Hi,

Is it possible to use 16% fuel instead of 25% for .21 on-road engine? Will it do any harm to it? Thanks!

Yes, most pros do for mileage. You just have to be careful on your tuning and maybe pull a head shim.

usually .21 onroad engines come stock shimmed for 25%...
Nova stock shim is 0.7mm for 25%, how many shims for 16%?

.21's are generally shimmed for 16% already. Read the insert in the box. Add a 0.1mm shim for every 5% increase in nitro content.

for .21 Nova engine they are come stock shimmed for 25% only their .12 engine come stock shimmed for 16%

So do u mean others .21 engine are come shimmed for 16% only?

Thanks!

Novarossi .21 on road engines come shimmed for 25%. That = .70mm of total shims. You can use 16% at this clearance with no problems, only adjusting the tune. It is also possible to remove a .1mm shim to use 16%.

Novarossi .12 engines come shimmed for 16%. That = .3mm of total shims. You can safely use up to 20% with this clearance. To use 25% you should have around .5mm of total shims.

This will get you in the ballpark.

I thought I should ask here instead of just opening a second thread...

When humidity comes into play... If you have a very high humidity should you remove shims or add shims?? What is the reason behind this? (So I can reason it in the future).

Thanks guys.

If you are going into high humidity, you should add shims to the head. The humidity is taking the place of the oxygen in the air. Water does not compress like air. Inside the combustion chamber the water droplets turn into steam. This creates heat, and can make detonation worse. You are safer to decompress the engine in high humidity, and heat.

Thanks for that reply Grinder! Exactly what I was looking for!

So, going into more specific...

For a .12 engine... up to what relative humidity (50%, 60%, ..etc) should I start adding shims and in what increment? (0.05mm, 0.1mm)

Thanks!

I would go up .1mm from stock for 70% humidity or more. Sometimes I have gone up .2mm if it is very hot, and humid. This applies to .21 engines as well. The best thing to do is track test if possible to see what is working best for that day with your engine. If you can keep track of the weather data, you can get back to that correct setting easier in the future.