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Compression Ratio
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Author:  Beanie [ Wed Mar 25, 2015 8:01 am ]
Post subject:  Compression Ratio
Hi All

Got a question regarding the Compression Ratio ..

what is the ideal number for a mini , assuming that there is no detonation can we keep bumping it up , is there any adverse affect on the comp ratio to the rest of the motor ..

im talking in the realms of 14:1 and higher ,


Cheers
Beanie
Author:  TheMiniMan [ Wed Mar 25, 2015 8:20 am ]
Post subject:  Re: Compression Ratio
16:1 was pretty normal for methanol fuel-ed minis back in the old speedway days

we`ve always run 13.5:1 on our race engines (*running Av-Gas)
altho i understand quite a few other mini racers prefer 12 or 12.5:1 -ish

rings & their ring lands can be hammered,,,
big end brgs can get hammered,,,
head gaskets can cop it
but as a race engine we usually pull them down every 200 Race miles & re-new all the soft stuff
(ie: rings bearings, valve seats etc) so they`re all simple collateral damage & if we replace all that stuff every 200 race miles then there`s no reason the engine won`t last & last & last
unless something else turns to sh!t in the mean time of course :-)

safe for road fuel is approx 10:1
safe for avgas is about 13.5:1
safe for methanol is approx 15:1
the new E85 i havn`t played with myself yet but a bundle of the Improved production guys
that i build engines for are using E85 & are running 13:1 with no problems
& also using significantly more ignition than they would with other fuels & all seems well so far in the last couple of years

i`m not sure where you`re going with the question to be honest tho?
Author:  Beanie [ Wed Mar 25, 2015 8:34 am ]
Post subject:  Re: Compression Ratio
Race motor that im running at the moment is at nearly 14.9:1 , running on E85 , but its something that i have put together with parts from all over the place , old Mahle flat top pistons , a very nice head , and a Vp5 cam , i recently killed some conrods ( after the car ran out of oil due to the oil filter coming loose ) so im doing a rebuild with new rods and pistons , but trying too see if im going in the right direction with high comp or barking up the wrong tree so to speak ..

Cheers
Beanie
Author:  Rookiepilot [ Wed Mar 25, 2015 9:46 am ]
Post subject:  Re: Compression Ratio
Beanie, are you running hardened value seats with E85? As Matt said above, my car also runs 13.5:1 on Av-Gas, but I'm finding it harder to buy it now, unless you buy a 205 litre drum or buy it from the circuit. There must be a Compression ratio figure where the costs of rebuilds out weigh the benefits of increased HP.
Author:  Beanie [ Wed Mar 25, 2015 10:48 am ]
Post subject:  Re: Compression Ratio
Rookiepilot wrote:
Beanie, are you running hardened value seats with E85? As Matt said above, my car also runs 13.5:1 on Av-Gas, but I'm finding it harder to buy it now, unless you buy a 205 litre drum or buy it from the circuit. There must be a Compression ratio figure where the costs of rebuilds out weigh the benefits of increased HP.



Im not running hardened seats but i should look into it

I made the switch to E85 and wont look back , 6 laps of phillip island with a standard radiator and no oil cooler and the water temp only reached 83 degrees , its knock resistance is fantastic

and then there is the cost

avgas cost $3.20 p/l or so
E85 $ 1.20 p/l and its at my local servos

love the stuff
Author:  FNQ [ Wed Mar 25, 2015 6:57 pm ]
Post subject:  Re: Compression Ratio
In the states (as in USA) mini racers use 14.5-15 BUT their octane rating is equiv to RON 113 as far as i can tell.
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