Straight 6 vs. V 8

[MPMurphy]

Original Questions

Grizzly... The longer crank in the I6 does not increase nor improve the torque. The torque applied to the crank is generated by the force in the connecting rod acting at a distance from the center of the crankshaft. Longer stroke engines create more torque for a given rod force due to the bigger moment arm between the center of the crank pin and center of the main bearing journals. The Ford V10 might have a small displacement, but it makes a lot of power because the engine turns at high RPM. Power moves the load, not just torque. Power is torque multiplied by RPM, so an engine that makes lower torque can make high power by turning very fast. High power engines that turn at high speeds tend to produce small amounts of torque at low RPM's, making them not as pleasant to drive as an engine that creates a lot of torque at low RPM's, like the Cummins B engine. I hope this helps, let me know if anything is not clear...

 

[Hohn]

Does the longer crank of the I6 help develop more torque then a V8 shorter crank?
Another question. The Ford gasser V10 seems to be a popular engine on class B motorhomes. The total displacement size is not particularly big, that means that the bore and stroke must be small. What does that do to the torque?

Can anyone answer my two questions?

Torque is force acting at a distance. The distance part of this STROKE (also think of it as lever arm length). The Force part is Combustion Pressure. Since combustion pressure varies so greatly, we use something called BMEP-- Brake Mean Effective Pressure, or how much average pressure there is pushing down on the top of the piston.

Pressure itself is a ratio: Force per unit of area. If I have 200lb pushing on 1sq in, I have 200psi. But if that same force acts over 200 sq inches, I only have 1 psi!

By now you can probably see how increasing the bore size will increase torque because it increased the surface area acted upon by the combustion pressure.

For example, the same 200psi acting over 11sq inches instead of 10 sq inch means you get 2200 lb of force instead of 2000-- a whopping 10% increase.

So, if you have the same amount of cylinder pressure, increasing EITHER bore or stroke will increase torque output.

An inline 6 doesn't inherently make more or less power or inherently have higher or lower efficiency than a V-8. Cylinder configuration matters very little overall in terms of power production. More important is displacement, volumetric efficiency, and such.

You can reasonably compare the "thermal area" of two different engines configuration of similar displacement. The "thermal area" is just my term for the sum of all the surface area that can lose combustion heat. It's basically the surface area of a cylinder (inluding cylinder head and piston crown) times the number of cylinders. This area has a lot to do with how much heat is converted to power instead of lost to engine coolant.

In general, a given engine design becomes more efficient as it gets larger in scale. So if you take the exact same engine design (all geometries) and double its displacement (again, all in scale), efficiency overall will improve.


Now let's address rod length and stroke. The individual measurement of either really doesn't matter. Rather, it is the ratio of one to the other, typically between 1. 4 (on the very short end) to 2:1 on a long rod engine. The ratio of rod to stroke (R/S ratio) is what determines geometry, so one only matters in relation to the other as far as geometry goes.

R/S matter because it determines the exact shape of the sinusoidal wave pattern followed by the pistons as it oscillates up and down. A lower ratio gives a steeper slope to the line and increases dwell time at either TDC or BDC.

In application, R/S changes how the engine breathes and how the components experience stress. A Shorter R/S (shorter rods for a given stroke) generally let the engine breathe better because the piston will accelerate faster away from TDC and "suck" harder on the intake. You could run a bigger cam with shorter rods and still have the same idle.

But shorter rods also increase the amount of force applied to the cylinder walls instead of the piston, increasing friction. Shorter rods are under more stress because of the higher rate of acceleration on both ends.


Sorry I have to go-- ask away and I'll answer when I get back.

jh

 

[HRDROKN]

Pressure itself is a ratio: Force per unit of area. If I have 200lb pushing on 1sq in, I have 200psi. But if that same force acts over 200 sq inches, I only have 1 psi!

For example, the same 200psi acting over 11sq inches instead of 10 sq inch means you get 2200 lb of force instead of 2000-- a whopping 10% increase.

This doesn't make sense... if (by your example) the first part is true, then 200psi acting on 10sq. in. = 2000lbs of force and the same 200psi acting on 11sq. in. = less than 2000lbs... . ?

Now let's address rod length and stroke. The individual measurement of either really doesn't matter. Rather, it is the ratio of one to the other, typically between 1. 4 (on the very short end) to 2:1 on a long rod engine. The ratio of rod to stroke (R/S ratio) is what determines geometry, so one only matters in relation to the other as far as geometry goes.



R/S matter because it determines the exact shape of the sinusoidal wave pattern followed by the pistons as it oscillates up and down. A lower ratio gives a steeper slope to the line and increases dwell time at either TDC or BDC.

jh

Respectfully disagreeing... . you have some of this theory backwards. It is the longer rod and/or the higher R/S ratio that slows the piston at TDC, effectively giving you less scavenging at valve overlap. Higher R/S ratios slow piston velocity (piston acceleration) and makes for a more reliable bottom end... or it can allow the tuner to increase redline of the motor. Dwell at TDC and BDC or not the same (think it has to do with rod angle at BDC?). This increase in dwell (as i said before) lets combustion pressure build to a higher level before accelerating the piston toward BDC. Inherently inline motors have greater R/S ratios as they have greater deck heights.

 

[hammersley]

And don't dorget that any engine has only so many

Revolutions before overhaul... ..... they are just like grains of sand in an hour glass - there are only so many until they are all gone... ... the more you need to use - higher RPM - the sooner it wears out :{ I remember an actual HONEST ad (if you can believe that) maybe 30 years ago from Cat regarding a D-9... they were making the point in their ad that "..... every engine wears out A LITTLE BIT - each time it is used - even a CAT!" That's the only time I can remember anybody telling us that... ... ... .

 

[Hohn]

This doesn't make sense... if (by your example) the first part is true, then 200psi acting on 10sq. in. = 2000lbs of force and the same 200psi acting on 11sq. in. = less than 2000lbs... . ?



Respectfully disagreeing... . you have some of this theory backwards. It is the longer rod and/or the higher R/S ratio that slows the piston at TDC, effectively giving you less scavenging at valve overlap. Higher R/S ratios slow piston velocity (piston acceleration) and makes for a more reliable bottom end... or it can allow the tuner to increase redline of the motor. Dwell at TDC and BDC or not the same (think it has to do with rod angle at BDC?). This increase in dwell (as i said before) lets combustion pressure build to a higher level before accelerating the piston toward BDC. Inherently inline motors have greater R/S ratios as they have greater deck heights.

200x10=2000
200x11=2200

What's so hard about that?

Let me restate. If pressure is constant, a larger area means more force. If Force is constant, a larger area means less pressure. Got it?

With longer rod, the piston is higher in the bore at every crank angle between TDC and BDC. This INCREASES dwell at TDC, and DECREASES dwell at BDC.

Short rods have the opposite effect. My apologies for the oversight-- rod effects at TDC and BDC are opposite.

Inline engines don't have any "inherent" higher R/S ratios. They CAN have them because of the taller deck heights, and they generally will. It's more correct to say that this is an advantage of the inline, but it's not "inherent" per se. After all, one could build the exact same geometry in a Vee, it just takes a taller deck height which in many cases is impractical. It's a subtle but important distinction.

jmo

 

[HRDROKN]

Ahhhh..... very well restated :-laf

 

[rperker]

The V configureation helps in balance more pulses per crank revelution. Shorter block, 3 in a row verses 6 in a row. Also a lighter block. GMC went to the extream in the 60's with V6's. They built a V6 in the neighborhood of 500 ci. , they had pistons like coffee cans. This was at a time before turbo Diesels realy took off so the only way to get torque was cubic inches. GMC even bolted these big v6's one behind the other. They had some awsom torque and hp. They could pass anything but the gas pump some trucks didn't have mpg's but gpm.

 

[SGibson]

The longer rod would only have the piston "higher" in the cyl. if the same piston was used, normally the pin is moved up in the piston when a motor is stroked. the main reason the v-10's came out was because it's hard to get a large bore motor ie big block chevy, Mopar, and "385" ford to run clean. I'm w/Justin on the torque theory, old stlye inline sixes with thier tall deck hts lend themselves to be engineered to be torquers by being undersquare, but it's just a flatter torque curve, the actual power is not dictated by cyl. bank conf.

 

[DieselLady]

Revolutions before overhaul... ..... they are just like grains of sand in an hour glass - there are only so many until they are all gone... ... the more you need to use - higher RPM - the sooner it wears out :{ I remember an actual HONEST ad (if you can believe that) maybe 30 years ago from Cat regarding a D-9... they were making the point in their ad that "..... every engine wears out A LITTLE BIT - each time it is used - even a CAT!" That's the only time I can remember anybody telling us that... ... ... .

And this point is exactly why I bought the Cummins in the first place. Logicly if the engine turning slowly so will everthing else attached to it. So everything wears out slowly. Good point.

Regarding the long stroke on a long throw. It seems to me that if you have a long stroke you will have to have longer rods and throw. Otherwise it can't reach to the otherside of the crank. So don't they go hand in hand. I realize you can adjust this to a degree but for the most part I wouldn't think you could have a long stroke and a short throw or vise versa.

 

[DieselLady]

Does the longer crank of the I6 help develop more torque then a V8 shorter crank?

Another question. The Ford gasser V10 seems to be a popular engine on class B motorhomes. The total displacement size is not particularly big, that means that the bore and stroke must be small. What does that do to the torque?







Torque is force acting at a distance. The distance part of this STROKE (also think of it as lever arm length). The Force part is Combustion Pressure. Since combustion pressure varies so greatly, we use something called BMEP-- Brake Mean Effective Pressure, or how much average pressure there is pushing down on the top of the piston.



Pressure itself is a ratio: Force per unit of area. If I have 200lb pushing on 1sq in, I have 200psi. But if that same force acts over 200 sq inches, I only have 1 psi!



By now you can probably see how increasing the bore size will increase torque because it increased the surface area acted upon by the combustion pressure.



For example, the same 200psi acting over 11sq inches instead of 10 sq inch means you get 2200 lb of force instead of 2000-- a whopping 10% increase.



So, if you have the same amount of cylinder pressure, increasing EITHER bore or stroke will increase torque output.



An inline 6 doesn't inherently make more or less power or inherently have higher or lower efficiency than a V-8. Cylinder configuration matters very little overall in terms of power production. More important is displacement, volumetric efficiency, and such.



You can reasonably compare the "thermal area" of two different engines configuration of similar displacement. The "thermal area" is just my term for the sum of all the surface area that can lose combustion heat. It's basically the surface area of a cylinder (inluding cylinder head and piston crown) times the number of cylinders. This area has a lot to do with how much heat is converted to power instead of lost to engine coolant.



In general, a given engine design becomes more efficient as it gets larger in scale. So if you take the exact same engine design (all geometries) and double its displacement (again, all in scale), efficiency overall will improve.





Now let's address rod length and stroke. The individual measurement of either really doesn't matter. Rather, it is the ratio of one to the other, typically between 1. 4 (on the very short end) to 2:1 on a long rod engine. The ratio of rod to stroke (R/S ratio) is what determines geometry, so one only matters in relation to the other as far as geometry goes.



R/S matter because it determines the exact shape of the sinusoidal wave pattern followed by the pistons as it oscillates up and down. A lower ratio gives a steeper slope to the line and increases dwell time at either TDC or BDC.



In application, R/S changes how the engine breathes and how the components experience stress. A Shorter R/S (shorter rods for a given stroke) generally let the engine breathe better because the piston will accelerate faster away from TDC and "suck" harder on the intake. You could run a bigger cam with shorter rods and still have the same idle.



But shorter rods also increase the amount of force applied to the cylinder walls instead of the piston, increasing friction. Shorter rods are under more stress because of the higher rate of acceleration on both ends.





Sorry I have to go-- ask away and I'll answer when I get back.



jh

Okay you can call me a girl, but this one's a "little to technical" for me. I am kind of a "get the general picture" kind of gal, if you know what I mean. BUT, where you say that" increasing the rod and stroke length doesn't really matter", Well try turning a lug wrench with a short bar. This I do understand and I have to think that increasing the stroke and rod length must matter. Leverage matters. What am I missing here????

 

[HRDROKN]

Regarding the long stroke on a long throw. It seems to me that if you have a long stroke you will have to have longer rods and throw. Otherwise it can't reach to the otherside of the crank. So don't they go hand in hand. I realize you can adjust this to a degree but for the most part I wouldn't think you could have a long stroke and a short throw or vise versa.

Within design limits you can have a longer or shorter rod for a given stroke. The distance from the rod journal (big end) to crank centerline dictates the stroke. Rod length (or lack of) is a product of deck height (how tall the block is) and where the piston pin (or small end of the rod) is located in relation to the piston top, combined with some other design factors. It is common in racing motors to increase the rod length without changing the stroke and either the deck height needs to be increased, pistons tops milled or piston pin centerline raised... . clear as mudd!

 

[SGibson]

long stroke=increased piston speed=increased wear,

 

[Hohn]

Okay you can call me a girl, but this one's a "little to technical" for me. I am kind of a "get the general picture" kind of gal, if you know what I mean. BUT, where you say that" increasing the rod and stroke length doesn't really matter", Well try turning a lug wrench with a short bar. This I do understand and I have to think that increasing the stroke and rod length must matter. Leverage matters. What am I missing here????

Of course stroke matters and so does rod length. But as far as "short rods" versus "long" rods go, it's just the RATIO that matters.

For example, a 6" connecting rod in a Small Block Chevy is somewhat "long" while a similar length rod in a Big Block is actually "short".

This is because keeping the same rod length as stroke increased drops the RATIO down.

Ratio is what determins angularity and geometry, not just stroke OR rod length but rather BOTH in concert.
jh

 

[Hohn]

The V configureation helps in balance more pulses per crank revelution. Shorter block, 3 in a row verses 6 in a row. Also a lighter block. GMC went to the extream in the 60's with V6's. They built a V6 in the neighborhood of 500 ci. , they had pistons like coffee cans. This was at a time before turbo Diesels realy took off so the only way to get torque was cubic inches. GMC even bolted these big v6's one behind the other. They had some awsom torque and hp. They could pass anything but the gas pump some trucks didn't have mpg's but gpm.

We had one of these in the bus I lived in as a child (Partridge Family style, lol!)

478CID in a V-6!! Gas guzzler, and not too reliable, either. Ours grenaded at a most untimely location in the Black Hills. But that was like 27 years ago... .

 

[DieselLady]

Within design limits you can have a longer or shorter rod for a given stroke. The distance from the rod journal (big end) to crank centerline dictates the stroke. Rod length (or lack of) is a product of deck height (how tall the block is) and where the piston pin (or small end of the rod) is located in relation to the piston top, combined with some other design factors. It is common in racing motors to increase the rod length without changing the stroke and either the deck height needs to be increased, pistons tops milled or piston pin centerline raised... . clear as mudd!

:-laf :-laf You can say that again :-laf :-laf But still interesting.

 

[hammersley]

Inertia??????

I once had an engineer - not an ME but a CE discuss his dad's trucks (they lived on a dairy farm) and he thought that the big six's with the long strokes maintained their speed on a long hill better. Now I haven't seen any data on this has anybody else?

 

[cojhl2]

<< once had an engineer - not an ME but a CE discuss his dad's trucks (they lived on a dairy farm) and he thought that the big six's with the long strokes maintained their speed on a long hill better. Now I haven't seen any data on this has anybody else?>>



No doubt about it!! On the Weat farm we had a Ford Flathead and a Binder KB7. The ford would start at the bottom sounding like it was gonna fly over the hill (field) but would be in the lower gears almost immediatly. The old Binder with it's exhaust coming up thru the fender (for fire protection) would finish in the same gear as it started. Sounding impressive as it got to the combine with that beautiful exhaust sounding like it was no particular effort.

 

[Vaughn MacKenzie]

i think some of the benz diesels they had in the unimog's were setup like that with 4 mains... doesn't the ford flathead v8 only have 3 mains???

The Packard Super 8 has 9 main bearings in their straight 8. Pretty much all of the automotive straight 8s in their time had 5 main bearings, that's what my '40 Buick has.

 

[DieselLady]

Wow, I can't even imagine how long a straight 8 would be.

 

[Joe G.]

It's really fun to take the head off a straight 8. The Buick had overhead valves so it was heavy and awkward. Some old straight 8 engines were flat heads so it was not so bad.