Phoenix64

iTrader: (3)

What are the advantages or disadvantages of each?

It seems to me that the engine with fewer cylinders would have an advantage in terms of friction with the air in the ports, and with valves, as well as internal friction. Also seems like it should be lighter overall, yet most race engines and high performance engines seem to favor more cylinders. v10 or v12.

gametech

iTrader: (1)

Fewer cylinders means cheaper and simpler, and possibly more compact. More cylinders means much lighter pistons and valves, which allows more revs. Since most racing classes are displacement limited, the increased rpms are necessary for them to make more power. Apparently the friction must not be increased with the added cylinders?

Big-DEN

Disreguarding engine dimensions...

More cyllinders == more combustion strokes on a revolution

Normally means more power for a given displacement.

tahoe

More cylinders = more valve area for a given displacement!

//

Phoenix64

iTrader: (3)

I thought one larger valve would move more air more efficently than 2 or 4 smaller ones?

N4cer

iTrader: (11)

Nope. That's why a 4v engine is an advancement above a 2v.

01WS6er

iTrader: (2)

As stated above the more cylinder the less weight plus more combustion on strokes.
Very few factory V8 turn 8500 RPMs if any. The BMW M5/M6 V10 revs to 8500 this year and is going to 11,500 next year. The whole valve train weight less than 5 lbs. The is the same design the the F1 car runs and turns to 20,000. The more cylinders the more RPMs, due to less rotating mass.

Sharpe

iTrader: (7)

John_D.

iTrader: (2)

This would be true for a given "cylinder" displacement.

But not true for a given "engine" displacement.

.75 liters * 8 cylinders = 6.0 liters
and
.60 liters * 10 cylinders = 6.0 liters

In this case all we did was spread the same 6 liters over more cylinders. We still get 6 liters worth of combustion per revolution.


The gain is in being able to rev higher (since each piston/rod assembly is sized smaller).

Big-DEN

John_D.

A V12 of 6.0L with a given valvetrain architecture SHOULD be longer than a V8 of 6.0L or given valvetrain architecture.

Revving higher is more a result of lighter valvetrain architecture required for each cyllinder.

John_D.

iTrader: (2)

I'm not sure where you keep going with the engine dimensions. I don't think that's relevant to what anybody has been saying so far. Unless it's a comment on additional mass the vehicle has to carry around.

I just wanted to make the point (because some other people jumped on the more "strokes" wagon too) that increasing the number of strokes in itself doesn't get more power out of the engine -- because the individual cylinder displacements are reduced.

Yes I agree, valvetrain size (mass) comes down with the downsizing in bottom end parts too. We definitely need the valvetrain to continue to reciprocate, as well as needing the bottom end to survive the tendency for its parts to disassociate from each other at those higher piston speeds.

Big-DEN

John.

Engine dimensions mattered, because it determined where the engine can fit.

Just like you are not going to be able to use a turbo charger thats 6 ft tall.

Or if an engine was 5.0L of cyllinder displacement but had the external dimension of a 4 foot x 4 foot x 4 foot box, your not going to be able to install it anywhere.

A V12 should be much longer than the similar displacing V8, and that enhanced revving is more about the smaller and lighter valve train and not the smaller lighter piston/rod.

The V12 argument has come up, and the two reasons for increased power were:

lighter and higher revving individual cyllinders
increased combustion strokes on a revolution
v12 also reduced vibration

12secSS

iTrader: (30)

More cylinders actually equates to more torque, hence why V8s have more torque then a comparable I4. The more combustion cycles you apply to the crankshaft will result in more torque, since the crankshaft is not allowed to slow down and wait for the next combustion to occur, rotation wise. Much like spinning a wheel by turning it every quarter turn vs every half turn. Although this is not related to Horsepower, Big-DEN had the right concept, but the wrong "power" term. It is more torque, not more power (HP).

ProdriveMS

Given that both engines are 6L, the V12 may have more combustion events per revolution at equal rpm, but the combustion events are less powerful than the larger cylinder displacement V8's. It's a wash in the end.

The main reason to go with the V12 is the ability to rev higher and therefore make more power.

Assuming that the V12 has a 60 degree bank angle, and the V8 is a 90 degree with a dual plane crank, then the V12 will not have less vibration. Primary and secondary vibrations cancel themselves out on both engines. Neither requires counterbalancers.

From an all out racing standpoint, there are some drawbacks to the V12. While power may be greater, the 60 degree bank angle means that cg is higher than with a V8. The V12 will be heavier and longer. The powerband will be "peakier" and slightly more difficult to drive than the V8. Everything must be balanced against the higher power.

Al

John_D.

iTrader: (2)

I've never seen a 5.7 or 6.0 I4 so I'm not so sure I can use that comparison to draw anything from.

Pretty sure the crank is not slowing appreciably between firing strokes, especially at high rpms, in either case. Considering that a v8 intake valve is opening and closing about 50 times per second already, at 6k rpms.

mzoomora

iTrader: (36)

More cylinders also equals a smoother running engine since the ignition strokes are more equally spaced (every 30 degrees rather than every 45). Harmonics are better as a result also.

12secSS

iTrader: (30)

I wasn't referring to displacement issue, but as you stated cylinder size. Much like a 5.7L V8 SBC and a the 4.3L V6, I believe there was even an I4 based off of the SBC. The later two did not have the torque output of the 5.7L.

Given that friction is at play here, the crankshaft (actually rotating assembly) is always slowing after the max cylinder pressure has been achieved, after a complete burn cycle. More cylinders will always result in more torque per revolution, given a specific bore and stroke size.

ProdriveMS

Imagine that. A 4.3 doesn't have the torque output of a 5.7. Now there's an outcome I never expected

After two revolutions, roughly the same energy has been transferred to the crank in the V8 as in the V12. In the V12 though, the energy came in smaller doses every 60 degrees while in the V8, it came in larger doses every 90 degrees. Because the same energy has been transferred, the torque will be the same. (Everything else remaining equal- a lot of assumptions have to be made here regarding equal friction, vibration, etc.)

Yes, the crank will slow down more in the V8 between firings. The difference between max and min angular velocity at a given rpm will be greater in the V8. Torsional vibration may be higher in the V8 as a result.

Al

12secSS

iTrader: (30)

The 4.3L and the 5.7L share the same bore and stroke, 4.00" Bore x 3.48" Stroke. I think the I4 version (literally a chopped in half 350 SBC) was a 2.9L. But we are digressing from displacement to cylinder volume.

Oh, I totally agree with this ... if the displacement is the same between the comparing engines. Assumptions are a bad thing when theoretically comparing two engines of either equal displacement or equal cylinder volume, who knows how in reality it would translate, as each would require different design heads, intake, cams, etc ... You can have two 6.0L engines, but based on many variables ... one might rev strongly to 10,000 rpms while the other will be a torque beast upto 5000 rpms.

BJM

iTrader: (1)

A V12 of a given displacement will have smaller bores than the same displacement V8 (stating the obvious). Burn time is reduced since the longest flame path length is smaller. Knock resistance will also improve, everything else being equal. This should enable running higher compression with no other changes.