steven74

V2Rocket

Walnut blasting is to clean the carbon **** out of ports and valves on DI engines that don't have the constant mist of excellent solvent (gasoline) raining down on everything.

Your exhaust ports probably have some carbon on them making them black as a result of combustion. It would only be really heavy if you had a serious oil burner on your hands.

It's like when guys pull the head on their engine and spend all kinds of time cleaning the pistons...it's all going to be "dirty" again in about 10 minutes of running, so why bother?

Droops83

I work at a shop that specializes in Porsche and BMW/Mini, and we also do some Audi work. Porsche tends to design effective breather systems, so we do not see too much of an issue with carbon buildup on the intake valves/ports of their DFI engines. However, on the DFI BMW N54 engines, Mini N14 engines and anything VW/Audi we see lots of carbon buildup, and do a fair amount of walnut shell blasting (which works as well as pictured above).

I remember seeing BMW literature from the 1980s advocating the use of walnut shells to remove hard carbon deposits from intake valves, which tended to form as a result of the poor early formulations of the unleaded gasoline of that period (the oil-based carbon the builds up on the valves/ports of DFI engines tends to be softer and gummier, much of it can be manually removed with pick tools).

Anyway, the main reason for using walnut shells is that it can safely performed on an engine installed in the car. During the service, the engine is turned over until the valves of the intake port being worked on are closed; the goal is to vacuum out as much of the walnut media as possible, but if a small amount end up in the combustion chamber, it is not the end of the world as the shells will quickly burn up.

As for a 944 head that is disassembled and removed from the car, simply use a soda blasting cabinet. I have one at my shop and use it all the time on aluminum cylinder heads. It instantly blasts off any hard carbon buildup. I understand that your concern may be the delicate nature of the ceramic exhaust ports used in the Porsche 951 heads (also used on the 964/993 heads). Walnut shells might be "softer" than grains of baking soda, but there are other variables such as air pressure, duration of blasting, etc. The bottom line is that 30+ year old ceramic port liners are susceptible to cracking no matter what, but if there is heavy carbon buildup, you will need to blast them anyway to find out!

Hope this helps, good luck with whatever you decide.

Droops83

Your last comment about about cleaning carbon from piston tops is not 100% valid . . . . while it is true that pretty much any engine will show a light layer of carbon on the piston tops on after a certain amount of running, there can often be a significant amount of "3D" carbon buildup, especially in the case of an engine with excessively worn valve guides and requisite oil consumption. This not only increases the compression ratio, but carbon buildup can absorb some of the injected fuel and in extreme cases cause hot spots and pre-ignition.

So, a bit of time spent cleaning each piston top (while at TDC and by hand/solvent, NOT with any kind of air tool or grinder!) is worth it.

Now, if said carbon buildup is due to excessive blow-by from worn out piston rings/bores, then the whole thing needs to come apart and be cleaned and/or replaced!