My GB and 25% nitro

wha-tah-hey wrote:The GB has a standard head, 1 gasket, 2-bypass BW cylinder.
I've been running it on the Cox 15% I had on hand, picked up some 25% yesterday and tried it on 3 different props.
I was surprised to find it picked up only 2-300 rpm over the 15%.
My experience with larger engines is (very roughly) 250+ rpm per 5% increase in nitro.
Is there something I don't know about running 25% in a Cox?

Are you using props that will let the SPI kick in... this is an open exhaust, right? In the warm Alabama weather you have what I'd guess is as high compression as you can get without a different head. I don't trust Cox heads much anymore, though I don't run enough to really have a credible opinion these days either. But I am still firmly convinced that the Merlin or Norvels are the best common heads for them. The N/G always outran everything else. Have you checked the deck height of the piston?  You'd pick up some RPM by boring the venturi and inlet hole to BW size.... Ah it's too late for me to think... ~.083, does that sound right? Run a wire down your fuel vents and blow out the spraybar with Brake cleaner or alky or something.
I'm shot, let us know what you find tomorrow.
Rusty

I'm not looking for more performance, in fact I plan to reinstall the original GB cylinder.
25% seems to be the go-to mix for Coxes but I see no reason for the extra cost and fuel consumption unless there's a need for more power, which apparently requires more than a simple fuel change.
I'm just surprised that all that nitro added only a few hundred rpm, given the increase I see in larger engines as nitro goes up.
Is my limited experience an aberration or is a minor rpm increase the usual result when going from 15% to 25% without additional tuning?

I'd lean more towards an aberration.

I also have a pair of product engines so I'll try them on 25% and see how they do.

Well, it certainly doesn't prove anything other than what happened in this instance, but it sure generates some head-scratching.
GB, BW cylinder, standard head:
15% N, 5.5x4 Windsor prop - 16.9K
15% N, 5x4 TD prop - 17.4K            
25% N, 5.5x4 Windsor prop - 16.9K
25% N, 5x4 TD prop - 17.3K

I ran a product engine (slit exhaust, 2 bypass) with these results:
5.5x4 Windsor prop
15% N, standard head - 15.2K
25% N,      "         "    - 15.6K
25% N, Hi-Comp head, 1 gasket - 14.8K
25% N,     "          "   , 2   "      - 14.7K
25% N, " " , 3 " - 14.6K

I'm not concerned that the GB runs better, but the result with each engine is puzzling.

I know there are many here who's experiences are different to mine, but I once had a well respected guy, high up in our state aeromodelling scene, tell me that anything above 15% nitro is just a waste of nitro & money. I've not done any testing to confirm, but I would suggest that I have seen no noticeable performance difference between 15% & 25% in the small amount of Cox flying I've done. I do think that it may effect the ease of starting.

I've read that it also makes needling easier.
My personal experience with larger engines does show a definite increase in peak rpm with additional nitro.
Thus, in a stunt engine not normally run at peak, the engine can be richened to bring the rpm back down to desired launch rpm, the greater fuel flow meaning less heat but with a commensurate increase in fuel consumption.
One of those "no free lunch" deals.
In any case, as I understand it a 1/2A engine of this type will pick up 1-1.5K in the air.
With max power output in the 13-16K range, a launch rpm of ~14K should allow the engine to unload to that max range and still have some breathing room left.
Most all the results shown for the GB would then indicate a need to either richen the mixture with these props or perhaps use a bit larger prop for best performance.
But that's just my understanding, still to be tested in the air, and that soon I hope.

Did you put another head gasket in before calling it quit?

Phil

I didn't, Phil. After seeing a slight loss after adding a 2nd, I didn't think to try a 3rd.
Is it likely to make the difference? "Over the hump" so to speak?
I can always try that tomorrow.

I would expect the RPM to noticeably increase with a 3" pitch prop. 4" pitch is a bigger load, at least on the bench. You never can tell with these fickle engines, but I don't think lowering the compression on 15% will improve it.
Rusty

I understood Phil to be suggesting a 3rd gasket for the 25% test and I'll try that.
But you're certainly correct, Rusty - I didn't list it but I also ran a 6x3 APC on 25% with peak (IIRC - notes are in the shop) of 17.5-18K.
But again, I'm looking for performance in the air to stay in the power band, not simply max rpm.

If you tried 1 and 2 gaskets, with a loss between 1 and 2 it probably will be a loss between 2 and 3 unless something else is going on like dirt, or creases, or hardened gaskets.  Ideally at least one gasket should be new in the stack.  The gaskets do work harden while being squished, and annealing them is generally not worth it.  Gaskets are cheap, reasonably easy to get, and the torch may blow them out of your work space. (use a small torch in a fireprof space, heat to about 1000F, dullest red in dim light, and cool by any means)

It is still worth trying 3 gaskets, but I am not optimistic.

You can try facing down the head on some wet/dry sandpaper on glass.  Taking more than just scratches out is dicey, but you can blacken the mating surface with sharpie marker and gently thread it together without a gasket, and see if there is a low/high spot as you go.  The sharpie should evenly get marked by the steel cylinder.  (400 grit minimum)

No I do not recommend facing a standard compression head till it becomes a high compression stand-in.  It can be done, but it can also ruin a good head.  Drop in plugs are a much better option.

Phil

wha-tah-hey wrote:I understood Phil to be suggesting a 3rd gasket for the 25% test and I'll try that....

That's fine, especially if the weather is cold.

I ran the product engine with 3 gaskets (1 new) and another 100 rpm loss.
I added the info to the chart above.
I guess 'til I find out that I'm doing something wrong, 25% nitro is a waste of money for my engines and uses.
Now if Hobbico would only make 15% available!

Have you taking this engine apart and cleaned it ? / and if so how is it needling for you touchie or sloppy /// sounds like some air leaking going on ?? those no.s are not that bad and I think the 35% would have been a bigger and better jump for performance , if you bore the venture the fuel milage will lack but up you Rss . I have the red back plate with new BW Pis/Cly kit , Mylar Reed , 4.5x4 prop 19.5-20K 3oz. 35' 25 roundie rounds . I don't sweat a few hundred Rs

Yes I cleaned it, Eric, and needling is solid.
I was just surprised by the small increase in rpm going from 15% to 25% - I'd expected a good bit more.
But I'm happy with the way these 2 engines run - still have some issues to work out on the other product engine though.

Thinking today about my test results and everyone's input, I realized two things that prompt me to offer my apologies.
I realized that my thinking, focused on running my Bee at it's best, was fine as long as that performance would do what I wanted it to do.
However, if it would not then I would definitely need the higher performance everyone offered advice on.
But I didn't give that advice proper consideration until I realized that the key fact I overlooked is that upping performance as suggested did not put my GB past it's power band (as simply using a smaller prop to gain rpm might), it also raised the power band to make the extra rpm do more work - precisely what I might need.
So, apologies extended, advice valued and it's a good day for me - I learned something.
Thanks again to all.

wha-tah-hey wrote:Thinking today about my test results and everyone's input, I realized two things that prompt me to offer my apologies.
I realized that my thinking, focused on running my Bee at it's best, was fine as long as that performance would do what I wanted it to do.
However, if it would not then I would definitely need the higher performance everyone offered advice on.
But I didn't give that advice proper consideration until I realized that the key fact I overlooked is that upping performance as suggested did not put my GB past it's power band (as simply using a smaller prop to gain rpm might), it also raised the power band to make the extra rpm do more work - precisely what I might need.
So, apologies extended, advice valued and it's a good day for me - I learned something.
Thanks again to all.

Hey Terry, thanks for saying that. It was apparent to me that you wanted to be conservative with many of the suggested power boosting methods, and nobody can fault you for being careful. So no apology necessary. I can understand anyone not wanting to Hotrod an engine. But Cox Bee engines vary from one-another so much that it is frequently impossible to know why.

If you or I were an engine tuning hobbyist, we could try things that, if not the problem, might render it permanently more anemic or needing new parts. Nothing that radical was suggested. For instance, we've not touched on a possible crank to bearing or crank to drive plate fit when at running temperature. Polishing the crank might wake it up. Or make you need a new crank. If that is it, it will probably gradually fix itself if the tightness is lateral, crank bearing to the race in the case. But it could be the nose doesn't have enough play and it grows with heat into the drive washer. Make sure the end play is no less than .022". Check if that's the right number, it came from paul G's article. If that's so, you'll need to shave a few thousandths off the nose. Norvels do that too. Taake a close look at the connecting rod for the slightest bend... but you'd have caught that when you checked your deck height to piston crown. I think I remember you did check that

Sorry, I don't remember; was this one broken in on pure or high castor? Varnish in the cylinder? 50/50 oil will fix that after a while. I stick with my recommendation of a Merlin Drop-in and 3-5 gaskets to begin. Those will light with your spring type pocket igniter too.

And since you've called off the dogs, Lol, I extremely recommend high nitro fuel. I run nothing but 35%, but 25% is ideal for your needs. Yeah I know people have great results with 15%, but your SPI engine is craving enough RPMs to get that extra boost of cold air flowing. And the engine will run cooler with high nitro. With all that volatile power juice rushing through it, it's a little flying refrigerator. Nitromethane is worth every penny of the small extra cost. That's my story and I'm sticking to it.
Good luck,
Rusty

Well, I'm not coming over to the "hot side" yet, Rusty, 'til I find out (quite likely) that my stock(ish) GB won't do the job, but suggestions noted, recorded, and I will check those things because they're things that should be made right in any case.
And it was broken in on Cox Super Power years ago then put away 'til now.
Many thanks for the kind, encouraging words.