Piston and cylinders on Cox Intl

John Goddard wrote:Hi Matt
Doubtless one of the experts will chip in soon but meantime...
The lightened piston belongs in a Bee type motor (such as the
Killer Bee) you'll notice
The cylinder is slatted as well.
The other (teeDee type) has a MUCH larger exhaust port.
IMO the Td needs a stronger piston as it's a much faster
Engine by vitue of it's rotary induction.
I've tried many different permutations of cylinders and pistons.
One of the fastest combo's I ever achieved was one of Bernies lightened
Pistons in a TeeDee cylinder and Td 049 bottom end.
It lasted 4 runs......

Well you answered my next question about using the lightened pistion with a td cylinder........

The piston in the first set is lightened by shortening the skirt of a regular Cox piston. This was done mostly to give the ”slit” cylinders SPI (sub piston induction) but it also increases the rpm because of the lighter weight of the piston. The piston in the 2nd cylinder set was not shortened or lightened because the open exhaust of the ”TD” style cylinder already has SPI. I have tried one of those shortened pistons in a TD style cylinder, it didn't work well because it had too much SPI. You can lighten a piston without shortening the skirt, but that is something that you would need to do yourself and is not yet offered by any of the sellers online.

Admin wrote:The piston in the first set is lightened by shortening the skirt of a regular Cox piston. This was done mostly to give the ”slit” cylinders SPI (sub piston induction) but it also increases the rpm because of the lighter weight of the piston. The piston in the 2nd cylinder set was not shortened or lightened because the open exhaust of the ”TD” style cylinder already has SPI. I have tried one of those shortened pistons in a TD style cylinder, it didn't work well because it had too much SPI. You can lighten a piston without shortening the skirt, but that is something that you would need to do yourself and is not yet offered by any of the sellers online.

i thought the piston in the reference was also lightened by cutting the inside of the piston on the bottom part. that's what it looks like in the photo from the link. happydad

Matt:

As to dealing with Cox International I belive it accurate to say that the owner (Bernie) is not only a sponsor of our forum but a highly respected vendor to the Cox hobbies. In fact I've never heard a complaint directed toward Bernie.

Buy from Bernie with the confidence he deserves.

Admin wrote:The piston in the first set is lightened by shortening the skirt of a regular Cox piston. This was done mostly to give the ”slit” cylinders SPI (sub piston induction) but it also increases the rpm because of the lighter weight of the piston. The piston in the 2nd cylinder set was not shortened or lightened because the open exhaust of the ”TD” style cylinder already has SPI. I have tried one of those shortened pistons in a TD style cylinder, it didn't work well because it had too much SPI. You can lighten a piston without shortening the skirt, but that is something that you would need to do yourself and is not yet offered by any of the sellers online.

Admin is correct.
Before 1996 Cylinders had open exhaust ports. Some were ground lower to create sub port induction(SPI) when the piston skirt rose above the bottom of the opening to allow a cold air charge to enter, and this was a high performance cylinder. Other cylinders did not, but the difference was not apparent to the casual observer. There were all sorts of different internal porting configurations. At this time, all pistons were the same height, whether used in an SPI cylinder or a non-SPI cylinder. Later cylinders all had the exhaust ports cut to the same height. To achieve SPI, the piston had to have a short enough skirt to clear the bottom of the exhaust port and allow SPI. Enter the short(lightened) piston. They figured a lighter piston would raise RPM, plus allow SPI, so that was two enhancements in one. They no longer had to cut two different cylinders, the same one would go on all engines. These cylinders all had the same porting, which was developed based on Tee Dee porting with slight differences, and were arguably superior at feeding the combustion chamber. These new cylinders also appeared with slit exhaust for safety reasons. That's all the slit was for, to prevent morons from getting burned. Some power can be gained by removing the center bar of the slit.

But throw a short piston on any engine with a new type(slit exhaust) cylinder and you have the benefit of both SPI and a lighter rotating assembly. A formidable gain. Maybe. Some people (me) would argue that a lighter piston will deliver higher RPM but at the cost of a reduction in torque. Commence arguing. But whether or not lightening was just a side benefit of a new convenient method of achieving SPI, now they can say, "Hey boys, this high performance piston is lightened!!!" As for me, high RPMs are great on Youtube, but I like torque for my aerobatics. I use Tee Dees anyway. But on Bernie's site you will see that the piston/cylinder set in your second link, the Tee Dee set, is described as their most high performance set.
http://coxengines.ca/cox-.049-tee-dee-cylinder-and-piston.html
It uses a tall skirted piston(not lightened) and a cylinder with old style SPI created by the low cut exhaust port. BTW, that is why admin said putting a new short piston into a cylinder cut for SPI will give too much SPI and perform poorly.

I'd use that Tee Dee cylinder/piston set to get the best performance from any Bee. It may or may not be faster than the others, but engines are funny that way. Some run great and some are ordinary, no matter what kind of fancy enhancements you do. The one you have in the first post will certainly fly your planes nice and fast too.

happydad wrote:i thought the piston in the reference was also lightened by cutting the inside of the piston on the bottom part. that's what it looks like in the photo from the link. happydad

You're right, I think that might have started with the Killer Bee. I don't think all short pistons are lightened further by thinning the skirt.

John Goddard wrote:
I've tried many different permutations of cylinders and pistons.
One of the fastest combo's I ever achieved was one of Bernies lightened
Pistons in a TeeDee cylinder and Td 049 bottom end.
It lasted 4 runs......

Just in case the above has been misconstrued........

It wasn't a attack on Bernie, his products or his service,
All of which I believe are beyond reproach.
Which is why I've spent not an inconsiderable
Amount with him and will continue to do so.
Rather, some of his bits which whilst not made to be run
Together can be (ab)used to get silly RPM's
But not unsurprisingly go bang (piston) when pushed too far.

Thanks, Bernie, for supplying us with so many things to play with.

I probably have more Cox engines on hand than most members here.

That said I DO find that some perform better than others right from the start and without modification. Quirky things they are. But most important is that they be properly cleaned, maintained and kept in that condition.

In storage they need to receive an initial few drops of "after-run" oil and be kept each in it's own plastic bag. Periodically they should be removed from the bag, a prop attached and then "flipped" over a few times to keep the lube adequately distributed.

NOT doing this leads to "frozen" engines and resultant problems.

Using an SPI piston in a TD cylinder will actually reduce RPM (too much SPI) but we will again test such a combo by adding timing shims (reducing SPI) and using our head conversion that over-compresses the engine.

I would be curious what the results of this test are.