Straight 6 vs. V 8

[DieselLady]

I know that a Straight 6 will produce more torque than a V-8 but I don't know why. Can any of you super tech types out there explain exactly why this is????

Is is due to leverage and space limitations or is there some other "physics" reason for the inline producing more torque than a V config? Thanks

 

[Greenleaf]

I don't know if it will produce more torque or not... You'd have to compare apples to apples and thats impossible when comparing 6 vs 8... ... . or not?



Longer stroke would assume more torque... . or not???

 

[DieselLady]

Well yeh, it does produce more torque. The my 5. 9 cummins on the dyno will produce 361 hp and 812 ft lbs of torque at 1800 rpm. On the same day a 6. 6 Duramax produced 474 hp and about 860 ft lbs. In most dyno event a similar sized engine with a v-8 will produce more hp but not a whole lot more torque. I know that the Cummins has a longer stroke which would give it more leverage, hence more torque, especially at lower rpm. But I am wondering if there is another reason. Very curious.

 

[PSchwering]

Absolutly. The longer the engine stroke, the more torque. It really is that simple. IF you have a V-8 with a similar stroke & displacement the torque would be similar, but V block engines tends to have a much shorter stroke, Which reduces torque, but it allows for a speed increase(RPM's) over long stroke engines. This results in more power. Our CTD's have a long stroke compared to others, Witch limit's our RPM range, but gives us great amounts of torque. More torque allows us to do the work at a lower RPM, thus increasing fuel economy. (Not the only reason we get better fuel economy!!)



This is a pretty basic overview, But to the best of my knowledge, accurate. P...

 

[ezcurra]

Yes, the bore/stroke ratio is what determines the engine's output for the most part. But an I-6 engine tends to have more main bearings than a V8, and that makes the engine more robust. It is no surprise that most of the toughest (not most powerful) gasoline engines are inline engines.

 

[HBarlow]

I generally agree with the previous posters. The long stroke inline six makes more torque at lower rpm because of the additional leverage provided by the long arm (connecting rod).



Another factor that was touched upon but not emphasized and should be considered is when comparing the torque output of a V8 engine with the torque output of an inline 6 of similar displacement operating at the same rpm the inline 6 will produce considerably more torque than the V8 at any engine speed from idle speed up to the upper end of the engine's operating range.



You will very rarely see a manual transmission 6. 0 Ford or Duramax for that reason. They are absolute dogs to drive loaded with manual transmissions. Our Cummins engines produce approximately 400+ ft. /lbs. of torque at idle and maximum torque of 500 to 610 rpm, depending on model, at 1600 rpm. That makes driving our Dodges easy when heavily loaded. We can simply put the shifter in 1st gear and let out the clutch to launch. No need to rev the engine and slip the clutch. Try that with a 6. 0 Ford or Duramax and the engine stalls and dies. The V8 diesels produce very little torque until they are revved up.



Another consideration is loaded operation at highway speed. A typical diesel-powered pickup truck running at 60mph on the highway, depending on gearing, has engine speed of approximately 1600-1800 rpm. The Cummins engine is producing maximum torque at highway speed. The Ford 6. 0 and Duramax engine is operating at far below their maximum torque band which occurs at approximately 2200 rpm. The Cummins is capable of pulling steep grades in overdrive while loaded at that speed, the Ford or GM will have to downshift one or two gears to maintain speed. That means more wear and more fuel burned by the V8. Some will have differing opinions here but keep in mind that I'm speaking in generat terms.



When light truck diesel engines were "passed out" Ford and GM got the short end of the stick.



Harvey

 

[DieselLady]

I generally agree with the previous posters. The long stroke inline six makes more torque at lower rpm because of the additional leverage provided by the long arm (connecting rod).



Another factor that was touched upon but not emphasized and should be considered is when comparing the torque output of a V8 engine with the torque output of an inline 6 of similar displacement operating at the same rpm the inline 6 will produce considerably more torque than the V8 at any engine speed from idle speed up to the upper end of the engine's operating range.

I agree with this from experience, but is it just because of the longer stroke creating more leverage? I realize that longer stroke translates into larger crank and more leverage, but if you made a V-8 with the same stroke and crank, only in a v8 config, with the same displacement, would it create the same torque???? And if so, Why??????

 

[JasonCzerak]

I generally agree with the previous posters. The long stroke inline six makes more torque at lower rpm because of the additional leverage provided by the long arm (connecting rod).

.....



That makes driving our Dodges easy when heavily loaded. We can simply put the shifter in 1st gear and let out the clutch to launch. No need to rev the engine and slip the clutch. Try that with a 6. 0 Ford or Duramax and the engine stalls and dies. The V8 diesels produce very little torque until they are revved up.

... ...





Harvey

HA! yeah I know this with the 6. 0's!!! I had a a little race car, like a mini F1 car in a medium size trailer (20 feet?). . new truck work at the time had 3000 miles on it. No more then 4000 pounds with tools and trailer and car I'm sure.



6 speed transmission, and since like I said before, it's new, at one light the truck wouldn't slip into 1st easly, but would into 2nd, I even gave it a little extra RPM but the motor fell on it's face and stalled after rolling about 1 foot.



This is something I would easly do with the 3500 dodges!!! Even the SO 03 could just MOVE.



Never pulled anything with the duramax, but did cruise around town with one for a weekend before buying a new truck.



I obviously picked the dodge.

 

[JasonCzerak]

I agree with this from experience, but is it just because of the longer stroke creating more leverage? I realize that longer stroke translates into larger crank and more leverage, but if you made a V-8 with the same stroke and crank, only in a v8 config, with the same displacement, would it create the same torque???? And if so, Why??????

You can ONLY compare apples to apples. So, V8 to a I8, V6 to an I6 would be really all you can do.



Inorder to get a V8 the same stoke with the same displacement, you would have to have a much smaller bore. Less bore = less torqoue as well...

 

[HeavyHauler]

To make a V-8 with the stroke of a Cummins (roughly 4. 75") but with the same displacement, you would have do decrease the size of the bore in the V-8. You are dealing with 2 more cylinders in the V-8. Decreasing the bore size is going to hurt your power output because you are losing the surface area of the piston.



To make a 359ci V-8 with a 4. 75" stroke, the bore would have to be downsized to 3. 009" compared to a bore that is rougly 4" in the Cummins.

 

[rperker]

rperker

longer stroke does produce more torque no matter the configuration, I6 V6 V8 . The reason the I6 or V6 for that matter will have more torque than the same displacement V8 is each cylinder is larger in the 6 than the 8. When each cylinder fires in the 6 it has more power than the cylinder in the 8 because it is larger. The V8 will spin faster, alittle better balance, less rotating mass (smaller pistons) and yes usually shorter stroke. The V8 diesels drive more like the gas outfits most people are used to.

 

[Dieselnut59]

This is EXACTLY why I do not understand the thinking of Jeep, who is dumping the 4. 0L inline 6 for the 3. 7L v-6 in the '07 Wrangler!! Go smaller AND with less torque? In a vehicle that is designed to USE torque? I do not understand. The only SAVIOR to the Jeep Wrangler will be the diesel WITH A MANUAL TRANSMISSION. They had better offer it or the Wrangler will become just another gutless pavement pounder/grocery getter like the Liberty. whose highlight is the diesel option.

 

[DieselLady]

longer stroke does produce more torque no matter the configuration, I6 V6 V8 . The reason the I6 or V6 for that matter will have more torque than the same displacement V8 is each cylinder is larger in the 6 than the 8. When each cylinder fires in the 6 it has more power than the cylinder in the 8 because it is larger. The V8 will spin faster, alittle better balance, less rotating mass (smaller pistons) and yes usually shorter stroke. The V8 diesels drive more like the gas outfits most people are used to.

So does this mean that and I6 and a V6 of the same displacement and stroke will produce the same amount of torque at the same RPM? I was always under the assumtion that the I6 would do better.

 

[HBarlow]

DieselLady,



Well, I'm getting in over my head here. I'm not an engineer so maybe a fellow TDR member who is will set us straight.



I will stick my neck out a little further and add this thought. The inherent smoothness of most inline sixes results, in part, from the fact that power pulses occur every 60 degrees of crankshaft rotation (360 degrees divided by 6 = 60 degree separation. This may also increase effective torque output. I'm in unfamiliar territory here. A V8 engine produces a power pulse every 90 degrees because of the 90 degree V configuration and the number of cylinders.



Harvey

 

[DieselLady]

Well see, now you are giving me something good to think about. I would have thought that the V8 would be every 45 degrees, 360 divided by 8. So how did you come up with 90 degrees. I kind of feel like I am asking stupid questions but I just love hashing over this kind of thing and coming away with a better understanding of it. Like I said before. Everyone always says that the I6 has more torque but I have never heard anyone explain why. So here I am. I also never thought about the fact that the V8, having close to the same displacement obviously has to disperse that between 8 cylinders instead of just 6 so each stroke has less power to move the crank with. Learned 2 things so far.

 

[HookMeUp]

DieselLady,



Well, I'm getting in over my head here. I'm not an engineer so maybe a fellow TDR member who is will set us straight.



I will stick my neck out a little further and add this thought. The inherent smoothness of most inline sixes results, in part, from the fact that power pulses occur every 60 degrees of crankshaft rotation (360 degrees divided by 6 = 60 degree separation. This may also increase effective torque output. I'm in unfamiliar territory here. A V8 engine produces a power pulse every 90 degrees because of the 90 degree V configuration and the number of cylinders.



Harvey

Actually it's every 120 degres for the 6 (720 divided by 6 = 120). The crank shaft rotates twice.

 

[DieselLady]

That makes sense. That would make the 90 degrees on a V8 understandable too. But that would seem to favor the V8 for torque. Wouldn't it????

 

[TTfordcummins]

I'm new to tdr but i am not new to diesel's... I won't lie i was a powerstroke guy.

But after the second rebuild i converted to the true power house. but i know a little about both . somebody quote me if I. m wrong but the big three light truck diesel's

are all four cycle. that means that you divide the number of cylenders by two.

wich means I-6 fires 3 times every revolution to the v-8's 4. wich would not explain

the horsepower increas and the torque decrease of the v-8. I think what the other member's said about the smaller cylender is the reason for the differance . But if you compare a stock 7. 3 55. 6 cubes per cylender to a stock 5. 9 59. 98 per cylender . They have

a very comperible horse torque ratio . Don't get me wrong i'm not comparing the two

there is no competition with the cummins . trust me I know first hand . like I said stock to stock . after that there is no touching the mighty cummins ...

 

[HBarlow]

I better leave this discussion before I further embarrass myself. Again, I'm no engineer. I might discover that some "facts" that I have subscribed to for many years are not facts at all but simple misunderstanding and misinterpretation.



I think power pulses in a V8 occur at 90 degree intervals because the two cylinder banks are separated by 90 degrees.



I hope an engineer will step in here and complete this discussion with actual facts and a scientific explanation.



The TDR has many members who are not geniuses but have knowledge and experience in a wide range of topics. I learn from many of them. I rarely sit down to a session on the TDR forums that I don't learn something of value. Sometimes it happens when I least expect it when someone who is not a professional engineer, scientist, or Ph. D. makes an offhand comment that lights a light for me.



Harvey

 

[TTfordcummins]

harvey thats what we are here for is to share what we think we know...