6 cylinder or 8 cylinder?

[ISAF2009]

Torque doesn’t matter on long grades mechanical advantage does. I have towed the same 20k dump trailer with my ram 3500 which has 385 horsepower and 935 ft/lbs and my buddie’s Ford F-350 with 440 horsepower and 860 ft/lbs and the ford SMOKED my Ram. my ram is geared at 4:10 the ford was geared at 3:73. The size of the cylinders has almost nothing to do with engine size anymore because of the prevalence of forced induction. An engine is an air pump it’s displacement refers to how much air it has the ability to displace and mix with fuel, burn it, then evacuate it. I am looking at a 2015 ford escape with a 2.0 inline 4 that will displace much more than the 1989 ford f-250 with the 460 in it.

[Sforeaker]

I don't know if what I'm describing has an engineering name for it, but here's what I'm wondering.

If you have inline cylinders, during the power stroke each rod is always in the same position, essentially vertical. If you have six cylinders means six power strokes. If each power stroke turns the crank X times and always from the same physical position.

Take the same crank and put it in a V eight configuration. Now you have more power strokes as well as each of them closer together to turn the same crank the same amount of times.

It's like only applying power to a bicycle pedal when pushing down, versus pushing on the pedal from multiple points throughout the crank rotation.

To me, this means you have more frequent and consistent delivery of power in a V8 then an I6.

Based on a previous post showing power = torque x speed, it would appear this is why V8s, overall, are more powerful.

Thoughts?

[TJones]

Originally Posted by ISAF2009

Torque doesn’t matter on long grades mechanical advantage does. I have towed the same 20k dump trailer with my ram 3500 which has 385 horsepower and 935 ft/lbs and my buddie’s Ford F-350 with 440 horsepower and 860 ft/lbs and the ford SMOKED my Ram. my ram is geared at 4:10 the ford was geared at 3:73. The size of the cylinders has almost nothing to do with engine size anymore because of the prevalence of forced induction. An engine is an air pump it’s displacement refers to how much air it has the ability to displace and mix with fuel, burn it, then evacuate it. I am looking at a 2015 ford escape with a 2.0 inline 4 that will displace much more than the 1989 ford f-250 with the 460 in it.

Torque actually does matter when climbing grades. It is torque which is applied to the wheel that produces movement.

In theory engines with identical hp but different torque would perform the same provided that the gear ratio for each engine was always optimal for the wheel torque needed and desired travel speed.

This is not the case in the real world and the engine must work within a range of rpms in a limited number of gear ratios to provide the desired travel speed. This is where torque curves come in. Higher torque at lower rpm improves startabilty as well as prevents / delays down shifting on grades. A flatter torque curve allows the engine to produce more consistent power output across the operating rpm. An engine with a more "peaked" curve over a narrower rpm band would require more gear ratios and more frequent shifting.

Actually displacement refers to a volume calculated by multiplying the cylinder diameter by the distance of the piston travels. (Bore x stroke) You are referring to charge air. The volume of charge air will very depending on temp, engine rpm, boost pressure ect.

Between the 2.0l escape and the f250 with the 460. The 2.0l may use more charge air given the turbo but the 460 will still have more torque.

Ted

[ISAF2009]

You are correct an 8 cylinder engine moves the crankshaft 90 degrees during firing while a 6 cylinder moves the crankshaft 120 degrees.

[BeNimble]

Straight six engine are naturally balanced unlike pretty much any other engine, which needs counter-balancing. They are not compact at all and heavier, which for a bus is not much of an issue, BMW has had great success with them in sports cars.

Torque is what makes vehicles move -- the torque at the wheels. gears increase torque at the expense of speed. With enough torque you don't need gears, like electric motors in a Tesla have a SINGLE gear, not 5 or 6 or 10. And the Prius has just a single gear too, because it uses the electric motors torque at lower speeds.

[ISAF2009]

Actually displacement refers to a volume calculated by multiplying the cylinder diameter by the distance of the piston travels. (Bore x stroke) You are referring to charge air. The volume of charge air will very depending on temp, engine rpm, boost pressure ect.

Between the 2.0l escape and the f250 with the 460. The 2.0l may use more charge air given the turbo but the 460 will still have more torque.

I understand what displacement refers to I was simply making a point that static displacement doesn't matter when we discuss engine power. We are discussing diesels with forced induction. Static displacement doesn't dictate power, dynamic displacement and volumetric efficiency does. All things being equal the 2.0 turbo in the escape will do the same work as the 460 because they are both 245 horsepower. Torque is too often used to sell the public engines that have mediocre performance. I.E. Harley Davidson. Ideally you have a good balance of horsepower and torque Like the 6.7 powerstroke (which I don’t own) rather than lower horsepower and higher torque cummins (which I own). If you put both in front of a trailer on a hill you will notice a big difference, the cummins is a slug compared to the ford. There is a reason why tanks run on turbines and use gearing.

Ted[/QUOTE]

[TJones]

Originally Posted by TJones

Actually displacement refers to a volume calculated by multiplying the cylinder diameter by the distance of the piston travels. (Bore x stroke)

Ted

Correction:

Actually displacement refers to a volume calculated by multiplying the cylinder cross sectional area by the distance of the piston travels. (Bore x stroke)

Ted

[Booyah45828]

The flatness of the graphs depends on it's scaling. If someone has the time, they can convert the scania data to ft lbs and then overlay all 3 on the same graph. But, I've got a feeling they'll all be relatively the same.

In my eyes, the torques are close enough to the same, but the 100+ hp is huge. Yes, hp is just a calculation vs a measured figure like torque, but the HP gap being so large shows that there is way more torque available between peak torque and rated rpm. The dd16 drops 250 ftlbs between 1200 and 1800 rpms, the cummins isn't clear but looks to drop as well, whereas the scania GAINS around 200 lbft of torque in that area.

Peak numbers are great, but it's the area between the peaks that matters.

[rossvtaylor]

You all are getting really deep! I appreciate the depth and breadth of knowledge shared by the members here. Personally... 8 or 6?...I'd have to answer 42...that's the right answer.

[EastCoastCB]

Originally Posted by Mgenin

Hi there, I am looking to buy my bus and convert. I currently live in a 40 ft D444T 8 cylinder international. I am having a hard time finding buses for sale with 8 cylinders. Most I find have 6. More cylinders = more power right ?

I’m concerned that if I get a full sized (40 ft) 6 cylinder bus I’ll have issues driving mountainous terrain, ie not enough power.

What are your thoughts on this? Anyone built out a 40 footer with 6 cylinders and have experience driving around in the west ? Help please !

In the diesel world the straight six is king.

[s2mikon]

Does anyone here remember the Mack e9? A company I worked for had 2 of them and at 500 hp they would haul the mail! They made the new 3406 cats we bought seem puny. But they wanted better fuel economy so we had to relegate the Macks to back-up status. Shortly after NM went to low emission diesel the injection pumps on both of them failed and they cut them up for scrap. I missed them.

[Booyah45828]

Yup, for a time the big dog engines were v8 diesels.

Then fuel prices skyrocketed, and big displacements were out, and the big v8's went away.

[Mgenin]

This has all been informative. Can you explain to me what no replacement for displacement means in other words?

[Mgenin]

Originally Posted by rossvtaylor

You all are getting really deep! I appreciate the depth and breadth of knowledge shared by the members here. Personally... 8 or 6?...I'd have to answer 42...that's the right answer.

I appreciate the simplicity here ! Ha

[s2mikon]

Generally, the more cubic inches the more foot pounds of torque. Positive manifold pressure increases effective cubic inches. Generally you can take 2 250 HP engines and the one with more displacement will have more torque.

[TJones]

Originally Posted by ISAF2009

Actually displacement refers to a volume calculated by multiplying the cylinder diameter by the distance of the piston travels. (Bore x stroke) You are referring to charge air. The volume of charge air will very depending on temp, engine rpm, boost pressure ect.

Between the 2.0l escape and the f250 with the 460. The 2.0l may use more charge air given the turbo but the 460 will still have more torque.

I understand what displacement refers to I was simply making a point that static displacement doesn't matter when we discuss engine power. We are discussing diesels with forced induction. Static displacement doesn't dictate power, dynamic displacement and volumetric efficiency does. All things being equal the 2.0 turbo in the escape will do the same work as the 460 because they are both 245 horsepower. Torque is too often used to sell the public engines that have mediocre performance. I.E. Harley Davidson. Ideally you have a good balance of horsepower and torque Like the 6.7 powerstroke (which I don’t own) rather than lower horsepower and higher torque cummins (which I own). If you put both in front of a trailer on a hill you will notice a big difference, the cummins is a slug compared to the ford. There is a reason why tanks run on turbines and use gearing.

Ted

[/QUOTE]

I have heard that people really like the performance of the powerstroke 6.7. There is alot of original design in the engine.

Ted

[BeNimble]

The replacement for displacement is magnetics!! i.e. electric motors.

[Bon Voyage]

Here’s a couple videos I watched a little while ago. Kinda related to topic but kinda not. Unfortunately he doesn’t look at 8 cyl engines. The second video will make more sense if you watch the first one first.

https://youtu.be/82rxavW0A3c

https://youtu.be/VQJ0kYFF9T4


As to the question of reliability in a theoretical world a 6 cylinder engine has fewer moving parts and therefore hopefully fewer points of failure than a 8 cylinder. But some 8’s are more reliable than some 6’s; and some 6’s more reliable than 8’s.

[mmoore6856]

Originally Posted by rossvtaylor

You all are getting really deep! I appreciate the depth and breadth of knowledge shared by the members here. Personally... 8 or 6?...I'd have to answer 42...that's the right answer.

the answer to lifes most important question and it did not take a million years

[ChuckRatto]

I have a 6 cyl Cat engine making about 215hp. It in a 32 ft and we tow an additional 7000lbs behind for a total groos weight just under 30,000 lbs. Have toured 11 western states. It goes up okay, just wishing I had an engine or drivline retarder for the 6-7 % downhills.