Combustion chamber finish

[axspeed]

GC,
What finish is the best for the combustion chamber, valve heads and piston crown?
I have to admit that all of the engines I've owned since my 50cc days, I have very carefuly worked the parts mentioned earlier to a mirror finish.
This is down to being instructed to do so by a local man who races a grasstrack sidecar. I never questioned it, just did it - the naievity of youth.
His reasoning behind it was that it prevented carbon build up(which it appears to do) and reflects a little of the combustion heat away from the components, increasing efficiency.
Any veiws on the subject would be appreciated.
Regards, axspeed

[Guy Croft]

Axspeed, hi

first thing to say is you can't stop carbon and sulphur compounds from adhering to metal under the influence of heat and pressure with water present. It's impossible. Even F1 heads 'carbon up'.

You can mimimise the amount of these particulates by getting the engine fuel and ignition really well optimised, making sure the guides and rings are in good condition - it's mainly oil that causes the fouling.

As for cc finish you have to remember that the alloy most heads are made of is full of tiny fissures (holes) that act as the key for the stuff sticking to them, and though you can polish it doesn't stop that. Same is true of the exhaust ports. However, the gas action across the cylinder during exhausting and overlap scrubs the particulates clean on every stroke in those regions so the layer should nbever really build up to anything thick enough to cause a problem (usually pre-ignition) on a good engine (as described). Were it not for scouring the ex ports would foul up totally in no time.

And, of course - what's wrong with a fine layer of deposits? Bearing in mind they are going to adhere whatever you do. I am pretty sure that to an extent they provide a smooth surface for gasflow in themselves. I mean, it kind of 'finds its own layer'. This is not unique to engines - the phenomenon occurs in turbines, turbochargers, guns too. In turbochargers they can run forever and day on good quality fuel. Big units, eg: on ships, run the very lowest grade of heavy oil and have to be 'washed out' periodically to clean the turbine blades. (You see the at ports, clouds of black smoke coming from the funnels as they go thru that water-washing process).

The short answer regarding 'enginering practice' is I have run as-cast chambers where the surface is relatively smooth anyway, on those where it's rough to the touch I usually go to 80 grit polish then 120 for cosmetic reasons mostly. 80 grit can look really dire.

As important in my view is to relieve intrusions in the chamber - in other words sharp edges, bits sticking out, edges of valve inserts etc. These can only inhibit the development of the flamefront.

There are details of tools for doing this in the CVH thread.

GC

[sumplug]

Over time, carbon build up on pistons for example can increase the CR slighly and as you say Guy, the surfaces key to an even level.
My understanding is, if the head has a mirror finish, the fuel droplets fall through the chamber too fast and this reduces power. Some so called gasflowed heads that are commercially for sale are little other then heads with polished chambers. No gas flowing done hardly at all. Its all smoke and mirrors!!

Andy.

[Guy Croft]

It's worse than smoke and mirrors, believe me.

There shouldn't be droplets if it's atomised or mixed properly, goes back to my point about reduction in carbon deposition by calibrating the engine well to ensure as clean burn as possible.

All pistons and heads 'black up' within seconds anyway. I've borescoped enough engines after dyno test to know.

GC

[axspeed]

GC, Thanks for clearing that up, I'll certainly save my self a few hours effort by skipping that.
It does highlight which pieces are poor quality though, the porosity that's shown up occasionally is shocking. As far as pistons go, you can see the difference between OE and cheap replacements. You obviously get what you pay for.
Many thanks.