3 cyl. vs 4 cyl.

[john_bud]

Have you ever seen a lineup of connecting rods? When I saw this the GM rods were the smallest, ford rods significantly larger, and even figuring that the Cummins rods should be 33% larger they were huge, Cummins is an engine built to last, a medium duty engine in a light duty truck quite literally, so you pay more for a stronger engine.

As far as displacement and cylinders, a lot of small cylenders are better for a high RPM engine and a few large cylinders are better for low end torque on a slow rotating engine, think of Fords 5.0L V8 vs their 5.0L I6, same size, but the V8 is a dog with a loaded truck and the I6, while not fast, is almost unstopable.

Edit...something about ballance, Inline 6, Flat 6, Flat 12, and either a 60 degree or 90 degree V12 (cant remember what one) are all perfectly ballanced engines, no primary or secondary vibrations, you can ballance any engine with weights, but then you are using some of the fuel and power to move the weights, simply having more cylinders won't always improve ballance, but I do prefer the V8 rumble in many instances, the I6 sound good too

This being said I have heard that the inline 3 engines have similar ballancing to the inline 6, but with half the cylinders, perhaps we need to do some testing and teardowns

Napa,

You can't declare a winner based on rod size alone! The straight 6 design has only 1 rod per throw, so the rod can be larger. But, larger has more weight and that puts more stress on the crank at higher rpm's. Like everything else, what is a benefit in one way, is a deteriment in another. (although the cummins I6 is the current king of 3/4 and 1 ton diesel HP! ---> and I have a duramax)

Inline 6 cyl engine from ford (200 & 250, 240 & 300) all have the same issue. Crank breakage at higher rpm's. Tossing that heavy rod / piston puts some major stress into the crank. Great for low rpm's but not high!


I would be glad to do the teardowns. Just send the new engines to me....


jb

 

[nosliw]

Have you ever seen a lineup of connecting rods? When I saw this the GM rods were the smallest, ford rods significantly larger, and even figuring that the Cummins rods should be 33% larger they were huge, Cummins is an engine built to last, a medium duty engine in a light duty truck quite literally, so you pay more for a stronger engine.

i've had all of those trucks -

first was a 1995 dodge cummins 12V w/ 5spd
second was 1999 ford powerstroke 7.3 turbo w/6spd
and my current truck is a 2005 gmc duramax allison 5spd

the dodge had 230K when i bought it, sold it with 260K
the ford had 90K when i bought it, sold it with 95K
and the dmax i bought new, and now has a little over 20K

all three of the big three truck engines are great (let's pretend ford is still using the 7.3 )

now, the comparison of rods is really an inane point. why does the cummins have bigger rods? cause it has a bigger stroke. longer stroke tells me that the rods are experiencing more stress, and need to be built stronger.

does that mean my duramax rods can't take a beating? oh no! i know that the limit for the stock rods on a duramax are very high. sure, they'll start bending @ about 650hp, but for a mildly modified workhorse (like mine!) they'll last forever.

my current truck is my favorite by far. the dodge was slow, and it was a poor quality truck (in my opinion)

the ford was SLLLOOWWWW, but it towed great (dually, 6spd) and never let me down. the reason i traded was because i was eye-ballin' a duramax. it's all about the GM package; great diesel, fantastic automatic, solid, comfortable interior and it's good looking.

the duramax is ridiculously powerful, reliable, and comfortable. i have no trouble pulling 10K (about the highest i have gone) over the mountain passes around here, and all that power equates to smoking out the occasional fast-and-furious wannabe in their honda vtec also, they DO last. the reason you don't see the million-mile duramaxs around is because they only came out in 01. i have pictures on my computer that show the odometer reading 400,000miles +.

i know this is a bit off topic, but you started it just figured i'd throw in my twocents.

ps- cummins is the best engine of the three. i do know that. but what i like about my duramax is that i can plug it into a laptop and now have 500rwhp (upgraded tranny), while the dodges are a bit more 'hands-on'. i'd be wrenching all day.

have a good one

 

[NewToy]

A 3 & 4 cylinder engine with the same size bore will result in higher piston speeds in the 3 cyl. The higher piston speeds will result in accelerated wear of the piston and rings. This may be negligible wear but extra wear indeed. I know single cylinder dirt bikes with a longer stroke and smaller bore are sometimes close to the piston melting from excessive piston speed. The same displacement engine with a big bore/short stroke has appreciably less piston speed. These two engines even though they are the same overall displacement have completely different torque/HP characteristics.

John

 

[napabavarian]

Napa,

You can't declare a winner based on rod size alone! The straight 6 design has only 1 rod per throw, so the rod can be larger. But, larger has more weight and that puts more stress on the crank at higher rpm's. Like everything else, what is a benefit in one way, is a deteriment in another.

jb

My intent wasn't to compare overall engine build by rod size, but generally speaking a heavier duty engine will have larger rods.

As far as high engine speed use, if that is what you want then perhaps a Viper is more up your alley as a truck engine the Cummins has its place making gobs of torque at low RPM and returning excellent fuel economy for the job done compared to other options, if someone chooses to build a hotrod out of it and starts breaking stuff then that was their choice, hotrod anything and you will find the weak link.

Nosliw- As I stated the cummins rods were more than the 33% bigger than fords 7.3 rods, more like twice the size. If I was interested in spending $50k on a new truck it would be GM in a heartbeat due to quallity issues with Dodge and the ford 6.0's reputation, lets hope the 6.4 does better...and that is comming from someone who has never had a truck that wasn't an 8 lug Ford, but I have plenty of miles on all 3 in gas form at various jobs.

GM or Dodge will run strong, and the rods I saw may have been from a 6.5 GM diesel, very economical to run, but not nearly as robust.

Here is the first picture I saw

To be fair in my search I also found this:

The second picture looks more ballanced, Ford VS Dodge look well ballanced, with Dmax looking a bit smaller, but if you tow your 12,000# RV the Dmax would still look like a great choice, if you want to turn the fuel all the way up and find the bigest turbo you can cram under the hood...you are gonna break whatever you drive, I used to have a neighbor with a dodge that broke the transmission input shaft every couple months, granted he insisted on having the fuel turned all the way up, 38" tires, and not pulling his foot off the throttle during wheelhop granted the engine never hickuped.

Newtoy John- I concour 100% that the 3 cylinder will have faster ring wear at the same RPM, however in an economy situation where excessive speed isn't the goal the different torque curve you mentioned allows the engine to be run at a lower RPM allowing longer for the fuel in the cylinders to burn as well as a few other factors that relate to economy.

Back to the origional vibration question, a 4 cycle engine takes 720 degrees of crankshaft rotation, and only 1/4 of that is the power stroke, that would be 180 degrees of rotation, the trouble is that at the top of the stroke the piston would be pushing straight down on the crankshaft and at the bottom the same thing happens, half way down the piston gets the best leverage on the crankshaft, so if 2/3's of that 180 degrees of rotation are effective power stroke then for every 720 degrees of crankshaft rotation each cylinder produces power for 120 degrees.

In a 6 cylinder engine this can be ballanced so that every time one cylinder is ending its power stroke another is starting power stroke, cylinders traveling up and down in 3 pairs.

In an inline 4 the pistons often move up and down in pairs with the end cylinders together and the center cylinders together, this appears to be for ballance purposes, but means that it will have 2 120 degree power strokes, 1 120 degree lag, then repeat, so with the lag and power stroke combinatin it will be more prone to vibration from combustion, granted todays technology seems to have fixed many of these problems with computer controls and better mechanical ballancing.

The inline 3's that I have seen have had 3 cylinders that move up and down an equal intervals, so it can have 120 degrees of power and 120 degrees of lag repeating for a smoother engine.

 

[RobJ]

Napa,

You can't declare a winner based on rod size alone! The straight 6 design has only 1 rod per throw, so the rod can be larger. But, larger has more weight and that puts more stress on the crank at higher rpm's. Like everything else, what is a benefit in one way, is a deteriment in another. (although the cummins I6 is the current king of 3/4 and 1 ton diesel HP! ---> and I have a duramax)

jb

I go with the other guy, rods are sized based on the loads they are expected to carry. Not to be big just to be big.

 

[RobJ]

A 3 & 4 cylinder engine with the same size bore will result in higher piston speeds in the 3 cyl. The higher piston speeds will result in accelerated wear of the piston and rings. This may be negligible wear but extra wear indeed. I know single cylinder dirt bikes with a longer stroke and smaller bore are sometimes close to the piston melting from excessive piston speed. The same displacement engine with a big bore/short stroke has appreciably less piston speed. These two engines even though they are the same overall displacement have completely different torque/HP characteristics.

John

Well ok sort of kinda. The piston in the bike doesn't melt because of the speed of the piston. It melts because of the heat produced. If you design an engine that can't handle the heat it produces it will burn up. You can find some diesel truck owners that found this out the hard way. They installed a 150+hp chip and ran up a long grade with a heavy trailer for 45 minutes. The engine isn't designed to handle the extra fuel producing the 1000 degree exhaust temps, something has to give, piston, turbo, tranny, etc.

I don't think the wear and piston travel have a lot to do with each other either. Dirty oil, air, bad machining, poor assembly and other impurities cause the wear, mainly the dirty things.

 

[FULLPULL]

To me on a tractor 3 or 4 cyl the differance is the 4cyl run much smoother power than a 3cyl.I think the 3 has more knock and the 4 is just smoother power for the same hp rating.As for torqe the 3cyl may win the 3000 ford diesel has alot of grit the one my dad has has 3700+ hrs on it and shoved or dug dirt almost all of them.A TC40D has about the same hp rating as a 3000 the pto @ 540rpm, on the ford is 1800 rpm, on the TC40D is roughly 2500 to 2600rpm.My tc55 has a smooth running engine with the same high rpm to get the power 540@2475 rpm. I dont know what will wear you or the engine out most loud knockin engine or loud wound up engine . You can see this differance and more pronounced on the new skid steers. We got a Case with a 4cyl in it a large cubic inch so it takes less rpm to make smoother power than a 3cyl .At a idle the 3 rattles ya in the unit more vibration and noise.As far as lasting the 3cyl are proved to be tough as well as the 4cyl.

 

[_RaT_]

To me on a tractor 3 or 4 cyl the differance is the 4cyl run much smoother power than a 3cyl.I think the 3 has more knock and the 4 is just smoother power for the same hp rating.As for torqe the 3cyl may win the 3000 ford diesel has alot of grit the one my dad has has 3700+ hrs on it and shoved or dug dirt almost all of them.A TC40D has about the same hp rating as a 3000 the pto @ 540rpm, on the ford is 1800 rpm, on the TC40D is roughly 2500 to 2600rpm.My tc55 has a smooth running engine with the same high rpm to get the power 540@2475 rpm. I dont know what will wear you or the engine out most loud knockin engine or loud wound up engine . You can see this differance and more pronounced on the new skid steers. We got a Case with a 4cyl in it a large cubic inch so it takes less rpm to make smoother power than a 3cyl .At a idle the 3 rattles ya in the unit more vibration and noise.As far as lasting the 3cyl are proved to be tough as well as the 4cyl.

Did you happen to read the link Egon posted? By your post I don't believe so. You list things that may only be unique to you. The link posted what I deemed to be well explained and detailed reasoning on all the various cylinder configurations. While your experience may say a 3 cylinder "knocks" more then a 4 cylinder, many of us can site otherwise or at least detect no noticeable difference as is my case. I have or have had a Kubota 4 cylinder, a Kubota 3 cylinder, a Kubota 5 cylinder and a International 3 cylinder. Maybe Kubota figured out how to make a 3, 4, and even 5 cylinder diesel with no discernable knock between them. Egons link helps sum it up pretty clearly.

 

[FULLPULL]

Yea i noticed this more in the ford nh line.3cyl are tough no doubt the new 4cyl seem smoother running to me .I didnt get to read the link till now.

 

[dgl24087]

The only 3 vs 4 cylinder comparison I can make by the same manufacturer is JD. The 3 cyl. JD 1530 and 2240 diesels do knock more than the 2350 4 cyl. diesel. That being said, the 2350 is a newer tractor than the other 2. As for the Ford 3 cyl. diesel engines, they are a large bore, short stroke engine, whereas the JD 3 cyl. that I have experience with is a small bore, longer stroke engine. The Deere produces more HP out of a smaller cid., and yes, it is noisier and does use more fuel. I'm not going to say one is necessarily better than the other, I have a cousin who bleeds JD green, but purchased a 3 cyl. 3610 Ford last year to handle raking, tedding, brush hogging chores. As much as he hates to admit it, the Ford uses considerably less fuel than the JD 1530 3 cyl., likely due to gearing. The Ford is much faster in all gears than the JD. Both are excellent tractors, although I suspect the higher RPM Deere lugs down less than the Ford.