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 Raising Compression Briggs 12.5 HP

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Apprentice
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PostSubject: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeSun Oct 21, 2012 8:04 pm

I have a 12.5 hp briggs powerbuilt on my yardmachine and am thinking about putting a head from a 10 hp briggs on it to raise my compression and get a bit more power. I was wondering if any of you guys have done this and if there are any risks involved. Also do I have to change or modify any other parts on the 12.5? Very Happy
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeMon Oct 22, 2012 5:21 pm

The head from a 10hp (22ci) will not fit the larger 25/28ci blocks. If you have a 28ci block, using the head from a 25ci block will raise compression some.
The 11hp head is a direct swap for your 12hp head, no modifications involved.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeMon Oct 22, 2012 7:34 pm

I have a parts 11 horse I can take the head off of. Do you think the engine can handle the extra compression? I dont want to break any thing.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeTue Oct 23, 2012 5:55 am

It's not a huge bump in compression; Your engine will be fine.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeTue Oct 23, 2012 3:21 pm

I will try it out.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeTue Oct 23, 2012 5:59 pm

I changed the head and took it for a test ride. I dont really notice any difference in power or anything really. Im going to leave it on but I dont notice any changes.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeTue Oct 23, 2012 8:18 pm

take a sheet of sand paper and "mill" the head down yourself, even more compression:D
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeTue Oct 23, 2012 8:24 pm

How much psi can my engine handle. I want more power but I need this engine to last and be dependable. I need to buy a compression tester but I dont want to spend $40-$80 on one.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeTue Oct 23, 2012 8:33 pm

I dont know how much they handle but I thought you could make a compression tester if you got a pressure gauge, small check valve(hardest thing to find) and a fitting that would fit the spark plug hole so when you turn the engine over the check valve doesn't allow the compressed air back into the engine and you dont get a reading. I know of some people that did that but I don't know if that's exactly what they did.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeWed Oct 24, 2012 12:41 am

You can use a schrader valve ( used in valve stems for tires) as a check valve, just point the little nub towards the engine. ( i have a cheapo CanTrash compression gauge that uses this exact method). For the spark plug adapter, I've used actual spark plugs, by breaking out all the ceramics cores and conductor, and welding in a nipple. I;ve also done the same thing by breaking out the ceramics and threading the plug body to 1/4" npt, it depends on the size of the plug
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeWed Oct 24, 2012 6:38 pm

One thing to remember is if its the late 90's 12.5 powerbuilt motors( black flywheel shroud) have a weak rod casting - i have like 3 tractors that had those motors, all had thrown rods - 14HP OHV rods can replace those weaker rods.

Actually a better way to add more power - a better carb - like from a motorcycle or snowmobile - the plus is theyre a whole lot more adjustable then a stock briggs carb.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitimeFri Oct 26, 2012 3:27 pm

Yup thats the engine I have. I dont know if i want to get into swapping carbs. I think im going to keep this one the way it is and build up a tractor with more hp.
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PostSubject: Re: Raising Compression Briggs 12.5 HP   Raising Compression Briggs 12.5 HP Icon_minitime

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