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

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Vigilant1

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A prop extension is just an extension of the crankshaft/prop hub to allow the prop to be mounted farther away from the engine case. Many of them bolt directly to the "stock" prop hub. Just to keep things clear, if we are talking about changing or adding a "front" bearing to make it larger, to allow it to take thrust loads (either way), etc, then it would be good to be clear about that, because a prop extension doesn't necessarily mean a bearing, etc
 
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TFF

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As much fun as it is putting a stoker crank or bigger pistons in is, the prop adapter is the most important thing. It should be the number one task. If you are using the stroker crank, get one and start the prop adapter. Arrowprop has some airboat parts. Is this a source, or is a new one to be designed?
 

philr

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Yes Arrowprop was the one I am considering. Anyone know what the most common taper is on prop hubs?
1628002421163.png
 

rv7charlie

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Re: thrust bearings...
A 200 HP Lyc thrust bearing is a flat shoulder on the crank, running against a machined flat in the aluminum case. AFAIK, same is true for the big 6s, too. Just a data point to consider.
 

Chris Matheny

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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.
Yes I can but you would need to provide me with your desired bore, stroke, compression ratio, connecting rod length, intake valve size and port cross sectional area and length for them to be accurate. Head flow cfm would be helpful also.
 

philr

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Re: thrust bearings...
A 200 HP Lyc thrust bearing is a flat shoulder on the crank, running against a machined flat in the aluminum case. AFAIK, same is true for the big 6s, too. Just a data point to consider.
That is awesome to hear, so that is one thing to cross off the list as not needed at least for testing.
 

rv7charlie

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Just remember that it's next to the pressure fed long sleeve bearing on the output end, lubed by the oil exiting the sleeve bearing.
 

philr

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Just remember that it's next to the pressure fed long sleeve bearing on the output end, lubed by the oil exiting the sleeve bearing.
Are you thinking it may be a good idea to add a chase to directly feed oil to the thrust bearing? May need to look at pushing oil pressure up a bit.
 

philr

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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.
View attachment 113775
I am not sure what the stock numbers are but here are some custom cams from Zach Kerber Machine that he says are for under 5000 rpm.
1628114937879.png
 

Lucky Dog

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Late to this thread, but if you plan on building your Kohler to be direct drive, you'll need to ditch the heavy flywheel and devise a different ignition system. Power pulses cause oscillations between the mass of the prop and the mass of the flywheel and will probably break the crankshaft. Redrives resolve that issue. Forged alloy steel crankshafts are available with longer strokes - which would boost reliability and low end torque for a direct drive setup. Or, you can drive the prop from the flywheel side of the engine (Two European designs do this with B&S small block conversions), which eliminates the conflicting mass issue, but puts the drive loads on the flywheel side's smaller plain bushing.
 

Victor Bravo

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Welcome Lucky Dog ! Glad to see you here!

I saw your airplane at Brian Ranch before it closed down, my friend Paul Curley introduced us a year or two ago. Congratulations on a neat design.

We're glad to have you here.
 

philr

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Late to this thread, but if you plan on building your Kohler to be direct drive, you'll need to ditch the heavy flywheel and devise a different ignition system. Power pulses cause oscillations between the mass of the prop and the mass of the flywheel and will probably break the crankshaft. Redrives resolve that issue. Forged alloy steel crankshafts are available with longer strokes - which would boost reliability and low end torque for a direct drive setup. Or, you can drive the prop from the flywheel side of the engine (Two European designs do this with B&S small block conversions), which eliminates the conflicting mass issue, but puts the drive loads on the flywheel side's smaller plain bushing.
Hi, The flywheel will be lightened as much as possible to support the ring gear and magnets and I am looking for a forged stroker (3") crank but haven't found on. I think the engine would work good with a redrive but my plan is direct drive for simplicity. If you need a pusher setup it may work on the flywheel side but the thrust bearing is on the PTO end so tractor setup on the PTO end makes more sense to me. Redrives add a lot off complexity and I am not going that direction.
 

Lucky Dog

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Thanks Victor Bravo. My latest is at the covering stage. This one's powered by a modified Briggs V twin model 38.
 

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Lucky Dog

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@philr, I'll dig up a link to that stroker crank you need and post it here. Here's a pic of one of the the V twin conversions made for the for Igor Spacek's 250-pound SD-1. They convert three different V-twins The 24 hp one is based on the small block Vanguard. This 35 hp model, (Briggs 47? I'm not sure), but all use the flywheel side. I originally thought this was foolhardy, but there may be more to the story inside the cases. It may be worth an email to them to get some insight on the conversion.
1628197805649.png
 
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Lucky Dog

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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.
 

TiPi

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@philr, I'll dig up a link to that stroker crank you need and post it here. Here's a pic of one of the the V twin conversions made for the for Igor Spacek's 250-pound SD-1. They convert three different V-twins The 24 hp one is based on the small block Vanguard. This 35 hp model, (Briggs 47? I'm not sure), but all use the flywheel side. I originally thought this was foolhardy, but there may be more to the story inside the cases. It may be worth an email to them to get some insight on the conversion.
View attachment 113900
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.
 
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