airraptor

My Feedback: (66)

Qwk I have a few of these NIB and i will check this week for you.

1QwkSport2.5r

Thanks man. It's probably not designed into the engine - an unintended consequence of you will. Engines designed with a throttle and muffler shouldn't have SPI. This is something we'd see on a Cox TeeDee without a muffler.


Interesting to to see if many have it or only a few. The odd thing is how far apart these two examples I have were manufactured. The o-ring .20 dates to roughly 1986.

Mr Cox

I don't think that the Cox TD is much go by, they have a very different porting.

One of the best engines in the small class is the Norvel .074 RC, the power and throttling are in a class of their own. And guess what, it has SPI, muffler and an RC carb.

1QwkSport2.5r

There are several replies in this thread: http://www.rcuniverse.com/forum/glow...induction.html that coincides with what I've read in regard to SPI. Engines with it run okay with a muffler, but don't idle well typically, and the real gain is in peak rpm and only with certain props. The consequence on an muffler RC engine is ingesting contaminated air (exhaust). Whether other engines have SPI and a well sealing muffler doesn't matter - the concept doesn't limit SPI engines to mufflers or not, there just is no gain to be had using a muffler on an engine with SPI. In the case of the Sportster engines - the amount of SPI is incredibly minute, so I doubt there are any performance consequences to it, but perhaps if they were timed higher they may gain some power on top. Otherwise it seems the .20 Sportster isn't as nice throttling and idling as the .28 is, but that could be partly due to different break-ins or just runtime of the engine.

airraptor

My Feedback: (66)

Sorry qwk I havent had time to dig the engines out yet. I have a swap meet this weekend and for the first time I will be selling some stuff instead of buying lol so I have been working in garage all day.

1QwkSport2.5r

No worries. Thanks for keeping it in mind. I'm gonna sell a few things too to finance other projects. I just don't have much worth anything and the stuff worth anything isn't for sale no way no how... LoL.

Mr Cox

That's just an assumption, in real life the last thing that comes out of the cylinder, before the exhaust port closes, will be parts of the new charge of air and fuel that was meant to stay inside the cylinder. This is just wasted on a regular engine, but can be re-entered in engines with SPI or by using a tuned pipe etc.

In your case I guess the SPI opening is too small to have much of an effect, neither positive or negative. The throttling around idle can be affected, but mainly in terms of the needed amount of carb opening, on some engines this can be really small.

1QwkSport2.5r

Here's a pointless experiment I did this morning involving swapping .20 and .28 parts to make either a .22 stroker or a .25 non stroker. .20 P/L/R on a .28 case/crank makes a undersquare stroker and a .28 P/L/R on a .20 case makes a .25 with a huge head clearance.

https://youtu.be/Y7zMLYDfX0Q
Im not very good with making videos - my camera isn't that great and I don't bother editing my videos, so don't be a hater.

airraptor

My Feedback: (66)

ok I have two NIB .19s, .20 sportsters and two .28s
The only one to have SPI is the 20s I didnt measure it but looks around .003-.004.

airraptor

My Feedback: (66)

You should try both of those swaps in your video. get some aluminum sheet from hardware store and make a shim of sort between the cylinder and crank case. doesnt have to be pretty for now but would be interesting to see what it would do. long stroke versus short stroke kind of thing

1QwkSport2.5r

I am not sure my .20s have even that much - it's very very little.

1QwkSport2.5r

I very well may do that. The huge head clearance with the .28 cylinder on .20 case may very well lend to using a lot of nitro due to the lower CR, but I don't know how it will affect power. I have some stock I can use to make a shim for the .20 cylinder on the .28 case. I'll have to do a little digging to find it... My only concern with shimming the cylinder up is how much it's going to affect the port timing.

airraptor

My Feedback: (66)

Both of what you said is true but could be worth the try. maybe no head gasket if there is one on the 28 cylinder? and if dont care could also machine the case down on the 20 to get squish better. Cool either way that you are playing around with them.

1QwkSport2.5r

I'll mess around and see what I come up with. It's starting to cool off up here, so the experiment might wait. Got a lot of stuff to get done around the house before the frozen white crap starts to fly.

airraptor

My Feedback: (66)

yeah frozen pipes are not good. On mine the SPI might be less than what I said just didnt want to take a new one apart. might sell them one day

1QwkSport2.5r

Either of your .20s have any literature eluding to a .25 "hop-up" kit for the .20 sportster? Old K&B PN: 5673, new Mecoa PN:33-5603. Trying to find out if the "hop-up" kit was just a .28 piston and cylinder that you bolt onto a .20 bottom end - essentially what I did today, but the only question is whether they had a different rod in the hop-up kit or not - the .20 & .28 use the same Rod otherwise, but the piston is way below deck at TDC as I noted earlier.

1QwkSport2.5r

FWIW, here are some notes I just made...

To add info to my parts swap experiment from earlier -


.28 piston/cylinder on .20 case/crank - piston crown .049" below deck at TDC and .030" below bottom of exhaust port at BDC.


.28 stock engine - piston crown .016" below deck at TDC and .030" below bottom of exhaust port at BDC. Head shim .010".


.20 stock engine - crown .012" below deck at TDC and .022" above the bottom of the exhaust port.

.20 piston/cylinder on .28 case/crank - crown .021" above deck and flush with bottom of exhaust port at BDC. To remove SPI, cylinder would need to be shimmed .042" higher.

airraptor

My Feedback: (66)

none of the engines I have mention it. I do know neither one of the engines made much power but ran pretty good

1QwkSport2.5r

They weren't setup to scream, so of course they didn't make much power. They do make good diesel conversions though with the .28 being a bit more stout than the .20. I think I got 11,300 from a .28 and 10,800 from a .20 (the best readings I've gotten so far) using a 10x6 APC and Davis diesel conversion heads running my own homebrew fuel.

airraptor

My Feedback: (66)

that is pretty good as the manual said 9x4 wood prop around 13,000

1QwkSport2.5r

Well, the base prop is a 9x4, but whether they turn it at 13k is another story. My Sportster experience has been they seem happiest below 12k. I should try a 9x4 and see what I get from my little ones. I haven't run a .45 yet - I have a new one and a used one, though the used one might need a rod. Getting parts from Mecoa is not something I'm very interested in though - I cannot stand the idea of doing business with Randy Linsalato. He accused me of being a thief from the US Government.

FWIW - my .65 Sportster turns a 13x6 at over 11k and 12x6 at 11,550 (APC props and 5/22 fuel). Not bad in my book (especially since both of my .65s were free).

airraptor

My Feedback: (66)

Yeah I think those are good numbers for a 60 size engine. I dont know about meccoa as I never ordered anything from them

1QwkSport2.5r

Many have posted about ordering and not receiving the order or having it take weeks to months for the order to arrive. I personally think Randy's attitude/customer service is for the birds. I'd rather buy stuff off the secondary market than buy from Mecoa. I'd go so far as to make my own connecting rods before ordering an $8 Rod from him. Just my take. FWIW, all of my K&B engines were produced when K&B was still K&B, and not Mecoa.

airraptor

My Feedback: (66)

so are mine still have stickers in them and all. Even has the price list of all the parts like piston and sleeve for 8 bucks

1QwkSport2.5r

Mine too. I don't have paperwork for the .65s, but they came with the MIG-27 drones, so they were packed with the planes.