The 35S is a baffle-piston engine, the 35FP is a Schuerle ported engine.

I haven't used either one, but my understanding is that the 35S runs more or less like a Fox 35, while the 35FP is a more powerful, high-revving engine.  I don't know if the 35FP makes a good stunt engine, but if it does it'll do so on a bigger plane than the 35S can handle, and it'll do it with a flatter-pitch prop and at higher RPM.

The 35S is also more than 2 oz lighter than the FP.

Thanks guys, for all the replies.. I was going to buy an FP but your advice has changed my mind.. I have an max S so I know how good they are... Cheers  Phil

I don't have any OS 35FP engines.  But the engine in my soon-to-be-current-again-ride is a Magnum 36, and as far as I know, it's a fairly faithful Chinese knock-off of an OS 35FP.  I also have an FP20 and a couple of LA25's, which are Schnuerle ported, high-revving engines similar in design to the FP35.  They all fly great -- as long as you treat them right.  They really really want to run at a fairly constant speed in a wet 2-cycle, between about 9500 and 10000 RPM (faster than that for the smaller engines).  Put one in it's preferred zone, and it'll stay there all day.  Put the right prop on it for that speed, and it's a great engine.  Try to adjust one to a four stroke that blips up to a wet two stroke on the up-lines and tops of maneuvers, like a Fox 35 does, and sooner or later it'll go where it wants to go -- to a wet 2-cycle, between 9500 and 10000 RPM.  Do all that with a prop that's appropriate for that four-two break style of run, and you'll be going way too fast and using way less fuel than when you launched.

People who are expecting a four-two break style of stunt run call this "running away".

If you don't like all the "to my knowledges" and "as far as I knows", then dig through the engine discussions here -- the whole subject of using the high RPM Schnuerle ported engines vs. the old-style "stunt run" engines with a good 4-2 break has been pretty much beaten to death over the years.  You'll find that little of what I say at the top of this post is fresh material: I'm just channeling what a lot of people with a lot more experience and success at flying stunt have said, adding in what experience I do have with these motors.

Andrew, my modeling budget is pretty severely constrained right now, but if you want to get rid of one or two of those ratty old useless OS 35FP motors please send me a PM.  I may have to give you a regretful "no", but if I can afford your prices I'll put them on airplanes that are really a bit too heavy for the OS35, with 11-4.5 props, adjust them the way that I adjust my Magnum 36, and suffer the consequences.

Remember that the OS35-S has iron piston and steel sleeve.  That means you need a fuel with lots of oil.  Think of it as a replacement for a Fox 35, but with more power.  You use the same Fox fuel, at least 25% castor oil, and 28% is better.

I'm flying a nice 35-S in a Palmer T'Bird.  A perfect match.

Floyd

Phil
You  may want to rethink the low oil, The OS 35S needs 25% oil, you do not have to run all castor and I ran 50-50 in my for many many years, So a fuel that is 25% total oil 1/2 castor will help the motor last and run well, Remember the rod at the top and the piston pin boss is the weak points in these, they need lubrcation.
I used 10% nitro 25 oil 1/2 castor, and I still have ones from teh 70s in very good shape

Randy

I've been running 20% oil, 50/50 in a OS 35S with a Bristunt ABC setup since 2008.  It's well worth the cost of the upgrade parts to get away from the all castor fuel, in my opinion.

Hi Tim,  The 35FP isn't really related to the Magnum 36.  The Magnum 36 is a clone of the OS 32F with greater displacement, with (educated guess) likely zero parts that will interchange with the 35FP.  I've got 20/25FPs, 25Fs, a couple 32Fs and a Magnum 36, and they all run much much better than the 35FP.  While the rest all have a wide range of needle settings that result in consistent runs from start to finish of the flight, for flight after flight, the 35FP has a razor's edge needle that's always either a click too lean or a click too rich, with a run that often varies in the middle of the flight.  This isn't about 4-2-4 or high rpm/low pitch, it's about the consistency of the engine run. For whatever reason the 35/40FP, simply don't develop suitable power for the class of intermediate planes where they'd be useful.  Tamed down they're weak for their size, weight, and fuel consumption, leaned out they sag or develop too much power for a well built Twister size plane.  I don't need to dig for second hand info on the forum, since I was running the 35 & 40FP in competition back in 1993, and remember having conversations at contests with other people using them, and noting how our configurations greatly differed.  I found reasonable success with my setup, and had more problems when I tried other setups I'd heard about.  Eventually, after a few seasons, I was able to see from my score sheets how badly the consistency issues and random over runs were hurting my scores. 

I don't even need to present a convincing argument that the 35/40FP engines don't work out of the box.  All the varied setups, the magic quantities of head gaskets, or magic props that aren't at all consistent from one person to the next, combined with the fact that none of the "serious" competitive flyers use them, and the rework aftermarket which sprang up around these engines, already effectively validates my point. 

I've been strongly considering liquidating my stock in 35/40FPs, but while they aren't viable engines for future projects, they represent nostalgia to me.  They remind me of saving all my odd job, allowances, Christmas, and birthday cash so I could buy a finely machined jewel of an engine at the hobby shop.  It's just too bad they don't run as good as they look.  Fortunately at the time, I was just happy if the engine started easily, made lots of power, and ran until the tank was empty. 

Thank you Andrew and Bill.  It looks like I was mistaken.

In Philly we run a lot of FP40s, FP35s and Tower 40s (these are FP clones, the pieces interchange.) We use them in stunt competition. Effective power in Intermediate and Advanced. I have described how we set them up. I prefer FPs (and Towers, which I like even more than FPs) to the OS35S. Considerably more power, more durable, much more tolerant to an inadvertent lean run, especially true of the FP abn engines and the Tower abc engines. When FPs and Towers are used in typical 40 sized stunt planes the superior reserve power is very useful. If your engine is running away chances are you are using the needle to adjust speed, setting the FP or Tower to run too rich. As the engine heats up it will run faster. The runaway. Which is usually the engine finding its balance speed. The speed at which load, fuel mixture, etc. are working together. If your plane is too fast, use a smaller venturi. Don't try to slow the plane by needling a richer setting. If the engine is breaking too hard, add a head gasket or two. The successful engine mods that I have observed, hemi the head to lower compression. Far as I can tell the same effect can be achieved by adding head gaskets.

You don't have to machine the engine. You can add head gaskets. What does this gentleman do when he crashes his profile inverted and snaps the needle valve off. First off, the needle valves for these engines have not been available for years. Changing the venturi is simple once the needle valve is out. Everything I describe is basic stuff. Very basic. If the gentleman cannot do what I describe how will he trouble shoot a fuel tank where the pick up is misplaced or solder up a leak or... Backs of almost any metal fuel tank need to be popped and the tank inspected. Clunk tanks need to be assembled. Then there's the pin hole in tubing. How to clear a needle valve when dirt enters. Or how will the person clean out an engine that has crashed and sucked in dirt. Disassembly required. I am surprised that anyone who's done control line for a while would think popping a venturi in and out is a major complicated tuning. Besides that, these old OS35s, have not been manufactured for fifty years. Good luck finding a pristine example whose piston/cylinder fit is good as new. And so on. In any case, like I said, the operations that I describe are very basic.

The muffler thing isn't a big deal with the S35 - they were sold well into the 70s with the 703 muffler, and worked fine with it.  I have only ever flown them with this muffler, and found them to be very friendly.  The two issues I've had so far are a worn rod and a worn bushing (no doubt exacerbated by using an extension shaft in an Ares).  Both known problem areas.  Never had any issue with piston/cylinder life, running 25% castor.  I've got a 35FP but have never flown it in anything, so can't really comment on it.

Look. You guys advocate what you want. I debated this myself over the years and chose what I chose on the basis of my experience. My club members have chosen what they do. The engines I refer to work for me in local contests, at Brodak, even at the NATS. If this is a discussion about the choices of a practical cost effective engine to use for entrance into the hobby of control line sport/stunt, it makes sense to choose an engine that is currently manufactured. An engine this has proven itself capable time and again in stunt contests happening today. LA25 or LA46. Both available in control line configuration. Both are eminently practical and reasonable in cost. Debate if you like. I am stating my opinion. Everyone is welcome to their own.

It was stated above that the FP35 is a terrible stunt engine. Since I have a different experience, I talked about that. Jack Weston, a club member, has won many local contests in Intermediate and Advanced, using an FP35 in a Gieseke Nobler and an FP40 in a Vector. He was a mid pack finisher in Advanced at the Nats. On the local level he is now flying in Expert. I have followed his methods which largely left the engine alone. The venturi choices are standard OS venturis, supplied with the OSFP35 and OSFP4O control line engines when they were sold. A large venturi and a small venturi. Both still available. Sold by Tower Hobbies. The standard FP40/35 control line stunt engine was also supplied with extra head gaskets to lower the compression and adjust break. The supplied venturis were .065 and .083. These are standard OS parts supplied with the stunt configured FP40s and FP35s. I happen to like a venturi in the middle. So what. They're available from various sources. Cheaper than the OS part. Dan Banjok and Mike Palko have used these engines successfully in stunt. The original Fancherized Twister was powered by an FP40. Many modelers in my club use these engines in stunt planes. On and on.

We are describing the differences between FPs and OS35s. You described them on the basis of what your experience. I described them on the basis of mine. I hope you are not seriously telling me to deny my lying eyes. I do not like arguments like that.

You are assuming that this person in an inexperienced modeler. Sounds that way. If that is the case, he is better served by the currently manufactured OS offerings. The LA25 or LA46. The title of the thread is, after all, OS engines. Any old engine is a potential problem. I know this first hand as well. Having purchased a few NIB engines that looked pristine and beautiful but had poor compression or a damaged rod. Ebay purchases or other on line purchases. I believe a cast iron piston/steel cylinder engine can be damaged, as well, by corrosion. Store the engine poorly, that's what you get. Purchase of Bristunt ABC piston and cylinder is expensive and not currently manufactured. Although I read on Stuka Stunt that the individual who had them manufactured will do another batch, if he can find enough customers. As I said. Not cheap.

If we were face to face I'd let you know what I think about your last remark.

I don't know Dennis, I'm not the one suggesting a face to face confrontation over an online discussion.   

I just had an additional thought. OS35s engines were designed to be run without a muffler. .

Not quite so.  The OS 35S was sold with the Jetstream muffler, designed specifically for that engine.  The Jetstream muffler bolted onto the exhaust stack.  It was not a strap on.  In fact, an OS 35S with a Jetstream muffler (both inserts removed) powered a stunt ship that won the Walker Cup as the National Stunt Champion.  It was not the first muffler equipped stunt ship to win the Walker Cup, but I think it was the second one to do so.  That was a few years ago, but it really did happen.

Keith

Keith, I just looked at 2 35s, they have bosses at the corners of the exhaust stack that look like they can be drilled and tapped for bolting on a muffler. But, at least on these two examples, they are not drilled out. A friend gave me an Oriental with a mounted 35s. That engine does mount a tongue muffler bolted to the exhaust, attaching via two screws to those bosses. But I don't know if he drilled and tapped. I haven't seen any 35s stock in the box with these bosses drilled out. Did some engines ship that way? The 35s I bought in the early 70s did not come with a muffler. Did some of them ship that way. Or was the muffler purchased separately?

I do not remember if the 35S was sold with that Jetstream muffler.  Early on, with that Jetstream muffler, the engines came with those bosses drilled and tapped for the muffler.  The Jetstream was in two halves.  The one half bolted onto the engine with bolts from the inside of that half.  Then, the outside half was held in place by 3 small bolts.  It was not a very heavy muffler.  I wold not hesitate to use one now if I were to use the OS 35S say for a Classic design.  It is almost a perfect match say for the Chizler.

Keith

Here's a pic of the Jetstream borrowed from Ebay.  In this pic you can see how it would bolt to the inside of the exhaust stack, and also you can see the boss on the extension piece where the butterfly valve was mounted for RC, and the round boss on the muffler body where some RC versions of the muffler had another exhaust valve.