An example Kohler

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

karmarepair

Well-Known Member
HBA Supporter
Joined
Jan 13, 2011
Messages
648
Location
United States
A relatively mild, built, used, pulling engine. Started with a 25 HP Kohler Command, bored and stroked to 50.5 cubic inch. Mid West Super Cub 3" crank, Billet rods, 4000 RPM cam kit. Adjustable main jet carb. Adjustable billet aluminum flywheel. Listed at $2,795 (starting from a new engine, you could not build this engine this cheap), advertised 50 HP. Weight is unknown, but my educated guess (based on book weight for a "dressed" engine, and the reduction in flywheel weight, plus a guess at the removed shrouding) that its about 90 lbs as it sits. Don't know what RPM that HP number represents, but I'm guessing 4000 RPM, so you'd need a PSRU to get all of it, but with that displacement, you'll surely get North of 30 at direct drive RPM (the stock engine was rated at 3600 RPM). But for how long? With a forged crank, forged pistons, billet rods, heat dissipation of the heads will be the limiting factor. This engine has an oil cooler, and pressure lubrication.
50_HP_Kohler.jpg
 

karmarepair

Well-Known Member
HBA Supporter
Joined
Jan 13, 2011
Messages
648
Location
United States
Look at all those threaded mounting bosses for a belt reduction.... :)
Pretty common across all the V-twins, in fact, I think they are standardized. Figuring out where to anchor the upper pulley is a little harder, unless your PSRU is stiff enough to essentially cantilever it off the lower attachments around the PTO shaft. That seems to be the approach taken by Lone Star Hovercraft on their PSRU.
Certain people well qualified to know would consider this PSRU un-airworthy, given the overhanging bearings, and the lack of a tensioner able to adjust belt tension over a wide range of loads/temperatures.

In rebuttal, one of the PSRUs with the most FLYING hours, the Stewart Maximizer, commits both these sins. Maximizer VW Belt Drive Plans - Stewart Aircraft

The Valley Engineering Big Twin had a proper idler/tensioner. http://lh3.googleusercontent.com/proxy/3RGnvalaxQFTBnNDawKp6AVrizose_uBtc0zpZ-1PhlqqbirbjUfnIZAYyBcCm5nkji9JRdj5fur3Ep9lcx9xYwjaTcCyRTc9NQ
 

plncraze

Well-Known Member
HBA Supporter
Joined
May 11, 2006
Messages
2,113
When Don Stewart did his reduction drive a friend was looking over his shoulder telling him to "do it right." Stewart said it was with goals of simplicity and reliability.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,813
Location
US
A relatively mild, built, used, pulling engine. Started with a 25 HP Kohler Command, bored and stroked to 50.5 cubic inch. Mid West Super Cub 3" crank, Billet rods, 4000 RPM cam kit. Adjustable main jet carb. Adjustable billet aluminum flywheel.
Do you know what model carburetor he's using and how the induction is set up?
 

karmarepair

Well-Known Member
HBA Supporter
Joined
Jan 13, 2011
Messages
648
Location
United States
Do you know what model carburetor he's using and how the induction is set up?
MWSC sells a "breathed on" stock downdraft carb with an adjustable main jet, and a larger fuel delivery tube. That's what this "stock appearing" engine is using, with a billet venturi adapter to a 3 inch outlet K&N filter. Rework Downdraft Carb for 25HP Command The Stahls (the family behind Midwest Super Cubs (Kohlers) and Mid West Horsepower (briggs mud boat engines) occasionally advertise used carbs like this on Facebook for about $200. If you're handy and clever, you can mod the carb yourself using their adjuster, main jet, and some machine work Mixture Adjuster
The stock carb won't flow enough fuel when you turn up the wick. I wish there was a page of instructions on how to modify the carb, I haven't found one, yet.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,813
Location
US
MWSC sells a "breathed on" stock downdraft carb with an adjustable main jet, and a larger fuel delivery tube. That's what this "stock appearing" engine is using, with a billet venturi adapter to a 3 inch outlet K&N filter. Rework Downdraft Carb for 25HP Command The Stahls (the family behind Midwest Super Cubs (Kohlers) and Mid West Horsepower (briggs mud boat engines) occasionally advertise used carbs like this on Facebook for about $200.
The photo on the MWSC site looks like a single barrel carb. Is that how the engine in the OP is set up, with one single barrel feeding a split induction runner? That seems unlikely, so I'm probably missing something.
 

Matt G.

Well-Known Member
Joined
Nov 16, 2011
Messages
1,265
Location
Kansas, USA
The photo on the MWSC site looks like a single barrel carb. Is that how the engine in the OP is set up, with one single barrel feeding a split induction runner? That seems unlikely, so I'm probably missing something.
Most likely. Stock carb is a single barrel downdraft Keihin (sp).
 

karmarepair

Well-Known Member
HBA Supporter
Joined
Jan 13, 2011
Messages
648
Location
United States
The photo on the MWSC site looks like a single barrel carb. Is that how the engine in the OP is set up, with one single barrel feeding a split induction runner? That seems unlikely, so I'm probably missing something.
That's exactly it. And the plastic intake manifolds flow better than the cast aluminum ones.

MWSC and/or Zach Kerber sell two barrel carbs/manifolds, and dual carb setups, but at the RPMs I can imagine an aero conversion turning, and the amount of valve lift (high lift needs heavy valve springs and steep cam ramps, that degrade reliability on these flat tappet engines) that can reliably be run for long periods of time, I think such carburetion is unnecessary.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,813
Location
US
That's exactly it. And the plastic intake manifolds flow better than the cast aluminum ones.

MWSC and/or Zach Kerber sell two barrel carbs/manifolds, and dual carb setups, but at the RPMs I can imagine an aero conversion turning, and the amount of valve lift (high lift needs heavy valve springs and steep cam ramps, that degrade reliability on these flat tappet engines) that can reliably be run for long periods of time, I think such carburetion is unnecessary.
It's surely possible to get enough air and fuel through one properly sized barrel, but I think the issue would be balancing the mixture going to each cylinder in the "lumpy" induction flow (270 degrees, bang, 450 degrees, bang, etc) of these V-twins. It would be interesting to put a wideband O2 sensor on the exhausts of these single-carb Kohlers and see how the mixture compares across both cylinders. B&S cuts costs to the bone, but they still put dual carbs on their twins. I doubt they'd spend the extra $10 if it wasn't needed. But, maybe Kohler found a way to make it work well enough.
 
Last edited:

karmarepair

Well-Known Member
HBA Supporter
Joined
Jan 13, 2011
Messages
648
Location
United States
So then stock 45.5 to 50.5 c.i. displacement. Do they bore the stock liners or install larger liners?
They bore the stock liners, and I think 3.4 inches is as far as they dare go. Or you can put in new liners, but the same limit applies. You have to have enough wall thickness on the liners to resist the shrink fit. SOME blocks can go bigger by running without liners, and Nicom coating the bores.
 
Last edited:

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,266
Location
Port Townsend WA
So 828 c.c. About the same as the stock Briggs 810 that is lighter and cheaper. (but is vertical shaft)
 

karmarepair

Well-Known Member
HBA Supporter
Joined
Jan 13, 2011
Messages
648
Location
United States
So 828 c.c. About the same as the stock Briggs 810 that is lighter and cheaper. (but is vertical shaft)
You're not wrong. But...

I'm not comfortable with all the Demon Tweaks needed to convert that engine to Horizontal.

And if one accepted the premise, and wanted to hang an 810 on a firewall and make Oshkosh 2022, the only way I know you could do it that quickly would be to send about 5000 Euros to Czechoslovakia where Igor Spacek will sell you a complete Firewall Forward, the only way he'll sell his developed, flying setup, due to the modifications needed inside the engine itself, and all the bits and bobs needed outside the case to make it all work.

Using the "midsize" Kohler (CH18 through CH750) you could source all the parts from the US (even a PSRU if you go that way) and be in the air in relatively short order for the same money or less, with a full firewall forward weight maybe 2-4 kG heavier (the Briggs 810 flipped Vertical to Horizontal needs an external sump and electric oil pump the Kohler doesn't)?

You Do You; me, I'm All In with Brand K. The cover for the engine test stand is almost done, the engine mount is next, but I have a Sonex to finish and fly, so don't hold your breath for test results from me. Capable experimenters like TiPi are going the other way, with Brand B.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,813
Location
US
You're not wrong. But...

I'm not comfortable with all the Demon Tweaks needed to convert that engine to Horizontal.

And if one accepted the premise, and wanted to hang an 810 on a firewall and make Oshkosh 2022, the only way I know you could do it that quickly would be to send about 5000 Euros to Czechoslovakia where Igor Spacek will sell you a complete Firewall Forward, the only way he'll sell his developed, flying setup, due to the modifications needed inside the engine itself, and all the bits and bobs needed outside the case to make it all work.

Using the "midsize" Kohler (CH18 through CH750) you could source all the parts from the US (even a PSRU if you go that way) and be in the air in relatively short order for the same money or less, with a full firewall forward weight maybe 2-4 kG heavier (the Briggs 810 flipped Vertical to Horizontal needs an external sump and electric oil pump the Kohler doesn't)?

You Do You; me, I'm All In with Brand K. The cover for the engine test stand is almost done, the engine mount is next, but I have a Sonex to finish and fly, so don't hold your breath for test results from me. Capable experimenters like TiPi are going the other way, with Brand B.
I sure couldn't say one approach is hands-down better, it probably depends on a person's priorities and the way they estimate the risks and challenges in each approach.
It's good to have options, and different ways to get the job done.

Three observations:
1) Getting the Kohler modified (the new forged crank, upgraded rods, etc) will also be some work. The path has been blazed by pullers/racers, but their priorities aren't the same.
2) Tipi is doing the Full Monty: Fip the engine upside down AND mount the prop directly to the PTO end. It is ambitious, and requires a lot of new work compared to the approach on the other Spacek conversions.
3) Not sure, but I think the SD-1 conversions of the B&S 810cc just use the electric oil pump to prime the engine-driven mechanical pump. That might also be done with a bulb from the cockpit. They don't use a dry sump, but may have an aux gravity sump IIRC.
 
Last edited:

Victor Bravo

Well-Known Member
HBA Supporter
Joined
Jul 30, 2014
Messages
9,669
Location
KWHP, Los Angeles CA, USA
What if some clever sod made a simple 90 degree gearbox (using COTS gear sets) that mounted on the "vertical shaft" Briggs or Kohler engines? You'd wind up with a prop speed reduction unit, a higher thrust line, and equal or easier mounting of the engine itself. This would apparently allow a greater selection of engines, I'm always seeing comments here that the vertical engines have XYZ advantages in power or weight or features.

With the cylinder heads "pointing" forward, the aircraft's cowling would look like the Fieseler Storch's Argus engine cowling, which is kinda funky-cool.

But... mounting the vertical shaft engine so that the rocker box covers were at the rear (against the firewall) would allow a pretty clean tapered cowling, maybe looking like the typical Gipsy or Menasco inline cowls on so many classics.

The only complex issue would be some method of stealing a tablespoon of Billski's brains applied to getting the resonant frequencies outside of the engine's operating ranges as he has mentioned.

Has anybody explored this configuration?
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,266
Location
Port Townsend WA
You Do You; me, I'm All In with Brand K. The cover for the engine test stand is almost done, the engine mount is next, but I have a Sonex to finish and fly, so don't hold your breath for test results from me. Capable experimenters like TiPi are going the other way, with Brand B.
Yeah, my experiments are in a different direction other than the 810. No guarentee any of my experiments will work. Thanks for the update of your proposed conversion.
 

billyvray

Well-Known Member
Joined
Aug 17, 2005
Messages
889
Location
Newnan, GA
I do not offer this as a way to use a vertical shaft engine on and aircraft, just an example of how it's been done in hovercraft and airboats.
They use a v belt and two extra pulleys for hte 90 degree change. The belt is long enough it can twist easily. I'm not sure how one would tighten this configuration up the be a front-mounted tractor engine.

1632415728726.png 1632415739156.png
 
Top