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simple fi / cr question

 
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yourconfused
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1992 Subaru SVX

PostPosted: Fri Feb 27, 2009 12:59 am    Post subject: simple fi / cr question Reply with quote

So I tried the search and didn't find what I was looking for. I then went to google and just got destracted, but found no answer.

Why is it advantageous to lower the compression ratio when going with forced induction? I got the answer years ago in an engineering form, but forgot cause I tent to forget things. Anyone able to help me out?
Thanks,\
Shaun
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87IrocTim
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PostPosted: Fri Feb 27, 2009 1:10 am    Post subject: Reply with quote

well plain and simple, not always an advantage. the goal is to run as high of a compression as possible compared to the boost you will be pushing, with detonation.

when your motor detonates, its like hitting the top of your piston with a sledge hammer. it is basically when the air/fuel charge ignites long before the spark plug fires, which when the piston is moving up at high speeds, and that gas ignites early, you can imagine the shock it places on everything.

pre ignition and detonation is most commonly caused by too high of a compression ratio, and timing too far advanced. with forced induction, you are greatly increasing your compression ratio, making it much easier to detonate.

but the main reason why people lower their compression is best explained like this....

say you take a motor with 10:1 compression, add 3 psi of boost, and have a total of 12:1 compression, it will not make as much power as a motor with 9:1 compression, 6psi of boost with the same total compression.

but you dont want to go slam your compression all the way down just to jam a bunch of boost in it, because a very low compression motor does not run efficiently until it is getting boost.

the best way to go about it is really matching up the heads to the psi you want to push and where it will make power, along with the right camshaft to also peak at the same point, and the highest possible compression you can run with that setup without detonating. theres tons more to it but i need to go to bed Laughing

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SMOKEmUP
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PostPosted: Fri Feb 27, 2009 10:09 am    Post subject: Re: simple fi / cr question Reply with quote

yourconfused wrote:
Why is it advantageous to lower the compression ratio when going with forced induction?

You can run more boost.

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96capriceMGR
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PostPosted: Fri Feb 27, 2009 5:53 pm    Post subject: Reply with quote

Keep it simple.
You can only build so much cylinder pressure before things will ignite before the spark.

If you are feeding pressurized air in then you are partway to that point before the piston even begins to compress the mixture.

Oversimplified, but you get the idea.

Maybe it would help to know that diesels run by compressing the mixture till it ignites just due to the heat and pressure, that is why no sparkplugs, just glowplugs used to give the heat part of the equasion a boost at startup.

These days though we are running a LOT more compression with boost than you used to be able to get away with.

A lot of things go into determining how much cyclinder pressure you can get away with before it will ignite without spark, and a lot of those things have been improved in recent years.
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clay
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PostPosted: Sun Mar 01, 2009 10:09 pm    Post subject: Reply with quote

Increased compression ratio helps with thermal effenciency. Lowering the compression ratio but running more boost improves volumetric effeciency. There is more power to be gained in this case from increasing volumetric effenciency than thermal. Clay
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yourconfused
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1992 Subaru SVX

PostPosted: Fri Mar 06, 2009 9:17 am    Post subject: Reply with quote

clay wrote:
Increased compression ratio helps with thermal effenciency. Lowering the compression ratio but running more boost improves volumetric effeciency. There is more power to be gained in this case from increasing volumetric effenciency than thermal. Clay



Thanks a million. this was the little bit of info I was looking for.
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