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Old Jul 25, 2006, 08:11 PM
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castor straight

Hi,

I am new to the hobby, and have got a trainer equipped with a Super Custom .46 2-stroke engine. The engine currently runs fine on 10% nitromethane fuel, but this is expensive from my local model shop at 4/$7 per 500ml (i live in the UK). Would the engine run well on castor straight (80/20)? This is much cheaper and i can get it cheap either mixed or the separate substances.

Thanks.
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Old Jul 26, 2006, 04:18 AM
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Staffs, UK
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You don't say who your local model shop is but you're being ripped off. I too live in the UK and I've never paid more than about 15 quid per GALLON (4.5 litres) for fuel. See http://www.modeltechnics.com/ for the proper retail prices. It sounds like you may be buying CAR race fuel which is not only very expensive in small quantities but it's also very bad for your engine because it doesn't contain nearly enough oil.

Anyway my SC .46 doesn't like straight fuel (tricky starting) but it's fine on the (less than 10/gallon) 5% nitro fuel that I use for almost everything.

Steve
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Old Jul 26, 2006, 05:49 AM
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Dublin, Ireland
Joined Mar 2002
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You could try using castor straight with your existing Super Custom.
but with less nitro the engines compression will drop somewhat.
So you would have to put in a hotter glowplug to compensate for this, and minimise the compression reduction.
With a Model Technics M7 or M6 glowplug, or an Enya No 3 glowplug things should be ok for summer use.

If you were to get a high compression engine eg MVVS that would be an ideal solution, because these engines are designed so as to not need much nitro or any at all. And straight would be perfect, with maybe 2% - 5% nitro in wintertime, with the normal glowplugs.

Also - as mentioned by a previous post - start buying your fuel in gallons, not those tiddly little bottles. It is better value that way.
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Old Jul 26, 2006, 07:32 AM
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One more thing,if you decide to use no nitro fuel,then remove one head shim.
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Old Jul 26, 2006, 10:37 AM
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Adelaide, South Australia
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I run every engine I own on 80/20 all castor and never have a problem. It doesn't hurt to take out a head shim so long as the head still seals properly but I never bother doing that because there's not enough gain in compression to make it worthwhile. Worthwhile gains can only come from some machining to get the compression up to around 13.5:1 and then they honk but there aren't any off the shelf engines with compression even close to this.

The nearest thing I've got to the SC would be my Bluebird 51 and it's an absolute delight on 80/20 fuel box stock standard.
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Old Jul 26, 2006, 03:12 PM
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Istanbul, Turkey
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Hi ANZAC, do you mean ıf I could make my engine(using no nitro fuel) compression ratio
around 13,5/1 I could feel a significant power improvement?
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Old Jul 26, 2006, 06:19 PM
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Thanks for the advice.I am currently using model technics duraglow 10% (18% oil). I managed to get a gallon for 15 at a different model shop. i bought the previous expensive bottle of fuel from antics. perhaps it may be easier to use nitro fuel than removing shims! The duraglow fuel is the only fuel i can easily obtain in the local area.
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Last edited by jcow; Jul 27, 2006 at 06:58 PM. Reason: i described fuel wrong
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Old Jul 27, 2006, 09:51 AM
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Quote:
Originally Posted by turk1
Hi ANZAC
Thanks for the compliment
Some engines react differently to others as far as raising the compression but when I did it with my Enya 60X (in small increments and checking the gain each time) I ended up with 20% more HP on the same prop. The main needle had to be opened about another 1/4 turn but the idle mix had to be leaned drastically because it was running far more efficiently at idle. I could have raised the compression even further if I'd decided to play around with colder plugs.

The trick with doing this is that you have to measure the exact volume of the combustion chamber (including the squish volume) to know how far down the head needs to go and staying aware of the squish clearance so a decent depth guage is needed. None of this is really necessary though because they'll run just fine stock standard.

All engine manufacturers know that America is their largest market and that the Americans love nitro so they have to make the engines with a low enough compression to keep them happy. Pure nitro will only handle about 6:1 while pure methanol can handle around 17:1 so when you blend the two the more nitro you add the lower the compression has to be to avoid detonation.
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Old Jul 27, 2006, 10:18 AM
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Istanbul, Turkey
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Thanks for info Downunder.But do you think stock connecting rod can handle this too?
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Old Jul 27, 2006, 08:39 PM
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Everett Wa.
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Turk,
It has been many years since a 2cycle model engine was designed that could not handle CR of 15:1. I have have only 1 engine fail a rod in comprssion (other than hydraulic lock) and that was an old ST .15 (2.5cc) on diesel. All my other rod failures, and there have been many, have been in tension .

As you shape the combustion chamber you want to keep at least a 0.2mm piston to head clearance. Anything tighter than 0.15mm and a sonic shock wave might start. these shock waves will blow out any glow plug.

On 80/20 fuel I like to set my crank timing to close early say 45 to 48 degrees ATDC. This helps with the throttle responce.
The old PICCO's had very high CRs and early cranks. I use to love to run circles (with my Piccos) around the way my fellow Americans would try to run OS engines on nitro.

Just to prove my point wrong, I could get the old black head Rossi .40 (6,5cc) to run great on FAI fuel and it had a late crank 62 degrees ATDC. It also had a very mild exhaust timing 148 degrees IIRC.

Norman,
Did you mean to say that your cylinder pressure would drop with less nitro? I don't see how lowering the nitro content would lower the CR.


Konrad
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Last edited by Konrad; Jul 27, 2006 at 08:50 PM.
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Old Jul 28, 2006, 03:18 AM
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Thanks Konrad.Nice info.
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Old Jul 28, 2006, 10:55 AM
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Compression gives almost no load on the rod compared to all the other loads it has to handle. The worst of these is piston acceleration/decelleration which is around 2000+G at normal revs. You can certainly feel the difference when hand starting but it's far from difficult to flick over compression.

Lowering nitro (even to zero) has no affect on CR but without nitro the ignition point is retarded which is why a hotter plug should be used to advance the ignition back to where it should be. Raising the CR also advances the timing so it's then possible to start using cooler plugs to keep the timing where it should be.
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