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Compression height

Old 08-02-2020, 08:58 AM
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Lightbulb Compression height

Has anyone played around with compression heights on their engines? my rtr 8ight came with .014" worth of shims making the compression height .040" i removed one shim bringing it down to .031" I will be testing it as soon as my new clutch bell comes in. i feel like the stock gearing is way too tall at 17/52. most race buggies seem to 13/48 ish range it seems like to me. i ordered a 15tooth bell so my new ratio will be 3.71:1 which is a lot closer to the 3.69:1 a 13/48 set up would give you. Anyway, any experiences anyone has had with changing compression height of your engines would be greatly helpful and interesting to read
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Old 08-02-2020, 09:25 AM
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Maybe doing some search? There are some engine tuning topics to find where compression is talked.
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Old 08-03-2020, 03:17 AM
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Why do engines have such large squish clearences manufactured into button?

How do you adjust engine timing on nitro engines?

Head shimming and nitro percentage

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Old 08-03-2020, 02:45 PM
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A good chart for head shimming at different altitudes would make a nice sticky.
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Old 08-03-2020, 03:04 PM
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There is nothing wrong to try something bij taking out 0.1 or 0.15mm but if there are issues with like blowing plugs and losing top RPM you have to know why. but no way you can go low on the head clearance or else the piston will hit the button head at higher RPM.
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Old 08-04-2020, 01:28 PM
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Thanks for the links. 2 of those 3 i had already read through but the more the merrier lol. I was hoping to have some personal experiences with changing compression heights shared. i have a firm understanding of the functionality but lack the trial and error part of testing it. i have started though. with any luck itll dry up enough to run some this weekend.
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Old 08-04-2020, 02:39 PM
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The functionality of the head clearance starts with adding some space between piston and head to take care that the piston will not hit it due flex and play in the system. At higher rpm the krankshaft will bend a little due the forces the piston creates.

Then the total volume is making the compression. Compression, nitro content, glowplug number and outside temp are working together for the ignition timing. Changing one of these will shift the ignition timing. Glowplug and outside temp in a small factor, headshims and nitro content in a larger factor. So when one is changed you know with which one you can correct it.

The bending of the crankshaft at higher rpm has also a function. At hiher rpm the piston gets higher, getting closer to the head, resultinh into a higher compression, that will work as a distributer advance
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Old 08-04-2020, 06:33 PM
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Funny, I just asked this same question to Adam Drake personally. I asked him what he would recommend as a range to start and he said in an email, "In 1/8th off-road typically you'll be in the .018 to .024 range". I definitely trust his advise, so I'm adjusting all my engines within that range. Keep in mind you might have to then play around with glow plug temps because you'll likely be advancing the timing if your removing shims.
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Last edited by shotime; 08-04-2020 at 06:52 PM.
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Old 08-05-2020, 01:05 AM
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The head clearance is different per brand but also today with 16% and 25% buttons you also have to think different when using the other button although 25% configured engines do rum perfect on 16%
A friend had once an issue with his OS and ruind his P/S set. At the end comparing with another engine it seemed to hold on the 0.6mm clearance was a bad choice because the good engine had 0.8mm (not sure if he was talking about shim stack or real clearance)
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Old 08-06-2020, 07:04 AM
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from what ive read novarossi engines often come factory in the .024" range which is why they typically ship with a colder plug. i have seen p3 plugs in those show signs of detonation. Awesome info thanks for sharing!
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