Hi all -

Some may have searched, as I have, for data on no nitro compared to some nitro in various engines.
I didn't set out to do a rigorous test sequence, so I'm simply offering results as noted on the bench from some of my engines.
These pretty well run the gamut of c/r and technologies for plain-bearing sport engines.
Strictly empirical of course, but I selected common points. YMMV.
These engines were run during the past year or so, different days, temps, humidity, only 690' ASL and my old Carl Goldberg test stand clamped to the porch rail were the same.  
Fuel even varied +/- 1%ish, in 16 oz batches, as I remix Morgan's 15% for ~9% nitro, adding straight castor to 25%; I mix locally obtained methanol and straight castor for 0%.
I've had no problem with starting either mix.

I make no recommendation for any particular mix; you may well have particular requirements.
Factors other than rpm certainly enter into that selection.

I hope you find it at least interesting, maybe even useful.

regards,

Terry


                                          Nitro/Meth/Oil
                           0/75/25                        9/64/25

                           Veco .19
8-4 MAS                 14.5                             15.0K
8-5 APC                  13.3K                           13.8K
9-4 APC                  11.4K                           12.0K

                         OS .20FP steel
9-4 Zngr                 12.6K                        (n/r) (not run)  
9-6 MAS                  9.9K                              (n/r)
                                                                                        note: I do fly these OS .20s on 9% fuel.      
                         OS .20FP ABN
8-4 APC                   13.1K                            (n/r)

                            O&R .23
                     #1             #2                 #1          #2
9-4 APC          11.8K        11.6K               12.1K     11.9K
9-6 APC          10.2K        10.4K               10.7K      (n/r)

                         OS .25FP ABN
9-4 APC                  13.7K                            15.0K
10-4 Zngr                11.0K                            11.7K

                        McCoy .29 1952
9-4 Zngr                    8.5K                           8.5K

                           Enya .35-III B
10-4 Zngr               11.5K                            11.7K
10-6 MAS               10.6K                            10.8K

                              OS .46LA
11.5-4 APC            10.4K                             10.8K
12.25-3.75 APC      10.1K                             10.4K
 

My feeling from lots-o-reading -- backed up by only a minuscule amount of real testing on Cox 049 engines with hand-modified heads -- is that just changing the fuel in a given engine without changing the compression isn't telling anything at all like the whole story.  Changing the amount of nitro in the fuel really changes the optimum compression ratio -- at low nitro level, engines like more compression, and visa-versa.

If you were really going to do a fair test, you'd start with engines that have compression a bit high for no nitro, you'd experiment with head shims for each fuel until you found an optimum combination, and you'd publish the best results for each fuel/compression combination.

Even then, just getting the numbers from bench runs won't tell you how fit the combination would be for a good stunt run, or suitability for team race, or whatever.

My feeling from lots-o-reading -- backed up by only a minuscule amount of real testing on Cox 049 engines with hand-modified heads -- is that just changing the fuel in a given engine without changing the compression isn't telling anything at all like the whole story.  Changing the amount of nitro in the fuel really changes the optimum compression ratio -- at low nitro level, engines like more compression, and visa-versa.

If you were really going to do a fair test, you'd start with engines that have compression a bit high for no nitro, you'd experiment with head shims for each fuel until you found an optimum combination, and you'd publish the best results for each fuel/compression combination.

  Depends on what you mean about "fair test". Terry's experiment is very much representative of how nitro changes are typically used in stunt. In almost all cases, stunt engines are *far* below the limit of problems so adding nitro typically moves them closer to the optimal compression.

  I treat nitro like an oxidizer - if you are short of oxygen, add nitro. Almost no one who knows what they are doing is changing the compression every time they change the nitro, they have a baseline system and adjust the nitro to get the same performance based on the air density.

   Brett

Brett and Tim, you flatter me unduly - "experiment" is too fine a word.
As I said initially, I wasn't running a test about anything but my engines, and my only interests were - Do they run? How well? How about on no nitro?
That's all I was trying to find out and those are just the notes I took.

My post was made only because I've run across similar questions, with no posted results in the responses.
MOF, it was prompted by a question on another forum, by one who I am absolutely confident knows way more than I do.
When it comes to real, more in-depth experimenting, I leave that to you guys a whole lot more knowledgeable and much better equipped to carry them out.

BTW Tim - I did note that I run nitro in the OS engines for flying, and they do great.
I just didn't make note on nitro bench runs for some unknown reason.

Terry

My approach to some testing comes from a different angle because I don't use nitro at all but every engine I've ever run on zero nitro, ranging from several different types of Cox .049's up to a 1.20, have behaved perfectly with one flick starts and easy tuning from full 4 stroking to peak revs.

My testing involved changing compressions to get the most power from zero nitro which almost always means a fairly large increase because almost all current engines have compressions low enough for nitro to be used. Initial tests were done with an Enya 60X but, to avoid having to change shims or machining the head, I made a variable compression cylinder head so I could quickly alter the compression while still running. Starting with the same compression as standard and noting the peak revs (which happened to be exactly the same revs as with the original head) I kept raising the compression a bit at a time, check revs, increase compression, repeat, until it reached a point where there was no more noticeable increase in revs. That occurred at 13.5:1 which gave me a basis for other engines. The increase in revs, using an old square tipped Master 12x6 prop, from 11700 to 12500 indicated a HP increase of 22%. A similar increase in compression using a new head button on my Enya 61CXLRS gained 26% more HP with the 13.5x5 prop I use in my SV-11 where peak revs went from 8800 to 9500 but in actual flying where release revs are 7800 the main benefit was that the 4 stroking range was extended by several hundred revs and now gives a much nicer 4-2-4 run.

In a somewhat different area, I made a similar increase to my son's car engine and it then had the same performance (using 80/20 fuel) to other identical car/engines of other competitors using 25% nitro except he could run for 8 minutes on a tank to their 6 minutes.