I was just wondering how many degrees of crank rotation there are between sparks on a small block chevy.

Furthermore, why is it generally accepted that V8 engines are less economical than 4 cylinder engines. To my knowledge V8s generally displace more and thus require more fuel, creating more torque and the need for more fuel. Theoretically, wouldn't a 5.0L V8 and 5.0L I4 use both the same amounts of fuel and air?

Just doing some thinking,

Jason

 

89,

There is a 90° crank rotation between each cylinder firing. This is only 45° of camshaft and distributor rotation. Oddly, some newer engine with the DIS ignition system fire eight times every revolution since the coil packs are timed by the crankshaft instead of the camshaft. The extra spark is on the exhaust stroke of every cycle, and is neither detrimental nor beneficial, although Porsche initially "sold" the idea claiming it helped keep the plugs cleaner.

Your presumtions about efficiency and fuel economy are correct, to some extent. In a theoretical world, two engines of equal displacement should use the same amount of fuel to propel the same vehicle ubder identical circumstances. Unfortunately, the real world does not completely fit the theoretical model. An engine with more cylinders necessarily has more parts, and thus, more losses due to inefficiencies and friction. Conversely, the engine with more cylinders will produce more torque at lower RPM than the "other" engine. Because of these factors, a simpler engine would be more fuel efficient at a given speed if the gearing were identical. However, since the more complex engine produces more torque at lower RPM, th edifference from losses can be nearly negated by the lower gearing of that engine to take advantage of the greater torque. The four cylinder will operate at 3,000 RPM at 60 MPH, creating 35 HP. The V-8 would operate at 1,600 RPM to produce the same 35 HP output at 60 MPH. The V-8 will only use slightly more fuel, but more, nonetheless.

In the real world, one example that sticks in my mind is the late '80s and early '90s Chevy and GMC C/K trucks. A half-ton pickup with the 4.3L V-6 got no better highway mileage than the same truck with the 305 V-8, and only one or two MPG better mileage in city driving. Even more unusualy, the 305 and 350 V-8s got the same mileage numbers, so why not spend the extra few bucks for the bigger engine?

In another example, my '96 SS is EPA highway rated at 27 MPG. Put a weenie little ***** engine in that 4,400 pound car and see if it would get any better. Then see if it would push it along at 150 MPH - didn't think so...

------------------
Later,
Vader
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"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0

 

I don't know about the spark, I think that it would be 45 degrees between cylinders on any v-8.

I think that the 5.0 V8 will make more power than the 5.0 I4 because there are more cylinders firing on each rotation. Also the V-8 is closer in firing 45 degrees vs. 90 degrees (I4)

hope this can help you.

 

Thanks for the help Vader, if you don't mind I'd like to pick your brain a little more.

I was just wondering why the engine with more cylinders would produce more low end torque than an engine with a lesser number of cylinders assuming equal conditions and equal displacement. I came to this conclusion thinking that each explosion would produce a peak torque on the crankshaft at less than 90 degrees of crankshaft rotation after the sprak. Assuming constant force the peak torque would be applied at 90 degrees after TDC due to the levarage provided by the angle between the force applied down the connecting rod and the crankshaft. But, there is also a loss of compression as the piston travels down and energy is used so peak torque has to occur at less than 90 degrees of crankshaft rotation after TDC. Am I right about this?

So what I'm getting at is that for an I4 and a V8 engine of the same displacement, each V8 cylinder would hold half as much air/fuel as a single I4 cylinder. So it would seem to me that one firing of an I4 cylinder would be equal to two simultaneous firings of two V8 cylinders. So, since the V8s do not fire two cylinders at the same time, and given that my above paragraph is correct, the I4 is getting the torque of two V8 cylinders and the V8 is getting one V8 cylinder plus possibly part of the next cylinder. If the above is correct the I4 should produce more low-end torque. But this doesn't seem right.

So now I'm wondering how exactly engine torque is measured. It seems that the I4 would have more peak torque, and both engines would produce the same total torque after firing all of their cylinders once since they have the same displacement (not taking into account friction, etc). It can't be the maximum instantaneous torque on the crankshaft because then the I4 would have to prevail. So how is the torque of an engine calculated?

I also considered that maybe the V8 fired more fuel/air per turn of the crank but this can't be b/c both engines fire all of their cylinders in two turns of the crankshaft.

I hope I'm making sense to you and I don't sound too off base, sometimes I get a little confused trying to think about all this myself. I myself am a fan of big cube engines, but it would seem to me that if anything the I4 would produce a little more torque than the V8 on paper, but in reality given that each engine fires all it's cylinders once they'd produce the same total torque.

Any help you can offer Vader is appreciated,

Jason

[This message has been edited by 89BlwnRs (edited December 20, 2000).]

 

Don't know if I'm right here but just for the sake of discussing....

Your assumption that firing an I4 cylinder will make twice the torque of firing one V8 cylinder only holds water if both engines are making the same torque numbers.
And that's where they disagree, because the V8 is firing two times for every one time that the I4 fires, and that sounds like making more torque...
I don't know if this is true, just trying to figure it out with you.....

 

Just for fun, i'll jump in too...
I hear your theory blown, about how both engines are theoretically burning the same amount of fuel/air...however, i think the 8 ends up making more torque because of the greater consistency of the firing time of the engine. Hmm, kinda weird phrasing, i'll try again.
The 4 cylinder fires it's BIG cylinders only 2 times per crank rev. The 8 fires 4 times, albeit, each fire is half of the size of the 4. However, because there is a cylinder firing every 90' instead of 180' of crank rotation, and even though every single 4 cylinder fire has as much fuel, do to the fact that it does lose compression and peak torque as it spins around after it fires.
Oh christ, if that doesn't make sense...i give up....lol
...ed

 

I hear what you're saying Ed, and everything you said is true, but since the V8s do not fire two cylinders at the same time the peak torque would have to belong to the I4. So I'm guessing that they measure engine torque differently and not just as the peak torque on the crank. As far as losing peak torque and compression, the V8 has that happen too, and the firing of the second cylinder will not happen at peak torque of the first, and even if it did, the peak torque of the second cylinder would not coincide with peak torque of the first. I'm a little confused,

Jason

 

I'm sorry about the incorrect numbers the I posted, I forgot that the crank is twice as fast as the cam.

Anyway, this is a very interesting question. I think that the torque would be a constant of the length of the stroke.

hypotheticaly if a 305" engine had a stroke of 5"(A) and another had a stroke of 3"(B.
Engine A would have much more torque than that of engine B. But that torque would peak very rapidly whereas engine B will have lesser torque but over a much broader range.

Now to make this argument simpler, lets make the V8 and teh I4 have the same stroke, and the same cubic inch value.

I think that the V8 will have more power due to the fact that the timing allows for two pistons to fire for every one I4 piston. This will make for a better saturation of torque for every revolution.

I don't know, but this is my .02 cents.
Anyway if you would like to figure this out I would reccomend this book, it is all about engine design. It is old (from 1933 (copywrite)) but all of the principles apply to today's engines.
The name is Automotive Engines by Heldt.

Mine is brown and about 1-1/2" to 2" wide and 4" by 7" to 8". Hope this book can help.

 

Ace,

Thanks for the info on that book, I'm gonna head to the library this weekend or next and see if they happen to have a copy. Thanks for the help,

Jason

 

I've been doing some furhter thinking and this could be how the power could come out. For reasoning and ease of graghing, lets say the two engines produce the same amount of torque.

I4:

|\__|\__|\__|\__|\__|\__|\__|\__ and so on

V8:
|\|\|\|\|\|\|\|\|\|\|\|\|\|\|\|\


that is all because of the firing of the pistons in a revolution. the spikes are approximations on when each cylinder would produce peak torque.

hope this is right.. If it is I hop this helps with this discussion.