Lean = broken piston?
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Senior User
Joined: Aug 2006
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From: east dubuque, il.
Lean = broken piston?
I have a coworker that has a 94 camaro, 383, supercharged. He put the supercharger on two years ago and has nothing but problems since. On Friday he blew her up again while on the dyno. It has dished pistons, and it broke the outside top edge off. He claims that at high rpm's it was not getting enough fuel and caused this. I disagree and would like some input from guys in the know.
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Blew off the top edge of the piston above the top ring land, right?
He needs to upgrade pistons.
Late model Dodge Hemi engines do this too, as will a lot of newer engines without blower pistons. New stuff puts the top ring as close to the top of the piston as possible. The eliminates the dead area around the cylinder above the ring, a great place for unburned hydrocarbons to lurk, and then cause emissions to rise.
With the puffer on there, cylinder pressures are up to the point where the piston just isn't strong enough to hold it in that place.
Now it is possible that a lean condition overheated the piston, leading to this failure. Inspection of the piston and the rest of them as well should tell that tale.
He needs to upgrade pistons.
Late model Dodge Hemi engines do this too, as will a lot of newer engines without blower pistons. New stuff puts the top ring as close to the top of the piston as possible. The eliminates the dead area around the cylinder above the ring, a great place for unburned hydrocarbons to lurk, and then cause emissions to rise.
With the puffer on there, cylinder pressures are up to the point where the piston just isn't strong enough to hold it in that place.
Now it is possible that a lean condition overheated the piston, leading to this failure. Inspection of the piston and the rest of them as well should tell that tale.
Post Fiend
Joined: Jun 2001
Posts: 5,228
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From: Running Springs CA
Supercharged, and he blames mixtures? That lean=hot myth will never die. Very lean mixtures are too weak to produce cylinder pressures that can break pistons, but supercharging plus slightly rich "maximum power" mixtures put the engine in a danger zone. A/F mixture gauges are not very expensive and a must for running a heavily modified engine.
He does need to go way overrich at high power to cool things down, but will lose some hp on the dyno. We use super rich mixtures in aircraft for takeoff and climb, then lean way back for cruise. EGT gauges are used, since leaded avgas kills lamda sensors.
Jim
He does need to go way overrich at high power to cool things down, but will lose some hp on the dyno. We use super rich mixtures in aircraft for takeoff and climb, then lean way back for cruise. EGT gauges are used, since leaded avgas kills lamda sensors.
Jim
Cargo Master
Joined: Nov 2003
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From: Duluth, Mn.
A wide band O2 sensor, EGT, and tuning, tuning, tuning.
I'll agree, better pistons are in order. But I have seen forced inducted engines brake pistons because of Lean conditions.
A computer logging program will give the info needed to tune the fuel ratio properly
I'll agree, better pistons are in order. But I have seen forced inducted engines brake pistons because of Lean conditions.
A computer logging program will give the info needed to tune the fuel ratio properly
Elder User
Joined: Mar 2005
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From: Cornelius, Oregon
Are you saying that a lean condition does NOT result in higher operating temps? I'd like to see some proof of that.
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Senior User
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From: east dubuque, il.
He blew it up this time while it was on the dyno trying to tune it. He says it was at 18 psi on the boost gauge. Apparently he had a restriction from a mod to fit the headers, so they took off the air inlet tube and tried to run it without knowing where the boost was going to be at.
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Elder User
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From: Northern MD
Like fonefiddy said, you've got to have a wide-band. How else are you going to get real-time air-fuel ratios? They might need a higher volume fuel pump, higher fuel pressure or larger injectors. But how will you know if you don't know what the air-fuel ratio is?
Hotshot
Joined: Feb 2004
Posts: 11,697
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From: Marengo, Iowa
That sounds like the cause there, detonation will cause all kinds of problems like that... Had one engine that plug wires had been reversed on, and blew two pistons, one had a hole straight through it, the other blew the lands out.
Post Fiend
Joined: Jun 2001
Posts: 5,228
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From: Running Springs CA
Pelican's Perch #18:<br>Mixture Magic
These charts are the key to the whole thing.
http://www.avweb.com/newspics/pp18b.jpg
http://www.avweb.com/newspics/pp18d.jpg
Note that cylinder head temperatures go DOWN as you go leaner. "Slightly rich" is where the head temp is highest. That is also where detonation is most likely. Peak EGT is the same as perfect A/F ratio.
Jim
Posting Guru
Joined: Jan 2006
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From: Malvern, Ohio
Lean mixtures burn slower and cooler. Why? because the molecules are spread farther apart causing the slower oxidation rate. The reason the leaner mixture burns/damages/melts parts is the slower speed of the burn transfers more heat to the surface it is above. The boundary layer with lean mixtures is thinner than of the rich fuel mixtures. Which is easier to break/interrupt and cause higher heat transfer that causes the damage etc. Detonation and pre-ignition can cause that break in the boundary layer btw. Rich fuel mixtures create the higher temperatures from the energy release because of adequate A to F to complete the oxidation/burn of the fuel, and, produce the optimum cylinder pressures for making power in gasoline engines.There might be other minute variables most likely, thats is what I know. For now any ways. But, that is my .02 cents.
Postmaster
Joined: Oct 2008
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From: Renton, WA
There are two forms of "detonation";
Pre-ignition in which the pressure of compression inititiates combustion, like a deisel. The higher the cylinder pressure, leaner the mixture, and lower the octaine, the more likely to detonate.
Spark knock, in which many of the above items come into play but instead a seperate explosion happens in a corner of the combustion chamber that is initiated by the pressure wave of the flame front initiated by the normal spark ignition. This is much more common, and has several added factors that cause it. Timing of course, and combustion chamber shape. In engines with wedge heads with a quench area (like chevy V8s) it becomes very important that the quench area of the piston matches the head, this helps remove these pockets. And because these explosions tend to inititiate on sharp edges that there are as few as possible.
So he needs good forged pistons with the proper shape. A GOOD polish job on the heads. A professional ear that can hear the knock, and a wide band O2 sensor.