Scary! What was this engine's application, and what were it's design objectives?

 

[iadr]

[quote:22a2c111f0]John's stroker B23 is a custom piece. [/quote:22a2c111f0]

Oops! Hang on here, Matt- JohnLanes' car is a V6-( the one we all love, err, not quite- it'a a b280) with Eagle (Premier) sleeves and heads. It has little or no offset grinding on the crank (pretty sure none) and purely custom rods and pistons. Gives 3.0 displacement. I don't recall him ever getting into compression ratio, turbo trims or that sort of detail- even boost level I forget if I heard, but it's agreed it works- a well sorted package.


[quote:22a2c111f0]Ian - I never answered you about the Atkinson motor, though: The piston moves faster near the bottom of the intake stroke than it does at any other point - I'm not basing this on the actual HTML movement, [/quote:22a2c111f0]

Good- I'm thinking the drawing is at fault in one aspect because the piston never reaches quench (ie up to the deck) on the combustion cycle. On second thought, that's entirely possible though. Gives further complexity...
The original usage was in replacing steam engines in stationary app's in England in (I'm guessing here) late 1800's. Mine hoists and so on.
I just thought it was interesting, because it appears to me that one could *adjust* many of the geometries which are fixed on a conventional engine.


[quote:22a2c111f0]but rather on the fact that the green connecting rod is at the "top of it's stroke" during that phase. The fact that the piston goes through four cycles within one crankshaft revolution makes it intriguing, but I think that idler link at the frame would suffer pretty badly in a modern, high speed application. Also the amount of piston deceleration at the bottom of the stroke is amazing - probably limiting this engine's top speed more than "toss mode" does in a conventional engine. Scary! What was this engine's application, and what were it's design objectives?[/quote:22a2c111f0]