That's not much of an increase over the stock CR, but I'm not sure how well the VQ35 holds up against the combination of a high CR and boost. I guess it comes down to what you're after. Is it numbers or driveability and longevity?

High compression is still high compression while off boost. The lower, boost-friendly CR might allow the engine to behave more nicely while off boost. When you drop the hammer, that lower CR will allow for higher boost. In the end, isn't that what all of us boost junkies are after... ...more boost? lol

'05 IP(or ?) 6MT... ...Never too soon to think about modifications!!

 

But, Ron, wouldn't the ECR be the same? The VQ won't know the difference between the higher CR.lower boost and lower CR/higher boost. Wouldn't the car run kind of rough at 8.5? I am interested in number, reliablility, and drivability. I think if I build the engine right (all forged components) then I can reliably run the higher CR and higher boost, getting the best of all worlds....

Even if I just up the boost to 12-13, thats still 5-6psi more then I've got now. Hopefully, by the time I buy the pistons someone will have something in between what is availible now.



Kevin
www.KPtechnologies.net
AIM: KPiersonTec
04 DG Coupe / VDC Memory
OEM Remote Window up mod
Procharged / Crawford / TS

 

Boost by itself is almost meaningless because density is what counts: Boost at what at outlet air temperature?

A PSI is 6.80% [15.7/14.7=1.0580272] more density IF the temperature stays the same. IT CANNOT OCCUR because compressing air raises temperature.

A rule of thumb is [for radial SC] is a 10-15F rise per psi [different brands have different efficiencies] up to 8 PSI BOOST. When you overspeed a SC the efficiency goes to Hell.

At 8 PSI 6.8 x 8=54.4% less drive parasitic loss and temperature rise [80F would subtract 7%, 120F subtracts 11%].

Going from 8 PSI to 16 PSI will not double the power gain due to nonlinear rises in temperature and SC driving HP and the need for a gigantic intercooler but you might get 470 HP at the flywheel.

The least boost with the highest air density alway results in the least stress.

 

Tech,

As always, great info. But, it has brought up a few more questions....

What is considered overspinning the compressor? The C2 compressor that I have is rated up to 24 psi at 80,000 RPM. The pullys supplied with the G35 kit keep the boost around 8psi, but I'm not sure what RPM. I know more heat will be generated, but will it make that big of a difference in this application? Also, the intercooler with the Procharger is fairly large, so it should help quite a bit I think.

With my current setup, stock internals and 8psi if intercooled boost, I am running about 440hp at the flywheel, based off of 380rwhp dyno and 17% tranny loss. I just can't imagine that if I raise the bar (hehe no pun intended) that I will only gain 30hp. Please keep in mind that we arn't starting with a stock car.

So far it has:
Crawford Headers/Plenum/Cats
JWT Flywheel/Clutch
PE 380cc injectors w/return fuel system/regulator
Procharger (intercooled)
TS Reflash

I am planning on finishing out the exhaust here soon, but buying this enigne has taken all my money for this pay period and a little of next weeks....

Oh, and by the way, I am on my way to pick up the engine and tranny right now!

If you haven't seen the engine in my car now check out: [url]www.kptechnologies.net[\url]


Kevin
www.KPtechnologies.net
AIM: KPiersonTec
04 DG Coupe / VDC Memory
OEM Remote Window up mod
Procharged / Crawford / TS

 

Well, I am officially poorer, but better prepared to run the hell out of my car. I just loaded up the VQ35DE and the matching auto tranny. First impressions - the tranny is much smaller then I had envisioned, but the only other trannny I've ever seen was on my Dads 1979 GM van. The engine was heavy, taking four uf us and a makeshift ramp to get it in the back of my truck.

It's kinda a strange feeling looking into my rear view mirror of my truck and seeing my coupe's plenum!

So, we need to get down to business.
NO stroker kit
Pistions and rods are a must, but as to what brand and c/r I still haven't decided... there are good arguements in both directions. What else should I consider throwing into the equation? And how much do you all think I could get for a lightly used G35/350Z auto tranny?

Thanks,


Kevin
www.KPtechnologies.net
AIM: KPiersonTec
04 DG Coupe / VDC Memory
OEM Remote Window up mod
Procharged / Crawford / TS

 

Here's a pic

Kevin
www.KPtechnologies.net
AIM: KPiersonTec
04 DG Coupe / VDC Memory
OEM Remote Window up mod
Procharged / Crawford / TS

311750-g355.JPG

 

one more

Kevin
www.KPtechnologies.net
AIM: KPiersonTec
04 DG Coupe / VDC Memory
OEM Remote Window up mod
Procharged / Crawford / TS

311757-g358.JPG

 

Nice

To answer some of you questions ..... it is IMPOSSIBLE to run high boost on high CR pistons... why do you think the low cr pistons are used?

Well here is your answer. A while back there was a long thread on the z forum (you have to register to view the forum) about this - one of the best ones i have seen. Read it from the VERY beginning, the initiator asked the same questions you just asked.

http://www.my350z.com/forum/showthre...499#post970499

G

 

Gurgen,

I am running 8lbs of boost now on stock internals. If I run more boost I'll either
1. bend a rod
2. fry a piston ring
3. burn a valve

If I upgrade these three components why wouldn't I be able to double the boost? I don't want extremely high boost, just 16psi.


Kevin
www.KPtechnologies.net
AIM: KPiersonTec
04 DG Coupe / VDC Memory
OEM Remote Window up mod
Procharged / Crawford / TS

 

Hey Kevin

You'll need to read that thread: here is an excerpt from my post:

quote:
--------------------------------------------------------------------------------
Originally posted by slay2k
Even though the compression is less due to the lower CR, more air due to the boost can make the pressure equal (or higher) inside the combustion chamber. As a result, without the intercooler, why would the temperature be lower at the same pressure (151 psi) ?
--------------------------------------------------------------------------------


Here is why: Say a 151psi pressure is built up at TDC in the chamber. Lets also assume for a second that the intake air is equal in both cases (100% intercooler efficiency). In an NA motr, the 151psi was achived by taking the itake air pressure of 1 bar (14.7psi) and compressing it 10.3 times as the piston moves from it's lower most position to the top position (TDC) - 10.3 x 14.7psi = 151psi. In a low CR motor (8.5:1 CR), the air coming at the same temperature (as earlier assumed) but now at increased pressure (more moles - molecules - of air). At 3psi boost, 17.8psi of incoming air (while the piston is at the bottom) the is compressed to the same 151psi (8.5x17.8psi). The temperature is not a funtion of the final pressure at TDC, but the order to which that initial mass of air was compressed! Hence, it is directly a function of the piston CR.

It basically boils down to the fact that the side-effect of air compression by the turbos/SCs is remedied by the intercooler, while the air compressed by the cylinder (certainly a necessity) CANNOT be remedied the same way. Thus, the only way is jsut dial back the compression (lower CR pistons) and just increase the MASS (i.e pressure) of air coming in at ideally the same temperature.

Read it! You need to know this. It will tell you wy you can't really boost more than 10 psi on or motors, but you can boost upto 18-20 on the 8.5:1 CR pistons.

It's a good thread.