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Post  balogh on Thu Jul 04, 2019 3:54 am

I have 2 slit exhaust cylinders, non-SPI, but 1 booster flute in each of the 2 bypass ports. I think these cylinders are converted to TD049 cylinders by Bernie and Matt, by having the middle bar between the slits removed, and the bottom slit edge lowered to make it SPI.

I  have 2 shortened and lightened pistons bought from Bernie, that will turn the non-SPI slit exhaust cylinders to SPI, and given the lightened piston weight, may add some rpm too. So the cylinder piston combo will have chances to perform like a TD049 combo, except the effect of the narrow, slit exhaust

My question is, does anybody have a reference rpm measurement that will show what impact the slit exhaust, owing to the choked exhaust flow, will have on the performance, compared to a similar, no-SPI but full open exhaust?

I figure that even in a fully-open exhaust cylinder most of the exhaust gases should have left before the bypass port is opened by the piston top, otherwise the charging of the cylinder would be hindered by the cylinder back-pressure. And, in a slit exhaust cylinder, the bottom edge of the top slit maybe level with the top of the bypass port, so I assume the narrow slit exhaust will not obstruct the discharge of exhaust gases?  
All in all, because of the lightened piston skirt I expect equal or better performance than  the TD049 cylinder...maybe wishful thinking...
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Post  Ken Cook on Thu Jul 04, 2019 5:54 am

Keep in mind that some of the shortened pistons were cut too short. This offered too much SPI. This caused hard starting and very rough running. Bernie offered a exhaust timing shim which was copper and went under the base of the cylinder for SPI elimination using a muffler. That shim worked very well with the skirt that was too short. I was using this combination on a TD engine. I feel it was equal too if not better than a stock TD.
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Post  balogh on Thu Jul 04, 2019 6:08 am

Thanks, Ken,

I noted this very generous SPI on the cylinder with the shortened piston, and when put on a SureStart engine, it really was not quite powerful, so a timing shim under the cylinder really seems necessary.

I will experiment with some shims - and leave maximum one under the glow head to retain compression - and hope to approach at least the TD performance.

Thanks for the advice in any case.
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Post  944_Jim on Thu Jul 04, 2019 7:30 am

But if you shim the cylinder, this raises the glowplug too. So how much power loss will there be due to lower compression? I would suspect that a glowplug may need to be shaved down to compensate, or run with fewer (or zero) head gaskets.
Please try to tach the engine as you test. I noted the iPhone tach thread, and ran one on my Android for a while, if you don't have a tach...
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Post  balogh on Thu Jul 04, 2019 8:01 am

Hi Jim,

yes I do use an Android app on my phone, the RPM Gauge, and that is not an issue.
When reducing the excessive SPI under the short piston, I had to put 3 thick shims under the cylinder base - thus still having a narrow SPI - , and this really has left the piston top almost 1 mm below the cylinder top on TDC...so even if I did not use one single head shim, the compression ratio would be far off.

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I am thinking about using a Nelson turbo head that typically requires 2-3 shims, and having only 1 may reinstate the needed compression ratio...
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Post  aspeed on Thu Jul 04, 2019 9:24 am

If you have access to a lathe, you can take off some material from the cylinder and maybe a bit off the top too, so the plug doesn't bottom out on the plug step. That is of course once you have settled on the number of shims you will want. Those newer style cylinders are easy to mount in a collet or chuck.
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Post  balogh on Thu Jul 04, 2019 10:21 am

Thanks aspeed, unfortunately nobody near me is equipped with a lathe so I could not adjust the cylinder length.

I just tached the engine with the short and lightened piston, reduced SPI and the best I could make of it with Bernie's 5x4 COX prop, 60/20/20 fuel is a tad over 17k.

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.it does not impress me though..my home assembled TeeDeeReedy ( a Killer Bee clone) happily swings the same prop on the same meal near 19k and unloads above 20k in the air when pulling my Lil Roughneck.

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Post  JennyC6 on Tue Jul 09, 2019 1:12 am

balogh wrote:Thanks aspeed, unfortunately nobody near me is equipped with a lathe so I could not adjust the cylinder length.

I just tached the engine with the short and lightened piston, reduced SPI and the best I could make of it with Bernie's 5x4 COX prop, 60/20/20 fuel is a tad over 17k.

Slit exhaust COX cylinders. 20190710


.it does not impress me though..my home assembled TeeDeeReedy ( a Killer Bee clone) happily swings the same prop on the same meal near 19k and unloads above 20k in the air when pulling my Lil Roughneck.

Slit exhaust COX cylinders. Screen19

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Off topic, but can you drop a link to that audio tach? I tried searching for it but I couldn't find it.
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Post  balogh on Tue Jul 09, 2019 4:12 am

This is a free app that you can download from Google Play for Android phones ...others, like Rusty, have also found it for iphones on App Store
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