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Head shimming trx 3.3

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Unread 4-18-2012, 1:31 AM   #1
REVO-5309
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Head shimming trx 3.3

so im thinking to get a better result out of my trx3.3 when running 30% nitro I should add a shim on top of the factory one.

what does the shim increase do?

does it make it easier to tune?

do you loose power when adding shim?

when increasing nitro should a shim be added?
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Unread 4-18-2012, 2:00 AM   #2
TK5310
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Ok, so I'm new here & have SFA experience with nitro RC engines, but a wealth of experience with engines in general, including 'normal' 2 strokes.

Shimming the head up drops the compression & increases the quench distance.
Increasing the quench distance in a 2 stroke is usually a bad thing, squish velocity & therefore burn velocity suffers, requiring more ignition timing (hotter plug) & pushing it closer to the detonation threshhold.

Decreasing compression usually costs power unless it is 100% necessary in order to keep compression below the det threshhold.

I would think that running 30% should not require a drop in compression & certainly will not benefit from an increase in quench distance.
Wost case if the engine is running hotter than you'd like or is detonating (not sure what that'd sound like on a nitro engine, but it's usually like a gang on knomes rolling tin cans full of marbles around in the crankcase) a switch to a slightly cooler plug should pull enough ignition timing out to bring it back into the 'safe' zone.
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Unread 4-18-2012, 9:33 AM   #3
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Quote:
Originally Posted by TK5310 View Post
Ok, so I'm new here & have SFA experience with nitro RC engines, but a wealth of experience with engines in general, including 'normal' 2 strokes.

Shimming the head up drops the compression & increases the quench distance.
Increasing the quench distance in a 2 stroke is usually a bad thing, squish velocity & therefore burn velocity suffers, requiring more ignition timing (hotter plug) & pushing it closer to the detonation threshhold.

Decreasing compression usually costs power unless it is 100% necessary in order to keep compression below the det threshhold.

I would think that running 30% should not require a drop in compression & certainly will not benefit from an increase in quench distance.
Wost case if the engine is running hotter than you'd like or is detonating (not sure what that'd sound like on a nitro engine, but it's usually like a gang on knomes rolling tin cans full of marbles around in the crankcase) a switch to a slightly cooler plug should pull enough ignition timing out to bring it back into the 'safe' zone.
Your explanation is bang on, below is what detonation looks like, unfortunately it is very hard to "hear" if the engine is detonating. The best way to check is to pull the head button off and take a look.

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Unread 4-18-2012, 5:24 PM   #4
REVO-5309
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well I know mine didn't look like that after I was running 30%

question is there are 3 shims to a pack, 0.2, 0.3, 0.4mm. would like to know which one is stock and when I go to 30% nitro would it be a good idea to jump it up to the next one? or even add one?
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Unread 4-18-2012, 5:48 PM   #5
TK5310
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Basically you want the thinnest shim (tightest quench) possible without running into detonation.
If it was mine I'd stick the .2mm one in & keep an eye on it.
But if you wanted to be safe then use the .3.

---------- Post added at 5:48 PM ---------- Previous post was at 5:45 PM ----------

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Your explanation is bang on, below is what detonation looks like, unfortunately it is very hard to "hear" if the engine is detonating. The best way to check is to pull the head button off and take a look.
Aha, they speckle just like any other engine. You should also see det as a drop in power, especially when the engine is hot. If she's fine on warm up & for the first couple of hard power runs but then goes soft it's probably det.
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Unread 4-18-2012, 6:02 PM   #6
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Unless you running above 30% nitro content you will seldom need extra shims. Even with 33% you commonly don’t need an extra.
All this based on a good tune and fuel delivery system.
http://buy.traxxas.com/product_info....oducts_id=7689
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Unread 4-18-2012, 6:42 PM   #7
REVO-5309
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mmmk, well I will run the motor in on the 0.30mm shim, and after a gallon or so ill try out the 0.2 if I can be bothered. by then the picco 26 JLR should rock up so ill have that to play with.

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