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Post by JA-Moo on Nov 13, 2015 12:42:42 GMT -5
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Post by Walms on Nov 13, 2015 14:32:49 GMT -5
Interesting, I wonder if the Saab is liquid cooled and not subject to the head temp variations we get on the triple? EGT's would be a nice addition tho!  |
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Post by Walms on Nov 13, 2015 14:35:31 GMT -5
A frugal guy would say I would only need 1 EGT sensor if all head temps are monitored.
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Post by JA-Moo on Nov 13, 2015 15:20:49 GMT -5
Actually, no you can fry a motor without the htg showing a problem. The video shows this......
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Post by Jim on Nov 13, 2015 15:47:35 GMT -5
The Saabs are liquid cooled. I don't blindly accept the guy's ideas. I've put many hours on a water cooled 2-stroke outboard motor, some of which was at full throttle, but most of which was at various mid throttle settings. Mysteriously, I never melted a piston. It's a premix engine, about 750cc and 55 HP, built in 1967, and still runs fine.
The oil, by the way, has always been additive-free SAE 30, mixed around 32:1. The manual says you can go to 50:1, but says for heavy duty use like water skiing, more oil is a good idea. That engine is one of the reasons I have never bought high priced 2-stroke oil.
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Post by shiloh on Nov 13, 2015 17:15:38 GMT -5
Wow, she must be a smokey ole thing,burning engine oil?
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Post by Jim on Nov 13, 2015 17:40:25 GMT -5
Not at all. Next to zero actually, except on cold starting. The companies that sell 2-stroke oil say some true stuff, but they also say a lot of hype, so everyone thinks he has to buy their expensive products.
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Post by JA-Moo on Nov 13, 2015 23:40:18 GMT -5
EGT tells the story. Hard traction and soft are two totally different things. A low hp motor pushing a car around up hills and such, you can see where the differ loading makes the combustion temp go through the roof and melt pistons.
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Post by Jim on Nov 13, 2015 23:51:09 GMT -5
A little research told me that melted pistons were very rare among 2-stroke Saabs.
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Post by JA-Moo on Nov 14, 2015 0:52:16 GMT -5
Well, Europeans were driving them...... wink wink.....
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Post by Walms on Nov 14, 2015 6:29:00 GMT -5
"On or off the gas" Sounds like he just doesn't have his carbs jetted correctly for part throttle. I've never heard of cruising being the most taxing thing you can do to a 2 stroke.. His video might show a decent technique for saving a motor that is running way too lean though!  |
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jyrgnorway
2nd Gear
H2B CAFE, H2B, H1B, F8 BISON, Z1000ELR, RD500
Posts: 468
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Post by jyrgnorway on Nov 14, 2015 7:50:26 GMT -5
A little research told me that melted pistons were very rare among 2-stroke Saabs. You are absolutely correct, as long as the water cooling was ok. We have a saab 2 stroke in our club, and I have owned a dkw 2 stroke myself. This video is nothing but nonsense, as the on/of throttle position has nothing to do with those old 2-stroke cars if engine system is ok. On my 34 hp 750cc triple water cooled DKW, you could run full throttle forever, or part throttle for that matter. And a liquid cooled head temp as described in this hillarious video, vs a plug temp sensor for our triples is 2 totally different things. And an EGT for our triples can fool you, as less advanced ignition timing (or higher exhaust duration) will show higher numbers on the egt due to more of the gas expansion happens in the pipe header instead of combustion chamber. (hotter ex pulse) That means, less heat is transferred to cylinder/piston. And that means, when egt shows lower values, more of the gas expansion happens inside combustion chamber, thus temp of cylinder/piston is actually higher. This is the case when engine acts as a pump, prior to powerband. BUT: When in powerband, hotter exhaust pulse as in less advanced ignition timing (higher numbers on egt) also creates hotter returning wave supercharging the cylinders, and both plug temp sensor and egt will show same symphtoms. So, when we are running in cruise speed below powerband, an egt will actually show cooler temp when the temp inside combustion chamber rises, and vice versa. At least, that is what I believe regarding 2-strokes. "Well, Europeans were driving them...... wink wink....." Well, an American was driving this Saab, and he is far off......wink wink..... So, let's bring this further: The saab on the video has not a very efficient supercharging effect at all, as it has a "NON" expansion 3:1 pipe stock. That means, the engine "works" as a foulstroke, that means as a pump at all times. So, if he retards the ignition timing, the egt will show higher values, and lower values if he advances timing. BUT: The temperature on piston crown/combustion chamber will INCREASE by running more advanced, when the egt shows LOWER values (I have explained why earlier, it has to to with how much of the gas expansion takes place in combustion chamber vs pipe) So, when running a 2-stroke in piston pumping mode, as in cruising with this saab, the higher egt value indicates a cooler temp inside combustion chamber, not hotter. (like in ignition timing changes, not in throttle positions off course......the more hp the engine produces, the hotter the ex gases) At least, that is what I think. I am sure there will be comments on this one. Best Jørgen |
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Post by Jim on Nov 14, 2015 8:59:05 GMT -5
A friend of the family who was into cars when I was a kid bought one of the later Saab 2-strokes that had a bit of power. I rode in it, and saw him drive it many miles, with no odd problems like damaged pistons. He drove it like any other car.
Jørgen, the 2-stroke outboard motor I was talking about also has no tuned exhaust. A lot of the engine power output control is done with spark advance. The boat's throttle lever actually mainly controls the points plate, and a lever and cam push on the carb's throttle butterfly shaft. Coming off of idle, the throttle cam opens the butterfly as the advance moves. Then in the midrange, the carb butterfly opens very slowly as the spark is advanced. Then at the very end, coming to wide open, the carb is quickly opened from about 1/2 open to fully open, and the spark is retarded slightly, while fuel economy drops a lot. The engine is a reed valve type. I've always wondered how a triple would respond to a similar system, using advance more than the carb butterfly to control power output in the midrange. There is no burbling, the outboard runs like a 4-stroke at all times, unless you're in neutral and rev it, when you can hear the 2-stroke "ring ding ding". I wouldn't be surprised if the Saabs used a similar system to make it run as smooth as a 4-stroke.
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Post by JA-Moo on Nov 14, 2015 13:05:22 GMT -5
If one actually looks into Mr. Donney's videos, seems he has actually done a lot of performance work with those little engines. Building a Land speed car and such. Lot of hop up vids also. So it's a bit novice to make an assumption that his video is nonsense. Also, the average "Joe" would not have this kind of instrumentation.
My original "point" was to show the comparative of reaction time of the CHT and EGT. You can literally melt a motor (air or water cooled) before there is any indication on the CHT, and the EGT will show it.
My guess, is Mr. Dunney may have been a bit too general in his statement, and with being a performance tuner, there are different parameters which effect a motor than on stock motors. Stockers are relatively mildly tuned and jetted, so they can run 1000's of miles. But as with any tuning, sometimes the tuning dictates compromises, with a little too lean or rich mixture in certain throttle positions for best performance. We all know a 2 stroke can stick at any throttle position given the correct circumstances.
Also the combustion temp pretty much is what it is, no matter where it is burning, only where it is burning the longest changes with timing changes. So it is of little matter where in the motor it occurs, max EGT temp has to be respected or pistons will melt.
And one last thing, deto actually shows a colder temp on an EGT.
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Post by pipe welder on Nov 16, 2015 17:51:54 GMT -5
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