An example Briggs 38 - Lucky Dog version

[karmarepair]

Hosted from a thread about KOHLERS:

@Vigilant1: 100 hours so far. We're not concerned about the engine. Teardowns show zero wear or stress on the moving parts. We are more interested in the drivetrain and mounting parts going forward. Redrive uses four 3VX V belts at 1.67:1. The engine drives a Quicksilver single place, and thus is limited to a 52"propeller. Using a PSRU multiplies the torque, and allows the host engine to produce its maximum power at exactly the RPM you want to spin the prop.

We use a three blade Ultraprop at 15 degrees. Using the Ultraprop graph as a reference, we get 33/34 hp at 4500 rpm (measured at the propeller) at a pressure altitude of 5000 msl. Dyno tests at the crankshaft will show a higher number, but all that counts is how much torque is available to move air. The airplane's performance closely approaches the Rotax 447, so our figure is honest. (Remember, we're measuring HP at the prop, where the mighty 447 would not reach its advertised numbers either.) Similarly modified model 38 engines are available off the shelf with (dyno-tested) power outputs of 35 to 40 hp. Impressive, considering it starts at 23 hp.

Pricing & Ordering | Competition Aircraft $350 for the Ultraprop Lucky Dog is using.

Fuel burn is 1.8 gph and the engine never runs hot. Even on 100-plus degree days, the CHT rarely hits above 300f. Engine tune is mild: seven-degree advance on the ignition, stiffer valve springs, smoothed out casting roughness in the cylinder heads, and we switched to a cam that raises the torque to peak near 4500 rpm. We also jet the carb one step richer (#112, both sides). High compression pistons (about 90 bucks each with rings) would further increase torque, but we left them out because we wanted to minimize the engine mods to see how far we could get without going on a spending spree or exceeding garage-mechanic skillsets. Oh, and flat tappet engines absolutely need zinc-based racing oil to run long and hard. We run Valvoline VR1 20/50.

Briggs Model 35 Vanguard V-Twin Carburetor Main Jet-231660 Drill to suit?
One of these Briggs & Stratton Model 38 Vanguard V-Twin Camshaft
some of these EC Dual Valve Spring Set for GX390/420 & V-Twin Engines
Pistons if you must 793647 Briggs Model 38 Piston W/Rings but they may require new rods as well 6275 ARC Billet Rod, Briggs Vanguard 4.134

Because the Quicksilver flies slow, we left the stock fan shroud (4 pounds) and flywheel (14 pounds) in place, but it's doubtful that it needs the big fan. A smaller, 6-pound aluminum flywheel from ARC racing is the go-to for mower racers,

6611-VG ARC Billet Flywheel, Vanguard Adjustable, so that gets your ignition advance.

who say that their highly modified model 38s are still hard to keep warm enough to attain maximum power. Quicksilvers are probably the draggiest bird in the sky, so this little Vanguard could easily push or pull a more efficient plane around at speeds beyond FAR 103. In that scenario, I'd suggest a 1.8:1 reduction spinning a 60 inch prop. I'll report back on that later this year as we are putting this configuration in motion now.

Generally, we chose the Vanguard model 38

386447-0444 Small Engine Warehouse, there are other -Dash numbers with fuel pumps, fuel tanks, smaller output shafts, etc.

because it is one of the lightest V twins in its class, it's made in Japan, it holds up well in racing configuration, and there are many performance upgrades available. We prefer 3VX v-belts because they won't jump the pulleys if they get slack. Micro-v drives, however, can be more compact and lighter weight - plus, the pulleys seem to wear longer. All reasons why they have become the go-to, but alignment must be more precise (thin, wide belts) and the 5mm groove requires spot-on tension. Ace Aviation has a well-designed, lightweight micro-v redrive, made in India, that bolts right onto Industrial V-twin's universal face-plate pattern.

Redrives | aceaviation

Most good belt-drive PSRUs run around 800/900 bucks - which we discovered, is a fair price.

In the next post in this thread, I'll cost this out, at least for 2021 prices.

 

[karmarepair]

 

[Vigilant1]