I6 versus V8

[Hohn]

Others have mentioned the desirable features of the I-6 balance, bearing size, and room for a long-throw crank.



But you need "different horses for different courses. " How many I-6s are there in racing??? Top Fuel? F1/Cart/Indycar/IRL?



For pure acceleration, a Vee engine is much better, and the more cylinders it has, the better. For racing, a V12 is pretty close to "perfect. "



Now, I don't know if any of you have driven a car with scary rev gain, but THAT's why these engine designs are used. Simply put, with less crankshaft rotation between firing pulses, the engine will rev quicker. Revving quicker is a BIG component of acceleration, even if the dyno shows the same "hp. "



If you had two engines of identical displacement, compression ratio, etc, to where the ONLY difference was an I6 vs a V12 (bore/stroke and all geometry was proportional), the V12 would offer MUCH faster acceleration. Why? The more frequent firing pulses mean the engine can gain RPM much faster. Also, the V12 will breathe much better, even if the valve size is proportional. This is because the total valve area of 48 valves will be much greater than for 24V.



Inline 6 engines are best (or close to it) when priorities are pulling power, engine life, and efficiency. If pure acceleration is you goal, then a Vee (even just a V8) is superior.



That's what makes it all the more admirable that a CTD can produce the acceleration numbers it can-- it's NOT designed for accelation in ANY way! No one designing an engine for max acceleration would spec 1) I6 design 2) oversquare long stroke, 3) relatively short rods. A revving engine should have a shorter stroke, bigger bore and longer rods.



If you want an example of an engine born to rev, look at the Chevy 302, the Mopar 340, and the mopar 383.



Justin

 

[RustyJC]

If you want an example of an engine born to rev, look at the Chevy 302, the Mopar 340, and the mopar 383.

A well-built 289 Ford does quite well in this regard with a 4. 000 bore and a 2. 875 stroke.



Rusty

 

[NPloysa]

Little off topic...



Hohn, I understand your point about V12s gaining RPM much quicker than an I-6, but why can two-cylinder motorcycle engines gain RPM so fast? I'm curious!



Nick

 

[FloridaEd]

As a mechanical system, the in-line engine has always been a superior design. It cools better than a V-engine. It is better balanced. The power implulses from the power stroke are more evenly distributed in ONE plane. Plus, it's one heck of a lot easier to work on. That said.....



It's a longer engine and design requirements, particularly on cars, dictated a preference for V-engines in a small package. V engines tend to rev higher, too. But marketing is why we have V engines.



Long ago, I drove Dodge Slant 6 in-line engines in multiple cars and had great luck with them. Lasted 150,000 or so miles back in the late 60's and early 70's, which was really good back them. I also drove Ford pickups with an in-line 240 and 300 cubic inch 6's. Great engines, had the torgue of a V-8 with much better fuel mileage and lower cost. The one problem with the Ford I-6 was its ability to lug down to about 1,000 RPM and pull a load. All the drivers would hook up a Bobcat to one and lug it around in 3rd gear (3 was all we had then!) and tear the motor mounts apart because the power impulses would rock the engine back and forth at those low rpm's.



But it would last way longer than a V-8.



The smoothest engine I have every messed with was my Uncle Charlie's 1930-someting Buick Straight 8... . an in-line 8 engine with the sorriest dang carburator that God or General Motors ever created. But when adjusted correctly, that engine had no vibration... smoother than can be believed. Suicide doors was just an added benefit.....



If you want to use a vehicle for any type of work, you will be more pleased in the long run with an in-line engine. And the longer you run it, the better pleased you will be.



Florida Ed

(now Alaska Ed for a couple of months... . longing for that 3500 SLT LWB Dually that I'm gonna' buy in the Fall)

 

[Hohn]

Little off topic...



Hohn, I understand your point about V12s gaining RPM much quicker than an I-6, but why can two-cylinder motorcycle engines gain RPM so fast? I'm curious!



Nick

The cylinders are still opposed at 90°, 45°, or at some angle. You don't see many inline twins, do you?



They have very light flywheels and light internals. That makes for fast revs. Their quick revs are more a case of sheer engine size (being small) than of engine design, strictly speaking.



Heck, a single cylinder two-stroke MX bike gains revs pretty quickly, but this isn't because of the points I posted above.



Justin

 

[Hohn]

It's worth mentioning that horizontally-opposed designs (ala Porsche and Subaru), are also "perfectly balanced", and in some senses, even better. They also make for a low CG, which really enhances handling.



Ever driven a low-slung Porsche to feel how great it handles (at least the newer ones)? The older 911s and such like to swap ends, going from understeer to oversteer with little warning.



JLH

 

[MacHaggis]

And again, thanks!

More of what I was lookin' for on both sides of the equation. Thanks!



Got those sendin' units calibrated on the tie-rod lube monitors, but got a new problem with the back lighting on the gauge. Needle for the right rod lights up real good, the one for the left in kinda faint.

 

[klenger]

Opposed engines are also used in piston aircraft engines.

 

[cyborg]

seven main bearings

 

[NPloysa]

Yup, used a lot in aircraft engine designs. Heck, the 04 1150GS BMW bike I took out yesterday has a Boxer horizontally opposed twin cylinder engine.

 

[RustyJC]

Heck, the 04 1150GS BMW bike I took out yesterday has a Boxer horizontally opposed twin cylinder engine.

And, staying true to the inline engine topic of this thread, my 03 BMW K1200GT has an inline 4 cylinder engine mounted longitudinally with the cylinders lying flat - the crankcase is on the right of the bike and the cylinder head is on the left.



Rusty

 

[jwgary]

I'm not buying it

Hohn, I don't mean to flame you or anything, but I am not buying your analogy of the I-6 vs. I-12. I don't think you could scale the two engines to perform the test you are referring to. You would also have several factors hurting each other. Yes, you will have more valve area (about 20%), but you will have 20% more cylinder surface area to loose heat. There will be 37% more total piston circumfrence to drag on the cylinder walls as well. I won't even try to calculate the diference in rotating mass (inertia), but I am willing to bet that the I-12 is lower than the I-6. All of those things put together might make you rev faster but it might not. For you to have a fair comparison, you would have to match horsepower, because they have different efficiencies and therefore different power ratings at a given mass flow rate of air and fuel. You might get a faster rev out of the I-12, but at a sacrifice of fuel efficiency to do it. Overall, you would not be comparing apples to apples. I love it when we can have discussions like these!

 

[lklauder]

I-6 Cummins vs V-8 P-Stroke

A couple of months ago I jotted down a few Cummins ISB and the PStroke features that interested me as follows. Some of them of them relate to the cylinder configurations and some to vendor specific incidentals (no particular order):



6 cylinders vs 8:

8 cylinders give less torque fluctuation at a given rpm (such fluctuation can cause body shake and gear tooth unloading but engineers have become pretty adept at controlling those problems);

6 cylinders allow better thermodynamic efficiency due to more favorable volume to surface area ratio, an advantage reduced a little because a 6 will have a longer stroke than an 8 so that its rpms will be a little lower giving more time for heat transfer to the walls;

I like efficiency more than I dislike torque fluctuations.

The V arrangement makes the engine shorter and perhaps a little lighter and for packaging in a pickup the length is significant. The Dodge has the cowel crowding over the # 6 cylinder of the Cummins.

The in-line arrangement allows for wider big end and main bearings but I have not seen posts about failures of those bearings in the PStrokes.



freedom from the disadvantages of exhaust gas recirculation:

controlling NOx gets paid for one way or another; it would be nice if low sulphur diesel would allow future converters to transform the NOx and C back to N2 and CO2.

Having hot exhaust contribute heat to the coolant on the engine is not attractive. Having exhaust soot clog control valves is not attractive. Having new devices that that can fail and compromise engine performance is not attractive. (Maybe Kyoto will get people thining more about CO2 and efficiency and less about NOx. )



glow plugs vs heater grid in manifold:

I do not like glow plugs because they do not last. They also render the combustion chamber a little less clean.



In a pickup application a large V engine with all the air handling, electrical, AC, water, etc makes access for maintenance a nightmare. For good working conditions one would like to lift the cab or pull the engine - either of which is expensive; The forward tilting one piece hood, fender, and grill molding used on medium duties such as the F650 would solve the problem. But I suppose that trying that on a pickup would mess up crash test results, detract a little from the cosmetics, and raise the cost and weight.



Use of engine oil as hydraulic fluid for powering injectors works on paper and evidently comes close to working in practice. Same can be said for solenoid operated high speed valving of 30,000 psi diesel fuel. But in either case the rare failures can destroy engines if not caught in time. Proximity of fuel and oil in the oil powered injectors allows some failures to cause dilution of engine oil. Oil shearing is likely not an issue with the new piston style medium pressure pump. A rumored price of $654. 00 per electrically controlled hydraulically powered injector is not attractive.



Pattern of 6 head bolts around each piston is more attractive than pattern of 4 bolts. The 4 bolts could obviously be sized to apply whatever preload is selected but it is more aesthetic to have the forces applied to the head more uniformly around the cylinder periphery; heads are pretty rigid but there has to be some non zero deformation that is larger if the clamping forces are concentrated at fewer points.



Having a turbo bolt directly to the exhaust manifold is more attractive than having a "Y" pipe, and having the turbo on top of an automotive engine whose exhaust must be ducted downward is not attractive.



Variable geometry turbo compressor would be more attractive if it had fewer parts and were imune to the variable vanes getting stuck.



(A couple of the posts to this thread allege that the I-6 configuration is better balanced than the V-8. It was my impression that both engines are fully balanced through 2nd order with respect to forces and moments. Am I wrong? )

 

[WadePatton]

I like the sound of a big radial aircraft engine. Haven't figgered out how to get one of those in a pickup.



I'm with Hohn.



A point that I always raise in this discussion is that _IF_ v-8 diesels were better in any meaniful respect--durability, economy, or power (torque)--then that's what you'd find in OTR trucks.



Your witness.

 

[DFranks]

It's worth mentioning that horizontally-opposed designs (ala Porsche and Subaru), are also "perfectly balanced", and in some senses, even better. They also make for a low CG, which really enhances handling.



Ever driven a low-slung Porsche to feel how great it handles (at least the newer ones)? The older 911s and such like to swap ends, going from understeer to oversteer with little warning.



JLH

YUP!, I had an 1988 911 and when taken to the edge, it was one of the most unpredictable cars I have ever driven. I literally monitored tire wear weekly after 50% wear I would dump the tires.

Otherwise it was fun car.