Head shims for Nova RS12T5S1
 

I m confuse of whether to add 0.10 shim to my S5 or not. My LHS and most of the guys here told me if I want to use 25% nitro with this engine I should add the shim or if i don't want to add shim use 16% nitro with it. I want to get the most of my S5 so will the performance be the same if I use 16% without shim compare to 25% with the shim.

I'm running 20% in mine without adding any. There should already be a 0.03 shim in there. You can try 25% as is and if seems to be running unusually hot, then try adding 0.01

Re: Head shims for Nova RS12T5S1
 

With the stock 0.3mm (aluminium) you are safe up to 20%.

If you want to go to 25% you may need to add a 0.10mm shim and is safe up to 30% nitro.

With a 25% or 30% nitro you may need a colder plug, like a Novarrosi #7 or in extreme cases a #8

How about performance wise? Without shim and 20% or with shim 25%? I also heard that adding shim will decrease the compression a bit so meaning loose a bit power, so will the performance be par with shim/25% and without/20%?

with anything in nitro content above 20% put the shim in because the nitro makes it combust more, you will loose a bit of bottem power and you will gain on the top, when using the shim. Make sure you insert the copper shim in between the silver one and between the underneath side of the head.


Jeremy.

Nope.... if you don't have a grasp on what you're doing, better to not tent to the devil... ;)

Adding and removing shims is another way of modifying the power output of the engine, if you start adding or removing shims, you need to change your carb adjustment and plug selection. This is used too when the wheater change or you run on very different heights over the sea level or very misty conditions (then on the air goes an extra dose of oxygen in form of water or more O2 percentage).

Is a nice way too of toasting your engine. If you put too much to the limit, you can start doing detonation and pit the piston dome and the combustion chamber and raise the engine temperature too.

My advice is to not touch this kind of adjustment unless you have at the side anyone who understands what is doing. Stick to the adjustments I gave you before.

I tried running 25% without extra shims and it had pre-detonation problems. I added 0.1mm copper shims and the engines runs superb now. I am using #5 btw.

what is pre-detonation? any syntoms to look out for?

Look for pits on the dome of the piston. Are very characteristic, first time you'll see them, you will be sure to know what's pre-detonation. Sorry, don't have an image of a pitted piston, I don't know one from many time.

Pre-detonation is when the air/fuel mix detonates when the piston is going up (before TDC - Top Dead Center). If this occurs, piston and conrod suffers tremendous stress and the power output is reduced. Is like you run on your 1:1 car a very advanced timing on the ignition (knock due to low octane rating on your fuel).

those burn marks in brown/black color on the piston head?

At WOT you should hear the popping sound from your exhaust. A correctly shimmed head will not have this sound. It should be smooth sounding.