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Wisconsin Engines

Wisconsin THD/TJD generic question

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Old 02-05-2007, 01:57:54 AM
Steve Dawkins Steve Dawkins is offline
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Default Wisconsin THD/TJD generic question

I'm aware that Wisconsin THD engines are "even fire" and the TJD models are "odd fire". On the THD's, the pistons travel up and down together whereas the TJD's are 180 degrees out of sync. What is the advantage of one style over the other for different applications? I've always been curious about this. I had a generator set that used the THD, and a Ditch Witch trencher that used the TJD. Thanks.

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Old 02-05-2007, 10:47:24 AM
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John Newman, Jr. John Newman, Jr. is offline
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Default Re: Wisconsin THD/TJD generic question

In a word: Balancing.
With both pistons moving together there has to be a substantial counterweight on the crankshaft to keep it ballanced. Even with this, it still has a tendancy to hop at certain RPM (involves harmonic frequencies which I am not qualified to attempt to explain) By using a crank with the throws 180 degrees apart, it is naturally ballanced with less crankshaft weight. Other than a loping idle, it runs much smoother at all other RPMs.
John Newman, Jr.
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Old 02-05-2007, 10:59:09 AM
Bill Geyer Bill Geyer is offline
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Default Re: Wisconsin THD/TJD generic question

Hello Steve, I don't think there is a whole lot of difference, but the THD gives a power pulse every revolution, this is beneficial for generators at a1800 rpm. The TJD has a period of no power pulse for 540 degrees, this favors operation above 1800 rpm, and is inherently balanced better for less vibration. Although the TJD is used on some 1800 rpm generators, I would think the THD would be a better choice. I think the THD production ceased well before the TJD.
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Old 02-05-2007, 11:15:28 AM
K D Redd K D Redd is offline
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Default Re: Wisconsin THD/TJD generic question

The idea was the TJD would make more HP. Turne out the increase was only about 1/2 HP. I was resently told by a Wisconsin dealer/collector who was picking up a VR4D (255 cid) a friend of mine had, that foundry who did Wisconson's casting and are now building the OHV engine used on concrete saw were starting to build THD again with better balancing. They found there was not any advantage to the 180 degree crank from the 360 degree.

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Old 02-06-2007, 12:39:05 AM
Steve Dawkins Steve Dawkins is offline
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Default Re: Wisconsin THD/TJD generic question

Thanks to all of you for your responses. I thought the difference may have been a speed vs torque issue or something on that order. You have put my curiosity to rest.
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