Straight 6 vs Bent 8. Torque difference?

I have always heard about the superior usuable torque of the straight six. Comparing the inline 6 to the V8, comparable displacement, bore/stroke, cams, carbs, etc, is there a significant difference between the two? Perhaps we should look at it this way, if we prepared two engines (6 and V8) of the same bore and stroke and so on, would the 8 have 30% more torque?

If the six does have more torque, does anyone have a logical explanation as to why?

Bill

 

Six cylinder engines are usually made to produce thier torque peak at a lower rpm. So the guy driving around town or pulling some wieght may think it has more torque that a V8. No. In your example with all diminsions the same the bigger engine would produce more torque through the rpm range. How many think of a Jaguar twin cam 4.2 six as a torque monster?

 

Power pulses per revolution, more cylinders in equally timed events will produce more torque at low RPMs with less flywheel to dampen. However a V8 is not a naturally balanced configuration, an inline 6 is one of the engine configurations that is. To make a V8 smooth it needs to be counter weighted and balanced and this adds mass which negatively affects low RPM power.

An inline 6 can make slightly more power because of this.

http://en.wikipedia.org/wiki/Engine_balance

 

Real life normal production straight 6 vs V8: the 6 just doesn't make much power, it wont' breath very well at higher rpm, mostly because they stuck a little carb on it, and small ports in the head, and a nice mild cam. The V8 makes just as much torque per cubic inch, but it also revs better, because it can breathe due to having more carb, etc.

So people say the 6 has a lot of torque, but that's only because it has no power, so the low end torque stands out as it's one feature.

We have a couple stock straight 6s in the stable, they are turds, but they do get the job done eventually.

 

What squirrel said;
I'll add. The rpm where torque peaks is about how the cam is ground, not how many cylinders a motor has.

 

Right. And more torque isn't limited by RPM as I understand the question. A 302 GMC may well make more torque that a 302 Z28 type Chevy at 1800 rpm but what is the torque peak? I think many people don't understand the mathamatical realtionship between torque and horspower. And tend to repete common misconceptions.

 

The v-8's have two heads that have the intake and exhaust ports on opposite sides.

Most stock early 6's (and 4's) have the intake and exhaust on the same side.
If you see most aftermarket heads, like the wayne or howard heads, 12 port heads
they have the intake and exhaust on opposite sides for better flow,
basically what almost every v-8 already has.





The carburetor is an important but small part of the equation.

TP

 

Yes; Torque is a component of Horsepower. If a motor puts out more horsepower than another, somewhere in its rpm range it also puts out more torque.

 

BLUE...your question was simple and streightforward. so let's take a 6 cyl. ford 300 c.i. inline, a 302 ford v8, and a 301 v8 chevy. the better design will win out. and the v8 chevy. is far better. but a 30% incress in H.P. well that's asking a lot. but i can say for sure and certain the v8 chevy. is superior...POP.

 

Amen.

I have ended many a conversation on this topic with banging my head on my desk repeatedly. The misconceptions about horsepower and torque are deeply entrenched, and almost uniformly wrong. Worse, they are forcefully and argumentatively wrong.

 

I believe the OP was asking about Useable torque. Useable all depends on need. A working truck does not need peak torque at 5000+ rpm like a 302 V8. A 300 inline 6, with a much longer stroke is much more Useable here. On a road course, the inline 6 would not even begin to keep up with the similar V8. Yes, inline 6s can be built to rev, but due to the longer stroke will never pull the high rpms as well as a short stroke V8.

Devin

 

Okay, guys, I am with a lot of what you've said, but let's keep this apples to apples and oranges to oranges. Assume both engines have equal breathing, cam, carb, etc.

If comparing a 6 and 8 with the same bore and stroke, the 8 is going to have 33% more cubes. My question was, will it have 33% more torque?

The comparison of the big six to the smallish 8's is a good one, but can we be sure it is a valid comparison? Are they cammed and carburated the same? Do the heads breathe equally? Shucks, how about the exhaust systems? Are they equal?

Bill

 

That's the thing, when you compare production engines you're comparing apples to oranges.

If you build a straight 6 and a V8 with the same ports, cam, displacement, etc you'd be hard pressed to find much difference in torque.

 

IF you could make everything else equal, like you guys are talking, the only difference in the inline 6 to V-8 or v-6 would be the rod lenght, and a longer rod makes more torque. This has to do with the angle difference that it sees as the crank turns.

My real life example of this is the GM 71 Diesels. The inline 6 and the V-6. Same basic engine but the inline 6 pulls way better than the v-6. Same cam timing, Same bore and stroke, same CR, same airbox pressure, Even with the same size injectors, the inline with the longer connecting rod is a better performing engine.

 

An engine with a long stroke would be expected to have more torque than an engine with similar displacement but a shorter stroke. A long stroke means the distance from the center of the crankshaft to the center of the connecting rod bottom end gets larger. It would be sort of like trying to turn the crank by hand - you would be able to exert more twisting force by having a longer lever, but not be able to spin it as fast. If everything else was equal (which it never is anyhow), long stroke = more torque -- short stroke = more rpm.

 

Think of a BMW inline 6 ,some rev to 8000 rpm and rival any engine for hp per cubic inch.Good breathing head obiviously.The late model GM inline 6 found in Trailblazers revs freely,makes good hi rpm power.Older US inline 6's were entry level engines as mentioned above.Cheap people bought them cause they were standard equipment and supposedly got good fuel mileage. For modern engines,length of the stroke has little to do with high RPM power.Some new 4 cyl Jap engines make power well over 7000 rpm and have long strokes for such small engines.

 

It isn't. Just curious. Ever wonder if something you have heard all your life is correct?

Bill

 

Hell no, unless the sixes are hemis.
Bill

 

1966 GM truck repair manual,power ratings. 292 inline 6 has 170 hp at 4000 rpm and 275 fbs of torque at 1600 rpm.283 V-8,175 hp at 4000 rpm,275 ft lbs of torque at 2400 rpm. A 292 is considered a torque monster,the 283 a HP engine.

 

Of course a 426 hemi v8 would have more torque than a little 6 - that's cause it's bigger! Long stroke always means more torque when (or if) everything else were equal. That's the physics of it. It would be possible to design a V8 with a long stroke to give more torque and it would also be possible to design an inline 6 to rev more with a short stroke, but they usually didn't - at least not back in the 60's and 70's. For slow speed pulling power in a truck - a 6 worked better then a revving V8 back then - that's for sure.

 

Stroke, bore, rod ratio, intake, exhaust, carb, lot's of variables to consider.

 

Engines do not produce "horsepower". They only produce torque or twisting force at the crankshaft. Horsepower is a mathematical construct that applies that torque over time. Lets say for instance that a high revving low torque engine develops 200 horsepower at 6000rpm. A low revving high torque engine might develop that horsepower figure at only 3000rpm. Assuming that gearing is appropriate for both they will both produce the same acceleration and top speed. There's the rub. Greater gearing reduction and more gear "splits" will be required to extract the maximum performance of the high revving low torque engine which increases complexity weight and cost. Practically speaking nowadays even huge motors can sport six-speed transmissions so that advantage has faded for small naturally aspirated engines.

Engines such as the one in the Honda S2000 sports car have rev limits upwards of eight or nine thousand rpm and need far more gearing reduction than high torque big inch V8s. They will however develop a given level of horsepower at far lower displacements and much lower consequent weight and weight is, well, you know. What you don't get is the lovely thunder rumble of big V8s and in rodding that is very nearly as important as actual performance. luckily what you don't get with V8s, vintage American iron at least, is the near fiendish level of complexity, and subsequent cost, that characterizes most modern high-output engines.

 

That's the way it was explained to me! Guess thats why I've always
prefered long stroke British single motorcycles. They really don't thump
however they Hammer. Lovely sound!
Bill.

 

just stick a lot of carbs and pipes on it. it'll look cool anyhow.

 

liltimmy, nice.

 

If a motor puts out more horsepower than another, somewhere in its rpm range it also puts out more torque.

Or not.
Engine #1: 61" DOHC 4 cylinder, 150 hp @ 10,000 RPM, 100 ft/lbs. peak torque
Engine #2: 265" SV 6 cylinder, 130 hp @ 3,000 RPM, 200 ft/lbs. peak torque

 

this adds mass which negatively affects low RPM power.


Crankshaft mass has no effect on steady-state power, and an L6 crank is heavier than a comparable V8 crank, not only for the same displacement but for the same bore and stroke: longer by 2 cylinders minus 1 rod width, with larger journals.
Both have balance weights.

 

Let's get rid of the "shallow rod angle produces torque", beginning with the 366 BBC truck motor, yes? Long rods in a truck have 2 purposes: thrust control, and long skirts.
The rod ratio question is far beyond the competence of everyone on the planet, or there would have been a single answer and a patent 75 years ago.

Longer strokes apply more leverage?
Yes, they do - with a weaker force, since the piston area is smaller. Results are equal to such a small degree that individual differences in design are far more important.

These are all errors categorized as "post hoc, ergo propter hoc" (the thing that occurred first caused the later thing), or "false causation".

 

Apples to apples, meaning all things the same. You would not be comparing a 6 to an 8. They are not the same.

Equal bore, equal stroke, equal compression, equal combustion chamber design, equal valve lift and duration, same overlap for each cylinder, then you would have the same torque rating for each cylinder, assuming everything is equal, fuel air timing, etc.

Crank design is different, block design is different, cam design is different.

There is no fair way to compare.

nice to think about though. Good food for thought.

 

YA what he said..