Effects of Nitro

Ok Len here's one for you. I have one of your OS 35 FP's. I noticed that the engine liked it a lot better when I went from 5% to 7 1/2% nitro. I'm swinging a B-Y&O 12-5 prop. I'm not going to say that if a little nitro is good then more should be better. I'm not going to fall into that trap. I then tried some 10%. Both the 7 1/2 and 10% had the same oil content-22 1/2 %. After flying several sessions, here is what I learned. The revs are faster, lap times are faster, and the engine is a little hotter at the end of the flight. I'll richen it up a little bit more to see if it'll run cooler with the right speed/lap times. My thinking was that the "extra" nitro would be called upon by the engine when under a heavy load..i.e. overheads. The engine does seem to run in a faster 2c mode than before and power is up along with line tension. Can one think that by adding some nitro, (and not getting carried away) it would benefit the engine run when "extra" power is needed? I guess maybe a lesson on how nitro is used in our stunt engines may be in order. Thanks Len and it sure is good to have you back and getting better. You are thought of very highly in the stunt circles! Pun intended! Take care.
Crist

Well, Crist, you have things pretty well sorted out in the use of added nitro. Most stunt engines should run well with 5% nitro. That is the way I set up the Stuka Stunt series of engines. In fact, in my instruction sheet I suggest that people save their money and not bother with higher percentages. All of this is general observations, however, and different people may like the different result obtained by straying (slightly) from the norm. As you rightly suggested, if a little more works, don't try for a lot more nitro. It simply won't work in a stunt engine. But if you plan to fly at higher altitudes or in hotter weather you may find that performance begins to fall off with the standard brew. Adding nitro allows the engine to burn more fuel. Burning more fuel (effectively) increases the power.

Adding nitro has the same effect as decreasing head clearance. Decreasing nitro has the same effect as increasing head clearance. So, usually, the more the nitro, the greater the head clearance will be needed for the engine to run the same. Still, there will be a range in which an engine will run well on 5% nitro, 7 1/2% and maybe even 10% without changing the head clearance. Randy Smith likes to go to a contest running 5% in the cool of the morning. As the day warms up he may change to 7 1/2%, and as it gets hotter still he may go to 10%--all without changing anything else on the engine (other than the needle valve setting.) Now this is with a pipe engine, but the principles remain the same. Usually adding a little nitro will give a sharper break and (as Matthew says) a little more punch on top. Of course, you can get too much, and that is where you may need to add head gaskets.

On the OS FP engine the stock gasket is .016 thick. Adding a second .016 is too much. You generally only need to go up .002-.005 in thickness. The Magnum .53 has .004 head gaskets which is nearly ideal. We now have some .010 head gaskets that will fit the FP 35 and FP 40 as well as the 35S engine. since the stock gasket is .016, one can replace this with two of the .010 for a total of .020. Or one can use one .010 and one .016 for a total of .026. Just don't get carriec away, because adding too much head gasket will kill off all of the power that you would have gained by upping the nitro.

As to running hotter, if you run with the same type of mixture using more nitro, yes, it will. But it will, also, be producing more power--especially where you want it, while aimed up and in the overheads. On the other hand, if you add a little nitro and run the engine a little richer, you can keep the same power as before but find that the engine runs cooler in hot weather because of the added amount of fuel and oil going through it.

So, adding nitro can make the engine run hotter or cooler, faster or slower, all depending on the set up for the engine.

But if you need a little more power on a hot day or at a higher altitude, why not go for it and try a little more nitro?

LAST EDITED ON Aug-01-00 AT 10:07 PM (EST)

During 1977/1978, I started a series of semi-formal tests of a couple of engine types and a number of then-available mufflers (I had no DB meter and just used tape recorders to try for a way to compare audio outputs). I never finished the series, although I had the intention to do so.

Before I got back to taping the results for both engines on all of the mufflers that I'd acquired, a burglar got away with almost all of the test hardware. But two aspects had become fairly clear. If a muffler reduced the noise appreciably, it reduced the power available. It also raised the temperature at which the engine runs.

The two engines I was testing were an OS Max-S 35 and a Fox 35. I was interested in keeping the Fox cooler, given its minimal cooling capacity when unmuffled. Increasing the nitro content to 10% without increasing the rpm's still made up the lost power AND lowered the engine temperatures (more fuel flowing through helped cool it more).

The OS engine seemed somewhat less affected overall by the mufflers that I used, as long as they weren't excessively restrictive. I had an OS muffler that had both internal baffles and restrictors on the internal passages and the outlet as well that really hurt power output immensely. I had a second one that had been liberally routed out with a Dremel and still muffled quite well without costing so much power.

I had to suspend the tests a couple of times while collecting the baseline data, and ended up failing to preserve even the preliminary results. I had it all on a floppy disk for one of my first PC's (pre-hard drive era, an "Apple II" type) and that disk became unusable! Anyway, I still will use 10% nitro fuel when using any muffler on any of the stunt Fox engines, and tend not to muffle those motors if I can get by with it.

Although many (maybe most) mufflers will reduce power, there are good and bad muffler designs. Unfortunately, the manufacturers' first attempt at making mufflers involved making them too small (for light weight) and too restrictive (for less noise). Even today most engines can benefit from the use of a good aftermarket muffler. For the Fox 35 and the OS 35S, these engines can actually benefit from a little restriction. Remember the "butterfly" that was put over the exhaust to partially close it while the engine was being throttled down? These made for an exhaust restriction that helped to keep the plug lit when the engine wasn't running full bore. So, when you are setting up for a 4-2-4 break, a little muffler back pressure can be good. When Art Adamisin designed his tube mufflers he found that the flat back worked better than the cone for this very reason. However, when we first had him make us a muffler for our 35 FP, he used the same chamber size as on the Fox and the 35S. It didn't work. Too restrictive. (Even though the size remained the same, we were generating far more power.) He increased the length 1/4 of an inch and then it worked. Same exhaust outlet size and same inlet (you can bolt the Fox onto the 35 FP muffler). When we developed the Magnum .53 engine we didn't know what to expect. But, remembering the experience with the 35 FP, I had Art make for us a variable length muffler to use for testing. It had the same size tube and the same opening, but the back section would slide into the tube and could be locked in one of 5 various positions to test the effect of smaller or larger chambers. The results were surprising. I tried it first on the test stand with the engine turning a 13 inch 3-blade prop of 5.5 pitch--the flying prop that we were using. It worked beautifully, but to test the effect of length I didn't fix the back piece into place. I fired up the engine at 8,000 RPM and, while holding the tach on the prop, I moved the tail stinger in and out. The result: The rpm never budged from one end to the other. So with the sizes we had chosen here, starting with the size currently used on the 35 FP and then working larger, restriction on the engine did not change. I could, however, play "Mary Had a Little Lamb" by moving the tail stinger in and out as on a trombone. The larger the muffler, the lower the tones. And the lower tones appeared to be more pleasant on the ear, even though there was no difference in sound level on the db meter. Subsequent tests in the air verified the previous conclusions. One other thing I did was to run the engine wide open with no muffler, using the same prop and setting the rpm at 8,000. I then took the muffler and held it against the motor and there was no drop in rpm whatsoever. None. Which verifies what Art has said for a long time that a good muffler should not appreciably reduce the output of the engine. Our Holcroft mufflers have undergone three changes since the beginning, and they use slightly larger diameter chambers and a somewhat tapered cone at the end. The Schnurle engine doesn't need or like the back pressure that can be beneficial to a Fox or 35S.

When we began designing tongue mufflers, we made them larger than both Art's design and also the old SST mufflers. Art's mufflers used two rows of 5 holes and the SST mufflers used 3 rows of 4. Part of the muffling effect on the tongue muffler is the use of multiple holes which allow a slightly different tone to come out of each hole. Well, these worked fine, but in hot weather, when we were being pressed for power, I drilled another hole in one of the mufflers. It helped, so I added another...and another. We now make all of these with three rows of 5 holes and now find that the restriction is about the same as on a tube muffler.

Another effect of nitro (and the way I use it) is to alter the "operating point" of the engine. As an example, at the NATS this year, I got some SIG 10% that was in fact 5% (although I didn't know it at the time). The results was that to get the same lap time, the engine had to run closer to the 4-2 point. My engine is set to 4-stroke the whole time. When I flew it early in the week, it was right on the break point in level flight, and went hard 2-stroke in the maneuvers. This was a real disaster. Later, I switched to 15% nitro. This lowered the operating point to a medium 4 in level flight, and a fast 4 in the maneuvers, just like at home.

Running 15% at home (sea level) results in it being way too deep in the 4-stroke, and then it loads up in the maneuvers. I normally run 10%. At least with my setup on the PA61, 5% just doesn't work, particularly in the heat, humidity, and altitude of Muncie.

Point being, that nitro is an easy tuning device, even with no compression changes.

Ted Fancher, also used 15% nitro at Muncie, and who can argue with success? Ted won the Nats. However, it should be noted that both Bret and Ted were using a pipe set up with lower compression than what I originally set up on the 35 FP. This engine is set up to use 5% nitro and use it well. Matt's engine that he used at this year's Nats was also set up to use 5% nitro. When he switched to 10% nitro he had to raise the head another .005 to about the same level as used by Ted and Brett. It helped that engine, but that engine is different from the 35 FP. Lowering compression and adding nitro are certainly ways that a modeler can use to "fine tune" his engine if he feels he is not getting enough punch at top. But, as originally stated, don't get carried away. If a little helps, a little more can hurt again. And all of these fine tuning efforts were done because of changing temperatures and changing altitudes, much of which will not affect the beginner and casual flyer. To them I generally recommend "stick with 5% and save your money."

>Ted Fancher, also used 15% nitro
>at Muncie, and who can
>argue with success? Ted
>won the Nats. However,
>it should be noted that
>both Bret and Ted were
>using a pipe set up
>with lower compression than what
>I originally set up on
>the 35 FP.

I don't know about the 35FP, but Ted and I were both running pretty high compression - .023 head clearance with the hemi-head. That sends Ted's compression tester all the way up to 125 psi. Comparing this to other motors, the Fox is about 35-40, and the stock GMA/JETT 50 is about 35.

We had discovered the need for nitro at last years NATS, where we didn't think of it until Thursday night. It wasn't even all that hot. At the team trials, I used 10%, and that was a mistake. It's truly astonishing how much the performance of the airplane and engine changes going from home (Northern California) to Muncie. Even if the temperature is comparable, the humidity and the moderately higher altitude *really* make a change. Unlike the third member of "F troop", David Fitz, either Ted or myself is big on taking notes, but we are for sure recording what we end up with after each trip to Muncie. Ted was pretty OK right from day 1, but I was pretty much junk until Thursday morning.

My plan from now on, in a new situation, is to adjust the nitro until the fuel consumption is the same. For instance, at home, I use 7.6 oz of fuel for a 6:30 flight. First flight on 5%at Muncie, I ran 9:30, and eventually I got down to 6.4 oz for a 7:10 flight. I put in 15%, the fuel consumption went back to 7.4 oz for 6:15, the power came back, the needle went right back where it was, and the airplane flew great.
This is not as arbitrary as it sounds - lower effective air density = lower effective combustion pressure = same as adding head shims in a constant density environment.

>Lowering compression and adding nitro
>are certainly ways that a
>modeler can use to "fine
>tune" his engine if he
>feels he is not getting
>enough punch at top.
>But, as originally stated, don't
>get carried away. If
>a little helps, a little
>more can hurt again.

True! With nitro, just like compression and everything else, there is a point of diminishing returns. 5% won't fly my airplane in Muncie, 10% is marginally better, 15% is way better, and then 30% (Cox Racing Fuel - don't tell Randy) is no improvement.

The point is that being willing to experiment, and even more importantly, being able to objectively evaluate the changes you try, is how you learn. Ted and I are still finding out stuff every single flying session!

Sorry to respond to my own post, but to be fair to Randy, I need to add a disclaimer on my results with the PA61 and nitro use. Ted and David developed, and I, a lot of the locals, and John Sunderland use a PA61 setup that *is not* the way Randy recommends. The key difference is the prop. We use the Brain Eather heavily undercambered props at generally smaller diameters, and Randy and most PA61 owners use a 13ish inch Bolly 3-blade. Our engines do not rev as high - I usually launch at 10000-10100, as opposed to a more normal 10800-11000 revs for Randy's recommended system,

I think that our system definitely works well here, but I am not entirely convinced that its any better or even as good as Randy's system back in the Midwest. Randy's system is very similar to what most of us used to run on 40VF's and the like, just 50% more! I think that the West Coast system is closer to the the last hurrah of the ST60's, except more powerful and more adjustable. I like the relatively small diameter prop I can use and still get decent performance. This gives me more controlled corners, and sacrifices overhead tension. The reduced overhead tension puts premium on getting the engine to come on at exactly the right time and exactly the right amount. I think that Randy's system is probably more forgiving in this respect.

And what we were using (first attempt to try a pipe and get an honest comparison between the systems) was almost exactly what Randy was running. His engine, his pipe, his venturi, his recommended pipe length--even the same head clearance he was using after we added that extra .005. We are "spoiled" and therefore seeking more pull up on top. So the direction we go and the direction Bret goes can take different forks in the road at times. It doesn't make either wrong or right. The setup of a plane is just as much choice as the design and the color scheme we may choose. And that is what makes this event fun. I probably wouldn't like Bret's setup with the lower tension up on top and he may not like mine because he isn't used to or feel the need for the additional tension that we trim for.

To each his own, and that is why we enjoy airing our ideas in forums such as this.

Thanks, Brett

LAST EDITED ON Aug-05-00 AT 02:08 PM (EST)

> I don't know
>about the 35FP, but Ted
>and I were both running
>pretty high compression - .023
>head clearance with the hemi-head.
>That sends Ted's compression tester
>all the way up to
>125 psi. Comparing this to
>other motors, the Fox is
>about 35-40, and the stock
>GMA/JETT 50 is about 35.

I can certanly understand the differences in engine run between sea level and the level at Muncie (or Reno, or wherever)and the need to adjust compression levels and nitro content in some engines to restore lost power. But I don't consider .023 high compression. In our .35 FP we run .016 clearance. Lew Woolard on his Silver Fox 40 runs .015 clearance. We set up our Magnum .53 at between .020 and .024 and this is approximately the range that Randy uses in setting up the PA 61. I have a feeling that the reading you were getting off of the compression tester was more a result of piston/cylinder fit than compression itself. If there is leakage around the piston, then the compression test won't be accurate. Many of these engines when running hot will regain their compression that they have lost while cold. We had an old iron piston 35 FP, for instance, that had a couple thousand hours on in. There was virtually no compression on the engine when flipping it cold. Yet it still ran very well once it got up to operating temperature. The one that throws me, however, is the Jett, as I would think the piston/cylinder fit here would be quite good as well.

With the PA .61 we were using, we started out with .021 clearance which worked well with 5% nitro, but lacked "punch" on top. Switching to 10% was a semi-disaster. It just wanted to run faster. We added a .005 head shim the night after the first round of qualifying and then with 10% the punch was back as were the desired lap times. Now that would make it .026, which was a lower compression than you were using, but allowed the 10% nitro to work well. In all fairness to your observations, we did not have the shims at the time nor the time itself to try .023. But we will.

Going back to the original question of this thread, if you wish to add nitro you may need to raise the head clearance--slightly. Putting two .016 head gaskets in an FP is way overkill. I had one sent to me to rework that had five .016 gaskets for a total of .080. And he wondered why he had no power.

If you wish to add power in order to gain a little punch on top or to regain lost power having gone to higher altitude or because you are flying in hotter weather, always go slow. .003 or .004 or .005 increments are the maximum you will want to make at a time. And you need to be careful that if you remove head gaskets you do not lower the head too much. There is a minimum head clearance that you will wish to maintain, and a maximum head clearance beyond which you will gain no benefit. If you do make changes make a log of what you did and what the benefit (if any) occured. Then you can always go back to "ground zero" if you need to. I walked around the Nats this year asking almost everybody what their setup was--amount of head clearance, nitro content, prop being used, pipe setting--and although most could tell me the latter, it was surprising to learn how many didn't know what their head clearance was.

If you are going to make changes, record what you changed from, what you changed to, and what the results of the change were. And only make one change at a time. If you make more than one, you won't know which of the two did what.

Leonard Neumann

> Lew
>Woolard on his Silver Fox
>40 runs .015 clearance.
>We set up our Magnum
>.53 at between .020 and
>.024 and this is approximately
>the range that Randy uses
>in setting up the PA
>61. I have a
>feeling that the reading you
>were getting off of the
>compression tester was more a
>result of piston/cylinder fit than
>compression itself. If there
>is leakage around the piston,
>then the compression test won't
>be accurate. Many of
>these engines when running hot
>will regain their compression that
>they have lost while cold.
> We had an old
>iron piston 35 FP, for
>instance, that had a couple
>thousand hours on in.
>There was virtually no compression
>on the engine when flipping
>it cold. Yet it
>still ran very well once
>it got up to operating
>temperature. The one that
>throws me, however, is the
>Jett, as I would think
>the piston/cylinder fit here would
>be quite good as well.

All the engines has good fits! The Jett in particular was very tight. The difference was that while the volume of the head itself was similar to the Nelson, the head clearance as delivered was in the .060-070 range. This implies a tremendously lower compression ratio than the Nelson. The Fox is in the same ballpark as far as compression ratio goes, which is pretty consistent with th readings we got. Note that I give George a lot of credit, this is exactly what he told everybody to do, and he followed through with it in his own product. Even though I thought it might be a little soft, it did run nicely, with the power of a really strong ST46, and would (and in fact, pretty much had to) swing a huge prop. With the low compression, the timing with small props was just retarded too far, and the power fell off compared to larger props. Which is, once again, just exactly what George said would happen.
As an additional comment on the Jett, that extremely low compression allowed the use of a huge venturi compared to what we typically run on the Nelson - the smallest was something like a .218 straight (no spigot), which would turn the Nelson into an hand grenade every time you touched the controls!

My buddy Larry "Joey Jr." Fernandez has a Jett 50 running piped, and once it got enough time on it, he has been very happy with the performance, even in some portly airplanes. The one that nearly folded at the NATS on Sunday weighs something like 70 oz on 690 square inches. It did take a long time to bed in fully, which is pretty typical for chrome liners, particularly for the Jett with it's rat-racer inspired cylinder taper.

Well Len, went back to the 7 1/2% tonight. The engine seems to run a lot smoother and less "ragged" now. Can't tell if there is any loss of power. The overheads are a little "softer" but the airplane is flying a little slower too! Guess I'll keep the 10% for a real hot day and run it a little rich. Still playing with some different prop sizes and pitches. Gonna try a 12-4 soon. Tried an 11-4 and that seemed a little too small. With the 11-4 I can lean it out for take off and then it'll richen slightly. I'll keep ya posted. Take care and hope to see you soon.
Crist