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Old 08-28-2001, 12:26 AM   #1
Scooby South
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Default Compression Question

What is the optimum Compression ratio that I should acheive for a full blown buildup.....8:1??? 8.5:1??? or even maybe 9:1
What would you guys think??
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Old 08-28-2001, 09:01 AM   #2
8Complex

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Depends what you're shooting for and what type of power you want. Higher compressions get you more down-low torque, while lower ones can handle more boost and get you more high-end power.
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Old 08-28-2001, 12:25 PM   #3
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Allrighty.. so let's paint the picture of a daily driven MY00 2.5RS running 7psi; capable of a 12 psi on race gas for those special days. I was thinking 9.0:1?? Sound good?

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Old 08-28-2001, 12:37 PM   #4
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Maybee having the lower compression would save a few trannies out there. Less torque = Less stress on gears?
Kevin
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Old 08-28-2001, 02:02 PM   #5
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Hey Danny...what compression ratio was your 2.2....Built motor...

I want reliability, I WILL have some sort of engne management Tec2, Ultronic or equivilant. I will also have forged piston and rods...I know the lower you go the more boost you can apply but I also wnat the bottem end torque...9:1 is sounding really good right now...
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Old 08-28-2001, 02:29 PM   #6
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If you have the 2.5, don't go any lower than 9:1 otherwise it will feel like a 2.0L at the low end. But, you'll be able to run LOTS of boost with 8.5:1.

Whatever suits your needs.
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Old 08-30-2001, 03:03 AM   #7
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I'm running 9:1 and got cams put in and it feels about the same as 10:1. I'm still running the same boost until I can get engine management and injectors, which will be real soon.
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Old 08-30-2001, 04:40 AM   #8
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Thumbs up Thanks....

Good...That means my research is paying off....

Hey 8....Are you getting this???
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Old 08-30-2001, 12:32 PM   #9
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I believe that 8.5:1 is the best comp ratio out there for a turbo engine, dependent on target boost pressures. If you are only looking at a maximum of 12psi the you can keep your stock compression ratio and just get good quality (venolia, arias) pistons. You can tune that to run everyday at 12psi on pump gas. ECU and turbo premitting...

However!

Why limit yourself?? If you are building the motor might as well make it so you can have all the HP you want! Get something in between 8.5:1 and 9:1. Its a real sweet spot. 8.8:1 would work really well. Then you can run 16 -20 (again, other equipment permitting) on pump gas and 28 on race (c16 anyone? ) if your block will take it..... Your transmission will if you are drag racing. Just don't stay in 2nd gear too long. The tranny isn't that weak. There are ways around holes in the case.

Just some thoughts
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Old 08-30-2001, 12:55 PM   #10
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Personally, I was thinking of building a block to give me good torque and run extremely fast at 12psi as well. I was thinking that 9.5:1 is about right on the ragged edge of safe vs. blown up, so I'll probably go with that and settle for 10psi. At that point I shouldn't have troubles with built Corvettes, while still being able to autocross the car.

I have a friend who having his DOHC EJ25 built by a very reputable tuner. Latest dyno test showed over 400hp at the flywheel at 15-16psi, I believe he is running 9.7:1 compression. Crazy crazy car, I will love to see it on the road/track. I'm sure everyone will get a report on this as soon as it is done, though I suppose it's possible he'll hit a few mag's too.
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Old 08-30-2001, 01:47 PM   #11
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If my car ever gets finished I can let you know how 8.5:1 is with off-boost response and lower boost settings.

I wouldn't hold your breath waiting for my info though as it may be a while...

Aaron
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