To Shim or not to Shim, that is the Question  Ron Lund

This is a discussion about head shims. If you have any thoughts, I would be glad to post them here if they add to the discussion.

I wrote:

Adding a shim will almost always smooth an engine out a little, but in cold weather, it may be a necessity. In hot weather, adding a shim will cost some power. Remember, when the air is hot, there's less of it. Less air means that doesn't compress as much. To keep the compression constant as it heats up, you would remove a shim (s). When it cools down, you would add a shim. The shims that come with the engines are thick and would cause a really big change in compression. With our thinner shims, you can fine tune your engine for any temperature.

Ben Minor commented:

The first and primary reason for shimming is to to manipulate the the timing of the combustion process. Increasing the compression ratio by decreasing shimming tends to advance timing and increase power, up to the point where the nitro content of the fuel causes pre-detonation. The opposite holds true for increasing shimming, up to the point where power just goes south. I think we're on the same page here.

The part where I lost you on your blurb on the website occurred when you mentioned adding a shim in colder weather. Traditionally, if there's ever a time that we shim our contest motors to retard the timing and smooth things out, it's in hot weather, not cold. When we get down in the 40's here in the late fall, I can run my mixtures MUCH leaner without inducing a ping/pre-detonation in the motor. Regardless of the decrease in air density, the increase in ambient air temp in hot forces the combustion process to kick off sooner in the combustion of the engine. Per the suggestions of Rick Mattie, I've often had to add a shim and drop back to as cold as an Enya 5 plug to create a sewing machine drone in wickedly hot temps. When air temps shift from cool to hot, we all expect our needle settings to become richer as fewer air molecules occupy each intake charge. We also know, though, that if we then try to lean that engine back down to the same relative mixture and level of smoothness seen when the engine was needled for cooler weather, it'll sure as heck start to ping a little, especially on higher nitro fuel with a picky exhaust. I know of no other ways in hot weather to get an engine correctly needled IF it starts pinging at anywhere near a reasonable needle setting (meaning it goes from kicking the tail from richness to kicking the tail from pre-detonation) than going to a colder plug, reducing the nitro, adding oil, raising the exhaust timing, or adding a head shim. Each of these choices retards the timing to reduce the pre-detonation. To lose a shim will make matters worse.

If appears that Ben and I disagree on this...sort of. The basic disagreement is when to add a shim. I would add it in cold weather, Ben would add it in hot weather. Will both methods work?  Believe it or not, the answer is "yes".  The difference is that my way would maximize power and require other adjustments to control ignition. Ben's way would maximize smoothness without having to change other things.

As the compression changes, because you've changed shims or the temperature/altitude has changed , the ignition timing changes also. Under more compression, the fuel mixture has a tendency to ignite sooner.  There are ways to control ignition timing, including glow plug selection, oil content and nitro content. If you remove a shim when it gets hot, you can also plan on changing some of the other factors. You would use a colder plug, such as the Enya 4 or 5, reduce nitro content or add oil. All of these things will retard the ignition. Naturally, the plug is going to be the preferred method, since it shouldn't cost you any power. Adding oil or decreasing nitro will cause a slight power decrease, but not as much as lowering the compression.

After this, Ben commented further:

Now we're on the same page. The part about pulling the shim AND going to a colder plug to help compensate is the difference between night and day in how it all reads. If someone JUST pulls the shim in hot weather without doing anything to help the timing advance, then problems are likely to occur. The main variable being how much power a guy can stand to lose when the weather gets hot. With the big blocks, less fiddling needs to be done. The smaller 60's and such which are running pretty well maxed out in cool weather will take extra TLC to push them harder still in hot weather.




Viperhead Owners

Start out with the stock .008 shim and the other .008 shim that's included with your head. Fine tune from there.

We have .002 and .004 Brass head shims for most .60 and .90 engines.
Just search the store for head shim.

Boil it Down
If you're losing power as the temperature goes up, reduce the thickness of your head shim and go to a colder plug. If you have to, add a little oil to your fuel or switch to a lower nitro fuel.

If you're power is satisfactory, but you're just getting a little hammering (pre-detonation) in the engine, try a colder plug.

If you're getting some pre-detonation and are more concerned with smoothness but willing to sacrifice power, add a shim.