Speedracer

If there is some degree of safety even on the stock internals, why not just go with forged internals but maintain stock or near stock compression? Why has 8.5-8.6 become so popular instead of 9.5-9.6? You would certainly maintain much better off-boost response with the higher CR as well as less boosted power loss compared to stock on a given level of boost.

1 2 NV

So they can run higher BOOOOOST!!

Lorca@Z1

As much as I have been enjoying our car with APS and stock bottom end, I have considered the same thing although the biggest issue is room to grow. With lower compression pistons like 8.5/6 there is more headroom to make big power safely and a larger safety margin at lower power levels.

It sure is fun to drive at stock compression though ;-)

azrael

They lower compression so they can run higher boost on pump gas.

Personally, 9:1 CR (maybe even 9.5:1) seems like an ideal combo, allowing you to hit ~600+ RWHP on pump gas with good off-boost power. I can't imagine running higher than 600 RWHP anyway. No matter what everyone thinks, there IS such a thing as too much power. Surface streets are NOT drag strips, and maintaining traction with that much power on tap is a tricky endeavor. Driving a car that can spin the rear wheels in every gear is not that much fun in the long run.

Just because you CAN doesn't mean you SHOULD.

sentry65

iTrader: (11)

just how much difference is there in driving experience between 8.5 and 10.3?

8.5 is supposed to be like less than 3% loss in power over 10.3

the main issue is there's more lag?

how much lag is there on a 8.5 CR setup?

is it huge are just barely noticably more than stock CR?



EDIT: just read phunk's post here:
https://my350z.com/forum/forced-induction/123407-rods-or-pistons.html

just as I thought, not a massive difference in lag

having 3.5 liters I think is really helping us to not have so much lag regardless of CR

barthelb

Anything over 500rwhp in our car might be just too much. But I don't know, im not there yet. Maybe if the torque curve is moved 500 might be as manageable as 450?

Speedracer

So.....where might I get forged pistons in the 9.3-9.6 CR range? How much?

weslutes

iTrader: (7)

I dont plan on running 600+ hp, maybe 350-400 + a 75 shot should be plenty but not near 600. I just received my CP 10:1 pistons the other day. They should be good enough for what I'm doing.
I believe smogging the car may be an issue if you lower the compression drastically but the law(CA) was moved to 6 years for the first check. Still...
I guess everybody else all ready accepted the risk of driving a car that is illegal(smog) by installing their SC/TT/ST, so who cares about compression. When/if you sell your car, you may have to take all the stuff off if you can't find a shop that will make you pass for a few extra bucks.
I plan on having my car for a long time and being able to smog no problem.

GurgenPB

iTrader: (2)

My sentiments exactly, that's why I went with a "calculated" (lol) CR ration of 9.3:1.

Speedracer

Passing emissions is one of my biggest hesitancies in going this route. Of course here in Massachusetts, it is now a simple OBD check test, so as long as the OBD system checks out I'm good to go.

35ounces

iTrader: (5)

speedracer you are considering going FI again?

G3po

iTrader: (8)

Hmm I calculated 9.003:1 as the optimum on 91 octane.

G3po

iTrader: (8)

For example , Arias will make any custom CR you want.I had them make me 9:1 with a custom spec crown design.
Check out forgedinternals.com.

Dirty Dave

iTrader: (8)

After blowing my motor the other day i wouldnt even hesitate to go with real lo compression just to stay on the safe side. I've heard that there isnt a real noticable difference from 8.5 to 9 anyways.

Speedracer

[QUOTE=35ounces]speedracer you are considering going FI again?[/QUOTE

Yeah, I am considering it again......this week. After owning a high power vehicle, no matter how much I try to convince myself that my Z is fine the way it is, or that I could just do a nice moderate NA build-up, I just can't get over the fact there are many faster cars out there. The Z just looks faster than it actually is and I just want it to have all the "go" to match the "show". That acceleration is SOOOOOOO addicting. I try to convince myself that I really shouldn't do this, but it keeps haunting me and haunting me. It's not even an issue of can I afford it. It is a question of should I spend the money. In fact, I wish I couldn't afford it because then it would be an easy decision. I'm so confused, and that is why I keep jumping from one idea and philosophy to another. Even my wife gave me the go ahead, and that makes it even harder. I wish she had just said no!!!!!!!

captj3

iTrader: (1)

Go for it.She may never give you the opportunity again...lol
[QUOTE=Speedracer]

GurgenPB

iTrader: (2)

Can you tell me how exactly did you do this? I am pretty curious. When I said I calculated, I really did not calculate per se, only took a very educated guess. I am planning to be runnign more than 91 octane when I push the engine to the max (over 550whp if I ever get there)... so effectively, 9.3 or even 9.5:1 is not a bad idea if you plan to run high octane gas at the track...which many many do.

Brandon@Forged

Lower compression ratio lowers the detonation threshold, allowing you to run more boost.

Given a certain situation, two exact same cars, same turbo kit on each car, one with a 10:1 CR and one with a 8.0:1 CR, both are running the same PSI. The one with the higher CR will make more power. The higher the compression the more detonation threshold (less boost to achieve the same power, however, cannot boost past a certain PSI that a lower CR piston could allow you to boost to), the lower the compression ratio, the less detonation threshold (more boost to achieve the same power or more).

G3po

iTrader: (8)

I was yoking, I just went with 9:1 , not too low , not too high. The actual crown design has more to do with anti-detonation characteristics than the actual static CR +- .5 point. You could make a flat top 8.5 piston with less knock tolerance than a well designed 9.5. Ideally you need to pay attention to the combustion chamber design just as a modern OEM FI would. A simple dish is "OK" but not usually optimized. The reason a simple dish is often used is because it takes less time on the CNC machine, not because it is optimally matched to the combustion chamber of choice. The crown and chamber ideally act together to raise the knock tolerance, which is basically the main concern in addition to light weight and strength. Also lowering the top ring does alot to improve land strength; however , at a slight penalty of higher emissions.

35ounces

iTrader: (5)

[QUOTE=Speedracer]LOL, I wasnt buying that thread of yours about the car being more fun stock than boosted.
I hear ya though. I'm trying to decide what to do too.