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Chinese tachs
by: geh3333 - Jun 29, 2015 23:31:52 GMT -5
via mobile
Post by geh3333 on Jun 29, 2015 23:31:52 GMT -5
Remember, these are 4-strokes so 2 revolutions of the crank = 1 complete cycle - 1 spark (and 4 revs of the crank = 2 sparks). The tachs I have are both non-adjustable, made for 4-stroke engines. At best I can get to about 8k rpm's. Trust me - that little COX-airplane engine is NOT going 16K (and also is not only going 4 K). Actually these scoots fire on every revolution. They fire at tdc each time it hits Tdc, but there is only ignition on every other spark. So every full revolution down then up , the plug fires. So every other spark is wasted. |
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Chinese tachs
by: geh3333 - Jun 29, 2015 23:37:46 GMT -5
via mobile
Post by geh3333 on Jun 29, 2015 23:37:46 GMT -5
Remember, these are 4-strokes so 2 revolutions of the crank = 1 complete cycle - 1 spark (and 4 revs of the crank = 2 sparks). The tachs I have are both non-adjustable, made for 4-stroke engines. At best I can get to about 8k rpm's. Trust me - that little COX-airplane engine is NOT going 16K (and also is not only going 4 K). Actually these scoots fire on every revolution. They fire at tdc each time it hits Tdc, but there is only ignition on every other spark. So every full revolution down then up , the plug fires. So every other spark is wasted. Each cycle is 4 strokes or 2 rotations, but every rotation the plug sparks. So 2 sparks per cycle. Once on the fuel compression stroke , and one as the exhaust valve opens to let out the gasses. The second is the wasted spark. |
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Chinese tachs
by: geh3333 - Jun 29, 2015 23:49:51 GMT -5
via mobile
Post by geh3333 on Jun 29, 2015 23:49:51 GMT -5
Here is another major issue. A 2 stroke tach is set for an every rotation fire. Many think these 4 strokes only fire every other revolution , however this is false. These 4 strokes fire every revolution just like a 2 stroke. So if our gauges are set to read every ignition fire instead of every true fire , then they are doubling the rpms to get the correct rpm. However by doing this they are doubling the true rpm.
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Chinese tachs
by: JerryScript - Jun 30, 2015 0:31:34 GMT -5
via mobile
Post by JerryScript on Jun 30, 2015 0:31:34 GMT -5
Are the tachs being discussed stock or after-market?
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Chinese tachs
by: geh3333 - Jun 30, 2015 0:45:43 GMT -5
via mobile
Post by geh3333 on Jun 30, 2015 0:45:43 GMT -5
Are the tachs being discussed stock or after-market? Mainly stock , because the aftermarket TACHS are usually adjustable. These engines do fire every rotation right? I'm just trying to free my mind here. I'm stuck on this now , lol. I'm probably just thinking to much , but the rpms at idle just dont really seem as high as what the tach says. |
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Post by JerryScript on Jun 30, 2015 1:01:48 GMT -5
Hook up a tiny tach and compare. Some circuits may be more accurate than others.
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Post by rcq92130 on Jun 30, 2015 1:22:18 GMT -5
Actually these scoots fire on every revolution. They fire at tdc each time it hits Tdc, but there is only ignition on every other spark. So every full revolution down then up , the plug fires. So every other spark is wasted. Each cycle is 4 strokes or 2 rotations, but every rotation the plug sparks. So 2 sparks per cycle. Once on the fuel compression stroke , and one as the exhaust valve opens to let out the gasses. The second is the wasted spark. Oh, that's right. Pickup on the flywheel (not valve train, and no gear reduction); no computer. Wonder what that does to the valves. |
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Post by rcq92130 on Jun 30, 2015 1:23:53 GMT -5
Here is another major issue. A 2 stroke tach is set for an every rotation fire. Many think these 4 strokes only fire every other revolution , however this is false. These 4 strokes fire every revolution just like a 2 stroke. So if our gauges are set to read every ignition fire instead of every true fire , then they are doubling the rpms to get the correct rpm. However by doing this they are doubling the true rpm. Not possible the tachs are doubling the actual engine speed .... otherwise they would be showing 15K or 16K at WOT. |
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Chinese tachs
by: geh3333 - Jun 30, 2015 2:02:38 GMT -5
via mobile
Post by geh3333 on Jun 30, 2015 2:02:38 GMT -5
Here is another major issue. A 2 stroke tach is set for an every rotation fire. Many think these 4 strokes only fire every other revolution , however this is false. These 4 strokes fire every revolution just like a 2 stroke. So if our gauges are set to read every ignition fire instead of every true fire , then they are doubling the rpms to get the correct rpm. However by doing this they are doubling the true rpm. Not possible the tachs are doubling the actual engine speed .... otherwise they would be showing 15K or 16K at WOT. Unless the actually rpms are 1/2 what they are reading. I mean if the engine is actually idling at 700- 850 rpms . then the stock TACHS double that. I know its a long shot but if I'm reading the exhaust right , hats what's happening. |
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Chinese tachs
by: geh3333 - Jun 30, 2015 2:08:44 GMT -5
via mobile
Post by geh3333 on Jun 30, 2015 2:08:44 GMT -5
For instance let's say these TACHS are reading every spark as if the engine was only firing on the compression stroke. The tach would have to be set to read that and double it , in order to give off the right reading. Now let's say the TACHS are set up that way . now these engines are actually sparking every rotation , so the tach would be doubling both sparks and giving off a double reading. If I'm feeling and hearing the exhaust right , then these engines are idling between 600 - 900 rpms when the tach is saying 13-1800 rpms. I know I could be wrong , but from what I've seen , I need to find out for sure.
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Chinese tachs
by: geh3333 - Jun 30, 2015 2:17:38 GMT -5
via mobile
Post by geh3333 on Jun 30, 2015 2:17:38 GMT -5
What I'd like someone to do is use their digital tach , and read the instructions to find out what setting is for what . the .5 setting would read half of the true idle. The 1 setting would read a straight forward accurate rpm. And the 2 setting would double the rpms. Find out for sure what the setting s do and test on each setting. I'm sire most install the digital tach and match it to the specified idle. Which I'm indicating may be wrong. Please remember I'm not at all certain or saying what I am suggesting is a fact . I'm just in a spot where I need to know for sure. Also we need to know for sure if the TACHS being used for the tests are for these types of 4 strokes that fire every rotation.
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Chinese tachs
by: geh3333 - Jun 30, 2015 3:55:41 GMT -5
via mobile
Post by geh3333 on Jun 30, 2015 3:55:41 GMT -5
I'd like everyone to read this . this is true , but yet it is also false. Because there are two different types of ignition systems used by four stroke engines. www.mobiusinstitute.com/site2/item.asp?LinkID=10013&iVibe=1&sTitle=Analysis%20Definitions. Now let's say when installing these TACHS , the Chinese and others are going by this method. This means the TACHS installed will read twice the true rpms. I still find it very hard to believe that when these engines are near stall , they are still idling at 1200-1400 or more rpms! You can actually hear each exhaust stroke clearly during this time and it doesn't seem even close to those rpms. |
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Post by pistonguy on Jun 30, 2015 9:21:31 GMT -5
I installed one of little Digi Tachs I got for fee at a trade show.
Shows Hours, RPM's, and on shut down shows max RPM for a few seconds.
Now adjustability on the Digi, Just wrap the lead around the spark wire.
The Digi indicates 200-300 RPM lower than Analog at idle (1,900rpm)
Gets progressively worse at high RPM"S, Digi indicates 500-600 Lower than Analog over 7,000 rpm.
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Post by rcq92130 on Jun 30, 2015 9:37:50 GMT -5
I'd like everyone to read this . this is true , but yet it is also false. Because there are two different types of ignition systems used by four stroke engines. www.mobiusinstitute.com/site2/item.asp?LinkID=10013&iVibe=1&sTitle=Analysis%20Definitions. Now let's say when installing these TACHS , the Chinese and others are going by this method. This means the TACHS installed will read twice the true rpms. I still find it very hard to believe that when these engines are near stall , they are still idling at 1200-1400 or more rpms! You can actually hear each exhaust stroke clearly during this time and it doesn't seem even close to those rpms. George - are you thinking when yuo are running WOT, and your tach is showing 7,500 ... the engine is really spinning at 15,000? |
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Chinese tachs
by: geh3333 - Jun 30, 2015 19:16:57 GMT -5
via mobile
Post by geh3333 on Jun 30, 2015 19:16:57 GMT -5
I'd like everyone to read this . this is true , but yet it is also false. Because there are two different types of ignition systems used by four stroke engines. www.mobiusinstitute.com/site2/item.asp?LinkID=10013&iVibe=1&sTitle=Analysis%20Definitions. Now let's say when installing these TACHS , the Chinese and others are going by this method. This means the TACHS installed will read twice the true rpms. I still find it very hard to believe that when these engines are near stall , they are still idling at 1200-1400 or more rpms! You can actually hear each exhaust stroke clearly during this time and it doesn't seem even close to those rpms. George - are you thinking when yuo are running WOT, and your tach is showing 7,500 ... the engine is really spinning at 15,000? Actually the opposite , when the tachnis showing 7000 rpms the engine is really at 3500 rpms. This would be closer to a cars rpms . I'll be satisfied if someone does a test with their digital speedo according to the directions. |
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