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Old 04-04-2004, 08:03 PM   #1
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Default switching from 20% to 30%, got a couple q's

I know this is in an onroad forum, and I'm talking about offroad stuff, but I thought this would be a good place to ask anyway.

I've been running O'Donnell 20% in all of my engines since I got into nitro, and I'm thinking about switching up to O'Donnell 30%. I'm mainly switching because I want to run it in my MBX5, but I figure it would make more sense to just switch both of my cars. I've got a WS7 (first version) in my MBX5, and an MT12 in my XXXNT. I know the tuning differences between 20% and 30%, but I'm not sure about head clearances. Should I leave both engines alone, or should I add a shim to one or both of them? If so, what thickness shim should I use? Also, I've been running an MC59 plug in the MT12 and an MC9 in the WS7 (and considering trying O'Donnell 77's in the MT12 and 99's in the WS7, but I'll probably stick with the MC's). Will these plugs be fine for 30%? Thanks.
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Old 04-04-2004, 08:54 PM   #2
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You may need to add 0.1mm shim for your each engine when you switching up to 30%Nitro.
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Old 04-05-2004, 10:40 AM   #3
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As far as shims go you shouldn't have to add any of them. they come made to run 30%. the best thing to do is run them. If it crushes the plug than you have to add a .01 shim. But you should have to. Also I would recomend the od 77 and 99 I have had good luck with them.
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Old 04-07-2004, 08:31 PM   #4
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Only do something if it doesn't run right.

They both should be fine like they are, but if not, just like the others said .1mm head shim and/or a new plug would be as drastic of a change that you should need beside the obvious carb tuning changes.
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Old 04-20-2004, 09:59 AM   #5
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Quote:
Originally posted by kt racing
As far as shims go you shouldn't have to add any of them. they come made to run 30%. the best thing to do is run them. If it crushes the plug than you have to add a .01 shim. But you should have to. Also I would recomend the od 77 and 99 I have had good luck with them.
If it crushes the plug you most likely will have internal engine damage at this point. Adding a shim to a damaged engine would be a mute point. Typically, you don't need to add shims unless the engine starts to detonate or prematurely ignite in the stroke. This is caused by excessive compression and can be relieved by the shim gap. It's usually more of a problem when the temps climb outside or you are running fairly lean engine settings. You'll know when it starts to detonate, it makes a distinct crackling noise in the mid to high rpm range. If you hear this shut it down immediately and add the shim. Inspect the plug and check the top of the piston for pitting at this time as well. If the plug is blown or the piston is pitted, tear the whole engine apart and inspect it for damage or particles. Detonation for a sustained time usually results first in a blown glow plug, which can damage an engine in a hurry if the coil gets stuck between the piston and a port. Not hard to do at 40000+ RPM's.
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Old 04-21-2004, 09:16 AM   #6
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Quote:
Originally posted by terry sturchio
If it crushes the plug you most likely will have internal engine damage at this point. Adding a shim to a damaged engine would be a mute point. Typically, you don't need to add shims unless the engine starts to detonate or prematurely ignite in the stroke. This is caused by excessive compression and can be relieved by the shim gap. It's usually more of a problem when the temps climb outside or you are running fairly lean engine settings. You'll know when it starts to detonate, it makes a distinct crackling noise in the mid to high rpm range. If you hear this shut it down immediately and add the shim. Inspect the plug and check the top of the piston for pitting at this time as well. If the plug is blown or the piston is pitted, tear the whole engine apart and inspect it for damage or particles. Detonation for a sustained time usually results first in a blown glow plug, which can damage an engine in a hurry if the coil gets stuck between the piston and a port. Not hard to do at 40000+ RPM's.
Very true...

Remember that to get the longest life out of your motor you will need run it in again with the new fuel. remember that when you change the fuel, you are changing the equasion insitde the engine, ie: higher or lower temp combustion, more or less oil, you get the picture I am sure.
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