Author Topic: Cox Sub Piston Induction (Read 3651 times)

pat king · « **on:** November 24, 2012, 08:40:26 AM »

What is the optimum or normal distance on Cox engines from the bottom of the exhaust port to the bottom of the piston at TDC for SPI? I am going to set up the mill and cut out some slit exhaust sleeves to open port. I thought I would drop the bottom of the port while I am at it to make them SPI. Most are dual bypass sleeves, a couple have single boost ports. I have some 3/32" carbide end mills and some 1/8" carbide end mills, so once I get going I'll chew on a bunch of them. I don't want to keep cutting and trying on the dyno to see where the bottom of the port should be. All else fails I'll use a feeler gauge to check a couple of old stock SPI Sleeve/Piston assemblies I have.

Thanks, Pat

john e. holliday · « **Reply #1 on:** November 24, 2012, 08:43:53 AM »

Maybe I have a fluke of a Cox engine with the little slit exhaust. It is out performing all my other open exhaust Cox's. Gives good needle and economy as well as speed with a 6X3 nylon Top Flite prop.

pat king · « **Reply #2 on:** November 24, 2012, 09:10:56 AM »

I think all the slit exhaust sleeves are dual bypass. My early Babe Bee engines were all single bypass. At least one of those had SPI. The slit exhaust should make for a more manageable engine due to the exhaust restriction. I am looking to extract more of the power potential out of the engines. I will eventually be assembling a race engine for Skyray team racing we do at our 1/2A meet every year. The engines must be reed valve with an 8cc integral tank. I have two of the modified Piston/Sleeve assemblies on the way from Cox International.

Pat

kenneth cook · « **Reply #3 on:** November 24, 2012, 09:44:35 AM »

Hello Pat, I do have slit Cox cylinders that are indeed single bypass. These engines were from the 80's Babe bee. As for the SPI on the cylinders, some of these that were available from suppliers had too much SPI. I indeed purchased some of these. I recall when setting up Tee Dee's using the Kirn Kraft under the cylinder shims, You only needed a bit of light under the cylinder for high performance. Too much causes hard starting and really doesn't work well. I also think that rather than lower the exhaust cutout and run the risk of burrs in the cylinder, shortening the piston skirt would be ideal. You essentially would be doing the same but also lightening the weight of the piston. This also doesn't remove the bluing from the cylinder. I never have done this so it's just an observation. Good luck, Ken

pat king · « **Reply #4 on:** November 24, 2012, 12:28:48 PM »

I have slit exhaust cylinders that are single bypass and dual bypass. I will be opening some up to get rid of the exhaust restriction. Since I will have the cylinder in the vice I can give it a touch of SPI. I am well aware that too much SPI can hurt performance. If anything, I'll err on the side of too little. I hope that someone can tell me how much is optimum. I will remove the burrs with a fine stone. If I really am concerned about the blueing, I can re-blue them. This is an exercise in looking for a modest improvement in performance of Cox reed valve engines. If I want to go for maximum power I would make a small modification then run the engine to see how much it helps or hurts performance. I would think that an engine can be optimized for power output, but suffer all the problems of any super trick racing engine.

Pat

Tim Wescott · « **Reply #5 on:** November 24, 2012, 01:44:01 PM »

I haven't done much engine modification of any kind, but everything I've heard, and what little I've done, says that you're making a tradeoff between cost, ease of handling, reliability, and performance.

So I don't think there is a universal compromise between ease of handling (i.e. starting) and performance -- only what works best for you.

Ken's comment about Kirn Kraft shims sure resonates for me -- why don't you cut the exhaust ports so that you have no more, and possibly less, SPI than a "regular" Cox reedie, then make up some shims and start experimenting? If you're going to go that way, make sure to start out with high compression heads and head shims, too, because shimming the bottom end is going to lower the compression at the same time that it makes the engine want to go faster, so you'll need to shim the head to match whatever you just did to the cylinder timing.

pat king · « **Reply #6 on:** November 24, 2012, 02:57:13 PM »

I haven't done any modifications to any glow engines. In 1964 I built and raced a 301 cubic inch V8 that produced 415 HP at 8900 RPM, naturally aspirated on pump gas. That engine had the problems of all significantly modified engines, idle was at 3000 RPM, it wouldn't pull below about 5000 RPM.
I am not looking to modify cylinders for maximum power, it is mainly that I am offended by what I believe was a method of cutting down exhaust noise. Engine design changes that reduce efficiency to achieve a political result suck! I realize that we are extremely discriminated against because of the exhaust note of our engines.
I just received a pair of assemblies from Cox international. They differ by .003" . I will use the lesser of the two to set my maximum gap between the bottom of the piston and the bottom of the exhaust port at TDC. To do this right I will measure deck height, piston height, and top of sleeve to the shoulder on the sleeve. That way the SPI distance can be cut to match the set of parts and minimize variation due to part tolerances.

Pat

Tim Wescott · « **Reply #7 on:** November 24, 2012, 04:42:19 PM »

Just as a check you might want to measure the transfer and exhaust opening time for an open-exhaust cylinder vs. a slit type -- there's no reason that they should be the same.

AFAIK the slit-type exhaust ports were made to make the engines less likely to belch fire, and to make them more amenable both to mufflers and to exhaust throttles - sub-piston induction through the exhaust port only works well if there's fresh air available when the piston clears the port going up, after all.

kenneth cook · « **Reply #8 on:** November 24, 2012, 06:21:35 PM »

I found my instructions for the Kirn Kraft shims. Basically, there was vast differences in the stroke of these engines. It wasn't uncommon for the crown of the pistons to come up above the landing for the glow plug. To combat this problem, various shims of different thickness were added to raise the cylinder and optimize the deck height. It's been noted that higher performance numbers were gained by the alignment of the exhaust port facing forward. I believe this was more suitable for the fuel entering the internal bypass. The instructions however stated, that optimum performance was gained by retaining .012" sub induction with the piston ATDC. Ken

pat king · « **Reply #9 on:** November 25, 2012, 05:57:06 AM »

Tim,
It is understandable about fresh air being available, otherwise we have the infamous EGR setup.

Ken,
Thanks for the .012 dimension. I doubt I will go that far. When I do the race engine hand starting is as important as airplane speed. The fastest airplane is not much good if it can't be started quickly during the pit stop.

Jim Thomerson · « **Reply #10 on:** November 25, 2012, 10:50:18 AM »

I have seen slit exhaust port engines where the piston never opened to bottom slit at bottom dead center, but partially cleared the bottom slit at top dead center. I wondered if this was the way it was supposed to be.

pat king · « **Reply #11 on:** November 25, 2012, 11:12:43 AM »

That combination has SPI, but very restricted exhaust. It most likely is due to poor dimensional control on the parts. The SPI will probably do no good with the severely restricted exhaust. It should be a real dog with the Cox muffler or throttle ring forcing exhaust gasses into the crankcase while the exhaust gasses can not get out of the chamber.
In full scale automotive engines the most cost effective performance improvement is obtained by reducing exhaust back pressure.

Pat

Andrew Tinsley · « **Reply #12 on:** November 25, 2012, 12:41:36 PM »

Hello Pat,
An awful lot of people think that the dual slit exhaust is very restrictive. I am not sure that they are correct. I have done some mild tuning with Coxes (enough to use the diesel type crank). My best performer had a Surestart dual slit piston cylinder, better in fact than a well fitted TD piston /cylinder! Worrying about the dual slit cylinder is one of the last things I would bother about in the tuning sequence!
I would defer to anything that Ken advises, he knows his tuning better than most.

Regards,

Andrew.

Ron Cribbs · « **Reply #13 on:** November 25, 2012, 08:32:21 PM »

.012-.025 is the SPI range. They are all different. Keep in mind if you reset your ball sockets then your SPI gap can decrease somewhat. I would make sure you ball socket is reset prior to making any measurements or cuts.

Ron

pat king · « **Reply #14 on:** November 25, 2012, 09:16:30 PM »

Ron,
Thanks for the input. I will not take any dimensions until I have the con rod to piston fit right. The two new sets I just received need the rod to piston fit adjusted. I feel that the first thing that should be done with a Cox assembly with the ball/socket small end is to adjust the fit.
I will be checking the performance and hot restart with various SPI dimensions before settling on a piston/sleeve assembly for the race engine. A very fast engine that will not hand start with one or two flips will not win the race. I can not give up any starting reliability for more speed. For the sport engines I will be looking to optimize the power output. They can be started with the spring or a starter.

Pat

Paul Smith · « **Reply #15 on:** November 26, 2012, 03:44:35 PM »

I don't like SPI because I feel that it might detract from suction and weaken fuel draw.

I feel Cox's biggest mistake was devoting too much of the circle to exhaust and not enough to intake. Compare the intake/exhaust ratio of a Cox to any other engine, especially the excellent OS products. On the OS you have three intake ports and one small exhaust port. On a Cox you two gapping exhaust slots and one or two meager in take bypasses.

Question: What happens when you change your Cox cylinder for one to two baypasses?
Answer: Power goes through the roof.

Question: What happens when you open up the exhaust more?
Answer: Nothing good. It was already three times too big.

If the new owner of Thimble Drome offered a cylinder with three intakes and one exhaust I would be willing to buy and try a few. No sub port induction please.

dennis lipsett · « **Reply #16 on:** November 26, 2012, 06:53:31 PM »

There was a time when you could get uncut cylinders from Cox.Perhaps if Bernie has some blanks he'd consider selling them to the experimenters in the group.

Dennis

kenneth cook · « **Reply #17 on:** November 28, 2012, 02:34:26 AM »

I feel Paul is absolutely spot on with his theory. I have a Tee Dee which is a rear exhaust designed for a tuned pipe. The cylinder has three bypass ports and one small exhaust port. Aside from that, it also has been plated. It certainly shows a lot of promise, but I have never been able to locate a pipe. The pipe I have now is from a CS GZ .049 and doesn't fit without using a large silicone adapter. I just don't have the patience to experiment with pipe lengths. It does look neat sitting on the shelf however. Ken

john e. holliday · « **Reply #18 on:** November 28, 2012, 08:35:07 AM »

Maybe Mr Cox, Larry Rengar will back me up on this. The Meriwethers had obtained one of the proto type Tee Dee .15 for A speed. It had one exhaust port and several intake ports. Sent it back to Dale Kirn after Pappy Meriwether passed on and I got it with all the other CL related stuff the kid was going to put in the dumpster. When the Tee Dee did hit the market it had two exhaust ports and only two intake ports. I still have my Tee Dee that was put on the market. Don't know why they changed from the proto type as it was so much more of an engine as far as power.

Larry Renger · « **Reply #19 on:** November 28, 2012, 10:59:04 AM »

The 3 bypass, 1 exhaust Tee Dee .15 was produced and sold. It did not go as fast as the ones before or after it. Nice collectors' piece, though.

Incidentally, it vibrated enough that at least one free-flight caught fire on the climb as the wing/fuselage friction set it off.

There was no thought of pipes, as this was too early for them to be common (if done at all then).

Tim Wescott · « **Reply #20 on:** November 28, 2012, 11:20:15 AM »

If you're good enough to cut bypasses and exhaust ports in a blank cylinder, you're probably good enough to machine the whole thing.

Larry, do you know if Cox ever tried an engine with two bigger bypasses, and two smaller exhaust ports?

Larry Renger · « **Reply #21 on:** November 30, 2012, 08:54:44 AM »

No, I don't. I really wasn't involved with development of the Tee Dees except for instituting the Killer Bee bypass porting at the end.

don Burke · « **Reply #22 on:** February 25, 2013, 08:47:59 AM »

Quote from: Paul Smith on November 26, 2012, 03:44:35 PM

I feel Cox's biggest mistake was devoting too much of the circle to exhaust and not enough to intake.

The COX design is classic, original SCHNEURLE design from the 1930s

Reptoid · « **Reply #23 on:** February 25, 2013, 02:45:40 PM »

Quote from: don Burke on February 25, 2013, 08:47:59 AM

The COX design is classic, original SCHNEURLE design from the 1930s

Not quite correct

. Here's a quote out of the Two Stroke performance hand book that is confirmed by numerous sources:
The Schneurle's loop-scavenging method, patented in 1925, employed a single exhaust port flanked by two small scavenge or transfer ports, whose air streams were aimed to converge on the cylinder wall opposite the exhaust. Being aimed away from the exhaust, the transfer streams had a natural resistance to short-circuiting straight out the exhaust. Earlier designs had used deflector-dome pistons to keep the fuel/air mixture from going directly out the exhaust port.
By the way; the third bypass port or "boost" port was proven and made popular by a designer @ DKW motorcycle Co. in 1957.