Straight 6 vs. V 8

[jwgary]

I love Thought provoking threads like this one. I'll try to throw some ideas around in my head and see what I come up with. Any piston engine relies on the fact that you are increasing pressure on the piston head to drive a rod linked to the crankshaft in a circle. The engine will put out a variable torque based on the geometry of the piston/rod/crank arrangement and the variation in the pressure during the combustion cycle. The number you measure as torque is really an average torque that takes into account the peaks and valleys of the output (that's why there is a flywheel to keep all of that mass moving and smooth it out). At low RPM a larger bore longer stroke engine will produce more torque because you are looking at the output of only one cylinder at a time. As the RPM increase, you get more contribution from the other cylinders in the average. A smaller bore/shorter stroke could then put out more torque because the max RPM is higher.



The difference in the V6 and I6 are partly because of this averaging. For the engines to be essentially the same, you would need the cylinder banks to be at 120* from each other. If you think of the I6 that starts the power stroke at TDC every 120*, The TDC position for the next bank of pistons in a V6 would be 120* from the first. This doesn't give you much advantage for space unless you want a relatively flat, wide, short engine. With an angle of less than 120* you have a gap in power pulses. It's kind of like a V-twin on a Harley.



Now for why it's better for a diesel to have an I6 instead of a V8. The power stroke in a diesel is characterized by a constant pressure volume increase. Fuel is added throughout the stroke to keep the pressure (essentially) constant. In the Otto cycle (gasser) there is a constant volume pressure increase and then increasing volume and decreasing pressure as the stroke continues. The combustion event is fast compared to the diesel and therefore it's more of a pulse. The diesel can keep generating power throughout the entire 120* stroke until the next power stroke begins in the next cylinder. The gasser is constantly losing the force on the piston throughout the stroke and a shorter rotation angle (90*) is better for it. Guys who know a little more about diesel fuel timing, chime in here to tell us how many crank degrees the injector is open.

 

[Mundgyver]

One thing to consider in the discussion here is some history. In the beginning there was the inline I configuration. Why? Because they were easy to build and were the first developed. The benifits of TQ have been already lauded on and the fact that the engine is stronger by having a main bearing between every cylinder. The old cars were simpler back then.



As with all technology, it advanced notbecause it wanted too, but rather because it had too. People of the times got tired of driving a bread box style of car and wanted something with "Style". Those individuals wanted sleeker designs which required an engine with good power, but in a more compact design that would fit into the same space. More power at that time, meant more cylinders. The configuration from the "I" 6 to the "V" 12 allowed six more cylinders to be added into the same compartment that the "I" 6 fit into. But with that design, instead of a main bearing between every cylinder, they now had to have one between every two cylinder.



Anybody remember the Dusenberg So now we have the "Status Symbol" drawn into the pushing of the designs.



Study the history carefully and you will find that "Simple" is better



The "I" 4 and "I" 6 engines are the most reliable. To build a "V" 8 with the stroke of a "I" 6, the bottom end would be too wide to be effective in a automotive application. The crank diameter is key here. Think about it, The "I" configuration is very tall with a wide bottom to allow for crank rotation. Try and get this same stoke in a "V" configuration and your cylinder heads will be so wide apart that it is impractical for an automotive application. This is the main reason you will never see a "V" configuration with the same stroke as a "I" configuration.



As an example I will cite the Jaguar "V" 12. A great engine with a power application that feels very smooth. But it has very small cylinders and is an absolute beast to work on.



The "I" configuration is overall the most effecient design created and the most durable.



Just my perspective as an older kid with fun toys

 

[klenger]

A thread like this on rv.net would have turned real ugly 2 pages ago.

 

[DieselLady]

Wow, Now this is some great information. I was thinking that the angle would have to affect the way the power is applied to the crank and you guys are confirming that. I never thought about the fact that the diesel injects fuel for a longer period than a gasser. I suppose this would help explain the flat power curve of the diesel over the gasser too. Hummmm! Food for thought. Keep it coming guys this is great. Oo.

 

[Joe G.]

In my younger days I helped on a job where we overhauled a '34 Lincoln V12. This thing was huge. '34 was the last year that Lincoln was built in one spot, not on an assembly line. The engine was nothing like later V12s. One cylinder had a split that exposed the water jacket so we had the cylinder bored and a sleeve put it. The pistons were very large but the crank end of the rod was so big it had to be taken out the bottom. That ment that the wrist pin had to be punched out on top to release the piston. That was a big job. The car had a suspension like a truck. I have no idea how much torgue that V12 made but it was a lot because the car went pretty good in spite of being so heavy. Beautiful thing when the owner finished the restore.

 

[HRDROKN]

An advantage of the I6 design is that it does (as previously mentioned) provide more room for a longer stroke and unless I missed it... . it also provides for having a longer connecting rod (aside from the stroke). I would think that I6's typically have greater "rod ratio's then V's as they have more room (deck height). A longer rod (for a given stroke) will move the piston through TDC slower thus giving more time (dwell) for combustion pressure to develope and exerting a greater force on the power stroke.

 

[DieselLady]

So longer stroke means more leverage and more time or dwell for the fuel to burn hence more power during each cycle. Plus a better angle. Okay, and I am learning that the V config has interference problems if you start increasing stroke length so they make it up in RPM. Kind of simplified but now at least when someone tells me that the inline 6 produces more torque, I will be able to back it up. This has been a great learn. Thanks for all the great info. I will have to come up with another question and do this again sometime.

 

[Vaughn MacKenzie]

Lots of good discussion in this thread

Wouldn't this be applicable to any V-engine, where the crankshaft has 1 throw for two cylinders? I alway thought that this was a serious limitation to V-block design - with a 90 degree V, TDC for the front two cylinders will be 90 degrees apart. Obviously, if you made a crankshaft with a seperate throw for each cylinder, this would not apply, but most V engine cranks I have seen have 1 throw for two rods. I am going to have to study this... . P

There are two kinds of V6s . . . 90* V6 (which is essentially a V8 minus 2 cylinders) and the 60* V6 (where the banks are closer together such as the GM 2. 8 V6 and Ford 2. 9L).



Modern V6s have an even firing interval like inline 6s, both 60 and 90* V6s fire at even 120* intervals. How? With splayed crankshafts like, these pictures. The rod journals are essentially split. A 90* V6 needs a 30* splay, a 60* V6 needs a 60* splay.



The larger pic is a Mercedes V6 diesel. One pic is a closeup of a 90* V6 rod journals. Here is a great article, says the most naturally balanced engines are the Boxer 6, Inline 6 and V12: http://www.autozine.org/technical_school/engine/smooth3.htm

 

[hammersley]

One huge advantage of Inline vs V-8 is

the Inline has more (7) main bearings for 6 cylinders rather than (5) main bearings for 8 cylinders like the V-8 has Always nicer to have more bearings than pistons.....

 

[Joe G.]

Almost all I6 engines have a 153624 firing order. I say almost because I had one that didn't have that firing order. A boat I had was equiped with a flat head Chrysler marine engine. The distributer got pretty bad so I replaced it. Knowing that all I6 engines had the same firing order I just pull it out plug wires and all. Then replaced it. It would not start! Really loud back fire. It really made the sea gull in Trinidad Bay nervous. After a day trying to get it to run I went to see an old mechanic that knew about marine engines. After explaining the symptoms to him he said "Oh, you have the backwards turning engine from a set of two. The firing order is backwards because the engine turns the other way. " Sure enough, putting the wires in the cap the other way made it run just fine.

 

[Vaughn MacKenzie]

Too funny Joe, good thing ya figured it out. Must have been one of those motors built south of the equator :-laf

 

[Joe G.]

Imagine that thing in a car. It would have fit by taking the marine manifolds off. Let the clutch out and you have three speeds backwards! There are some 671 marine jimmies that are backwards. I think that all it takes is a different set of gears in the gear case and a backwards starter. Of course, a screaming jimmie two stroke might start backwards once in while just for the he11 of it anyway.

 

[nickleinonen]

I think that all it takes is a different set of gears in the gear case and a backwards starter.

i think all it needs is a reverse rotation starter [CCW instead of CW] and the camshaft installed backwards

 

[Grizzly]

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?

 

[Joe G.]

i think all it needs is a reverse rotation starter [CCW instead of CW] and the camshaft installed backwards

There is a gear difference in the gear case to drive the cam shaft backwards. The stock gear case for the marine engines I seen apart have places to mount the different gears so the result is turning the cam the other way. The injectors in those old jimmies were cam shaft driven.

 

[PSchwering]

Okay heres another dumb question. What do you mean by Throw exactly? Is that where 2 rods share the same arm on the crank???? Can you do that?

Maybe I should clear up the original question too. Lets compare a V6 to a Straight 6 with the same displacement and same RPM and for the sake of comparing apples to apples the same stroke. Will the I6 still produce more torque or not? I am beginning to think that it just boils down to configuration and nothing else.

Absolutly. Here is a pic of a chevy 350 crank. Clearly, the front 2 cyl. both ride on the same journal. I belive this is an Early V-8 design, & newer ones have a seperate throw for each cylinder... Anybody know If the D-max or Powerjoke have this type of crank?

 

[Vaughn MacKenzie]

The diesel V8 cranks look just the same, just heavier, and each throw shares two rods. I have seen pics of DMax and PSD cranks and they just look like an oversized gasser crank.



Grizzly I think stroke length makes more of a difference on torque than rod length, but most of the time a long stroke engine will have a long rod. Otherwise cylinder wall loading gets too high and I think piston speed changes more dramatically moving up and down the bore.



The Triton V10 has 3. 55" bore and 4. 16" stroke so yes that's probably why it's a good RV motor. . . for a gasser anyway.

 

[gonzo2]

Imagine that thing in a car. It would have fit by taking the marine manifolds off. Let the clutch out and you have three speeds backwards!

The new Ski-Doo two strokes engage reverse the same way. Hit a button and the motor stops, turns backward and there you go.

 

[Grey Wolf]

the Inline has more (7) main bearings for 6 cylinders rather than (5) main bearings for 8 cylinders like the V-8 has Always nicer to have more bearings than pistons.....

Not all I6 Diesels have 7 main bearings, the 6-33 Nissan Diesel that I put in my El Camino is a case in point. As a matter of fact there were very few failures in this design, however it unfortunately has other weaknesses [/IMG]#ad

 

[nickleinonen]

Not all I6 Diesels have 7 main bearings,

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???