Briggs & Stratton 993cc "Big Block" engine conversation thread

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Vigilant1

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Jan 24, 2011
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7,804
Location
US
Ken, thanks for the update. That 85.6 lb engine weight (incl exhaust, prop hub, even oil) is great, it looks like you'll make or be darn close to your 100lb FWF target weight. This is going to get a lot of attention from folks who want a reliable, light 4-stroke engine for simple flying, and who don't need/want a starter or electrical system.
 

Ken Powell

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Dec 8, 2020
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Location
Bryant, AR
Need input from the more experienced flyers:
Briggs 993 - The Eiffel club calculator shows a club of 785mm / 30.91" to absorb 40.1 HP at 3600 RPM. Question: Can a club with a radius of only 15.5" be hand propped? I was not planning on using a starter but I may not have a choice.

Eiffel club prop​
Imperial​
Inches​
Club dimensions​
RPM​
Hp​
Length​
785​
30.91​
Diameter​
785​
2500​
13.4​
RPM​
3600​
2.06​
Thickness​
52​
2600​
15.1​
reduction​
1.000​
2700​
16.9​
prop RPM​
3600​
2800​
18.9​
Power​
40​
2900​
21.0​
X-section​
52​
3000​
23.2​
3100​
25.6​
3200​
28.2​
Power​
40​
3300​
30.9​
RPM​
3600​
3400​
33.8​
reduction​
1.000​
3500​
36.8​
prop RPM​
3600​
3600​
40.1​
length​
785​
30.89​
3700​
43.5​
x-section​
52​
2.06​
3800​
47.2​
3900​
51.0​
Inputs are yellow cells. Edges should be left square​
4000​
55.0​
4100​
59.2​
4200​
63.7​
4300​
68.3​
4400​
73.2​
4500​
78.3​
 
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Ken Powell

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Dec 8, 2020
Messages
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Location
Bryant, AR
My concern is that there simply will not be enough leverage to turn the prop fast enough.
I'm running 2 separate GM HEI's for each cylinder. For each cylinder:
One HEI at zero degrees (top dead center) and one HEI set at 28 degrees BTDC. Start on the 0 degree HEI then switch to the 28 degree HEI after the engine is running. Some sources say that the HEI will fire at 100 RPM and others say 200 RPM.
 
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Ken Powell

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Dec 8, 2020
Messages
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Location
Bryant, AR
Yes. There will be one 'raised steel' pip (reluctor) set on a rotating disk and set at TDC. The selected HEI will fire when it passes the selected magnetic pickup for each cylinder.
Here is a picture of a reluctor:
 

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TiPi

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Aug 25, 2019
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Peeramon (AUS)
If you still have your mechanical decompressor (fitted on your engine?), you will be able to flick it over TDC and get it going. Depending on your flywheel (lack off), you might need to start on the cylinder where the next one is closer in firing angle (should be #2).
 

Ken Powell

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Dec 8, 2020
Messages
68
Location
Bryant, AR
The 61E377-0027 generator engine did not have a decompression device on the stock camshaft. I did replace the cam with a Dynocams 275 - same lift but more duration and this cam does not have a decompressor either. Thanks for the advice to start on cylinder 2 - that makes sense.
 

Victor Bravo

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Jul 30, 2014
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Location
KWHP, Los Angeles CA, USA
Here is an interesting Briggs & Stratton small block conversion I saw FLYING this last Saturday. The guy's name is Dimiter or Dimeter (not Dimitri), the aircraft is a light 2-axis Quicksilver based at Black Butte ultralight strip about 20 miles east of Palmdale CA. It gets off the ground and climbs out, but it is not over-powered. The local pilots who have experience with the Quicksilver line tell me it's halfway between marginal and adequate, but I watched it take off and it seemed somewhat OK. It has been extremely reliable. The high drag slow speed Quicksilver may not be the right airframe to match the little Briggs, but after seeing it fly I would not hesitate to use this style engine on a smaller and faster airplane like the Luciole or SD-1, as has already been proven out pretty well.

Dimiter Briggs Quicksilver 1.jpg
Dimiter Briggs Quicksilver 2.jpg
 
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Stuffengineer

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Apr 7, 2020
Messages
50
The 61E377-0027 generator engine did not have a decompression device on the stock camshaft. I did replace the cam with a Dynocams 275 - same lift but more duration and this cam does not have a decompressor either. Thanks for the advice to start on cylinder 2 - that makes sense.
Briggs did not use a decompressor for these engines, hence pull start was not an option. Even the starter has a higher torque than the 23hp.
 

rv7charlie

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Nov 17, 2014
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Location
Pocahontas MS
Can you just cut a scrap 2x4 to length, bolt it on, and try flippin the engine through a compression stroke? (ignition/fuel off, of course) You only go through one compression stroke on an a/c engine when propping it. A/c engines are typically lower compression, but the pistons are also 4"-5" in diameter.
 

BBerson

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Dec 16, 2007
Messages
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Location
Port Townsend WA
Do you think it can be hand propped with a club that is less than 31" long (15.5" radius)? I have serious doubts.
16” radius is about the preferred spot to grasp on my 48” prop. Get more spin with closer in. You might have a lack of inertia with no flywheel. I have a flywheel on my GX670. It helps get past the long gap in the firing order
 
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Hawk81A

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Sep 3, 2021
Messages
212
I have been told the stock Briggs magnetron won’t fire at hand prop rpm. I don’t know what ignition you have?
I've routinely checked spark on "Magnetron" equipped engines by hand flipping the flywheel. Not a lot of RPMs there. Dennis
 

Lucky Dog

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Aug 4, 2021
Messages
114
Here is an interesting Briggs & Stratton small block conversion I saw FLYING this last Saturday. The guy's name is Dimiter or Dimeter (not Dimitri), the aircraft is a light 2-axis Quicksilver based at Black Butte ultralight strip about 20 miles east of Palmdale CA. It gets off the ground and climbs out, but it is not over-powered. The local pilots who have experience with the Quicksilver line tell me it's halfway between marginal and adequate, but I watched it take off and it seemed somewhat OK. It has been extremely reliable. The high drag slow speed Quicksilver may not be the right airframe to match the little Briggs, but after seeing it fly I would not hesitate to use this style engine on a smaller and faster airplane like the Luciole or SD-1, as has already been proven out pretty well.

View attachment 127121
View attachment 127122
HA! I make these conversions. So far, just for single-surface Quicksilvers. Climb performance of the modified Model 38 Vanguard with the 52 inch three blade Ultraprop is between a Rotax 377 and 447. Going public this Fall. This one was the test plane. She's been putt putt putting in the sky for three years now. Less than two gallons per hour @43 MPH. She's not a rocket, but she flies wonderfully and worry free.
 

Lucky Dog

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Aug 4, 2021
Messages
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My concern is that there simply will not be enough leverage to turn the prop fast enough.
I'm running 2 separate GM HEI's for each cylinder. For each cylinder:
One HEI at zero degrees (top dead center) and one HEI set at 28 degrees BTDC. Start on the 0 degree HEI then switch to the 28 degree HEI after the engine is running. Some sources say that the HEI will fire at 100 RPM and others say 200 RPM.
I think the stock ignition WILL fire at hand-rop speed. The 23hp is the largest V twin Briggs sells with a rope pull starter and (I know this from experience) pull starting it is like ka-thunk-ka-thunk-ka-thunk - no faster than one could hand prop it - and it fires up. The magnets on the big block are farther out and thus will pass by the coils at a slightly higher speed, so my bet is that your 990cc monster will light up with the correct diameter club with the stock ignition. An aftermarket ignition with a Hall sensor should make prop starting a sure bet. Tipi is correct. You need to move the crankshaft just past the first compression to get enough inertia to pull it sharply through the second. We use that technique to start our higher performance small blocks using the smaller, lightweight stock starter motors.
 

Ken Powell

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Sounds like you guys think the short club will not be a problem to hand prop. I'm pretty sure the HEI will fire and that is easy to verify. I'll just follow Charlie's suggestion and use a short length of 2x4 verify that compression is not an issue.
 
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