illmatic94

iTrader: (12)

When to add or remove head shims. I’m runnin an os18tz and I recently changed to 30% fuel instead of 20%. Do I need to remove a shim?

UrabusDenis

Without knowing anything about that engine, I'd safely say you should add at least a 0.1 mm shim, maybe even 0.2mm.

illmatic94

iTrader: (12)

Okay . The engine itself is made for 20%. Should I be swapping out for a colder plug as well?

Z'Stein

Leave the stock shims in place and try a cold plug.

fyrstormer

iTrader: (1)

There is no direct relationship between the percentage of nitro in your fuel and the shim stack you should run. Also, the engine isn't "made" for any percentage of nitro, it's just pre-tuned at the factory for a certain percentage of nitro.

A lower shim stack increases compression, which has two effects: 1) The engine produces more torque, and 2) the carburetor will have a narrower tuning band -- it will be harder to get a tune that doesn't choke the engine and also doesn't overheat the engine. You can offset the second effect somewhat with a colder plug, but that in-turn will make the engine less likely to idle reliably, because the plug will cool-off too much at low RPM and it won't provide a reliable source of ignition heat. Conversely, a higher shim stack reduces compression, which has two effects: 1) The engine produces less torque, and 2) the carburetor will have a wider tuning band, though it might end up being so wide that you can't achieve full operating temperature without starving the engine and risking damage from lack of lubrication.

I run all my engines on 30% nitro, and for a long time I assumed that meant I needed to run an extra shim, but eventually I figured out that was an incorrect assumption. The only time you need to increase the shim-stack is if you can't get the carburetor tuned to avoid choking or overheating without using a plug that's so cold the engine won't idle properly. You could even run a lower shim stack for even higher torque than the 30% nitro fuel can provide on its own, as long as the engine has enough cooling capacity, but that's something you should only try once you're very familiar with the engine.

So, keep the shim stack and the glowplug as-is and see how it runs. If you can tune it to run fast and idle reliably without choking or overheating, then you don't need to change anything. However, since this is the on-road forum and I assume you're going to be pulling high RPMs pretty much constantly, you may need a medium-cold or cold glowplug to keep the engine from overheating. If the colder plug doesn't help either, then you'll have to try a taller shim stack.

Roelof

And here you are compleetly wrong.....

It is all about ignition timing, more nitro will shift the ignition to a sooner moment you can correct with adding a shim or use colder plugs.

That most RTR engines are made for a wide range of Nitro has to do with a lower compression and hotter plugs. The 18TZ is a competition engine running on a higer compression and colder plugs giving a much smaller window of the Nitro percentage.

RCTecher12

iTrader: (1)

I set all of my truck engines to .4mm head spacing checked with the head installed with a dial indicator. I use 10-30% Nitro and adjust the heat range of the glow plug to suit the fuel used. Too high of a compression ratio and too hot of a glow plug will cause ignition to be too soon. Colder plugs will adjust the ignition timing later.

Higher compression advances ignition as does hotter glow plugs. Leaner fuel mixtures will advance the ignition as well. So, the big picture is you have to balance compression, glow plug heat range, and fuel mixture. Even the actual fuel composition will affect ignition timing to a point.

Roelof

If it works, it works.... but 0.8mm clearance for every engine is not the way to find the best/right performance. Every brand of a competition engine has determed the volume of the combustion chamber with a low as possible head clearance for an optimised power.
On a fresh engine a clearance of 0.3mm is possible, at high revs due the low play and flex the 0.3 will almost become zero giving a higher compression. Most brands handle about 0.5mm~0.6mm clearance for an .21 so there is room for some tolerances and play before the piston will smash the head.

The low clearance will create power and the flex does advance the ignition so in every rpm range the ignition is on the right time. Messing up the head clearance far from standard so yo can run every fuel is just loosing performance.

fyrstormer

iTrader: (1)

I'm not wrong, your English just isn't good enough to understand what I was saying. (no offense intended.) A "direct relationship" is a mathematical term. Yes, there is a relationship between the percentage of nitro and the required shim stack height, but it's an indirect relationship that depends on several factors, not a direct relationship that depends on only a single factor. You aren't required to increase the shim stack height when you increase the percentage of nitro in the fuel -- you may need to adjust something, and the shim stack height is one possible thing you may need to adjust, but it isn't guaranteed. It depends on the overall configuration of the engine and its operating conditions.

As for the part about using colder plugs...yes, I said that. Go back and read my post thoroughly instead of looking for an opportunity to argue.

Roelof

Again, on low compression engines you can almost run any fuel,

On competition engines you barely can not go higher with the nitro content. I have seen many Novarossi engines shimmed that tight for the use of 25% fuel and an 8 plug that even a 7 plug or 28% nitro (mixed on the limit with a nitro max) gave detonation and a damaged plug.
The 18TZ is a competition engine so a 10% higher nitro does directly relate to adding a shim. If not, you overheat the engine and blow plugs.

RCTecher12

iTrader: (1)

Most of the engines I’ve had in my hands were generously shimmed from the factory. Some competition, some basic ones. Doing it “right” involves determining the actual bonafide compression ratio. Is there any factual correlations between a given specific CR and a specific Nitro percentage?

snuvet75

iTrader: (16)

From my experience, running w/o enough shim does lead to premature detonation almost always in Novarossi. And the tuning window does decrease with higher %fuel. And temp goes crazy high. When taken apart, it is very obvious about detonation as the top of piston becomes glazed dark. And the plug goes out like no other.

RCTecher12

iTrader: (1)

I’ve used Neal’s advice to shim the head for .4mm post break-in. I have been taught over the years to not run tighter than .2mm/.008” to prevent the piston from hitting the head, but I may experiment with tighter head spacing with a couple engines. I do agree that the head volume and shape must be taken into account when setting the head spacing and choosing a fuel mix.

illmatic94

iTrader: (12)

I’m curious on that concept to. He sent me an engine that was “modded” and told me to swap out the shim after break in.

illmatic94

iTrader: (12)

Can you actully hear detonation?