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shimming for nitro

Old 08-06-2014, 07:54 PM
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rcbence
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Default shimming for nitro

About car engines....let's say the engine is built for 20-25% nitro. Which most are I believe. Now you drop to 10%. What thickness shim is needed to fully compensate the lack of power and retarded timing? Lrp .28 for example says..... .1mm shim for 25%. No shims for 16%. And .15 for 30%.

When I measure the shims lightly I get .2mm. If I squeeze hard I get anywhere from .05 to over .1mm. I'm assuming they figured how much they squish when the heads torqued.

If it runs good at 20% and a .1mm shim for example. Will it now run about the same on 10% nitro with .1mm taken out? Is lrp correct? It seems like they take or add .1mm for every 10% nitro or so.

Also, if a .1mm shim is taken out for 10% nitro....would you still want to run the cold r5 plug that came with it? Or is a hot or medium plug needed despite the removal of .1mm?
Old 08-07-2014, 12:50 PM
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Lou Crane
 
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rbence,

Try it without a 'shim.' See how it does. Of course, you won't get the same power as with higher nitro fuels, but it may run as well as you want.

Plugs are another mystical art. It may be a good idea to try a plug one or two 'steps' hotter Your r5 spec sounds like there may r4 and r3 versions. Start there...

Luck, and let us know your results?
Old 08-07-2014, 06:37 PM
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rcbence
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Well I tried my firestorm on some 2yr old 10% omega fuel. Started with stock 3 shims and r5 cold plug. It took a hit on the acceleration. Idle would drop to a low idle (not rich).

Top end once tuned properly reved out OK.

Then took 1 shim out same r5 plug. Power seemed to improve....although the idle dipped still. Wasn't quite as snappy as 20% byron still. Both fuels have 15% oil by volume. Yep omega cheats you 2% or so. Byron cheats you about 3%. Its the rtr 16% oil that I added 2% castor. After adding it came at 15% oil.

The last step I took was using an old os 6 hot plug.

That seemed to do the trick. Power was snappy and very similar to 20% nitro. It could probably be better with fresh 10% fuel. I can't use all that power on loose dirt anyways.

I might pick up some mc 55 plugs. I believe they are medium hot. Os plugs are a hair shorter it seems.

At this point I'm not sure taking another shim out will help. Maybe I'll try it. If there's not much power to be had I'll keep it at 1 shim removed.

HSN seemed about as touchy with the 20% after the shim and hotter plug. Which I think means its close to where it should be.
Old 08-08-2014, 07:58 AM
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rcbence
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Sort of figuring out what I want to run when I get my methanol. 20% nitro would be simple I suppose. But 1 gallon of nitro would make ten gallons at 10%. And the powers there with just a shim and plug change.
Old 08-08-2014, 08:34 AM
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Here's a few things to note. Hot plugs give better idle and midrange properties and handle richer mixtures better but don't give too performance. Cold plugs have a less than stellar idle and midrange qualities and don't tolerate rich mixtures as well but give better peak performance. Cold plugs have more platinum in the element which aids to the catalytic reaction.

Compression ratios for lower nitro need to be higher. Think of nitromethane as a supercharger. The compression ratio would be too high if you supercharge (lots of nitro) an engine with a high compression ratio. 20% nitro in an engine with a 12:1 CR would be bad news. There I'd use 5% nitro instead. Conversely, 20% nitro in an 8:1 engine should run pretty well.

My European engines with high CRs get 5% nitro but my low CR car engines thrive on 20-30% nitro.

If you really like to tinker, make a degree wheel and degree an engine. This measures when and for how long the various ports open and close. Measuring the compression ratio is a good idea too. High compression, high timing, and lots of nitro = big power.

I use hot plugs in everything using 5-30% nitro and have very few issues. MC59 or Enya 3 are my goto plugs.

In model airplane engines, I've noted a 200rpm average increase for every 5% I add. In car engines, I didn't notice a huge improvement going from 20% to 30% other than throttle response was better and fuel consumption went up a bit.

Last edited by 1QwkSport2.5r; 08-08-2014 at 08:37 AM.
Old 08-08-2014, 11:04 AM
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rcbence
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Cool. I'll look into getting the mc59. If your not having big problems with hot plugs on up to 30% nitro then I'll stick to hot for 10%. Or even 20%. I'll probably skip the timing....if it doesn't work well I'll just stick with what I know works.

Last edited by rcbence; 08-08-2014 at 11:36 AM.
Old 08-08-2014, 01:13 PM
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Rafael23cc
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the answer to your question was in your opening statement:

About car engines....let's say the engine is built for 20-25% nitro. Which most are I believe. Now you drop to 10%. What thickness shim is needed to fully compensate the lack of power and retarded timing? Lrp .28 for example says..... .1mm shim for 25%. No shims for 16%. And .15 for 30%.


Let me re-arrange this a little:

For fuels around 16% use no shims.
For fuels around 25% use the 0.1mm shim.
For Fuels around 30% use the 0.15mm shim.

So in reality your engine was built for 16% nitro (no shims). I bet that is what confused you. Sorry, can't help you measuring the shims other than you do not squeeze hard but then "hard" is relative.

Rafael
Old 08-08-2014, 03:07 PM
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rcbence
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That's much cleaner. Thanks. I'll report back when I mix up my own fuel on 10% nitro for both my trucks
Old 08-12-2014, 11:23 AM
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rcbence
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So I got what i needed. The test fuel contains 10% nitro. 16% klotz benol. Then the rest methanol. Same hot plug and minus 1 shim.

Compared to 10% omega it might of had more power. The things I noticed though...is my temp was 20f higher or so. But it was hot (95) and a bit humid, so maybe it wasn't a fair test.
Also the idle liked to come down to a real low idle frequently. I know my lsn is not too rich. Great power out the hole and it revs up a bit than dies on the pinch test.
Omega measured out to 15% by volume. So I'm within 1% total oil. Only difference being its 70/30 where I'm 100% castor.

Think I'll just stick to 20% nitro as it was designed for and see if that gets it idling right.

You guys think its just not hot enough at idle?
Does castor oil run hotter than synthetic? Any real world tests appreciated.
The omega only dipped on the idle very few times.

The tune was very consistent. Though last time I checked with the omega i was 260ish in temps. Yesterday I was upper 270s even low 280s for a bit. I'm not tuning by temp just wondering if its the straight castor causing.

Before I switch to 20% nitro I'll run it again today on the 10%. Its low 80s today. That's about what it was when running the omega.

Last edited by rcbence; 08-12-2014 at 11:26 AM.
Old 08-12-2014, 08:58 PM
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rcbence
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Added some nitro to my quart. 20% nitro 16% oil with all shims in and medium plug. Idles good now. Doesn't do the half idle lol. Runs great easy to tune and 240-265 or so is what it read after a good tune.
Gonna stick with what works. Love that klotz smell too. The smoke trail too. Didn't like how there was very little smoke at wot on a racing fuel.

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