B&S 49-series (810cm3/49ci) - TiPi's conversion for aircraft use

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TiPi

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Aug 25, 2019
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297
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Mackay (AUS)
Oil system

I think I have figured out my way of changing the oil supply. There are a few options:
  • close the port in the crankcase cover, drill & tap a hole in the oil pump cover and connect the suction line to that
  • cut a hole and add a fitting/line to take the oil supply line out through the oil drain hole
  • cut a hole and use a fixed line (or hose) and connect to the inside end of the governor shaft hole. Tap the outside hole end for a M14x1.25 fitting to take a 10mm hose from the tank
I have opted for the last one to keep the front of the engine clean (an elbow fitting or better a bend will protrude quite a bit in front of the engine where I plan to have my front engine mount).

This is what it looks like before "cementing" the tube in (high-temp metal epoxy):
CIMG2325.JPGCIMG2326.JPG
The pipe is an alu pipe 12mm OD and 10mm ID. The bend radius could be a liitle bit thighter but that's what the pipe bender comes in (62mm diameter). I'll blend the outlet a bit more after the epoxy has cured.
I have also found another assembly error! The flywheel end main bearing is slightly off-set from the oil supply hole. It is about 1/3 covered and at this stage, I'll leave it. If I can find a small enough Dremel burr, I might open it up a bit. This oil line only supplies oil to this bearing, the bigends are supplied from the PTO bearing.
CIMG2324.JPG
And the final product:

The tube enters the oil pump inlet in the same location as the original set-up. I'm using the governor bore as the passage from the outside.
CIMG2331.JPG
The pump end of the pipe has been blended and provides a slighly larger passage than the original rough-cast slot.
CIMG2329.JPG
 
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TiPi

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297
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Mackay (AUS)
Carburetor :(

I have made an adaptor to put the Nikki on my flow bench. The first test is with all the OEM plumbing for the air filter and 2 straight pipes after the carby. The results are:
1590646020865.png1590646976157.png
There is a distinct difference in flow between the 2 barrels. At closed throttle, the difference is 10-13% (RH lower than LH), then it evens out and at WOT it is around 2% (RH lower than LH). The rather large difference at closed and near closed throttle is probably one contributor to the poor idling performance.
I'll try to identify the reason for the difference at idle, most likely a difference in throttle plates or shaft. A simple swap of the plates should show a difference.
This is the setup on the flow bench. I also want to map the signal pressure at the main jets to see if there are measureable differences due to the poor design of the discharge nozzle (mixture variations).
CIMG2328.JPG
A bit more trivia on the carburetor: This is the pressure drop (vacuum) at the main jet eg this is the negative pressure that pulls the fuel through the main jet. It includes the bleed air from the emulsion tube. The LH main jet has a bit more suction, that translates into 2% more fuel at WOT. As the LH barrel also flows a bit more air, that should keep the mixture even.
1590738228377.png
The plan now is to even out flow at WOT and idle, main jet pressures and the idle circuit. Then see how the carby performs on the engine.
One thing that is not re-created on the flow bench is the pulsing nature of the actual intake stroke. The flow bench provides a steady flow and some of these parameters might be different on a running engine.

If it doesn't, I'll seriously look at an EFI option.

A bit more on the carburetor: I swapped the 2 throttle plates and the difference was very minimal, still quite a bit of flow difference at closed throttle (measured from closed throttle in 1/2 turn steps):
1591008906057.png
Then I checked the throttle shaft, both flats machined into the shaft for the throttle plates are parallel to each other.
Next was the actual bore diameters. What I found is a small difference in diameter, both vertical and horizontal (about 0.04mm), which seems "nothing".
Then I calculated the difference in ring area around the throttle plate, et voila, I have pretty much the same difference in area as I have in flow (~15%).
With an idle air screw and an idle mixture screw, no problem. With the Nikki, I might try to drill a small hole in the throttle plate of the smaller bore (they are brass, if I stuff up, solder it closed and it is back to the way it was). The area difference amounts to a 1.5mm drill bit. It might need a 2mm due to the flow difference of a small hole vs a ring opening. I'll try the 1.5mm first and see how it works out.
The flow difference at WOT will be addressed with another polish of the slower venturi.

The hole in the throttle plate ended up being 2.5mm. This gives me an equal flow rate from 0 (fully closed) to 3 turns out which would be more that fast idle, even with a prop. The pilot signals at the idle fuel jets are also equal, so I should have the same air volume and the same fuel volume between the 2 barrels. Live testing will show!
CIMG2332.JPG1591065549097.png
 

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TiPi

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Blue-printing & assembly

It is time to start the blue-printing and then the assembly of the engine. I have built a small fixture for the conrods to measure the small & big ends in order to calculate & verify the crank balance. This is necessary as I'm polishing the conrods (remove/minimise any stress risers) and might change the weight slightly. The original conrods, pistons & pins are remarkably close (conrods 0.5g, piston and pin 1g).
This is the set-up:
CIMG2334.JPG
I also drilled 3 holes at the bottom-to-be as oil drain holes back to the heads:
CIMG2335 inked.jpg
Welch plug for the original dipstick hole (not installed yet), need to make a support for the case before pressing it in (sits on an angle):
CIMG2336.JPG
The crankshaft and camshaft endplay has been confirmed. This engine was fitted with a cover gasket (0.45mm) where the Vanguard engines are now only using a gasket sealant. The endplay without gasket is 0.09mm, specs are 0.05-0.76mm, so I'm at the lower end of the specs (good).

Update 25/6/2020
The crankshaft has been cleaned-up (all flashing removed, semi-polished, all corners radiused) and statically balanced. I still need to do the dynamic balance at one of the engine shops in town.

Currently working on making a jig/fixture to machine the intake manifold flanges. The port axis and flange surface have a 25deg difference. The original manifold simply has a 25deg corner slightly rounded, with a round hole in the gasket trying to cover an oval hole.
The exhaust flanges are straight, so easy to make.
 
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TiPi

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Final touch-up on the cylinder heads

The final results of the porting work: The flow of the intake ports has improved by around 20% at valve lifts >4mm
1592211768939.png
Another way to show the difference is the product of the flow rate multiplied by the time over the course of 2 crankshaft rotations (in my case, in 10deg increments at 3,600rpm). The area under the curve is the theoretical aspirated volume without any dynamic influences (a relative comparison before/after).
1592212150542.png1592212171882.png
A couple of things are visible here:
  • the EX flow did hardly improve with a port opening increase from 25 to 26mm, 3-angle valve seat cut and radiusing. The port turns very turbulent from about 3-4mm of lift, severly restricting the static flow (EX port is flowed from valve to port)
  • the total area under the IN curves are identical between #1 and #2 even though the shape of the curves is slightly different
  • No futher gains can be made without major surgery eg cutting the existing port away and weld in a port on a more declined angle and larger valves (a 36mm IN valve could be fitted with the existing seat)
    1592212793784.png
This is what the ports look like now.
The IN port is 28.2mm (up from 25.5mm at the narrow end) for the full length, the seat throat increased from 28 to 30mm.
The EX port has increased from 23.5mm to 26mm, the seat throat from 26 to 27.5mm.
Each head lost a total of 50g of material.
CIMG2350.JPG

With all the porting work completed, the valve seats cut and lapped, I considered ways of improving the airflow around the head for improved cooling. The heads have only 2-3 actual cooling fins, the rest is mainly a passage thrught the centre of the head. The fins have a number of partitions that prevent the air from flowing freely between the fins. I drilled a total of 13 holes of 7mm diameter to allow some airflow in the depth of the fin space.
CIMG2354.JPGCIMG2353.JPG

I also had to plug a hole in the combustion chamber where the EX stud hole was drilled a bit deeper on one head. I elected to plug it with some high-temp epoxy and a SS exhaust stud. As the existing hole was 5/16", I tapped it for M10 and made some M10-M8 stepped studs. The other stud is just M8.
CIMG2337.JPG

What remains is the shave 1.0 (#2) and 1.2mm (#1) of the head base to restore the CR. My aim is 9.0:1, a little bit higher than the OEM 8.5 but still on the safe side (the dynamic CR will increase above that with the better volumetric efficiency).

Update 26/6/2020
The heads have been shaved (1.0 and 1.1mm) and both have now a combustion chamber volume of 39ml. This results in a 8.8:1 compression ratio with the OEM head gasket and can be raised with thinner copper gaskets (9.1 with 0.8mm and 9.3 with 0.5mm).

On the suggestion from Vigilant1, I have also removed a bit more material from the head cross fins to open the air passages between the fins a bit more. I also rounded some corners a bit more as well. No polishing, just rough cut with a 6mm burr to keep the air turbulent.
This should provide a less interrupted path for the cooling air to pass between the fins.
CIMG2363.JPGCIMG2359.JPG
Similar work was done on the few fins located on the block. This engine really doesn't have a lot of cooling fins, not on the heads and not on the block.
 
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TiPi

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297
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Mackay (AUS)
Intake & exhaust

With the assembly of the engine nearing, I started thinking of the intake & exhaust piping. The photos below show a mock-up of what I'll be building for the test stand and hopefully, will also be suitable for the SD-1. I need to build a firewall to properly place all gear (engine mount, MGL RDAC, solenoid, battery etc). The cardboard background is the firewall, everything on the engine will fit within that outline. The prop axis will be slightly higher, on the upper black line. The the dots on the lower line and the bottom line are the locations of the engine mount bolts.

With the engine heads-down, the carby is now at the lowest point of the cowling (still above the floor line), so a gravity-feed fuel system should be possible :). I'm working on some mods to make the Nikki useable.
CIMG2372.JPG
The exhaust pipes could run as a parallel set and exit below the floor under the firewall.CIMG2376.JPG
This is the intake pipe flange, cut from some alu bar. The intake pipe is a SS shower grab rail.CIMG2377.JPG
I counter-bored the intake ports for easier alingment of the intake pipe. The pipe has on OD of 32mm, ID of 30mm and the port is 29mm.
CIMG2378.JPG
The exhaust port will accept the 25mm grab rail pipe without counter-boring.
 
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TiPi

Well-Known Member
Log Member
Joined
Aug 25, 2019
Messages
297
Location
Mackay (AUS)
Interlude

My project will be on hold for at least 4 months, we are moving house soon and I need to pack up 20+ years worth of "stuff" in my shed. At the other end, there should be a nice clean workshop waiting for all these goodies.

To keep my followers entertained for this time (well, for 9 minutes anyway), here is the latest video released by Vanguard on their engine production:

And this one is an informative side-by-side comparison of the small-block B&S (38-series), Predator 670 and Kohler 745 EFI engines by Lonestar Hovercraft:

I have also received my non-OEM carburetors. They were advertised on ebay as "CARB FITS BRIGGS & STRATTON 810cc V-TWIN ENGINE 49M977 NIKKI 796606 799511 MOWER".
Unfortunatley, this is hog-wash! The carb throat is only 26mm which is for the smaller 38, 40 and 44-series (the 49 has 28mm). The carbs are also not well built, the throttle shaft has quite some movement (doesn't seem to have a shaft seal) and the choke plate doesn't fit the carb opening. Trying to get a refund now.
CIMG2382.JPGCIMG2383.JPG

The larger carby (54/61 size) bought off Amazon is also finally on the way. I ordered it back in April, it was listed as "in stock" and still was not shipped by last week.
 
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