Monday, December 8, 2008

New Engine Break In and AFR

On many occasions over the past couple years I've heard the term "break in map". I've never really understood what this meant so I decided to ask around. I was really surprised at the answers I got...the most common was "I like to run it richer during break in so the engine won't overheat". This, under no terms, is the correct way to break in an engine.

As I've said before on this blog, I'm not here to debate how to do engine break in. There are dozens of opinions but I will say confidently, running it rich in the beginning is not the way to do it. Doing so will permanently damage the engine causing power loss you will never get back. The idea is to run the engine as little as possible to get close to a proper desired a/f ratio so nothing is damaged, then run it hard during the break in once you acheive that desired ratio. Outling break in procedure is important to explain why rich mixtures during break in is a critical mistake. Here's why...

The piston ring seal is really what the break in process is all about. Contrary to popular belief, piston rings don't seal the combustion pressure by spring tension. Ring tension is necessary only to "scrape" the oil to prevent it from entering the combustion chamber. If you think about it, the ring exerts maybe 5-10 lbs of spring tension against the cylinder wall. How can such a small amount of spring tension seal against thousands of PSI (Pounds Per Square Inch) of combustion pressure ?? It can't.

So how Do Rings Seal Against Tremendous Combustion Pressure ??
Rings seal against the combustion pressure from the actual gas pressure itself. It passes over the top of the ring, and gets behind it to force it outward against the cylinder wall...which is why they are beveled. The problem is that new rings are far from perfect and they must be worn in in order to completely seal all the way around the bore. If the gas pressure is strong enough during the engine's first miles of operation (as in WOT), then the entire ring will wear into the cylinder surface, to seal the combustion pressure as well as possible.

The Problem With "Easy Break In" ...
The honed crosshatch pattern in the cylinder bore acts like a file to allow the rings to wear. The rings quickly wear down the "peaks" of this roughness, regardless of how hard the engine is run. There's a very small window of opportunity to get the rings to seal really well ... the first 20 miles. If the rings aren't forced against the walls soon enough, they'll use up the roughness before they fully seat. Once that happens there is no solution other than bore, new rings/pistons, and start over.

What compounds the problem is the overly rich fuel mixture I mentioned above. The rich fuel mix will dilute the oil. Dilute the oil too much and you'll wash the oil from the cylinder walls because the rings aren't seated and fit tightly to prevent the mix from staying above the top ring. Remember the rings aren't sealed in the beginning, so the raw fuel washes down the cylinder and piston skirt. When you do that, you accelerate the wear on the ring and cylinder wall before the ring has time to seal...then you don't build the cylinder pressures you should and the engine will NEVER reach its full power potential....EVER.

This will probably be my last blog post for 2008. I would like to wish everyone the best during the holidays and a happy new year. See ya in 2009!

Happy Motoring!
Kevin

4 comments:

George said...

EXCELLENT Kevin! ABSOLUTE TRUTH Told there! That's about the Best Advice/Info I've seen on this subject in MANY Years! Keep up the Good Work/Info, It Really SHOULD be of Great Help to MANY!
Hope You have a Great Christmas and New Year. Hopefully Things will only Continue to get Better and Better With ALL the HeadQuarters People. Even Though I've Not Met you yet, I know Doug and Jo, and they are Pretty hard to beat for Being VERY Good "People", so You Couldn't be far off That Mark! ALL the Best Kevin.

Hogman

Dale said...

Hey Kev - Just read this post. If a motor is run too rich during break in, will premature wear on the piston rings show up as lower pressure numbers from a compression test?

Dale (Tinker)

HEAD QUARTERS said...

yes it will.

George Chabot said...

Kevin - I have just read this and it confirms something I always said about break-in but never fully understood - I said "Ride it like you stole it." which seemed right but you have pointed out the WOT puts gas pressure behind the rings and breaks them in while the cross hatching is still effective which is a valid reason to run it and not baby it. It also makes sense about two rich an AFR can wash the oil off the bore and dilute it, too. Thank you, Kevin