thought i knew ****, but guess not :o ...I was investigating the best way to adjust my valves and saw the v8 firing order is 1-8-...etc., when it occurred to me, only one cyl at a time is actually firing. What am i missing? A 4 cyl doesn't have to drag around the 4 extra pistions that the v8 has to. So, why/how is it that an 8 cyl has more power? Or does a v8 double up on on the power strokes. And, if not, why not?

be gentle...

 

In one rotation of the crankshaft, all eight cylinders fire once. So, the piston on the power stroke doesn't have to "drag around" the other cylinders very long before another piston is on its own power stroke. I am no guru, but I think that's how it works (obviously over-simplified)

Temvette72

 

Makes sense...all cyls fire in one rotation, so a v8 has 4 extra power stokes providing the extra power. THANKS!

 

In TWO rotations of the crankshaft, every cylinder fires once.They basically fire 90* apart, so you have one firing every 1/4 revolution versus every 1/2 revolution of the 4-banger you described.
As for more power, you're "inhaling" twice as much air and fuel, twice as many explosions per crankshaft revolution.

 

So why don't the v8s also double up on the power strokes? Wouldn't they be that much more powerful?

 

Come on guys... it's called a four stroke for a reason... intake, compression, power, exhaust... each cylinder fires every two revolutions, or four power strokes per revolution...

Lets dig out those 7th grade Physics books...

GUSTO

 

Thank you- I thought I had lost my mind for a second reading that.

 

Given that we have now established the correct operation of a 4 stroke engine..... the original question regarding a V8 having more power than a V4 is yet to be answered satisfactorily.

If we are talking about equivalent engine capacity.... then there may not be that much difference in peak power.... power will still be ultimately determined by the efficiency of the overall engine design... in other words, hp/cubic inch.

There are V4 (motor cycle) engines which are more efficient than v8's, and if they were the same capacity, the V4 would produce more power.

However, the reality is that V8's are generally much larger in capacity, and hence have more power simply due to that fact.

 

It would be the same power, just delivered two strokes at a time. The spacing out of the firing at 90°? stated vs. at every 180° leads to a smoother running engine.

 

In motorcycle racing, lower displacement four cylinder engines make equivalent power to the larger displacement V-twins, in many cases. Superbike racing used to be 1000cc V-twins against 750cc fours, and they were each competitive.

The answers are not just really simple, one being smoother power application, higher rev ability, etc. etc. Engine displacement is a rough approximation of the amount of combustion that will be available for any one revolution of the crank, so in a perfect efficiency world, the number of cylinders shouldn't make that much difference.

But I'm no engineer.

 

Two-stroke engines fire the cylinder on every cycle; four-stroke engines fire on every-other cycle. A two-stroke engine of the same size will out-perform a 4-stroke engine [almost] every time. Of course, it's drinking a lot more fuel, since it combusts on every stroke. Four-stroke engines are smoother and more fuel efficient [when making the same power]; two-stroke engines are smaller and simpler, and make more power for their size.

 

Some comments:
1. Nobody (almost) would ever build a "V"4. The best configuration for a four cylinder is inline (like one bank of a V8). The Saab Sonnet is the only example I can think of with a V4. The benefit of an inline configuration is the compact size and ability to reduce NVH (noise, vibration, & harshness) characteristics associated with an unbalanced engine.

2. An I4 versus a V8 in the same displacement (assuming volumetric efficiency is equivalent) will mean more power in the I4 due to the reduced frictional losses of a V8.

3. V8s are only balanced in the first order, I4s are not balanced at all - although studies show that you don't need a balance shaft under 2.2L of displacement. The best engines for overall smoothness are inline 6s. They are balanced in for both first and second order harmonics (as would a V12 given that it is two I6s bolted together). Flat boxer 6s (like Porsches) are also balanced in both first and second orders. The downside for I6s is weight and that for the boxer 6 is package. V6s are often seen as a good compromise.

4. For any given engine, a 2-stroke will be more smooth than a 4-stroke from a balancing perspective given the increase in power strokes. The general rule of thumb is that a 2-stroke will be only 2/3 as efficient (and powerful) as a 4-stroke PER CYLINDER FIRING. This means that 2-strokes will generally be about 30% more powerful overall compared with 4-stroke engines - but they don't breathe as well - and they are more polluting given their lack of efficiency.

5. Engines are nothing more than air pumps - process more air and you can have more power. Displacement is a key element of increasing airflow - which is why an 8 cylinder will have more power than a smaller I4 (but one with equal cylinder size). Airflow is also an ingredient which is why well flowing 4-valve per cylinder engines typically perform better at peak HP than 2-valves. Improving flow improves volumetric efficiency for otherwise similar engines.

 

I have a headache!!