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 186 Cast Iron barrel compression ratio

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johnSbantam

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Number of posts : 254
Localisation : New Zealand
Registration date : 2006-12-01

PostSubject: 186 Cast Iron barrel compression ratio   Tue Sep 08, 2015 4:09 pm

Do any experts have any thoughts about maximum CR for an old cast iron barreled 186 race motor?
Currently 12.3:1 geometric, with 175 degree exhaust opening running Avgas.
But difficult to get carburation right. Granted Taupo at 1650ft ASL, temp 4 -16 degrees poses some challenge !
It has been suggested that my CR is too high ?
We ran 14.3:1 in longstroke 125.
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dansofield550

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Number of posts : 362
Localisation : gravesend kent
Registration date : 2011-10-23

PostSubject: Re: 186 Cast Iron barrel compression ratio   Tue Sep 08, 2015 8:27 pm

about 8 or 9 to 1 seems the norm, aprox 23cc in the bowl part of the head, I always tend to
think that raising the comp is the easiest way to get a bit of extra power but has the highest risks of things going wrong,
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john bass

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Number of posts : 1737
Age : 88
Localisation : Bensberg, Germany
Registration date : 2006-12-06

PostSubject: Thermal Efficiency...   Wed Sep 09, 2015 4:19 am

Raising the compression ratio does bring increased power:  at 8:1 CR the thermal efficiency is 56%  and at
14:1  it is 65% -- an increase of 9% but with increase of CR there is an increase of engine friction drag which bringswith it  a power loss to the mechanical drag. So there is  a trade-off of >CR - power loss to drag .... Reduced engine  friction drag is of course the tuner´s top prioroity.  

Goes without saying, Ignition timing has to be modified with change of CR....

Thermal efficiency -- according to the Air Standard Efficiency curve -- at 9:1 CR is 58%  and at 12.3 CR is
63%  -- 5% better burning-of-fuel use ....  
.....  and if the fuel:air ratio is weakened by  10% lean there will be a 5% improvement ín thermal efficiency ...  

The latter is why a 2-Stroke engine -- particularly a Bantam Racer -- goes so much bettrer just before it seizes.

This  why most of us -- in those good old days -- ran our engines about 10 -- 15% rich....

With the latest Racing-Engine-Management-Systems where all engine data is monitored the A/F ratio and ingition timing can be rapidly  modified to momentarilly increase power to that Lean extra punch -- for that bit of extra acceleration -- and as quickly return to `Safe´ parameters before disaster occurs.

REGARDS THE LATTER -- THE Racing Bantams Don´t have that ENGINE MmANAGEMENT System yet -- DO THEY??

CHEERS!

JayBee.
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