What about the  OS Max FX 46?  I think I can tame it down by changing the crank porting.  Any other problem with this engine?

As changing crank timimg involves welding the crank and remachining then heat treating the crank. You might as well just save the origional crank and make a new one from scratch. But then why stop there. Obviously you have the numbers on the engine that lead you to believe changing the crank timing will get the results you want, so I don't understand the point of your question.

     I doubt that it needs it, but most of the crank mods for stunt involve making the duration longer, not shorter, so no welding. The RO-Jett, for instance, has the same crank timing as the Jett QM40 engine.

    Brett

knock the sleeve timeing down to something like 130 ex 110 trans .
Crank timing closeing up to 60atdc can work ok.

Dreaded theories on gas speed Vs transfer area are somewhat relevant,
for tourque / 4-2 switch .

Automotively , large ports give a top end circuit raceing engine with poor flexibility.
Smaller ports get same gas speed at mid-range to get flexibility / throttle responce
for rallying , of road / gravle road use.No good there if its only on or off .

The 21/46 being in the same case as a high speed 29 seems to bear this out .
Max timing of 140 / 120 on sleeve , better four stroke on the 130 / 110 .


Youd better let the lads see the pictures ? wot ho.

     I doubt that it needs it, but most of the crank mods for stunt involve making the duration longer, not shorter, so no welding. The RO-Jett, for instance, has the same crank timing as the Jett QM40 engine.

    Brett

Ahhhhh! Thanks Brett I did not know that. Please disregard my prior post. 

Can you please explain why?

   Because it tends to smooth out the transitions from 4-2.

     Brett

Brett,

 Can you please explain why?
 I'm making a new backplate/rotor for the MB engine. I have several reasons for having to do it, one being that the engine is very messy. It sprays some fuel out from venturi. At the moment intake timing is 180 degrees, open 45 ABTC, close 45 ATDC. This seems to be quite standard, least in the engines I've looked at. But the MB has a rear intake, long stroke and extremely small bottom end volume so the pumping efficiency is propably better than average.
 In a front intake engine I can just make a longer venturi to stop the fuel mess, but in MB there is not much room for a longer venturi because of exhaust adaptor.
 For this reason, I'm planning to shorten the intake duration ATDC. I'm thinking of going down in 5 degree steps. Nice thing with a rear intake engine is that I can easily make new rotors with differend timing.

 Lauri

Hi Lauri. Have you  tried a sharp edge on the bottom of the vent insert? I've had luck reducing blow back by not chamfering the bottom of the vent hole.

I spin it about 11,200 in the air and most people are in the range of 10000-12000 in in the air. The RO-Jett 61 has the same crank timing as the Jett QM40 engine, which is essentially the same as a 40VF.

      Brett

   

Don't doubt it Brett. It's just contrary info. Maybe the engines you speak of have large case/transfer volumes that allow them to ignore the extended timing. The QM40 spins in the high 20's and the vf was used in the low 20's in it's intended application so internally they must have some room. At 11000 they switch into "heavy boundry layer" mode..

Brett,

 Can you please explain why?
 I'm making a new backplate/rotor for the MB engine. I have several reasons for having to do it, one being that the engine is very messy. It sprays some fuel out from venturi. At the moment intake timing is 180 degrees, open 45 ABTC, close 45 ATDC. This seems to be quite standard, least in the engines I've looked at. But the MB has a rear intake, long stroke and extremely small bottom end volume so the pumping efficiency is propably better than average.
 In a front intake engine I can just make a longer venturi to stop the fuel mess, but in MB there is not much room for a longer venturi because of exhaust adaptor.
 For this reason, I'm planning to shorten the intake duration ATDC. I'm thinking of going down in 5 degree steps. Nice thing with a rear intake engine is that I can easily make new rotors with differend timing.

 Lauri

Hi Lauri

If your thinking about retarding the timing even more, I am sure you will find even more fuel blown out. I have found in almost every engine I have built, modified , design/ran that typical stunt engines work best with milder timing in the 45 to 51 degree retarded range, and with 180 to 190 degree total duration, (180 to 185 IDEAL) if you retard the timing to around 60 degree ATDC it will really blow a lot of fuel out of the induction system..whatever one you use. You will have to turn many more RPMs with retarded crank timing than you will with normal stunt timing, You will also find that a retarded crank makes for a ratty sounding/running motor  when under 9,000 RPM. I use 180 to 185 with the PA Engines, these have 45 to 48 degrees closings. I also use this same timing with dozens of homemade motors, When I ran the same motor with  50 , 55, and 60 degree timing they ran ratty and blew fuel out of the top of the venturies, they also wanted to run over 11, to 12,00 RPMs to get them to smooth out.
So try them out and see what works best for your setup

Regards
Randy

   They didn't ignore the extra timing, they ran *better* that way. Much better, in some cases. It tremendously smooths out the breaks and makes the engine much less aggressive in the maneuvers.

     The VF typically runs around 11,600-12,000 in the air.

     Brett

     

   

I'll take a stab here and say that the break got smoother because the vol eff went down.

Hi Lauri. Have you  tried a sharp edge on the bottom of the vent insert? I've had luck reducing blow back by not chamfering the bottom of the vent hole.

 Dave, If I understand your question right, then yes. We use a plenum chamber system, similar to what TR engines have. Venturi is a very short sleeve with M8x0.7 thread, screwed to backplate extension. Fuel nipple is right behind it. Difficult to explain such a simple thing.. I'll take some pictures when the camera comes back home.
 Randy, what do you mean by retarding? I tried to say than I want to make the intake to close earlier than 45 ATDC. Maybe you misunderstood what I was trying to say.
 I like to run the engine at about 6700rpm, and I'm looking for a constant-speed setting or at least a very mild 4-2-4 change. I know that many things in the flow dynamics are not the same as with higher rpm's, propably they are less critical at less rpm.
 Actually, at a rpm high enough, you don't need an intake timing at all! Resonance waves take care of it L

I'll take a stab here and say that the break got smoother because the vol eff went down.

   Perhaps, but given that we are trying to get 1/2 HP out of a (potentially) 2 HP engine, it doesn't make a lot of difference. Sufficient power is a given, it's a non-issue, and with the quality of the piston/cylinder fits the fuel mileage is generally good, anyway. The run characteristics are *everything*.
     I haven't seen any consequential ill effects of any amount of additional crank duration or earlier timing, up to the point of the engine wanting to start (and run hard) backwards, or blowing a lot of fuel out the venturi.

   I am not claiming that it's some sort of panacea, but certainly closing down the timing is not necessarily a good idea.

      Brett