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Tee Dee RC Conversion - Is it really this simple? Babe_b10
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Dieselized .049 with no Teflon disk or o-rings.

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Post  ThermalSniffer on Sun Sep 28, 2014 9:33 am

I have a stock standard 051 Tee Dee that I would like to convert to RC using the conversion set that Cox Engines sell. Do I simply get the RC carb and mount it in place of the standard venturi? Any difficulties or pitfalls noted by those who have gone this route?
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Post  Surfer_kris on Sun Sep 28, 2014 10:31 am

The dedicated TD with RC carb have a special cylinder without any SPI. This helps both for the throttling and for having a muffler.

On a regular TD you could perhaps shim a little under the cylinder a little to reduce the SPI, but I doubt you can remove all of it.
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Post  ThermalSniffer on Sun Sep 28, 2014 11:18 am

I thought the major discussion about SPI in cox engines was centered on engines with mufflers because those engines would ingest their own exhaust. Obviously, for steady control of fuel air mixture across the RPM range it would be better not to have SPI at all and a twin needle carb but that's not going to happen (at least not the twin needle carb).

On a TD equipped with an RC carb but without a muffler I would not have expected any useability problems even if it's mot ideal to have SPI altering the fuel air mix at different RPMs. At a constant RPM the SPI will lean out whatever mix was set at the carb but this leaning would not normally be noticed by the engine operator because he simply adjusts the needle valve until he hears the engine perform the way he wants. I am aware that on simple air bleed carbs like this the mixture is not evenly controlled throught the RPM range and that SPI would be another variable to contend with but ...

Are we not over complicating things by worrying about SPI when I don't intend to use a muffler or is it really that much of an effect?
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Post  balogh on Sun Sep 28, 2014 1:05 pm

I guess the real impact of SPI on a RC carb - even without a muffler - is that SPI counteracts to the throttle barrel, which, when at least partially closed, is supposed to richen the mixture by creating a stronger depression/suction in the carb , and thus draw in more fuel than with a fully open throttle..

Because the SPI will open the crankcase and reduce the depression inside, the richening effect of the closed throttle will be impaired.

Therefore the throttle effect of the RC carb on a SPI engine is only marginal to that on a non-SPI engine.

(I hope my Hunglish will not unable you to understand what I mean)
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Post  Surfer_kris on Sun Sep 28, 2014 2:37 pm

There can be a small amount of SPI on throttled engines but the throttle barrel will have to be closed more than if there was no SPI. The Norvel engines do have a little bit of SPI, and if you look at the throttle barrel at the idle position you'll see that it is more or less fully closed.

The TD has more SPI than the Norvels I believe, in its stock form, but it could perhaps be reduced a little by shimming the cylinder.

I have a stock .05RC that I have been flying with, and although it is nice with a throttled cox engine for a change, they are not very competitive compared to Norvel engines.

Putting a carb on a regular TD should give you a chance to alter the rpm, but I don't think it will idle very low. For flying this is probably fine though, as you rarely need a very low idle.
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Post  getback on Sun Sep 28, 2014 4:56 pm

Suction Per Induction as bringing air into the crankcase for premixed fuel/air + compression / ratio .... Eric Very Happy yeah you will just have to fun it and figure your mixture in to what RPM s you want to achieve!!
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Post  ThermalSniffer on Mon Sep 29, 2014 12:41 am

OK, thanks fellas. Seems I'll need to keep an eye on the effect of SPI and it may he prudent to get hold of a cylinder/piston that does not have SPI at the same time as the carb order.
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Post  Cox International on Mon Sep 29, 2014 11:32 am

Adding our carb assembly to a stock TD 049 / 051 works quite well.

It works a little better when a timing shim is added but compression is also reduced a bit.

If one wants maximum carb performance, we recommend simply changing the cylinder to one of our non-SPI ones. SPI makes very little difference on a TD engine (perhaps 300 RPM?) and one has to decide what is more important; Max RPM and decent throttle or max throttle and decent RPM.

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Post  fit90 on Mon Sep 29, 2014 12:14 pm

I have had very good luck converting .049 Tee Dees to R/C by adding a non-SPI piston/cylinder assembly, at least three head shims, a clamp on head, a hot insert style plug and the R/C carb. On a Master Airscrew 6x3 I am getting the idle down as low as a steady 5000 to 5500 RPM with great acceleration and top end comparable to a stock .049 Tee Dee on the same prop. Good luck with your conversion and let us know how it turns out.
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Post  Surfer_kris on Mon Sep 29, 2014 12:44 pm

I have only run the stock 05RC myself, below is a little video of a bench run that I did some time ago if anyone is interested in that. The engine does about 4000-17000rpm on a graupner 6x3 prop and 10% nitro.

The 05RC came with a low compression head, and that is what I used for the video, but it is a little too low compression ratio for 10% nitro.

The high compression head is on the other-hand a little too high in compression ratio with one shim (one could of course use more than one shim). I have had the best results with a turbo conversion head and a hot turbo plug from OS.

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Post  ThermalSniffer on Mon Sep 29, 2014 1:33 pm

This is very helpful advice, thank you guys. I was thinking of staying with a normal Cox head instead of a glow plug conversion that is unless those with experience of the RC carb think that the glow plug will make a tangible difference to my experience. What say you?
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Post  tubebass on Sun Oct 05, 2014 2:07 pm

I tried an R/C carb on a stock Tee Dee and couldn't get it to run worth a damn at either high speed or idle. Changing to a non-SPI cylinder and low compression head made all the difference, it now works great.
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