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SpiroMac
Samba Member
Joined: August 11, 2023
Posts: 2
Location: Germany
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 Posted: Fri Aug 11, 2023 2:33 pm Post subject: Piston shape - does it matter? (Will it run?) |
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I have an AP Motor that I bought used and when I pulled the sieve there were pieces of piston ring dropping out.
I ordered new pistons, but when they arrived they were the ones with a concave head. My old ones (they were upgraded by the person before me) were flat heads.
Do I need to send these back and order flat ones?
Thanks for the help! |
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SGKent 
Samba Member
Joined: October 30, 2007
Posts: 41031
Location: Citrus Heights CA (Near Sacramento)
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| Posted: Fri Aug 11, 2023 2:49 pm Post subject: Re: Piston shape - does it matter? (Will it run?) |
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concave are typically for buses. You will need to calculate the compression ratio to answer the question accurately/correctly.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin |
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bsairhead
Samba Member
Joined: October 08, 2008
Posts: 3581
Location: viroqua wi.
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| Posted: Fri Aug 11, 2023 2:49 pm Post subject: Re: Piston shape - does it matter? (Will it run?) |
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| If the deck height is the same your compression will be lower. |
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Wildthings
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Joined: March 13, 2005
Posts: 50353
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| Posted: Fri Aug 11, 2023 4:10 pm Post subject: Re: Piston shape - does it matter? (Will it run?) |
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| I run ethanol free fuel to eliminate problems with hot restarts and stale ethanol building up crud in the fuel fuel system and valve stems. Since the only ethanol free gas tends to be premium that means I can run higher compression. If you do the same then you should be able to run flat top pistons without getting engine knock. Just set your timing at 27-28° BTDC at 3800+ rpm and run with it enjoying the extra power and gas mileage. YMMV. |
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SpiroMac
Samba Member
Joined: August 11, 2023
Posts: 2
Location: Germany
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| Posted: Fri Aug 11, 2023 5:26 pm Post subject: Re: Piston shape - does it matter? (Will it run?) |
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| Wildthings wrote: |
| I run ethanol free fuel to eliminate problems with hot restarts and stale ethanol building up crud in the fuel fuel system and valve stems. Since the only ethanol free gas tends to be premium that means I can run higher compression. If you do the same then you should be able to run flat top pistons without getting engine knock. Just set your timing at 27-28° BTDC at 3800+ rpm and run with it enjoying the extra power and gas mileage. YMMV. |
Could that be what got me into this mess in the first place? I bought the motor used and it was put in for me, I’ve been running the cheap fuel in it. It started blowing out heavy smoke through the exhaust. Perhaps the compression was too high for the fuel I was using? |
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SGKent
Samba Member
Joined: October 30, 2007
Posts: 41031
Location: Citrus Heights CA (Near Sacramento)
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| Posted: Sat Aug 12, 2023 12:46 am Post subject: Re: Piston shape - does it matter? (Will it run?) |
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until you MEASURE the compression ratio you have no idea what it is. With flat pistons it is probably closer to 8.5:1. VW used 7.3:1 on late bays.
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