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Engine basics => Engine set up tips => Topic started by: ash on February 18, 2008, 02:36:04 PM
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I know some ST46s had it and some Jett engines have it (including ROJetts? I don't know...).
Is sub piston induction really a killer on muffled stunt engines?
From what I read all the downsides don't really matter to us anyway; throttle control, peak power etc...
Reason I ask is that I have an engine or two that could benefit from having the liner dropped and the ports tweaked. Problem on both is that the piston is already trying to lift its skirt over the bottom of the exhaust port, so any liner drop is going to result in SPI. A new liner is only the cost of a few good glowplugs, so I might do some experimenting for the hell of it, but I could use some opinions from people who know in the meantime!
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HI Ash
You do not want to have any sub piston induction if possible, You certainly do not want to add more to an engine that is just getting a small amount.
You will be much better off not doing this if you ever intend to run mufflers
Regards
Randy
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George Aldrich showed me an engine (don't remember what kind) which had subpiston induction. He told me he had run it in stock condition (no SPI) with a muffler. Then cut the bottom of the piston to give SPI. Ran it with muffler again and found no difference in performance. George was thus of the opinion that SPI was no problem with a muffler. I have no personal opinion on the matter.
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Years ago I reworked a 40 clone of the OS... Lowered the sleeve a bit to help the timing. I t had some sub port Induction. ran fine on the bench and in the plane (profile) with no mufler... with the tongue muffler it shut off like a switch when it went inverted... shimmed the sleeve back up and the problem went away... to echo Randy's comment... IMHO mufflers and spi are not good!!! n1 and as they say, your mileage may vary...
Thanks
Jim
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Thanks for that, Jim. Its always handy to know what actually happens when you add SPI+muffler.
Anyone else have any consequence stories of SPI+muffler?
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Thanks for that, Jim. Its always handy to know what actually happens when you add SPI+muffler.
Anyone else have any consequence stories of SPI+muffler?
HI as I stated above it's not a good idea to have much or any Sub port induction if your going to use on a sport or stunt plane.
I have personally tried this on about 6 motors and *all* of them had either lower power and / or, erratic running.
So if you feel your going to do this please let us know how your experience went
Regards
Randy
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I'm unlikely to do it, just needed the reasons why I should not bother trying it.
One of the motors (GMS76) has all the usual tricks applied now, just awaiting tests of the blowdown changes to see how that goes. I'll get a new liner made rather than drop the current one. Maybe after I have the ideal timed liner on hand I'll try the original liner dropped with SPI, but not before then.
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About 11 years ago I sold a SV-11 with a ST/56 in it and the large ST Muffler on it. The guy who bought it put a stub muffler on it and had trouble with it for a year. He then sold it to another friend and he put a tube muffler on it and it ran fine.Some of the ST/.56's have a lot of sub port opening. I now have that motor back after all these years and I have a tube muffler to go on it.I have never seen a single ring ST/46 with it but many of the ST/40 have it.
Ed
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Hmmm, interesting... I was contemplating last night after reading Ed's story and it occurred to me that the cutout during hard inside corners I was experiencing for a short time on the GMS76 began after shortening the stock muffler to save weight. Could be that the tiny amount of existing SPI (5 thou or less) and the short muffler were enough to upset it. I've changed alot since then, so I'm not sure if it was another change to the muffler or the much lower compression that helped it back to normal operations. Its running super great now, so the chances of further modification are slim. y1
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I have a feeling that spi is probably a more complex situation than we realize... In that many factors are involved in the running charactistics of the motor. I think it is safe to say, as I (and Randy) did, that spi should be avoided. Having said that, there are probably instances where motors with spi and tongue mufflers work great!!
I've seen several combinations that shouldn't work, that DO work HB~> HB~> HB~> and I've had some that should but won't ... HB~> HB~>
Keeps it interesting!!
Thanks
Jim
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From what I am reading here my STG21 40 that has SPI from the factory at least I guess thats how it came as I bought it used will not work with a muffler
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From what I am reading here my STG21 40 that has SPI from the factory at least I guess thats how it came as I bought it used will not work with a muffler
Hi Richard
Some 40s have spi in the 2 bottom corners others have it across the bottom, they will run with mufflers, its just better to not have it, if you have a choice
Randy
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Some years ago, I got into a discussion regarding sub-piston induction. The argument at the time was that this eliminated any residual crankcase vacuum just before the piston started compressing the crankcase volume..sort of-starting at zero pressure instead of having to first overcome any negative crankcase pressure.
Well, it was an easy experiment with my Orwick 64 spark ignition engine, which has spi by means of four 1/8" holes drilled through the case and liner. These are exposed by the piston at or near TDC. I got a tach reading with the holes open, and then taped the holes closed without touching any other adjustments. There was no RPM difference that I could detect.
Floyd in OR
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Some years ago, I got into a discussion regarding sub-piston induction. The argument at the time was that this eliminated any residual crankcase vacuum just before the piston started compressing the crankcase volume..sort of-starting at zero pressure instead of having to first overcome any negative crankcase pressure.
Well, it was an easy experiment with my Orwick 64 spark ignition engine, which has spi by means of four 1/8" holes drilled through the case and liner. These are exposed by the piston at or near TDC. I got a tach reading with the holes open, and then taped the holes closed without touching any other adjustments. There was no RPM difference that I could detect.
Floyd in OR
Floyd did you have a muffler on this engine?
Regards
Randy
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Randy; I have often wondered about this. On Anderson Spitfires, Super Cykes, and Orwicks, they use holes drilled in the case below the piston, and the piston does not uncover the exhaust port. Would these not work OK with a muffler as it would not be trying to stuff the exhaust gas back in to the case below the piston through the exhaust port.
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I recently ran a test on sub piston induction vs no sub induction. On a 15 Top F2D motor about all the sub induction did was knock down the peak rpm by about a thousand. Otherwise the motor ran exactly the same. Since the exhaust dilutes the incoming charge you'd expect it to reduce combustion temperatures and peak pressures and horsepower, just like Exhaust Gas Recirculation does in cars.
Obviously you want to avoid it if possible, but having a few thousandths of clearance for sub induction is probably not going to make a huge difference in how the engine runs. The only way to find out though is to try it.
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Randy; I have often wondered about this. On Anderson Spitfires, Super Cykes, and Orwicks, they use holes drilled in the case below the piston, and the piston does not uncover the exhaust port. Would these not work OK with a muffler as it would not be trying to stuff the exhaust gas back in to the case below the piston through the exhaust port.
HI Jim
What seems to be happening is ...with mufflers installed...the hot exhaust gases that are forced into the bottom of the case ,affect the run, and kills off some of the power, if the holes do not force hot gases back into the mix, it should not be any problem
Regards
Randy
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Thanks Randy; I know some of the guys are running mufflers on their sparkers, and they do seem to run OK. And there again, they have all been engines with the induction holes below the exhaust port in the front and back of the case and into the sleeve. The exhaust port is not uncovered by the bottom of the piston. I have thought about trying a muffler on one of my sparkers, but I think I would run low nitro fuel instead of gasoline, as it runs much cooler. Still love the sounds they make with open exhaust though. It just can not be duplicated. Kind of like a knuckle head Harley.