Red lining
 

Hardly a topic for a hypermile forum, I grant you.

I googled the consequences of redlining an engine, and one forum response suggested that the occasional foray into the red actually helped the engine, preventing the build up of carbon deposits. Opinions on this?

I've never done it, and with a red line at 8K and an average rpm of 2K I'm not likely to, but I'm curious. Is it simply a case that incursion over the line simply cuts the engine until revs fall below, at which point the throttle reengages? Is no damage caused?

I would think that it is plausible.
It is as plausible as those Fuel treatment, injector cleaner and all those products in a bottle that promise 30% increase in fuel economy.

It may work a little bit, but it is not worth to even try.

I doubt it, but what does work is the long trip for the short commuting car to get it hot every once in a while.
When you only do short trips the engine never gets hot. Over time some water may get into the oil from the vapour in the exhaust gas escaping through the gap in the piston rings.
High water content wil turn oil into a buttery slush, raising internal resistance and possibly blocking lubrication.
Taking that long trip will effectively boil the water out of the oil. Use grill blocks to speed up the process ;)

It's perfectly safe to allow the car the redline occasionally. The redline is determined by where the engine develops its peak HP, and the mechanical limits of the drivetrain. Most cars employ a rev limiter, which cuts fuel beyond a certain RPM, usually a couple hundred RPM after the redline. This is a safety feature that will not allow the engine to rev beyond its limits to prevent damage.

As far as cleaning out carbon, I would argue that you don't really need to go all the way up to redline to accomplish the goal. A few brisk accelerations up to about 60-75% of the rev range will work just as well. There are plenty of products like seafoam that are designed to do the same thing, without potential damage to your engine.

I like to think it does good, that way i don't feel bad about hitting 65MPH in 2nd every now and then when i have a highway onramp to myself. :rolleyes:

My uneducated opinion is that it will do little good, but couldn't hurt. I've hit the rev limiter on my car a few times at around 7,500 RPM. Maybe I average once per year. There is never a need to go all the way to the limiter though, unless you're racing.

It is possible to go over the limit in a manual transmission by shifting into a low gear while traveling too fast for that gear. I've never done it, but I'm sure someone has. It goes without saying, this could cause catastrophic damage to the engine.

It doesn't hurt, but it's not necessary. Just a few runs up into the powerband every once in a while helps keep the engine clean.

So is constant relatively low revving actually bad for the engine? I rarely cruise beyond 2200 RPM, and almost never go beyond 3000.

I expect that if revving does get rid of deposits and such, there would be evidence around of engines that weren't revved and have that kind of buildup.
Anyone seen that?

I'm willing to believe that the occasional revving helps the engine, just want to understand exactly what's happening.

Actualy yess, I have seen it. There is a groove of carbon deposit at the top of the block cilinder. But it's there because the piston rings don't travel to the very edge of the block. Going 7000 rpm instead of 2000 will not bring the piston closer to the edge so is will not change anything.
http://www.ms-motor-service.cn/ximag...1708201211.jpg
I have found that picture online, it is not accurate since the rest of the block is just too clean but it represent where it is. Usualy the groove is smaller than that and form a powder when removed.

Just guessing but I've read on here the GDI are having buildup problems on the intake valves because they don't get washed with the gasoline.

So with that in mind, I want to wash my valves and exercise the fuel system with a full throttle maybe 2/3 rev limit rpm blast, I have a 3rd gear right hand corner that goes up hill, I often go full throttle or close to but stay closed loop, and get to 55 mph then jump to 5th. Don't think it really hurts my mpg much, if I shift at 2k I can't accelerate as quickly so higher consumption rate longer than getting to 55 quicker in 3rd.

I've heard anecdotal stories about people who drive their car hard and never have problems, then sell it to people who drive conservatively and suddenly have problems sprout up. Something about racecar...

Hitting the redline in every gear up to highway speed is good once in a while, if just for cardio and a shot of adrenaline!

So much of this is hearsay left over from a long time ago.
 

When carburetors ruled the planet, the old "Italian Tuneup" did have merit. Running the engine up the power-band did produce benefits in cleaner valves, chambers and oiling. However, modern closed loop systems run cleaner in partial load ranges. Much cleaner in fact than the open looped full throttle rich setting. This is independent of engine revolutions. As someone pointed out, simply running the car at a constant and warmed operating cycle will benefit a commuter vehicle. Running the engine up to red-line does nothing at all. This is assuming the engine is running properly.

The photo of the in cylinder carbon above the ring band is true for pretty much all vehicles to varying degrees. The quenching effect of the piston wall and cylinder wall keep fuel that is trapped in this space from reaching combustion temperature.

The mention of gasoline direct injected engines fouling is a product of exhaust gas recirculation (EGR) systems and positive crank case (PCV) ventilation recirculation. The residue builds up on the back of the intake valve with no liquid gasoline to wash it off. High revs do little to help this situation.