forgive the newbie HP, cylinder question, but...

   / forgive the newbie HP, cylinder question, but... #1  

bx24d

Silver Member
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Jul 10, 2007
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197
Can someone explain simply the benefits of #of cylinders vs. HP. What impact does increasing either generally have? I'm in the market for upgrade and see 3 vs. 4 cylinder machines with various hp ratings.
 
   / forgive the newbie HP, cylinder question, but... #2  
Good question and I don't know the answer. I know that JD has 3 cylinder engines in fair sized ag tractors like the 5400 that make 65 hp. So I don't think its just a hp issue.
 
   / forgive the newbie HP, cylinder question, but... #3  
I think the longer stroke of a 3 cyl vs 4 cylinder of the same HP has more torque. Down side is, pistons have more move movement than a 4 cylinder causing more wear on cylinders. Just my thought....That is only if both engines had the same Cubic inches. Longer stroke will give more torque.
 
   / forgive the newbie HP, cylinder question, but... #4  
Some say the 4 cyl is smoother some say the 120 degree firing of the 3 cylinder is just as smooth or smoother. (both are very smooth to me!) Pretty much for every point there is a valid counter point.

You probably should go after the tractor specs in a "black box" mind set. Ignore how it gets the power, just look at the torque curves and compare. Then look at the fuel consumption for each. If you can find a dramatic decision making / breaking difference - well, you're a better man than I, Gunga-Din!

A bigger difference is the rpm where peak torque and peak HP are made. You want to have that point be lower for tractors.

A bigger concern is how the machine fits your body and your sense of ergodynamics. Only way to tell is lots of seat time.

Have fun,
jb
 
   / forgive the newbie HP, cylinder question, but... #5  
The number of cylinders doesn't really make any difference, what really matters is displacement. All other things being equal I'd choose a bigger displacement engine over a smaller one because it'll tend to be less "peaky" with it's power band.
 
   / forgive the newbie HP, cylinder question, but... #6  
I agree with the other posters. I'd bet on an equal hp 4cyl and 3cyl engien that the 3 cyl will have more torque.. Id also expect a 4 cyl to be smoother than a 3 cyl. A 3 cyl will hav emore unit wear on each part.. but a 4 cyl has more parts to wear. I'd call it even up.. get the hp you need and don't worry so much about the power plant...

soundguy
 
   / forgive the newbie HP, cylinder question, but... #7  
BX24D
The number of cylinders should have no bearing on your decision. There is no appreciatable advantage to the operator with either configuration. 3 having more torque with each firing, but fewer firings in a 720 degree cycle is a wash. 4 cylinders at 180 degree firing order means the worst possible design for vibration, 2 up and 2 down, never one coming up and around when one is firing, 3 cylinders eliminate this with their 240 degree firing, but with fewer per cycle, it too tends to vibrate, although usually less.
 
   / forgive the newbie HP, cylinder question, but... #8  
It seems like I helped a friend work on an older JD 4cyl that had some sort of counter rotating shaft that was supposed to cancel out vibration....but I may be remembering this wrong.
 
   / forgive the newbie HP, cylinder question, but... #9  
N80, Sorry I didn't mention those, seems they are used in many 4 cylinder engines, mostly larger ones. They effectively counter act the vibrations and work very well. Most of the auto builders use them in engines around or over 2000 cc's. Not sure which tractor engines have them and which don't. I imagine like most technoligies, that drift down into everyday use, most 4 cylinder engines today have them.
 
   / forgive the newbie HP, cylinder question, but... #10  
Yeah, every engine I've ever worked on has a harmonic balancer. Its attached to the crank and should dampen vibration caused by firing of the cylinders. I assume tractors would have them as well, but never took one apart to see. In older engines, the fly wheel generally has a lot of weight to help dampen the bursts of power from cylinders firing.

My opinion on the 4-cyl vs. 3-cyl debate- More is better, but will probably take more fuel to run. Cars often have same horsepower between high-end 4-cyl and mid-range 6-cyl, or high-end 6-cyl and mid-range 8-cyl engines. Even though horsepower is the same, it seems like more cylinders makes more power. Not sure why though.

Usually, if the pistons are smaller, the engine turns higher RPM's. So for a 4-cyl tractor, its probably going to run 3200 rpm for 540 PTO rpms, which is where the power band is. With 3-cyl, its probably more like 2700 rpms for 540 PTO rpms. And keeping the engine in the power band under load is the trick.

The higher rpms usually feels like you have more power. I don't know if you actually have more power, because in theory it should be the same. But I know that more cylinders and higher RPM's "feels" like more power in most cases. The trade-off is that at 1000 rpms or less, it feels like more cylinders has less power. So at low speeds, torque will be higher with bigger cylinders turning slower. And often people run tractors at low rpms, so I think that is why 3-cyl is more popular than 4-cyl. That's my unscientific opinion, but if you take some test drives, you can probably feel the difference yourself.

What you should do is analyze your intended usage. If you plan to run attachments all the time at high rpm's, get the 4-cyl. If you plan to use your tractor like a utility tool doing anything and everything, get the 3-cyl.
 
 
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