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Compression ratio.
https://www.ausmini.com/forums/viewtopic.php?f=2&t=18450		 Page 1 of 2
Author:  leyland brother [ Sun Apr 23, 2006 10:02 pm ]
Post subject:  Compression ratio.
I've just sent a message to a seller on eBay about the compression ratio of his mild engine, he replied a compression test showed 215psi.

What the???

Is that the right?
Shouldn't it say... **:1?
Author:  Spaceboy [ Sun Apr 23, 2006 10:05 pm ]
Post subject: 
nah, thats nice and high, which is good for POWA :lol:
Author:  leyland brother [ Sun Apr 23, 2006 10:16 pm ]
Post subject: 
Do you know how to convert that to **:1?
Author:  Spaceboy [ Sun Apr 23, 2006 10:21 pm ]
Post subject: 
no, but 215 psi is very good
Author:  Morris 1100 [ Sun Apr 23, 2006 10:22 pm ]
Post subject: 
You cannot convert the PSI figure to comp ratio very accurately but you can guess. :wink:
My HQ race car used to give about 230- 240 on the gauge and it was about 12.6:1.

(useless tip) If you want the gauge to read higher tip some oil in the hole. The more oil the higher the reading. :wink: (/useless tip)
Author:  leyland brother [ Sun Apr 23, 2006 10:57 pm ]
Post subject: 
So you could probably guess it would be around 11:1 ratio.

Not bad i spose!
Author:  Morris 1100 [ Sun Apr 23, 2006 11:00 pm ]
Post subject: 
leyland brother wrote:
So you could probably guess it would be around 11:1 ratio.
Or his gauge is stuffed! If it had 11:1 it would ping on nearly every fuel. And it would be hard on starters.
Author:  drmini in aust [ Sun Apr 23, 2006 11:04 pm ]
Post subject: 
It is not really this simple- if the motor has a long duration (ie sporty) cam, this will reduce cylinder filling at cranking speed, the result will be lower readings than with a stock one, at any given C/R. :lol:
Author:  Molina [ Sun Apr 23, 2006 11:05 pm ]
Post subject: 
My starter motor died recently and i had it replaced with a second hand item for $55. I thought my battery was sick since it would crank slowly and sometimes sound like it was struggling to turn the engine over. I tested the battery and it was apparently fine.

Could the sluggish cranking be a result of my higher than standard compression? I'm running 9.4:1 at the moment. Is that enough to make a noticable difference?

Sorry if this is considered thread jacking but i guess it's sort of on a similar topic. :?
Author:  drmini in aust [ Sun Apr 23, 2006 11:12 pm ]
Post subject: 
At 9.4:1 it should crank fine. Mine is running 10.7:1 and spins fine with a factory recond Lucas and a 550CCA battery.
Could be your starter is dodgy, methinks- I had one like that, allegedly reconditioned... :wink:
Author:  Molina [ Sun Apr 23, 2006 11:14 pm ]
Post subject: 
Difference is that i don't think it was reconditioned... should i have expected a reconditioned one for $55? That seems pretty cheap.
Author:  drmini in aust [ Sun Apr 23, 2006 11:16 pm ]
Post subject: 
Molina wrote:
Difference is that i don't think it was reconditioned... should i have expected a reconditioned one for $55? That seems pretty cheap.

No, but it should spin fine under load. If not, take back if possible.
You can get an idea of how good the brushes in it are by spinning it with your fingers- if no drag at all, it needs new brushes fitted.
:wink:
Author:  1360 LS [ Sun Apr 23, 2006 11:41 pm ]
Post subject: 
My LS runs at roughly 10:1 I'm told. is that good?? im clueless at the end of the day when it come to technical mechacanic stuff
Author:  drmini in aust [ Mon Apr 24, 2006 6:50 am ]
Post subject: 
What you can run at boils down to fuel type, cam duration and intended use. 10.0:1 is reasonable.
I run 10.7:1 now on the street with an RE282, with this cam it could stand a bit more.

For an RE13 and street use with 98 octane fuel though 10.7 is about it, I found 11.0:1 a bit high... it pinged then. :cry:

For track work only you could go up more, as the motor never gets lugged.
Author:  1310/71 [ Mon Apr 24, 2006 7:13 am ]
Post subject: 
Molina wrote:
My starter motor died recently and i had it replaced with a second hand item for $55. I thought my battery was sick since it would crank slowly and sometimes sound like it was struggling to turn the engine over. I tested the battery and it was apparently fine.

Could the sluggish cranking be a result of my higher than standard compression? I'm running 9.4:1 at the moment. Is that enough to make a noticable difference?

Sorry if this is considered thread jacking but i guess it's sort of on a similar topic. :?


I would also check/tighten all your earth leads and battery earth strap - you may have coincidently loosened one when changing over starters.
KB
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