Don't rev in neutral?

[Dirk73]

I have often heard this said, but just wondered specifically what causes the harm?

Is it the lack of combustion load on the crank/rods, or something to do with the cam & timing gears?
Dirk

[Bob Culver]

Lack of load also prevails on the overrun. The pistons accelerrte much more at top of stroke. At quite modest revs the total upward load exceeds max downrd firng load of low revs. If original crank, coast down 60 mph hills! And dont overdo the gear assistance for slowing.
Many mechanics annoyingly rev cars absurdly in nuetral. (always stand out of line of fan!)

[Tony Press]

"I have often heard this said, but just wondered specifically what causes the harm?
Is it the lack of combustion load on the crank/rods, or something to do with the cam & timing gears?
Dirk "

With an Austin 7 as with most cars it depends on whether the engine is hot or a cold and the rotational speed being used .

Nothing to do with combustion load that I can think of and the cams and timing gears are similarly loaded whether in gear or not unless you exceed the limit of the engine- unlikely with a standard road car.

Over revving when cold is not good - slight 'blipping' to get the engine warmed up should be OK

[Chris KC]

The dynamics of crankshaft loading are complex & I'm not even going to try. But 'wumping in the paddock' as Jack French put it implies high rpm on a cold engine and that has to be bad. Plus much potential for soaring over the red line.

Not to mention the risk of looking like a wally...

[Bob Culver]

Few appreciate just how great the inertia loads are. The firing load at full throttle and med revs is reckoned at about 100x the cr in psi so the load on piston can be approximated.
The big end, crankpin,  and not directly counterbalnced parts of the crank follow basic formula for centrifugal force. The accel/decel piston and little end is a bit more complex but is about 11/4 and 3/4 the corres centrifugal force at top and bottom stroke. (For a jowett engine the piston assembly exerts 800x weight upward at 4,000 rpm). So at small throttle and revs the total upward force from two piston assemblies, big ends crankpins etc soon exceeds the net down push from one firing piston under load. The inertia loads increase as the square of revs. So at 6000 more than double at 4000.

It is reckoned that at 7:1 cr for a typical square engine, full firing load down and upward load on conrod bearing are about the same at 4,000 rpm on our small bore low cr engines would occur at much lower revs..

[Robert Leigh]

Sorry to nit-pick, but aren't the loads horizontal rather than upwards on a Jowett?

[AustinWood]

I have heard that the highest loads occur at the top of the exhaust stroke. Here the piston has to reverse direction with nothing to works against unlike the top of compression. This presumably is no different revving in neutral to revving on load.

[Bob Culver]

Hi Austin

In the middle of a four crank the adjacent piston is firing, so at revs and full throttle the net "'upward" load on the crank is partly countered.
The downward load at bottom of stroke is not so great because the pistons change direction less abruptly.
(A concept which is not easy to grasp. Imagine a 3 inch stroke with a conrod 1 5/8 long. The piston barely moves for the lower half rev and executes nearly entire stroke during top half rev)

[dickie65]

Hi Folks,

I seem to recall that it is the torsional twisting on the bent wire crank that causes problems.
As you rev the engine you wind the crank in one direction when you un-rev it it tries to twist the other way, hit the harmonics then say goodbye to bent wire.

As well as the twisting you get bending in the unsupported middle.

(Think of a skipping rope that is going round then twisted into a helix , then try and change direction of the helix )

When you drive under load the twist remains more constant and is only being unwound on over run which again is normally a steady state.

You will always get the pulsing due to the firing order.

[Chris KC]

That sounds logical to me Dickie!