9 to 1 compression piston questions

[DJ Cool Breeze]

I got talked into getting a 9-1 CP pistons from jgy..they said i dont need anything extra to run this compression and that it will make more power...is anyone else running this compression right now?.???? and if so did you have to modify anything to have the car run with them in??? im new to the whole compression thing and im not really sure how it all works...maybe someone can explain it to me? like why 9 to 1 is better then 5 to 1

I also just got some eagle rods to go with the pistons....i could have just bought stock rods for the power goals i have (350) but id rather have peace of mind knowing that these rods are a bit stronger and can take a little more abuse.

Next up is a new oil pump/water pump and bearings...then im ready for the rebuild

[stopsnitchin]

9:1 compression will help your turbo spool faster. You dont need to do any mods except put them in. if you look at any high horespower drag car they usually use 9:1 compression. I also want 9:1 compression, but when I get my head ported Im going to have my head milled.

[silvia_ca18det]

i got 9:1 86.5mm bore CP pistons in right now and yes they help the turbo spool faster but you need to have a good tune to get the full potential out of higher comp pistons!

[DJ Cool Breeze]

Do i need to have my car tuned right away after putting them in? also without the tune will i notice a difference? I ask because i will be broke after this rebuild. so there is no way i could go to get it tuned right after i put them in.

Why do you spool faster with 9:1 compression? Im sorry if this is a retarded question. Im just trying to understand. why higher compression helps me spool faster.....

I must say, I do like the thought of my turbo spooling faster how much faster are we talking here? would i notice the difference on the stock t25 turbo?

[Hijacker]

what compression does in a cylinder is squish the fuel mixture more. the more you squish it, the more that mixture will push down on the piston when you ignite it. That's the basic, layman way I can describe it.

In non-forced induction applications, raising the compression of the pistons is a great way to get more horsepower out of a motor. In forced induction applications, you have to be careful. If you raise it too much, you run the risk of predetonation if you run too much boost. The reason is that you're compressing compressed air, and compressed air likes to heat up, and if it heats up enough in the cylinder, it'll light off without the help of the spark plug.

The big benefit you'll notice with running a slightly higher compression is more bottom end in power output. Of course, being the double edged sword it is, you'll limit top end some, but I think for a balanced motor, 9:1 sounds good.

Once you have the motor rebuilt and running, let us know how they run and try to get some dyno pulls to post up some empirical data.

As far as tuning goes, that's what dyno time is for. If you have any way of augmenting your A/F control, dyno runs will be the best way to optimize it, as well as positioning of the ignition timing.

[DJ Cool Breeze]

wow, thanks for breaking that all down for me. .

....I really like the fact it helps on bottom end power, as thats were id like to see some improvement.

Does having higher compression put a strain on the motor at all?

[Hijacker]

raising your compression is like any other power improvement: the way you handle it is the way it will effect long term reliability. If you put in 14:1 pistons and run 89 octane gas, don't expect to have the car idle for 3 seconds without blowing up.

So, if you apply the same theory of moderation and responsibilty to a turbo app with higher compression, don't turn your boost way up. More boost will increase the risk of predet. Of course, I can't give you a generic number of what PSI to tune your turbo to. That number is different with every comp ratio/turbo combination out there. You'll have to experiment to find that sweet spot where you get good power and response without sacrificing the motor itself.

9:1 compression isn't really all that high, to be honest. For instance, my friend is rebuilding the motor for his MX-6. It started with low comp 8.5:1 pistons, and he got new wiseco flat top pistons to go in. The old pistons would hit TDC about a centimeter short of the deck. The new ones are flat with the deck at TDC. You can imagine that the compression got bumped way up. But he's being responsible by figuring the best ignition timing and cam timing to run it.

And on the other end of the compression spectrum. A friend was telling me of some old porsches he had worked on that use dto run 6.5:1 comp pistons. Bottom end was very flacid, but by god that turbo could boost forever.

[DJ Cool Breeze]

this is a bit off topic, but i didnt want to start a whole new thread.

I was going to go ahead and order some rod bearings, but then i heard i need to messure all this crap to order the size bearings i need. So should i just wait on getting the bearings untill the motor is cracked open and is messured? also can i just go out and get some thrust bearings? or do i need to size that as well?

[silvia_ca18det]

their is a number writen on your block that indicates what grade/size your bearings are....crack the bottom open then order your bearings cuz you dont wanna have to turn the crank if you dont need to!

i made that mistake once i got some.025 under bearings and my crank was immaculate so i just stuck new oem's in!

[DJ Cool Breeze]

ok, thanks bro. I'll just wait untill the motor is taken apart.