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Discussion Starter #1
I remember reading an awesome post about compression ratio on a thread. I can't find it:/ Can someone explain how it affects HP or completely explain what differing CR would do?
 

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lower compression ratio you lose lower end torque but you can reach higher boost levels safer. higher compression you will have more power from low to high but will have lower psi and it's not as safe to run higher psi.
 

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Levee
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^^This, unless you are REALLY REALLY REALLY good at tuning and run straight alcohol and have a concrete bottom end for upgraded parts, your boost levels can't be that high. Most NA cars have higher CR's to give better power all around the power band.
 

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Same amount of air and fuel mixture in a smaller space makes the firing more forceful making more power and torque. But the burn needs to be slower which is why you need to watch how high you go with the compression before you have to run race gas or some sort of very high octane. 100+ at least. If not the gas burns faster which is too quick for the pistons to actually move fast enough and the force has to go somewhere. Correct me if i'm wrong, but this is my normal non-engineering way of understanding it. Lol.
 

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Discussion Starter #5
So raising the compression ratio is a ( for upgraded engines ) good way to raise HP? To do that you would need longer rods and ( curse my lack of knowledge ) higher out crankshaft journals? Would that make the crankshaft hit the block/something?
 

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Levee
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You don't need to worry about the crankshaft journals. You will need longer rods and different domed pistons to raise CR of the engine. There are more ways to do this but the basic and most simple way is just the rods and pistons.

Raising CR is a good way, but stroking it (longer rods) is the easiest and best way to get better Hp.
 

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I would agree with leveecius, you would also want to incorporate a bigger bore to. Thats what V8 gurus do to the bottom end. Up their compression ratio for higher end hp. Just be careful to have it all drawn out far as fuel, air, and spark.

Most of 4 cylinders that up the compression ratio, generally add bigger valves and valvetrain to accomodate the bigger compression. You have to remember if you get higher in compression to consider a different sized head gasket, yet along with longer cam lobes. This will help out tremendously for serious gains. I was thinking of staying N/A, but its a lot of parts to build it up right and keep as a daily driver.

The only downfall on having longer connecting rods is that you have a higher risk in breaking it, if its not tolerable to higher stroke. You have to weigh in all options: air/fuel ratio, heat of spark plugs, ignition timing (23 degrees after TDC), transmission, suspension, it all needs to be balanced for the power your wanting to throw into it.
 

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Discussion Starter #8
How dramatic would stroking it change power wise? And wouldn't longer rods hit the head? Or is that what you mean about different pistons?
 

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Levee
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How dramatic would stroking it change power wise? And wouldn't longer rods hit the head? Or is that what you mean about different pistons?
Stroking a motor pulls the pistons further down in the cylinder, as well as slightlypushing them up further. I've seen cars gain up to 40whp just by stroking it. However, when FI, stroking a motor gives well over double the power increase than an NA.

For example, a fully built EVO with a 2.0 and the same setup I have will hit probably 600ish whp, but my 2.3 striker will hit 800ish whp with the exact same setup. Stroking a motor allows for more air movement, which is where it is beneficial for FI setups.
 

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Levee
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So much time and money, its honestly not worth it
This all depends on his power goals. Sometimes, stroking a motor is worth it if you're looking to maximize the amount of power you can off the build you have. But again, it's more expensive, and unless you're FI, then it's not worth stroking it as the gains aren't really that much of a difference.
 
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