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Old 06-20-2003, 07:45   #1 (permalink)
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Compression For Dummies )(ie ME)

i need some enlightment please.
What is the benefit of increasing engine compression, and how is it done ? and when should it be done ? (whats engine mods are done to warrant the higher compression)
If someone can roughly expain it to me, because i know the concept of it, but dont actually know what i am talking about !

thanks

Luke
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Old 06-20-2003, 08:26   #2 (permalink)
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Re: Compression For Dummies )(ie ME)

I'll ramble on.Compression is what is needed to get power/ combustion. On an engine with a fair overlap cam where air resonates in and back out of port at low speeds. Makes V.E low so compression is built up to get some bottom end rpm power.
The more compression the better combustion can push piston down the bore.In most cases its done on higher reving engines where inlet manifolds are shorter [Gen lll] and as said slightly lumpy cam. As in XR6... See the compression off the 290 kw?
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Old 06-21-2003, 02:58   #3 (permalink)
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Re: Compression For Dummies )(ie ME)

** edited by xa-coupe ***

Compression Ratio is deemed by the swept volume of air/fuel of the cylinder bore by the piston from Bottom Dead Center (BDC) to Top Dead Centre (TDC) called the Stroke Plus the area of the Combustion Chamber.
Basically if the compression ratio is 10:1 the amount of air/fuel mix at BDC is compressed to 10% of its original size at TDC.

Raise the ratio, the air/fuel molocules have less area to move around in. They get hotter by bouncing into each other more. When they are ignited they rapidly burn faster than they used to creating quicker and greater combustion expandig pressure. I say greater because no four stroke engine ever gets complete combustion.This forces the piston down quicker, hence getting more power. This is a more accurate description.

To achieve higher compression ratio? Some ways:
*Lengthen Stroke of the Engine
*lower the block deck height
*change pistons-many ways
*Change combustion Chamber, shape, size.
*Machine the head

ColCam

Last edited by xacoupe; 06-21-2003 at 04:07.
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Old 06-21-2003, 03:48   #4 (permalink)
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thanks alot for that guys.
the reason i ask, is i am contimplating getting a 347 stroker kit for my XR8 and was told maybe a higher compression (compared to standard) would help too, but it sounds like it will already increase by doing this anyway.

thanks again.

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Old 06-21-2003, 05:53   #5 (permalink)
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Re: Compression For Dummies )(ie ME)

Quote:
Originally Posted by XR8fella
i need some enlightment please.
What is the benefit of increasing engine compression, and how is it done ? and when should it be done ?

(whats engine mods are done to warrant the higher compression)

If someone can roughly expain it to me, because i know the concept of it, but dont actually know what i am talking about !

thanks

Luke

What mods to warrant the higher compression??? !!!!!!
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Old 06-21-2003, 16:04   #6 (permalink)
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Re: Compression For Dummies )(ie ME)

oh yeah that was my other question.
but it seems like its pointless.
because as i said i am looking to stroke, and want to get some better heads, and thought that raising compression after i do this could give more HP, but by the sounds of it, stroke + heads will increase compression, am i right ?????
what compression did the au2 200kw 302 run at?
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Old 06-22-2003, 06:13   #7 (permalink)
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As you said, you don't have a lot of knowledge into what increasing compression ratio does, so don't leap into increasing it!

What has been said so far is correct, but what also is missing is some of the down falls.

Increasing compression can cause pinging or detonation if it is too high or other conditions are not optimum, then the need for high octane fuels to prevent this, otherwise engine failure is likely to happen.
High compression engines can be harder to start.
High compression engines can have less engine life.
High compression can sometimes increase engine temperature.

But as always, there is always pro and cons to anything. :s5
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Old 06-23-2003, 17:25   #8 (permalink)
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Re: Compression For Dummies )(ie ME)

Better off to lower your compression to 8.5:1 and bolt a couple of turbo's or a blower up to it. Much less ware on the engine.
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Old 06-23-2003, 21:13   #9 (permalink)
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Re: Compression For Dummies )(ie ME)

something that wasnt said yet is that stroking will effectively lower you optimum rpm. therfore if your peak hp was at around 5500 rpm then the stroked motor might peak at 4800 rpm which refers back to what was said about high compression being somewhat better for high rpm where you have less time to exchange fuel and air .
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Old 06-24-2003, 00:07   #10 (permalink)
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Re: Compression For Dummies )(ie ME)

Quote:
Originally Posted by BlownV8
Better off to lower your compression to 8.5:1 and bolt a couple of turbo's or a blower up to it. Much less ware on the engine.
Spot on :s6: I think EBXR8240 will agree.




Quote:
Originally Posted by 351 W +
something that wasnt said yet is that stroking will effectively lower you optimum rpm. therfore if your peak hp was at around 5500 rpm then the stroked motor might peak at 4800 rpm which refers back to what was said about high compression being somewhat better for high rpm where you have less time to exchange fuel and air .
You are probably right here, but the bad thing is stroking a 302/5.0L would most likely decrease engine life, most stroker kits have a very poor stroke to rod ratio, increasing side loads and piston speed higher than optimum.
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