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

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philr

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I am investigating using the Command Vtwin 25 hp block to build a stroker direct drive 3200 rpm max and around 35 hp. Looking into what the tractor pullers are doing with performance parts is informative but not really close to what low rpm mission for this engine. One vender is offering forged cranks up to a 3.120 stroke which fits the engine needing some clearance-ing at the bottom of the cylinder and uses the same connecting rods & journal size as stock. I believe the forged crank comes from the ch1000 engine which has a stroke of 3.1" and a bigger rod journal 44 mm, the stock CH750 crank has a rod journal 35.94 mm so you can see that if the 44mm journal size is used it would require more clearance-ing which may not be possible. This stock CH940- CH1000 forged crank is cost around $235 and comes with a 40mm PTO diameter shaft which is nice. The thrust bearing is only spacers on the PTO side of the engine. This may be changed to needle bearings but more investigation needed. Tractor setup only as thrust bearing is only on the PTO side. Comments from the tractor pull engine guy is that the compression ratio should stay under 8.5:1 for 91 octane gas and a 3" stroke crank with(EDIT 4.5") con rods should get us 30-35hp at 3200 rpm. I am not selling anything and everything I investigate will be shared openly. I want affordable 4 stroke power for my ultralight and see the need for affordableaviation. These engines are very easy to work on and many after market parts are available. Parts I plan to use Stock CH750 engine block, heads, stock cam, and stator. CH960-CH1000 crank machined to my specs. Weber light flywheel. Midwest Super Cub ignition. Bosch coils. Mikuni carbs. Planning to have the crank PTO end milled to accept a tapered prop hub which I haven't sourced. Connecting rods need to be 5" and can be from Midwest Super Cub if rod journal in machined down to 35.94 mm but I would like to keep it at 44 mm as this is the weak point in the crank. Further investigation needed to see if I can find 44 mm con rods 5" or slightly under and if the block can be clearanced enough and will it clear the cam??? Target weight 80 Lbs. If I am successful I commit to a tutorial others can follow for free. If you want to help please do.
 
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philr

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I forgot to state the displacement for the 2 options I am pursuing. 3.1" stroke = 852 cc and 3" stroke =824 cc.
 

Victor Bravo

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philr, that project would be a GREAT and very much needed thing to do. Half of this entire group would be very much interested to watch this program and be in support of it. Please move forward with this project, I'll be the first guy to download or request or trade or say thanks for the "manual" or "process documentation" you come up with on this.
 

philr

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Zack Kerber machining has this bored out and sleeved block, 3.405 max bore . With a 3" stroke you are over 900 cc. 3.405 max bore
1627936533879.png
 

ToddK

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If you serisouly have the time and knowledge to do this, you might figure out a way to pass a hat around. I would be happy to donate a few bucks to the cause.
 

Chris Matheny

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I ported a set of heads, installed Ti intake valves and copper beryllium seats for a local tractor puller running in the 45CI class (750cc) its turning 9600+ off the line and 10k mid track so these engines are capable of holding serious power. The heads need a lot of help in stock form. Let me know if I can help.
 

Rhino

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I'm gonna go out on a limb here and guess the CH750 reference has nothing to do with a Zenith airplane?
 

karmarepair

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I ported a set of heads, installed Ti intake valves and copper beryllium seats for a local tractor puller running in the 45CI class (750cc) its turning 9600+ off the line and 10k mid track so these engines are capable of holding serious power. The heads need a lot of help in stock form. Let me know if I can help.
The intakes you need at 9000 rpm and 3200 rpm are rather different. TiPi in his Briggs and Stratton build thread has posted his results on a mild port, and the results were kind of Meh. The posters time and money may be better spent elsewhere, like on increasing displacement and/or getting the weight off the engine while still providing at least spark if not DC current and electric start.
 

Chris Matheny

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I completely understand a port for 9000 rpm and 3200 will be different but the most critical part of a wet intake port is the short side radius and the cylinder heads on these engines have a sharp 90 there with casting flash hanging in the port at that location. I port and cam every engine I do taylored to the rpm it will make peak torque and HP. If you are stroking it and increasing piston speed and volume porting becomes more critical. Intake length and port speed are very good tuning tools.
 

philr

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I'm gonna go out on a limb here and guess the CH750 reference has nothing to do with a Zenith airplane?
You got it. Not sure who used that alphanumeric code first but this is about engines from K.O.H.L.E.R.
 

philr

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I completely understand a port for 9000 rpm and 3200 will be different but the most critical part of a wet intake port is the short side radius and the cylinder heads on these engines have a sharp 90 there with casting flash hanging in the port at that location. I port and cam every engine I do taylored to the rpm it will make peak torque and HP. If you are stroking it and increasing piston speed and volume porting becomes more critical. Intake length and port speed are very good tuning tools.
Ok we will keep that in mind. Cleaning out the casting flash seems straight forward. This thread is about sharing information on how the average guy could build this engine. I appreciate that some people make a living tuning engines but if you can show us with pictures, drawings or explain very well what to do to increase hp and torque at around 3200 rpm don't hold back. One of the tractor pull engine builders says you can add 10 hp by just adding a stroker crank. This makes sense to me because almost anyone can learn how. Please everyone posting here lets try to keep this illustrated with drawings and pictures if possible.
 

philr

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If you serisouly have the time and knowledge to do this, you might figure out a way to pass a hat around. I would be happy to donate a few bucks to the cause.
I have rebuilt a few engines and this is within my skill set. I have a line on a couple of 27 hp command pro engines in need of rebuild if anyone is interested $300 gets you a used block I will do the build for you if you provide parts if you want and I can work on a tutorial for others to follow as I build. I don't know how you pass a hat digitally any suggestions. Also I am in Florida, Winter Haven area.
 

philr

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Dec 26, 2020
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I completely understand a port for 9000 rpm and 3200 will be different but the most critical part of a wet intake port is the short side radius and the cylinder heads on these engines have a sharp 90 there with casting flash hanging in the port at that location. I port and cam every engine I do taylored to the rpm it will make peak torque and HP. If you are stroking it and increasing piston speed and volume porting becomes more critical. Intake length and port speed are very good tuning tools.
Can you provide cam timings that are tailored specifically to 3200 rpm and a very nice running engines at all rpm levels up to that? This engine has to be good at low idle and respond well when punched but also run well at all rpm levels.
 

Chris Matheny

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Just adding a stroker crank will increase torque but do so at a lower RPM unless the intake track, valve size and length was not optimized for the shorter stroke to begin with. Since HP=TQ*RPM/5252 you will likely make the same power but at a lower rpm. He may be correct though as the increase in stroke and swept volume will also increase compression ratio, very desirable in pulling, not as much for airplane usage.
 

TiPi

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Can you provide cam timings that are tailored specifically to 3200 rpm and a very nice running engines at all rpm levels up to that? This engine has to be good at low idle and respond well when punched but also run well at all rpm levels.
Hi philr,

I have mapped the cam lobes on my Briggs 49 and I think they are not ideal for the rpm range that we aim for. I'm looking at re-profiling them to the lobe profile that the Corvair guys are using, similar engine configuration (valves, displacement, power/cyl) and same rpm range. I would be very interested in the cam lobe profile that is used on the CH750 to compare to mine.

The B&S profile has too much lobe separation, compensated by very shallow ramps and a fairly large overlap. Going closer to the Corvair profile should improve low rpm (idle) quality and possibly a slight improvement in the max torque between 2,600 and 3,500 rpm.
1627992429816.png
 

blane.c

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It seems to me that these industrial engines will all need propeller extension as crank ends to close to the block for most applications?

While obviously extreme, it shows that the propeller extension/extension area can be used for other.

(Franklin engine for Sea Bee)

1627995558680.png
 
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philr

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Dec 26, 2020
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It seems to me that these industrial engines will all need propeller extension as crank ends to close to the block for most applications?

While obviously extreme, it shows that the propeller extension/extension area can be used for other.

(Franklin engine for Sea Bee)

View attachment 113776
The crank from the big block is available with I think ~10" outside of the engine and full diameter. This should be cut as short as possible to minimize gyroscopic forces while clearing intake and so on.
 

Riggerrob

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It seems to me that these industrial engines will all need propeller extension as crank ends to close to the block for most applications?

While obviously extreme, it shows that the propeller extension/extension area can be used for other.

(Franklin engine for Sea Bee)

View attachment 113776
Prop extensions serve three functions.
Since most automotive engines were designed to feed power straight into a transmission, they were never intended to absorb the extra loads imposed by propellers.
First, they allow mounting a (ball bearing) thrust bearing to absorb thrust loads. The best thrust bearings can handle both tractor and pusher loads.
Secondly, extra bearings strengthen the crank-shaft against gyro-scopic effects.
Thirdly, shat extensions can be used to improve balance. The Franklin engine illustrated above has a shaft extension to allow mounting the engine farther forward to cure a balance problem suffered by many light pusher airplanes. For comparison, the best Franklin engine replacement (for Sea Bees) is a Corvette engine with a shaft extension.
 
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