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timing vs. compresion

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timing vs. compresion

Post  newbie on Mon Feb 06, 2012 6:08 am

in lay mans term can you pls explain timing and compression when removing shimms or adding more to change compression/ and or timing? on a babebee cox .049?
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Re: timing vs. compresion

Post  Cribbs74 on Tue Feb 07, 2012 8:06 am

newbie wrote:in lay mans term can you pls explain timing and compression when removing shimms or adding more to change compression/ and or timing? on a babebee cox .049?

Not sure exactly what you wish to know, yes adding shims to the base of the cylinder will affect both. I have never messed with this myself. If I were to I would probably add shims and remove or add head gaskets till the engine ran at the most RPM. Perhaps someone here who has done this can comment?
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Re: timing vs. compresion

Post  RknRusty on Tue Feb 07, 2012 9:18 am

Shims are generally used to raise the exhaust port, which changes the timing, but the primary reason is to eliminate sub piston induction (SPI), so you can use a muffler or an exhaust restrictive throttle. There's no reason to shim an engine that doesn't have one of those, as it will reduce performance. SPI combined with mufflers work poorly together. If you can look into the exhaust port and see a gap under the piston at TDC, you have SPI. If you wanted to muffle that engine, you would add enough shims to close that gap.

Adding and removing copper head shims(washers/gaskets) is to adjust compression. With a standard head, generally use one washer for every 10% of nitromethane in the fuel. Using too few will blow the glow element. I use two shims with my 24% fuel and 3 with my 35% fuel. I also have killed a couple of glow plugs lately, so you may want to be safer and start with more. Other than that add some and remove some to see where the engine is happiest.

I don't have as much experience with high compression heads like for the Tee Dee and Killer Bee, but I stick to the same rule of thumb. I accidentally ran a high comp head with only one washer last night on a Black Widow and the glowplug was blown after the run. Higher compression on most Reedies makes them more difficult to needle, and I've found it's not usually worth the trouble. Rotary valve engines like Tee Dee and Medallion work nicely with High comp heads.

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Re: timing vs. compresion

Post  Mark Boesen on Tue Feb 07, 2012 11:05 am

Due to production tolerances, its often that the crown of the piston will sometimes extend above the inside edge of cylinder, the Shim is designed to raise the cylinder so the piston stops at top dead center and has nothing to do with removing SPI.
Your correct on the 'ball park' head gasket per nitro level, but on a dual port cylinder and larger intake Black Widow type fuel tank, you should see some improvement if you running a 5" prop or smaller. If it's hard to needle it’s a good indication of too much compression and you need to add a gasket.
I'm not even sure where you can buy cylinder shims anymore, Kern Kraft (aka Kustom Kraftmanship) used to sell them, maybe Combat Albert?

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Re: timing vs. compresion

Post  andrew on Tue Feb 07, 2012 2:41 pm

Mark Boesen wrote:I'm not even sure where you can buy cylinder shims anymore, Kern Kraft (aka Kustom Kraftmanship) used to sell them, maybe Combat Albert?

Bernie to the rescue: http://coxengines.ca/product.php?productid=115&cat=13&page=4

EDIT: Just re-read the description --- these may not fit anything other than TD's. However, the cylinder threads should be the same on all the engines, so it may depend on the amount of shoulder at the base of the cylinder.

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Re: timing vs. compresion

Post  Mark Boesen on Tue Feb 07, 2012 4:06 pm

Yea, Bernie! ...I wasn't thinking, I should of known : ^ )...

I also see they're now sold as shims to eliminate SPI (Sorry Rusty) however, I'd have to think raising the cylinder that much would have to mess with timing, but would still work and the fact that your using a muffler you'd not be that concerned with top performance anyway.

Another little know use, was using very thin shims to help align intake ports of the cylinder with the sides of case. I never used them for that, but many swore by it.
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Re: timing vs. compresion

Post  RknRusty on Tue Feb 07, 2012 6:04 pm

Mark Boesen wrote:...Another little known use, was using very thin shims to help align intake ports of the cylinder with the sides of case. I never used them for that, but many swore by it.
That makes sense. For speedsters, there is no such thing as a negligible gain, and obviously you'd want the ports to be symmetrically exposed to the fuel/air charge. I'll be trying that.

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Re: timing vs. compresion

Post  Kim on Tue Feb 07, 2012 6:23 pm

This is good stuff guys...and thanks to Newbie for starting the thread! Please consider joining!
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Re: timing vs. compresion

Post  fit90 on Tue Feb 07, 2012 8:22 pm

I have shimmed several engines (probably about 10 or so) with timing shims from either Bernie or Matt and without exception every engine lost power, typically over 1000RPM. The only reason I can think of to shim a cylinder is to keep the crown of the piston from going above the top of the cylinder at TDC. This is all just from me running the engines on a stand, not scientific or anything. Your results may vary.

Bob
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Re: timing vs. compresion

Post  Mark Boesen on Tue Feb 07, 2012 10:15 pm

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Re: timing vs. compresion

Post  Cribbs74 on Wed Feb 08, 2012 9:05 am

fit90 wrote:I have shimmed several engines (probably about 10 or so) with timing shims from either Bernie or Matt and without exception every engine lost power, typically over 1000RPM. The only reason I can think of to shim a cylinder is to keep the crown of the piston from going above the top of the cylinder at TDC. This is all just from me running the engines on a stand, not scientific or anything. Your results may vary.

Bob

Strange results Bob, have you measured to ensure that the piston is at true TDC? Maybe it's a tad BTDC and you are losing compression with the shim? Just a thought if you have been measuring then I don't know unless it's an SPI cylinder and you decreased the gap.
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Re: timing vs. compresion

Post  Cribbs74 on Wed Feb 08, 2012 9:09 am

Mark Boesen wrote:

Ha! I should have read this first. It explained what I was trying to get at!
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