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Octane relating to Compression 

 

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 Post subject: Octane relating to Compression
Posted: Fri Oct 10, 2008 11:44 pm 
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I've been wondering. What the maximum amount of compression I could have in my normally aspirated engine and safely run:

1) 91RON petrol

2) 98RON petrol

3) LPG (RON about 105, so I've heard)

And given that,

How much would I have to drop the static compression of each example for every 1psi of forced induction that may then go into the motor.

Which then begs the question. Should there be any difference in power between an engine that is running its maximum compression for any given fuel and a boosted motor with that fuel that has a lower compression ratio to allow for the forced induction?

I've done a bunch of searches with no luck.

Any insight appreciated

Cheers

 

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Posted: Sat Oct 11, 2008 1:26 am 
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91 RON you can safely run 9-9.5:1
98 RON you can go upto around 10:5 - 11:1
LPG upto 12:1

obsviously cams, tuning and timing play a big part, so do spark plug heat ranges and combustion chamber shape
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Posted: Sat Oct 11, 2008 8:27 am 
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Thanks for that, gives me somewhere to start.

 

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Posted: Sat Oct 11, 2008 9:22 am 
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Quote:
Which then begs the question. Should there be any difference in power between an engine that is running its maximum compression for any given fuel and a boosted motor with that fuel that has a lower compression ratio to allow for the forced induction?


depending on the levels of boost involved quite a lot of difference. even a high compresion engine wont operate near full 100% volumetric efficiency, especially a long stroke motor like our falcon sixes for instance. iirc these engines run at around 80% VE. with hi comp and fuel, cam and spark changes, you might increase that a bit, but nowhere near what forced induction does. when you go into boost you are increasing the engines VE past 100%, as its fitting in more air to the cylinder than what the engine is capable of. This is what increases power and torque as there is more air mass to be heated and thus expand in the resulting bang. Long stroke motors love FI because it overcomes the natural hurdles inherint in that setup, as rpm rises long strokes struggle to maintain efficiency in standard form, imagine a long thin bike pump, and a short fat one, which one would be easier to pump at high revs.

Im relativley newbie to this concept and I hope I havnt made too many glaring errors, but thats one beginer to another :)

 

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Posted: Sat Oct 11, 2008 10:01 am 
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66 coupe wrote:
91 RON you can safely run 9-9.5:1
98 RON you can go upto around 10:5 - 11:1
LPG upto 12:1

obsviously cams, tuning and timing play a big part, so do spark plug heat ranges and combustion chamber shape

So a 9.8:1 head should run 95-98RON?

 

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Posted: Sat Oct 11, 2008 6:45 pm 
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FairmontXR6 wrote:
66 coupe wrote:
91 RON you can safely run 9-9.5:1
98 RON you can go upto around 10:5 - 11:1
LPG upto 12:1

obsviously cams, tuning and timing play a big part, so do spark plug heat ranges and combustion chamber shape

So a 9.8:1 head should run 95-98RON?


Yes it can BUT with good ECU and tune..Four valve heads can run that comp easy with good tune..

 

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Posted: Sat Oct 11, 2008 6:45 pm 
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FairmontXR6 wrote:
66 coupe wrote:
91 RON you can safely run 9-9.5:1
98 RON you can go upto around 10:5 - 11:1
LPG upto 12:1
obsviously cams, tuning and timing play a big part, so do spark plug heat ranges and combustion chamber shape

So a 9.8:1 head should run 95-98RON?

 

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Posted: Sat Oct 11, 2008 9:52 pm 
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Grimketel wrote:
Quote:
Which then begs the question. Should there be any difference in power between an engine that is running its maximum compression for any given fuel and a boosted motor with that fuel that has a lower compression ratio to allow for the forced induction?


depending on the levels of boost involved quite a lot of difference. even a high compresion engine wont operate near full 100% volumetric efficiency, especially a long stroke motor like our falcon sixes for instance. iirc these engines run at around 80% VE. with hi comp and fuel, cam and spark changes, you might increase that a bit, but nowhere near what forced induction does. when you go into boost you are increasing the engines VE past 100%, as its fitting in more air to the cylinder than what the engine is capable of. This is what increases power and torque as there is more air mass to be heated and thus expand in the resulting bang. Long stroke motors love FI because it overcomes the natural hurdles inherint in that setup, as rpm rises long strokes struggle to maintain efficiency in standard form, imagine a long thin bike pump, and a short fat one, which one would be easier to pump at high revs.

Im relativley newbie to this concept and I hope I havnt made too many glaring errors, but thats one beginer to another :)


That definitely fills in one of the bits I was missing, Cheers. I'm planning a little boost for my VSI LPG 5.0 EL and I'm thirsting for knowledge. Got this link given to me today which you may find interesting.

http://www.modularfords.com/forums/showthread.php?t=51059

Thanks again

 

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