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Old 11-16-2003, 06:22 PM   #1
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Default Engine Shim Tuning

Okay guys onto the black art of engine tuning ! How does one actually go about tuning an engine by shimming it ? What do you gain/lose etc. I am talking particularly about performance on track - not about shimming it to match nitro percentages or to decrease/increase compression to solve pre-detonation or other problems.

Also does anyone know what nitro fuel % the Novarossi RS and NS .12 series came originally shimmed for ?
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Old 11-16-2003, 07:11 PM   #2
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In my experience I have found that just by lowering the head you gain more power especially in lower rpm range, idle quality can suffer, the engine also ran cooler. I've ran my deck height really low before and didn't have idle problems but every engine is different. I don't run any big tracks so I am not sure how it effects top end performance but would assume that a higher deck height would be faster for top rpm speed on bigger tracks. With the copper shims for me I'd lose .001" head clearance for every .004-.005" in the shim(s) after the head was tightened. The guys with a lot more on-road experience than me should know how it effects the top end speed better than I do.
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Old 11-16-2003, 07:25 PM   #3
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Yes you are right a decrease in head shims or an increase in compression ratio will increase torque because as the compression ratio goes higher the actual ignition timing occurs sooner. This is the same effect that happens with a colder plug except in the instance of a colder plug the ignition is slowed until a greater point of compression build occurs. however there is a point of diminishing returns where detonation occurs or engine temps can soar this is where a colder plug can also help.
Consequently the same effect occurs when you increase the head clearence. 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 the timing does not occur to late in the cycle as at this point you end up overleaning the engine to get it to rev properly and the engine life will suffer dramatically.

on a 12 engine I would only advise going .004 thousanths over or under .018. so that is .014 or .021. I have run as low as .008 but this was with tremendously increased exhaust timing and to regain static compression ratio. to little and the piston can hit the head as you have to take into account rod and piston growth relative to their tempratures.

On marine engines we could get away with .003 on a 21 but we were water cooling it. that won't fly consistantly on a car engine without a special head that has a larger combustion chamber to start with.

Savant you can safetly use 30% on the stock head clearence. using an odonnel 99 plug you can go 40% If you dont overlean it the engine life will not suffer dramatically.

I should also comment that my 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.

there you have it.
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Last edited by Motorman; 11-16-2003 at 07:34 PM.
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Old 11-16-2003, 08:15 PM   #4
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I just bought an STS D3 and tore it down before running it. I Noticed That it had no Head shims. I went ahead and Broke it in like that and Ran it today first time. After a Few more Tanks I had it turn ON, Switch action that I've Heard about, is that from being too Far advanced and How much do you think I should shim it? I was Thinking .2 mm .008" I think thats the conversion.After awhile I was able to get it regulated back but Had to Really richen it up then had problems with it being too rich. Is a very fine line between the switch action and being too rich. Do you guys think the shims are the answer?
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Old 11-16-2003, 09:03 PM   #5
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when you say switch action do you mean the temp soared?

you need to physically measure it. guessing won't get it take it apart again and look for the head shim again. sometimes it fits so tight to the head button it looks like it is part of it. I would bet it is there if not you need to measure it and set it to about .018 thousanths. measure the shims, never trust the package.
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Old 11-16-2003, 11:27 PM   #6
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Duh.... ok it looked like a machined alum step. .012" alum and
.008" Brass shim so .020".
The switch action that I was taking about was a High rpm idle that I couldn't decrease with the idle adj or end point on my radio.
and yes it was followed by soaring temp.Low end mixture too lean? Sounds right now that I'm on the keyboard. I was trying to Richen the High end because the rpm's were so high then it would die on accelleration (bogged down) Too Rich....
I think I got it now Just hadda Read it and look at it a few times to see.
So here is my mistake I think...... I leaned the low speed too much causing it to Rev High then trying to compensate and mistaking the high rpm for idle adj or too lean High end I had a mental breakdown and screwed up what I should have left alone and left alone what I should have screwed up. Therefore Screwing up everything that I have been learning......
Thankfully I was able to run one more tank through it after I turned the right screws the right way and my motor will live. I tore it down after bringing it home and everything looks ok.no scoring, pits or discoloration.
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Old 11-17-2003, 12:22 AM   #7
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sounds like your on the right track.

one last comment.never assume that the head shim that is on the engine is the actual head clerance. in many cases you will find that the engine actually has .006 to.oo8 without any head shim. Novarossi's are .008 without one and the factory installs an additional .010 giving the engine .018 stock. this has not always been the case but 99% of the time it is. when in doubt measure the head button register and the piston to the top of the liner at top dead center and subtract. this is the only way to really know.
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Old 11-17-2003, 12:42 AM   #8
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I did that after i found the step was actually the shims.... D3 has a flat Head no step then has 2 shims .012" & .008" Measured them on my Dial caliper seperatly The buttonhead goes into the combustion chamber .050" then shimmed .020" back out.
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Old 11-18-2003, 10:10 AM   #9
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Quote:
Originally posted by Motorman


on a 12 engine I would only advise going .004 thousanths over or under .018. so that is .014 or .021. I have run as low as .008 but this was with tremendously increased exhaust timing and to regain static compression ratio. to little and the piston can hit the head as you have to take into account rod and piston growth relative to their tempratures.

On my P/S sets I am able to run engine from .125mm (.005) to .15 mm (0.006) shims and with 20% or less fuel with dramatic power increase overall, temperatrure will stays within 210-220 and no any detonation was seen so far ( ones by my mistake was hell of detonation in Texas, but there was my mistake with tuning). The engines has defenetly different timing setting from stock version, but I have never faced piston hit the botton.
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Old 11-18-2003, 06:51 PM   #10
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Default Detonation

When you guys say detonation...does that mean the plug is turning black because of retarded timing due to extra shims?
To turn off the switch do you remove shims to speed up timing?

Please advise.
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Old 11-18-2003, 06:59 PM   #11
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Default Engine Shim Tuning

Guys;

I'm new to running Novarossi engines (I'm normally a Picco fan) and would like some advice.

Several People at the track told me that NR based engines will detonate on 30% and anything less than about .024 Head clearance. On my Picco engines, I have always set clearance to .018-.019 on 30% and they ran great. I would like to increase the compression to get a little more grunt, but don't want to damage a almost new engine by too little head clearance.

Any suggestions or comments are welcome.
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Old 11-19-2003, 08:13 AM   #12
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Default Re: Engine Shim Tuning

Quote:
Originally posted by popsracer
Guys;

I'm new to running Novarossi engines (I'm normally a Picco fan) and would like some advice.

Several People at the track told me that NR based engines will detonate on 30% and anything less than about .024 Head clearance. On my Picco engines, I have always set clearance to .018-.019 on 30% and they ran great. I would like to increase the compression to get a little more grunt, but don't want to damage a almost new engine by too little head clearance.

Any suggestions or comments are welcome.
My sugestion is:If you want to increase compression ratio, you will need to use lower nitro content and you will have better power output with no detonation.
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Old 11-19-2003, 12:39 PM   #13
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I read all the above and just measured my engines. My NS3 has a clearance of 0.018" at TDC where the Rossi Pixy Black has 0.028"

I am having all sorts of problem with the Pixy overheating. Will removing the shim and reducing the head clearance help?

Cheers
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Old 11-19-2003, 02:40 PM   #14
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Default Re: Engine Shim Tuning

Quote:
Originally posted by popsracer
Guys;

I'm new to running Novarossi engines (I'm normally a Picco fan) and would like some advice.

Several People at the track told me that NR based engines will detonate on 30% and anything less than about .024 Head clearance. On my Picco engines, I have always set clearance to .018-.019 on 30% and they ran great. I would like to increase the compression to get a little more grunt, but don't want to damage a almost new engine by too little head clearance.

Any suggestions or comments are welcome.
You can go down up to .021" with a 30%, but plug selection will be critical (you're on the ragged edge for 30% Nitro). Better to go with a Nova 7Tf with those nitro percentages. You can go down up to .018, but will be quite risky and will be quite weather sensitive.
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Old 11-19-2003, 05:14 PM   #15
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Quote:
I read all the above and just measured my engines. My NS3 has a clearance of 0.018" at TDC where the Rossi Pixy Black has 0.028"
Ziggy
This is a problerm on the pixy drop it to .018. That is exactly why it is overheating the CR is to low. The thing is you cant go that low withoug machining the head button to get it to go there.


I've been keeping my mouth shut for a long while but I have to say somthing.
TG you need to let go of the marine mentality man. If it works for you great but I know I can make more power than you on 30% at .018 and 40% at .021 with zero detonation at 4.5 mm exhaust height. we do it every day no problems. What you shouldn't be doing is spinning people off towards unrealistic head clerance on their production motors. If its a hand built thats a different story but you do them a disservice by directing then toward toward a global meltdown of their hardware.

If you feel inclined to take .008 thousanths head clerance advice do so at your own risk that is all I have to say.
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