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Discussion Starter #1
Alright, I know I'm a "noob" and all, but I understand the basic concept of the internal combustion engine. So, with that in mind, why do so many people seem to be so obsessed about making their engine low compression when they're going turbo? Maybe if you're planning on running 20-30psi it would make sense, but on a smaller displacement engine with a fairly low boost setting what's the point of going low compression? You're only going to get even worse low end throttle response than you already do with pretty much ANY four cylinder Honda (or any relatively small displacement four cylinder). All forced induction is doing is increasing your compression ratio anyway, so if you start with a moderate ratio to begin with (9-10:1 or so) it seems you'd have better low end throttle response to carry you up to when boost peaks. Maybe someone could help me out with this as I could be wrong. By the way, this icon is hilarious:IHI:
 

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uve got the right idea. u dont really want a super low compression piston because when ur out of boost it will be a dog.

IMO on a d-series the best turbo compression is 9-1 because thats just a tiny tiny bit lower than stock compression. and u will still have decent 'out of boost' response. while also creating faster spool times.
 

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R2.....Zap Balls
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Mine is around 8.5 to1 compression bored .020 with wiseco pistons and eagle rods. It drives just as good or better than my stock compression did. Granted mine is running 20.5 psi, but for the most part I just drive it around in vac. You want to go lower on the compression to allow for a more aggressive tune and allows a little more room for error. The higher your compression when tuning, the more dead on perfect the tune has to be.
 

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Discussion Starter #6
Well, that's what I meant. I understand if you're running fairly high boost you'd want to lower your compression ratio. Kinda like how the stock Evo is running something like 19psi and it has like 8.5:1 comp ratio (I'm not sure if that's completely accurate). But with lower boost is what I was talking about, where you're not relying pretty much soley on boost for your power.

And when I said do "they" make stock comp ratio Z6 pistons I meant does anyone make 9.2:1, not 9.0:1...but I guess there's really not much difference.

I appreciate the responses...and the rabbit with antlers avatar.
 

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I appreciate the responses...and the rabbit with antlers avatar.
thats no rabbit!!.... thats a jackalope!! very very sneeky!!

the main reason people lower the compression is because they want a little room for error incase something happens with the tune. i just personally think people need to put more time and effort into the tuning of their cars and build a stock cr motor with built internals. in my opinion it would be a better way to build a motor.

my friend has a turbo awd talon. he was running on a stock block 20psi with built head and a 60-1 turbo. his car ran its balls off.... the stock cr was around 8.5:1 or so and when we put in his built shortblock with 9:1 cr.... HOLY SHIT!!! you could definately tell the difference with the .5:1 cr increase.
 

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Super low compression for those production model engines are done that way so they are safe for pump gas and any room for error.

In our race engines, B-series setups, we are about 11:1 compression and boosting 30-45psi. It all depends on the piston design. I personally dont like low compression pistons under 9:1 in any setup. I'm doing something like a 10:1 piston on my own D-series Turbo build. I think alot of people are just scared of high compression from all the rumors and myths theyve heard. As long as your tuning the motor properly, and making sure everything is right, a mild high compression is ok for turbo. There's even some racers that I know run 12:1 compression turbo motors.
 

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allwheeldriveturboboxer
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Super low compression for those production model engines are done that way so they are safe for pump gas and any room for error.

In our race engines, B-series setups, we are about 11:1 compression and boosting 30-45psi. It all depends on the piston design. I personally dont like low compression pistons under 9:1 in any setup. I'm doing something like a 10:1 piston on my own D-series Turbo build. I think alot of people are just scared of high compression from all the rumors and myths theyve heard. As long as your tuning the motor properly, and making sure everything is right, a mild high compression is ok for turbo. There's even some racers that I know run 12:1 compression turbo motors.
right. the main thing to remember is controlled cylinder pressures are directly correlated to power potential. as mentioned, if you have ideal atomization, tuned timing events, etc, and the mechanical parts can handle the stresses, you can and should run whatever you can to gain the highest controlled cylinder pressures.

my mazda has 9.5:1 bone stock and runs 16psi. this is because it has direct injection so there is less need for a "compression cushion".
 

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Discussion Starter #11
So, would it then be practical to stay with near stock compression when only going to maybe 10psi boost? If your target was only to get up to 10, would it be the better way to go?
From my personal research, the short answer is yes. My brother is going to be building my car while I'm in Korea for a year (thank you US Army) and I'll be staying stock as far as the motor goes. I'll be running probably 9-10 psi or so and I'm supremely confident that, according to my research, with a good tune I'll be fine for daily driving. Stock Honda blocks are already sleeved (as are most modern engines) and they can hold up to a lot more than they do stock as long as the tune is good. With the freeware that's available nowadays, it's too easy to build and tune something stable from a stock motor. Unless you're building something for 300whp or more there's really no reason to spend money on internals, although overbuilding is always a good idea reguardless of what your power goals are.
 

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Regardless of what compression you choose. Stock block or built block, the MAIN Thing you have to invest on is tuning tools(Engine management, and dyno time) I came from the old school FMU days, and we've learned so much in the last few years that I would never put together a turbo motor just relying on an FMU and throwing in a bigger size injectors.

No matter how much boost you do, invest in some kind of engine management, fuel controller, etc.
 

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Along with everything that has been said there's another reason
alot of ppl that have D-series go hybrid (i.e. Mini-me)
stock A6 pistons = 9.11:1 cr ......(w/ stock head and gasket)
ADD a Z6 head and Z6 gasket w/ A6 pistons = 9.96:1 c/r

**math time**
if stock A6 longblock was 9.11
Subtract 8.5:1 Pistons
---------------------------
Equeals .... .6 c/r


now subtract that .6 cr from the setup Most ppl run cause they want the advantage of VtAKK

9.96:1
- .6
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9.36:1 cr....which is higher that the original CR that the car came w/


put a Y8 head on a A6 block that has 8.5:1 pistons... and you got a NASTY TURBO HYBRID W/ VTAKK and a cr of 9.63:1

all info was found here... http://www.knology.net/~jediklc/dcalc.htm

i just wanted to take a different approach and make ya think for a minute... but that's the reason i bought my 8.5:1 Wiseco (now paperweights) back in the day. Besides it was my first Mini-me Turbo build.. and i was a Baller back then. :)

Dr. Mike
 

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Discussion Starter #14
Ok that makes complete sense and it's something I didn't even think about. The whole mixing and matching D series components would change your cr. Thanks
 
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