K.O.H.L.E.R COMMAND VTWIN CH750 based engine

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philr

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@philr, here's a link to ARC racing's lightweight aluminum flywheel for Kohler V twins:

And, here's the link to the 3" stroker crank. Midwest Super Cub has just about everything, including advice, that you'll need for Kohler V twins.
I had seen that crank a few weeks ago but couldnt find on MWSC lately awsome it is still available. The flywheel I am planning to use in from Zach Kerber Machine and seems to be 6 lbs but I hope it can be lightened. Billet Aluminum Ring Gear Flywheel
 

philr

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The SE33 is based on the (vertical shaft) B&S 49 Vanguard engine. PTO stub cut off, flywheel lightened by about 2kg, flywheel puller bolt holes duplicated for a 4 bolt prop extension.
The SE31 is based on the CH750 but rarely used now due to the weight penalty over the SE33, about 4-5kg.
I think I read your entire thread on converting the Briggs. Did you have a separate thread the SE31 and the CH750?
 

TiPi

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No, they were the early engines from Spacek and some French builders who converted their own engines. I decided early on that the CH750 is too heavy and started the B&S conversion. Spacek then followed my recommendation for that engine as well, turning it into the SE33.
The new 7000-series might be a better candidate as it is quite a bit lighter but I would stay away from cast cranks. I'm not sure how durable the 7000-series is, I have a 725 in my mower so I'll see.
 

philr

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No, they were the early engines from Spacek and some French builders who converted their own engines. I decided early on that the CH750 is too heavy and started the B&S conversion. Spacek then followed my recommendation for that engine as well, turning it into the SE33.
The new 7000-series might be a better candidate as it is quite a bit lighter but I would stay away from cast cranks. I'm not sure how durable the 7000-series is, I have a 725 in my mower so I'll see.
I think under 80 lbs is possible dry weight for the ch750 converted as laid out in the first post in this thread. How heavy was that k.o.h.l.e.r. and what engine was it?
 

Chris Matheny

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If you want a 3" crank from MWSC you better get on their list now, they only have a certain amount made a couple times a year so they can have a considerable wait to get them.
 

TiPi

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I think under 80 lbs is possible dry weight for the ch750 converted as laid out in the first post in this thread. How heavy was that k.o.h.l.e.r. and what engine was it?
Based on the CH750, from memory, around 36-38kg, FWF weight wth prop, exhaust, cowling, mount etc was around 44kg. The Briggs comes in just under 40kg FWF.
 

Vigilant1

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Oh i missed that they claimed 31 hp. No stated rpm that I can see.
Spacek also sells another version of the B&S 810cc engine with a claimed max output of 33 hp. The biggest differences from their 31 HP engine is a slightly higher CR and cleaned up ports.

It will be useful to explore the max continuous HP that these industrial engines will produce in real world aviation conditions. The high max specific HP that many folks are getting goes well above the generally accepted continuos HP levels of dedicated air cooled NA 4 stroke aviation engines (about 30 cc per HP). The smaller displacements may offer some advantages in this regard compared to a 200+ CI Continental (smaller heads = more surface area per unit of displacement), but a close watch of CHTs will be important. It's no big trick to make any engine produce a lot of HP (higher RPM, appropriate ports/breathing, higher CR), but if we want a reliable, long lived engine for aviation use, we should have realistic expectations.
 
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Chris Matheny

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I believe the higher specific output per cc is because of the increased rpm of these engines vs aircraft engines more than the increased volumetric efficiency of smaller cylinders. It also helps not having heads stacked together in a line as far as cooling goes.

EDIT, I see you meant smaller cylinders with more thermal mass and area to cool, not volumetrically more efficient.
 
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karmarepair

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The new 7000-series might be a better candidate as it is quite a bit lighter but I would stay away from cast cranks. I'm not sure how durable the 7000-series is, I have a 725 in my mower so I'll see.
7000 series? News to me..
"based on the Command Pro".
7000 Series | Kohler Engines Vertical shaft only. Consumer grade.
I think I'll stick with the Command Pro series, me.
 

Lucky Dog

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@TiPi, I did a deep dive on your engine pages yesterday. Lots of great research in there. EC Carburetors sells the adjustable type two barrel carburetor with screw-in main jets for Vanguards and stocks jets for them. That's what I am using now. I have developed a Vanguard conversion for single seat Quicksilvers that has been flying for two years in testing. Very promising so far.
 

Vigilant1

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EDIT, I see you meant smaller cylinders with more thermal mass and area to cool, not volumetrically more efficient.
Right. A smaller combustion chamber (including head and piston top) has more area relative to its contained volume than a larger combustion chamber would have. To the degree that HP correlates to displacement, the argument goes that a smaller combustion chamber has (relatively) more external surface area to get rid of heat. The counter-argument is that (relative to a larger displacement cylinder) the >inside< surface of the combustion chamber also has more area to take on heat (per unit displacement), and this >increases< cooling issues.

There's more on the "continuous HP is limited by heat rejection" discussion here, complete with a groovy table.

The thermal mass won't do anything for us in improving continuous HP. It's not even much use in short runs--as a practical matter the AL has very little heat capacity relative to the entire heat balance of the engine, it gets "used up" in a very little time and then we're staring at a CHT gauge as it goes into the red ...
 
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Vigilant1

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Oh i missed that they claimed 31 hp. No stated rpm that I can see.
I don't know, but Tipi posted this about the Spacek 33 HP version of the B&S 810 engine:
The 33hp is max, not continuous. From the German POH:
Max take-off power: 33hp @ 3,600rpm
Max continuous power: 30hp @ 3,300rpm
Fuel burn @ 75%power: 5.9 lt/h
Fuel burn @ max power: 7.1 lt/h
Fuel burn @ 55% power: 4.6 lt/h
 

philr

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I am not sure how much an oil cooler does but I plan to use one. I see them on lots of mowers with this engine not sure if they always have them.
 

Vigilant1

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I am not sure how much an oil cooler does but I plan to use one. I see them on lots of mowers with this engine not sure if they always have them.
Oil coolers are stock on (all?) these V-twins. They do a good job of keeping the oil from breaking down and help with head cooling. To have a more substantial impact on CHTs (and, thus, continuous HP) would require a much larger flow of oil through the heads, to piston squirters, etc. Probably a high volume, low pressure oil pump would be needed in addition to the stock high pressure, low volume oil pump (plus a bigger heat exchanger, and air to feed it).

With 750cc, I think it will require "heroic measures" to achieve more than about 28 continuous HP (i.e no more than about 415F CHT, measured under the plug). Like most of the air cooled 4 strokes, heat rejection will be the factor that constrains the available continuous HP.
 
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