Cox.049 timing change

F4D Phantom II Mon Apr 08, 2019 9:08 pm

. I have lowered the cilinder in the cranckase of my .049 surestart to change the timinig in hope to increase the torque to use a prop bigger than the 6/3 . I thought that that by lowering the cilinder and adding head gaskets to avoid over compressing the the engine I would have more volume of air and fuel inside the cilinder increasing torque.I`m using the diesel crank and TD hi comp head also will be trying .051 cilinder and piston with a galbreath Head with nelson plug. I`m not looking for speed but a lower RPM steady run with bigger prop for sport an stunt. Has anybody tried this before ? I have given the engine a few runs to heatcycle it but have not tried a bigger prop yet.All advice appreciated.
Juan

Levent Suberk Tue Apr 09, 2019 3:08 am

Add %10 nitro to fuel for one head gasket.

Surfer_kris Tue Apr 09, 2019 4:01 am

I have never lowered the cylinder, only raised it to reduce the amount of SPI. The effect on the timing itself is rather small and you are also changing both the transfer and exhaust timings at the same time.

In order to use a larger prop, take a look at the texaco engine for inspiration, or simply get a texaco engine I guess. I don't think they change anything to the cylinders, compared to the regular engines, only reduce the intake diameter and improved the cooling of the head.

As is obvious with diesel engines, a larger prop needs a lower compression ratio. A glow engine acts the same, try a few different numbers of shims and write down the needle settings and rpms, that way you can find the optimum compression setting. Then use one more shim than optimum in order to stay on safe side.

F4D Phantom II Thu Apr 11, 2019 10:49 am

Thanks for replying guy`s .
Juan

wmazz Sat Apr 13, 2019 10:35 pm

F4D Phantom II wrote:I have lowered the cylinder in the crankcase of my .049 surestart to change the timing in hope to
increase the torque to use a prop bigger than the 6/3 .

I`m using the diesel crank and TD hi comp head also will be trying .051 cylinder and piston with a galbreath Head with nelson plug.
I`m not looking for speed but a lower RPM steady run with bigger prop

That sounded like a good idea, lowering the cylinder and increasing the
power stroke. I have not tried it on a Cox, but it is common practice
for other 2 stroke engines.

You might try the Texaco cylinder, I don't see the need for large exhaust
and transfer ports of a TD cylinder. Someone else will know if the Texaco
cylinder uses SPI or not? If not, you might pick up some torque from SPI
if can you lower the cylinder enough.

Bill M.

F4D Phantom II Sun Apr 14, 2019 9:37 am

Hello Bill
I`m using a TD .051 cilinder and piston on my engine I still want the bigger ports and I have always heard that there is nothing like cubic inches. I`m working on a little P-40 plane at this time and haven`t worked on the engine idea. I will start with about 6 head gaskets and work from there. I will make sure my exhaust ports face fore and aft because I want my intake ports to be to the sides of the crankcase for better mixture flow. I will see how it handles a 7 x 4 Master airscrew, I have been looking for a cox 7 x 3.5 with no luck.
Juan

Dave P. Sun Apr 14, 2019 9:43 am

I got some parts from an old 1/2A speed freak's estate some time ago and one of the TD cases had been shaved to lower the cylinder. I can't really tell how much, maybe .010 or so. I haven't tried it and I've been curious about it. It doesn't appear that it was run very much. Anybody know if this was anything of a standard kind of mod, or was it a one-off experiment that didn't work so well?

The old adage "There ain't no replacement for displacement" still applies as far as I know, Juan.

wmazz Sun Apr 14, 2019 2:10 pm

F4D Phantom II wrote:
I`m using a TD .051 cylinder and piston on my engine I still want the bigger ports

The TD cylinder has SPI, and by lowering the cylinder, you are increasing the
SPI. There will be a point where too much SPI will cause problems.

I assume the TD has a larger exhaust port than other cylinders, and they
have a lot of transfer port area. That extra transfer port area is useful for
supporting hi rpm.

IMO a cylinder with partially blocked exhaust (2 slits/side) with no SPI is a
better choice.

I recommend you look at "Iceberg's" threads on his Texaco engines and his
quest for a 5 minute run time with 8 x 4 propeller.

Bill M.

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