blanks596

iTrader: (20)

Could someone explain how shimming, nitro percentage, and glow plug heat affects timing.

My understanding is that higher nitro percentage advances timing, while lower nitro percentage retards timing. Also shimming an engine retards the timing by increasing head clearance while reducing head clearance advances timing. So, the overall affect would be: advanced timing increases acceleration, while retarding timing decreases it. Also, I heard that using higher nitro percentage will increase acceleration at the expense of top end speed while lower nitro percentage would lessen acceleration while increasing top end speed.

afm

Engine Shim Tuning
By Dennis Richey

Experience dictates that just by lowering the head (more compression) you gain more power especially in lower rpm range, idle quality can suffer, but the engine also runs cooler . Also, that a higher head (less compression) will increase top rpm speed on bigger tracks.

A decrease in head shims (an increase in compression ratio) will increase torque because as the compression ratio goes higher, the actual ignition timing occurs sooner. However there is a point of diminishing returns where detonation occurs or engine temps can soar, and if this happens a colder plug can help. The colder plug will also increase torque, but ignition is slowed until a greater point of compression build occurs.

When you increase head shims (a decrease in compression), top end is enhanced as the ignition timing is retarded and occurs later. Generally a hotter plug is needed to advance the ignition cycle so that timing does not occur to late in the cycle, but at this point you end up over leaning the engine to get it to rev properly and the engine life will suffer dramatically.

On a .12 engine we would only advise going 0.10mm over or under 0.46mm, so that is 0.36mm or 0.56mm of total head clearance at sea level.
Generally on a .12 engine 0.10mm will change the compression ratio about 3/4 to 1 point..

We should also state that the comments with regards to plugs and head shimming are when both are used together. Used alone a hotter plug will rev harder and a colder plug will make more torque.

One last comment. never assume that the head shim that is on the engine is the actual head clearance. When in doubt measure the head button register and the piston to the top of the liner at top dead center and subtract, to determine proper shim. This is the only way to really know.

You can safely use 30% Nitro with 0.56mm of real head clearance, and If you don’t over lean it, the engine life will not suffer dramatically. For less Nitro % you can safely use 0.46mm of real head clearance.

Someone asked about plug fatigue the other day, plug fatigue can occur under high sustained RPM's, and this is where the plug stays shiny and the wire just fractures for no reason. When this happens it is usually not a carb. tuning or head clearance problem. If you see the wire start to pull out of the hole this means that the engine is scavenging really hard and when this occurs the engine is making serious power.

There you have it.

AFM

blanks596

iTrader: (20)

Wow, thanks for that great information, but I have one more question. What do you use to measure real head clearance? Also, are stock head clearances standard across brands-ie-are they all .46mm?

afm

Your Welcome.

Use a digital caliper (vernier), and as said before, place piston at TDC and measure distance to lip, then measure head button recess, then subtract and you get head clearance without shims. Now you add shims to get desired real head clearance according to Nitro% to be used.

Yes, that head clearance number is theoretical valid target for all .12 engines, that is why it is best to first measure clearance without shims on every engine, and then add shims to get desired real head clearance.

AFM

Nosram

Another way of measuring the gap is to insert a length of solder, 1/32" diameter, in through the plug hole. Angle the solder about 45° so that it is trapped between the piston and head. When the motor is turned over the piston will squash the solder to the same thickness as the head gap. Just pull the solder out and measure across the flats on the end of the solder. This is your head clearance.

Rich Browne

I have attached our instruction sheet of how we set our deck height.

afm

That's a neat instrument, do you sell the gauge??, or where can I buy one ??
It come prepared for turbo style thread??


AFM

MassiveMods

Rich .. where can i get one ?

razzor

Ive seen a tool like that ,it works great and accurate.
Would also be interested in buying one.

rmdhawaii

iTrader: (4)

The Gauge: http://www.empirercracing.com/gauge.html

Rich Browne

They are available in standard and turbo plug (Nova Rossi or Sirio only). Custom made and usually not in stock from local hobby shops - these are for the "ultra-racer"

Purchase from Ashford Hobbies:

http://www.ashfordhobby.com

I know Ace Hobbies had some in stock (They're closing soon - better call quick): http://www.acehardwarehobbies.com

I would prefer you purchase from these sources. I am happy to help with Tech Questions. Our phone number is listed on the Empire site.

nitro_moe

Hi everyone..

I was just wondering what the original shimming size is on a stock OS12TZ-3port? I'm planning to run 30% nitro and want to know how safe it will be on the life of my engine and its performance.

I'm currently on sea level with relatively low-med tempretures (between 10'c-15'c). I burnt three OS P7 pluga and a P6 within 3 hours of operation. The P6 only lasted a tank of fuel untill it was cooked.

I will be running on an open track with high speed cornering and few tight bends.

Thanks..

afm

OS 12TZ comes with one 0.1mm shim and one 0.2mm shim.....yuo need to add one additional 0.1mm shim for 30% nitro.

I run mine with P7 plug, and carb is set with high end needle at 1 3/4 open,
the low end needle flush with the ball link body, the mid range needle flush with carburator body (factory settings).

AFM

Sirio2127

iTrader: (1)

Hello, Say you shortened the stroke of an engine by 0.08 of a mm and the engine stock runs 0.65mm head shims and uses a 7tf plug sould i drop the head clearance by 0.05mm or 0.1mm or not at all or more than 0.1mm?.

Grinder

iTrader: (2)

I use mine constantly. It is the easiest way to accurately measure head clearance that I have found. Great tool. Thanks Empire!