I asumed we talk F20C here ;-)

But am I far from the truth?
BtW. today I saw the boost creeping time to time to 9psi in 6th gear at higher rpm!!!
WTF?

 

Yikes...

Careful with the creep there.

 

im just trying to 1st understand the theory here...

i want a turbo motor to run at an ECR of 19.4 and want the safest way to do it...

------------Compression---Boost--------ECR
Motor A--------11:1---------11psi------19.4:1
Motor B---------9:1---------17psi------19.4:1

either Motor A or Motor B... what would you suggest and why?

can i asusme both can be tuned for 0 detonation?
are you saying one motor is more prone to detonation than the other?...
which one? WHY?
is one motor gonna be "easier" to tune for timing?...is that it?
what am i missing?

 

It seems that once you get to a certain ECR, the higher compression motors have a tendency to detonate before the lower compression motors. This phenomenon can be tuned away more safely with the lower compression motor, due to it's ability to support more timing.

The big mystery right now is what level of ECR the lower compression motor becomes the better route. This is HIGHLY dependant on the engine tune, the engine itself, and a boatload of other variables that could only be figured out through testing.

The best thing that I can tell you is to look at what other people are running safely on their motors. The consensus seems to be 8-10 psi on stock compression before things start to get interesting. That gives you an ECR of 17-18. I have seen this number recommended from a number of members.

Please do not confuse the ECR of a turbo system with the ECR of a supercharger system. ECR is proportional to boost, not RPM. Some people might say that they had a supercharger running with a 12 psi pulley on stock compression with no problems. Well, up until around 7000 rpm, their ECR would be less than an 8 psi turbo system's.

I would venture a guess that you are above the "threshold" with a 19.4 ECR. A lower compression head gasket may be in order. But then again, if you can get a REALLY nice tune with the higher compression motor, you will reap the off-boost benefits.

 

I wish I had more data...

A Maximum ECR vs. Compression Ratio chart would be a perfect compliment to the last graph I made. It would tell you what a safe ECR would be for each particular compression ratio. Could add a third column for octane level, too.

 

What might have caused the boost creep? Didn't see 9psi for over 2 months, so why now? The temps. dropped here a LOT< might that be a reason?

 

The only things that matter are effective compression ratio, which has been covered nicely already in this post, and mass flow rate (which has not even been mentioned). The effective compression ratio determines BMEP (average cylinder pressure). More is better, but you are limited by detonation (varies with fuel octane, combustion chamber specifics, etc.). The engine is an air pump, if you can move more air, you can burn more fuel (while maintaining safe A/F ratio), release more heat from the air/fuel charge, and get more pressure to drive the pistons. Problem is its very hard to measure flow, so everyone resorts to talking about boost pressure, which is easily measured. Boost pressure actually indicates resistance to flow (backpressure). If you had sewerpipe sized intake ports and valves, even a huge turbo (or supercharger) would not make boost pressure although it would be flowing a ton of air (and therefore making a ton of power). The problem is that everyone assumes that boost pressure is equivalent to mass flow rate, and it is not. There is a relationship, but it varies from engine to engine, ambient conditions, etc. So boost pressure on your engine can be used to measure gains/losses on your setup. But the exact same relationships do not apply to all cases. In general, the backpressure offered by the intake tract is proportional to flowrate (probably close to linear curve), but that exact relationship is not known. Everything in the ECU is emperical though, so it doesn't matter if you can't do the math beforehand. Injector bandwidth is adjusted in the lookup tables until proper A/F ratio is emperically verified by wideband O2 sensor. Flowrate is transparent to tuner, but matters to engine.

 

i see.. i guess this is what i was looking for... when your getting close to the threshold... higher boost is easier to tune for, than higher static compression.
you can get better performance with the higher compression motor...especially off boost, but as is always the case... that motor is in greater potential danger...

snakeeater... regarding your post... the motor with low compression+higher boost is going to have more air in the combustion chamber... thus having a better flowrate, than the comprable high compression+low boost motor...

are you saying this, if tuned right for enough fuel, will yield more power?

 

I think we are beating this one to death. I'll throw this out there for anyone who cares to listen. (This is all based on reading members experiences and my own research so take it for what its worth to you).

An well tuned ECR of 17 should last you a very long time. Obviously more stress than stock, but the motor seems to be able to handle it. Of course driven like a jackass, the motor will suffer accordingly. This is probably a little below the "threshold" mentioned above, but its a (relatively) safe place to be. Anything above this and you are either going to want to look into lower compression or a build motor, or just occasional bursts of higher ECR with controlled boost.

My 2 cents. The remaining 98 are above.

 

Iceman,
The engine doesn't care where the chamber pressure came from, static CR or boost pressure. The turbocharger is the upstream 1st stage of compression and the piston squeezing from BDC to TDC is the 2nd stage of compression. Where the split in pressure ratios occurs between 1st stage and 2nd stage just doesn't matter. As you can see from the ECR chart, you can have the same ECR from low static CR with high boost, mid static CR and mid boost, or low static CR and hi boost. At steady load conditions, the engine will make the same power in any case.
The street drivers prefer hi CR (for increased low RPM torque and driveability) and then are limited to low boost (due to detonation with low octane fuel). The all out racers use low CR (since driveability is secondary) and hi boost (for max hp).
All else being equal, the higher the boost pressure, the higher the HP limit. The theoretical limit to cylinder filling is the boost pressure. And Boyles law says that for a given volume (which combution chamber is) there is twice as much mass of air at 2 atm than at 1 atm. So your 2.0L engine has turned into a 4.0L engine at 15 psig of boost pressure. Now, you wont get quite that much due to losses, since the chamber has only milliseconds to fill up at high RPMs, and chamber pressure will never quite equalize to manifold boost pressure.
To find the theoretical limits of your engine, first accurately calculate pressure ratio = [(boost psig + ambient psia)/ambient psia]. Then multiply it by static CR to get effective CR. Then multiply it (pressure ratio) by displacement to get effective displacement. Then multiply effective displacement by engine speed to get air flow rate. Then divide air flow rate by safe A/F ratio to get needed fuel flow rate. Watch your units on all these calculations.