924Board.org

[augidog]

A diesel sound I’ve just noticed that if I shift a little incorrectly and apply load on the engine I hear what seems to be a dieseling sound.
Any explanations?
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[Joes924Racer]

yea your shifting iat the wrong time and theres not enough rpms
and your putting a strain on your motor.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[augidog]

thanks pumkin head
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[Richard]

What you are hearing is pinging, a bad type of combustion event where pockets of fuel/air mix are igniting randomly instead of auniform burn. This can cause damage to the piston and make the engine run hot.

I don't know how much "bad shifting" it takes to make it ping, but engines can be doing it and you don't really hear it. You may be close to that point if you can bring it on easily. Cure is to retard the timing and/or increase the octane of the gas.

[1982Porsche924]

Yup, happends to my 944 as well, usually if i let the revs drop bellow 1500 and then accelerate without downshifting. I use 91 octane btw. Just shift later, keep the revs above 2000.
_________________
1979 Porsche 924 "S"
Engine: Euro pistons, Light Flywheel, Weber TB, Bursch Header, Cam Wheel
Suspension: Front coil-overs, adj sways, Solid bush. Thicker tors. bars, 5-bolt, BBS rims
Body: Fiberglass widebody kit, 931 Nose

[augidog]

I thought it might be fuel related.
I'm running regular now.
But I just fixed my timing and mix so I might go up to 89 octane because the sound is new with these recent adjustments.
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[Roger]

Here is my understanding someone please correct me if I am wrong.

Preignition or knocking & pinging is caused by the fuel ignighting to early either with or with out a spark. The fuel will not burn in pockets since once its ignighted the entire amount of fuel and air in the cylinder explodes in a fraction of a second. Usually the comression in combination with the added fuel from acceleration under load will cause the fuel to ignight before the piston has fully risen and the valves are closed. The early explosion of the fuel puts stress on the valve train and some burning fuel enters the exhaust system. Causing the pinging noise. The piston is forced downward prematurly by the burning fuel and this causes the knocking. As the compression stroke catches up with the burn rate of the fuel the knocking and pinging goes away. Overly advanced timing or a lean fuel mixture can cause this but the most common cause is a worn engine or low octane gas. You may or may not notice a lack of power depending on how bad the problem is.

Deiseling or run on is caused by the fuel ignighting without a spark. Heat and compression build togeather and the fuel explodes. This usually happens before the piston reaches the top of the compression stroke, and causes a knocking and pinging also. This will also cause the engine to continue to run after the key is turned off. This can be caused by low octane fuel but the most common cause is overheating.

Some people equate higher octane fuels with preformance. However the truth is that the higher octane rating only decreases the point of ignition. This means that it takes more compression and heat to burn the fuel. The burn rate is also slightly slowed down (talking miliseconds here). By reducing the burn rate, the heat and compression necesary to burn the fuel the knocking, pinging, and deiseling is controled. Since performance motors create more heat and compression they require higher octane gas. However simply running higher octane gas will not add performance to an otherwise healthly engine in good runing order.

Modern engines have a knock sensor that retards the timing to control knocking and pinging.

Shifting at to low an RPM will cause the engine to stumble possibly knocking and pinging. In addition you will notice a lack of power, and possibly a shudder from the clutch.
_________________
1981 924 NA

Some people are like Slinkies. Not really good for anything, but you
still can't help but smile when you see one tumble down the stairs.

[Joes924Racer]

pinging whats it sound like does it have a
metallic sound cause mine does it also if I let it
and I havnt checked the timing its a vey annoying
sound.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Sleykin]

It sounds like a diesel. It is also sometimes called dieseling. Though that is normally refering to the engine continuing to run after the ignition is switched off in a carburated vehicle with a mechanicle fuel pump.
_________________
Glenn Neff
Medford, OR
87' 924S

[Richard]

Pinging or knocking and pre-ignition are different. Pre-ignition comes from hot spots in the combustion chamber that act as a source of ignition.

Pinging occurs when the pressure wave which travels faster then the flame front ignites pockets of the air/gas mixture first. Instead of an orderly flame propagation and growth you get a helter-skelter event. It does not happen in the exhaust manifold.

Since the piston is usually still travelling up when pinging occurs, it puts a lot of force one the crank and rods, robs power, causes overheating and can destroy piston crowns.

Combustion in a engine is not an explosion but rather a controlled series of events. Chamber and piston design, timing, octane, load, heat, and piston speed all determine if pinging will happen.


retarding the timing and/or increasing octane content of gas is the usual cure.
shifting at the correct time don't hurt either.

[Smoothie]

[Richard]

I've never heard of the piston slapping the side of the cylinder as the cause of the actual pinging sound, I think it is just little mini detonations. Would be interesting to find out if that is part of the sound.

[Joes924Racer]

has this occured upgrade the octane of your gas I did when this was first posted and the prob. went away for good.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[geezergary]

Something you might want to keep in mind is the posability of a carbon
bildup in the head or and/or the top of the piston . If the buildups are
thick enough they can still be "red" hot on the next compretion stoke
and fire your gas/ox charge much before it should . That is not good!
There are lots of good products you can buy to remedy that , and
they are not expensive .
Also , that is the main reason for an engine to rattle on after you have
turned of the key ! , but there maybe others.
Please be aware that this advice is free and it's worth price! ( probably
nuthin ) but then again - - - G.

[Smoothie]