RE: Hot engines
 

@ rmenke

Tuned pipe header: The Alley Cat fuse offers so much space that it’ll be easy to mount the OS .91 VR-DF inverted and using a straight header (0 degrees) in combination with an inside placed tuned pipe. I recommend buying the rear exhaust header # 3890 by MACS products. It is a bolt on zero degrees header.
However- it turned out that the pair of stock M 3.5 header mounting screws delivered with the OS .91 are prone to shear-off. This can be avoided easily by using two M4 screws instead, having a steel grade of at least 10.9. There won’t occur any problems while cutting the “new” M4 thread into the aluminum of the OS .91 exhaust flange.
Those high strength M4 screws will survive all stresses during an entire engine lifespan.


Exhaust gasket: Please don’t use an exhaust flange gasket since heat transfer from the cylinder case to the exhaust header will be obstructed. Metal to metal contact is desired here.


Changing head screws: Again talking about changing screws. You’ll do your OS .91 a favour by exchanging the 6 stock M3.5 head screws by grade 10.9 M 3.5 screws. The original screws elongate by-and-by due to heat leading to a slightly loosened cylinder head.

Other than the above described screw problems the OS .91 VR-DF does not need any changes, presumed you don’t want to have it modded to run 24000 or more rpm in flight.


Fuel tank size and position: The Alley Cat tank bay, although reduced in size by the tuned pipe tunnel, will provide enough space for a 300 to 400 ml (10 to 13.5 oz) tank. If using the recommended bubbless tank you can position the tank nearer to the CG!
If you want to place the tank nearby or into the CG you can install the Perry VP-30 pump. It provides a strong enough pumpage, even for the OS .91.


Tuned pipe: If your pipe features same internal volume like the MACS # 1190 and a similar geometry, then you can use it for the OS .91. If your pipe is designed for a smaller engine size your OS .91 will run too hot potentially damaging your piston (up to aluminum melting –> piston hole).
If your club does have noise restrictions I would like to recommend the JetModelProducts JMP System 2 tuned pipe. This is a high volume tuned pipe with a fixed length resulting in a peak rpm of about 22000 rpm. Unfortunately it is no light pipe and the cost of the complete set consisting of tuned pipe, machined header, retaining clip, pipe hook and heat seal is US$ 170.

RE: Hot engines
 

Hello Cyclic Hardover,

Before installing a hot .60 I would consider to reinforce the DD airframe. What about a fully balsa sheeted DD? - Still light, but very torsion proof.

That would not take much time and would ensure safe operation and no more covering hazzle.


Because your OPS .60 FIRE has the high torque liner, I also recommend rising the exhaust timing and using a non-muffled tuned pipe like the MACS #1160. A 9*8 or 9*9 prop would be ideal then.

RE: Hot engines
 

well I must admit, as those in the know have acknowledged, this is good information... but lets say I did want to turn an os 91VR-DF up around 24,000 rpm, what am I looking at doing to the engine to achieve this?.......

roughly calculations tell me 24,000 rpm should be in the vinicity of around 220 mph given a super clean frame with correct wing..and calculated on the 9 X 10 prop...

and while I'm at it..I'll go for broke..Ive seen one topic on making carbon fiber props.

http://134.169.96.2/~modellflug/klau.../prop/prop.htm

is this the same procedure you use or are there other ways ..looks pretty straight forward.. though the hub thickness is something I'd like to look at more closely

thanks
HB

RE: Hot engines
 

Flight report and a video? I can see this coming. At least the video may help me find the remains! Probably look like a Quail shot down by a 12 guage.

RE: Hot engines
 

I does seem that most DD's do not survive for long. I somehow got well over 100 flights on my first one before it could not be repaired.

RE: Hot engines
 

The prop diameter was actually 10.5 inches... my error. However, the speed was correct.

RE: Hot engines
 

Very good idea. I'm sure there will be enough power to overcome a little extra weight.

RE: Hot engines
 

Bump...

I found this thread, while looking for nothing in particular....some interesting (flashback ) reading...

RE: Hot engines
 

Guys like SloMo wasted their words on Americans who are afraid/too lazy to do any "hot rod" work on their engines..plus doing any prop work to marry the engine to the plane.
The few REAL hotrodders that are still around on this continent are still building hotrod cars. Maybe in Germany automotive hotrodding is too restrictive and that is why they have so many "speed freak hotrodders" in RC..?

RE: Hot engines
 

All,
I am seeking parts for the OS 91 VR-DF to help a beginner in glow DF modeling.
Specifically, we need: Hi-grade M3.5 head bolts, Plug for the stock needle valve assembly ( a remote needle will be used), Machined throttle are to replace the stock one which breaks from vibration (JMP used to sell these but no longer in stock.)
Kindly replyor send me a PM. Thanx.

Art ARRO

RE: Hot engines
 

A vast selection of fasteners can be obtained from "Modelfixings" in the UK...I don't quite know where you can get decent quality metric stuff in the US. I do believe either Dubro or Sullivan carry some M3.5 hex bolts (and they are good, because I once used them to keep a muffler from rattling loose on an ST S90K engine).

The throttle arm should be no big deal...if all else fails, make one yourself from a suitable wheel collar, a small piece of sheet steel and some silver solder.
Take a look at the heli stuff, these guys drool over anodized alu bits...perhaps you can find a throttle arm there that fits the engine.

As for a plug for the needle valve...I'm pretty sure OS has something exactly for that purpose...maybe for another engine, but I'm 99.5% sure I once saw something like that as a spare somewhere.

RE: Hot engines
 

To plug off needle valve holes I've threaded the case and used nylon hardware to act as filler plugs. You can even try a mismatched thread [10-32 thread with a 10-32 nylon screw] to make it a permanent "one shot" deal.

RE: Hot engines
 

Dubro does: http://shop.dubro.com/c/car-trucks_f...g=3&pp=12&pg=3

I have made replacement throttle arms for a few engines from plastic servo arms.. dunno if that is a viable option here though.

Can you plug the needle valve opening by Dremeling off the needle from the needle valve and screwing what's left into place?

RE: Hot engines
 

I just realised Webra, and now West engines have alu throttle arms that could fit with no or litttle modification...
And servo arms are good for a lot of stuff... i once made a throttle arm for the slide carb on my OPS 60 SP VAE Speed engine from an old JR servo arm...works like a charm, no slop what so ever.

RE: Hot engines
 

Guys, thanks for all the replacement part options for the OS 91 VR-DF.
I know DuBro and others offer M3.5 bolts but don't know their grade or hardness. The stock OS ones stretch with running and this causes problems- like a loose head and a lean run.
Will try re-threading theneedle assembly in the carb and plugging the hole-good tip.
Also, somejet guys used to double up on the throttle arms and braze/silver solder themtogether but eventually these broke also. The machined arms from JMP really solved this problem.
Be aware the DF usage is hard on these partsdue tothe accumulated runtimefor this application. Typically we burn 22-28 oz per flight withmany flights logged. These are the known weak points of the OS 91 VR-DF, otherwise a fine engine,except for the ABN liner material.

Art ARRO

RE: Hot engines
 

Well, Modelfixings has them bolts in 12.9 grade...they're reasonably priced too, order a bag of them, that way you're set for life, lol ;-)

RE: Hot engines
 

Sorry to intrude. I normally don't come by here often, but I used to fly planes and thought I'd give a little friendly input. If you're lookin for OS 91, OS 65, OPS 67, OPS 80 parts a good place to look might be US boat racers and where they hang out. Like International Waters for sale forum. Many used to run OPS 67/80's (me included). and have bunches of parts accumulated that are now obsolete. There used to be a titanium rod made in Canada for the OPS67/80. Nelson used to make aftermarket P/S and rods for the OS 65 and 90. Maybe you can try and find some of these wonderful old parts for your endevour. Of course I see whole engines go on e-bay for very modest prices, sometimes for less than a P/S rod combination new. I got a ton of old style OS 65 parts, disc rotor style before the drum rotor, 7F carb neck style carb. These won't interchange with the drum rotor model, they changed just about every thing, though they look near identical. The boaters that used OS 46/65 and 90's used to cut the ports quite often and had virtually no trouble with the plating. 60% Nitro is the norm in US nitro boat racing. Rudy

RE: Hot engines
 

Also please be reminded that Novarossi makes a speed 60 FIRE for those wanting a more plug and play experience and don't mind being spendy. Check thier website. They've been know to have great mettalurgy and fit and finish. I've had trouble keeping up with all thier 21 models and variants, which has been the problem (for me at least) in getting parts, but usually the parts are available, I just can't seem to figure out which ones will fit.

RE: Hot engines
 

Hallo speedfreaks,

here some pictures from my aktuell OS 60 speed engine.

A very powerfull engine with Nelson AAC piston an cylinder and different intake systems. Always more than 14 mm intake for good breathing. Mostly parts are selfmade.

Have fun

Best regards Michael

RE: Hot engines
 

Michael, very nice..!
Can you tell me how much exhaust timing this engine uses..?
Do you use a inflight fuel mixture valve between the engine and the fuel tank..?
14 mm intake looks very impressive........[8D]

RE: Hot engines
 

Hallo,

the engines have 192 degree exhaust timing.
Also inflight mixture control from BVM.
30 % nitro, 20 % oil

25000 rpm on the ground.

Best regards Michael

RE: Hot engines
 

Thanks Michael..!
Do you think 192 degrees is a fairly "universal" number for speed .60 engines that turn the size of prop that you like to use..?
I ask this because I know a guy with a Picco .60 pattern engine who would like to modify it to the same level of performance as the Speed or Fan engines. The pattern engines are easy to find.
The hope is that with a little bit of work a low timed .60 FIRE could be modified to turn a 8.75x9 prop fast enough to make the small investment in time and money worth trying. I advised him to try 180 degrees to begin with and to use a 8x9 while testing it on the ground.

RE: Hot engines
 

I agree with you to start 180 degree , not more.

The biggest handicap is the small bore in the crakshaft. Mostly 12mm or less. This is to small for a good performing engine.

My front intake OS on the picture has a 17 mm gas passage !!

Best regards Michael

RE: Hot engines
 

Those pipes are a much "peakier" design than the standard MAC pipes I think!

OMG look at the size of the intake on the rear rotary version.. :) That's what it takes to flow 4l of air per second..

RE: Hot engines
 

Those are pretty sick looking set-ups![8D]