old cliche': "gotta burn the carbon out of it"

[BTF/PTM]

This is pure curiosity, I'm just wondering if there's any real science behind the cliche' of running a car hard to "burn the carbon out" and somehow remove deposits that may be there from loping it along or having it sit for a long time. I know it's an age-old saying, and I wonder if it has (or ever had) validity. Sorta the same as oil change and engine break-in science being largely old cliche's and hearsay. Thanks

[GreenZZZ]

The other cliche is spraying water in the carbs to steam clean the cylinders...

[jmortensen]

I have a bunch of Z owning friends, but I'll never forget when one took his Z to the other who is a mechanic because it was running a little rough. The mechanic friend asked: "When was the last time you really opened this thing up?" and the other guy said: "It's been a while." There were no cars in the parking lot, so the owner got in the passengers side and the mechanic drove, and they hauled arse around the parking lot like they were autocrossing for about 5 minutes. When they stopped the car purred like a kitten.

I would guess that what happened to the engine had more to do with the spark plugs than any deposits on the valves, but it definitely ran better after a little "abuse".

My brother used to put a little ATF down the carburetor on a used car after he bought it to clean out carbon in the combustion chamber and on the piston. That one also supposedly works. Makes a nice James Bond style smokescreen too. I believe seafoam does basically the same thing when added to the gas.

[BTF/PTM]

I've actually used the water method, but I did it on a fuel injected Honda and the water was sucked in through a very small tube connected to one of the manifold vacuum nipples. It did in fact blast a ton of carbon out of the tailpipe for a while, but admittedly I have no idea if if was the steam removing carbon from the internals, or just the severly vibrating engine straining with all its might to maintain 4,000rpm at WOT as it slowly drank a couple gallons of water breaking carbon chips off various pieces of the exhaust pipe itself. I got the advice from a friend who builds and maintains his own race cars, so I trusted the source. It was an experience, and I can definitely vouch that a small and steady feed of water will produce carbon from somewhere

I've also heard of the ATF method, supposedly it's an incredibly detergent fluid. But, as Jmortensen mentioned, it also creates ginormous clouds of white smoke. Haven't tried that one or Seafoam.

Jmortensen, do you have any theory as to specifically what happens to the engine after it's run hard for a while? Is it a thermal efficiency thing, maybe the engine straining brings the air/fuel ratio or work/heat ratio into better balance?

[jmortensen]

Best guess is that it cleans the plugs and helps more on an engine with leaky valve stem seals where oil is slowly leaking into the combustion chamber. At WOT you need a hotter spark to jump the gap. By sustaining a high rpm level you're going to work the plugs harder, and maybe that burns some of the crap off of the inuslator and electrodes. That's a guess.

I don't think you're going to clean the valves in 5 minutes, although I will buy that ATF or seafoam or Techron over a longer period can clean valve desposits to some degree.

[mlc240z]

Always heard this with carbed, non-ECU cars.

I can believe a benefit if running WOT will expose passages in a carb that do not normally get opened up around town (secondary jets, etc). Possibly clear some accumulated gunk that never has the chance to be pushed thru. Doubt this would apply to SU's, but who knows.

Seafoam, ATF, water or similar from all reports does clear the combustion chamber, don't know if a couple of minutes would be enough time tho.

[d240zx2]

Be mindful that if you blow out too much carbon, especially from the tops of your pistons, you'll be reducing your CR. ;-)

[Walter Moore]

You could also blow out more than "carbon"... but I digress.

[doradox]

In my former life I was an auto technician and owned my own shop in Tucson AZ. We would get customers, typically seniors, coming in with rough running cars asking for a tune up. I'd ask how long since the last tune and they'd reply "a year or so". I'd pull a plug and other than being coated in carbon it would look brand new. I'd then ask "how many mile have passed since the last tune"? They'd reply "2000". A years worth of one or two mile trips to the market or church or wherever. I'd make sure the choke and carb were working correctly and then tell them to go out on I10 and run at the speed limit for 20-30 minutes. They'd come back with the car running like new and as happy as could be.

On a carbureted car running at a high load for an extended period is the key. Combustion chamber temps need to be elevated for an extended period to effectively, and safely I might add, clean out the carbon. Short high RPM bursts or other short term high load schemes don't work nearly as well on anything but the lightest of deposits.

I've used the water and/or ATF, methods to dislodge chunks of carbon from engines that were knocking due to piston/head interference . They do work. Sometimes. And in that extreme case it's worth trying before pulling the head.

I don't recommend them as regular maintenance procedures but as the last resort before tearing down an engine.

Regular extended high speed (highway speeds) running is a good thing for your engine. For those that store your Z over the winter a highway blast should be the last trip you make before putting her up. You'll have the best chance of getting any water, unburned fuel, etc. that might be hiding in the nooks and crannies of your engine vaporized and sucked out through the PCV valve.

Steve

[BTF/PTM]

This turned out to be a very informative thread

I'll add on a part b of the question since it does seem to be in agreement that "burning the carbon out" works. For those who made the comments that a few minutes isn't enough, how long is enough, and would a highway blast require that the engine be jumped between lower gears than top (third in our case, I guess) to keep the rpm up? Also seems like the method Jmortensen mentioned would be safer, finding a parking lot or something and bomb around in it. By safer I mean mostly avoiding tickets.

[doradox]

This turned out to be a very informative thread

I'll add on a part b of the question since it does seem to be in agreement that "burning the carbon out" works. For those who made the comments that a few minutes isn't enough, how long is enough, and would a highway blast require that the engine be jumped between lower gears than top (third in our case, I guess) to keep the rpm up? Also seems like the method Jmortensen mentioned would be safer, finding a parking lot or something and bomb around in it. By safer I mean mostly avoiding tickets.

70-75 mph is plenty and you'll be turning around 3500 rpm or so. No need to run in third. Here in Indiana I can run 75 all day long and the troopers will never even blink an eye. Raving around in a parking lot may draw unwanted attention unless you have chosen carefully. Might be a lot more fun though. The idea is to get the combustion chambers hot and keep them that way. Running where you needn't ever close the throttle is the easiest way to do that IMHO. As far as how long? I usually say 30 minutes is plenty but I haven't done an exhaustive scientific study so that's always just been a guess. It's worked well for me and those I've suggested it to. Since you are doing nothing more than driving your car in a perfectly normal manner longer can't hurt.

Steve