REFLECTOR: Replacing low compression pistons for standard compression

[Scott Derrick]

John,

thank for the info. I believe 8.5:1 is the standard compression for 
IO520 and IO550. Not sure what Lycoming uses.

I had considered going with aftermarket 10:1 but they are very expensive!

Scott

-------- Original Message --------
Subject: Re: REFLECTOR: Replacing low compression pistons for standard 
compression
From: John Dibble <aminetech at bluefrog.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Date: 05/17/2012 12:32 PM

> Scott,
>
> My experience is that for low compression engines, CHTs run higher when
> high octane fuel is used. High octane fuel is for high compression
> engines. The high octane fuel burns slower and avoids detonation that
> the high compression causes if low octane fuel is used. Since high
> octane fuel burns slower, some of the heat of combustion may not get
> converted to HP in a low compression engine (where combustion is already
> slower because of the lower compression) and instead contribute to
> higher engine temperatures. 8.5:1 is still low and better suited for
> lower octane. Still it should be better than 7.5:1 with 100 LL.
>
> John
>
> On 5/17/2012 8:10 AM, Scott Derrick wrote:
>> I'm thinking of replacing the 7.5:1 pistons in my IO520 with Std 8.5:1
>> pistons.
>>
>> For the obvious reason of gaining 18 HP.
>>
>> I think somebody mentioned that they thought that the CHT's would drop
>> with that change because we are running 100 octane fuel?
>>
>> Can whoever mentioned that temperature difference elaborate?
>>
>> Scott
>>
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