It will work fine.

 

Sweeeeeeet!

 

I disagree completely. That is WAY too low static compression. Sure, you wil be able to run more boost without detonation, but ESPECIALLY with most of the turbos that guys run in budget HMT setups, this will cause a dramatic increase in the time it takes to spool the turbo, and when it does spool, the power will spike, which will make driving the car rough and also put much more strain on the drivetrain leading to broken axels or difs.

 

I should have been more verbose in my prior statement. Yes, those pistons will work fine, would I spend the money and time to swap the stockers for them on a low boost setup, no, unless I didn't care about the out of boost driveability. Given 2 engines, all being the same besides one is 9.0:1 and the other 8.0:1... the lower static compression motor will be able to make more power, (don't forget CFM) than the higher static compresison motor. As far as static compression and spooling turbos, the turbo will reach full boost at about the same RPM/load for either engine, just the lower compression one will take longer to get to that RPM/load.

 

^^i agree with him. basically think of it this way: how would it feel to drive around all day with an 8.1:1 compression? absolutely terrible. it would be slow and sluggish. now once you have a turbo, sure; once the turbo hits you'll have power up the ass, but until you get there, you'll be extremely sluggish. just go with the stockers. 9.1:1 is pretty much ideal for the compression you would want.

 

Yup. If you want to drop the compression a couple tenths, Cometic & Greddy make thicker head gaskets for the D16A6..

Most factory boosted cars with quick-spooling turbos are around 9.0/1 compression; You might say its comparing apples to oranges, but the '07 WRX STi has a higher compression ratio (8.2) than what P28's in an A6 yield and it makes 293 horsepower stock.

I know its not a whole lot like a D-series, but our little engines need more help from compression to wind up a turbo than a DOHC 2.5L boxer-style engine. :TU:

 

Cool guys thanks for the input... I didn't think about the effects of such a low compression ratio... stockers are about 9.1:1... perfect. Thanks again.

 

lower compression will affect out of boost power, and slightly less in boost on the same boost levels but for the record lower compression does NOT affect spool times

 

CoreyR, I didn't mean that higher compression spools the turbo.

Before your turbo is making significant boost pressure in low rpms, the engine is essentially naturally aspirated.

Like whats been said above, a small 1.5L engine running 8.1 compression is going to be asthmatic out of boost. When the turbo finally spools up the power delivery will be very sudden and harsh on the driveline.

 

it wasnt really directed at you. someone else said it would drastically affect spool times which is just not true. its been tested and proven that compression ratio of a motor does not affect the time it takes a turbo to spool. in other words if your spooling a turbo at 3500 on a 10:1 its not going to take any longer on a 9:1 comp. ratio.

but of course out of boost power will be slightly lower.

 

Really? Compression is one of the factors that affects cylinder filling and in turn, volumetric efficiency. The better the VE, the faster the turbo will spool. Also, higher static compression generally increases the velocity of the exhuast gases, which will help spool the turbine faster.

Have you driven a very low compression setup? (Anything less than 8.5ish:1) It sucks. The power delivery, as I have noted is very disconcerting and not smooth or easy to tune at all. To contrast that, a ZC engine with stock internals running at 9.5:1 can spool a regular T25 to 8 PSI at 2500 RPM with a good exhuast. If you dropped the compression, the car would not be able to accelerate the mass of the turbine as effectively because you are simply loosing mechanical advantage, and thermodynamically, because your exhast gasses will generally have less velocity and heat to accelerate the rotating mass of the turbo.

Also, the difference in spool times between a 9:1 compression engine and a 10:1 compression engine is not the same comparison as between an 8:1 engine and a 9:1 compression. Yes, they are numerically higher, but the thermodynamic effects are not directly proportional. If you had a 6:1 compression engine and a 16:1 compression engine, the power outputs would not be 10 times the other.

So, how do you back up what you say? I have riden and driven cars with low compression set up to boost high PSI. Once the turbo spools, it's great, but getting them to spool takes more time. If I had the time and money, I'd build an engine with low compression pistons, tune it, dyno it, and gather data on spool times, then put in higher compression pistons and do the same thing. Unfortuneately, I don't have the resources available to do that.

 

its been discussed alot and if memory serves jeff evans did a test. ill believe real life testing over theory.

i owned an SRT-4 8.1:1 stock. granted its a small turbo and large displacement but it had no problems having such low compression. most factory turbo cars have well under 9:1 comp ratio.

 

Which turbo cars are you refering to? Jeff Evans name rings a bell. I will look into this further.

One other thing I would like to note is that with a higher compression engine (on boost or not) you will usually have a significantly better BSFC. With fuel prices being what they are now, that is a concern to many people.

Small turbos will always love a large displacement engine. I don't think that the SRT comparison woul dbe valid unless you were using comparable displacement sizes. A larger displacement engine should always spool a turbo faster because there will be more air moved into and out of the engine per revolution. Also, as I have been in (not driven yet) a non-stock SRT, I can say that I really hate how it drives. Aside from it being a neon, the dynamic of the engine was rather annoying. It didn't wake up until you really got on it. If you raised the compression, I bet you would make much more torque earlier on in the power band, though you may start moving enough air to get the turbo out of it's efficiency band sooner. Seeing how most people drive thier SRTs, I bet Dodge lowered the compression just to keep the engines together. But, that's just my opinion and we aren't really discussing those.