I think that it was more a matter of questioning what your ability has to do with giving recommendations on a model of a car that is unknown to you.

I bet that Nissan of yours is a blast to drive.

 

I understand it just fine. You just can't seem to relate it to the subject at hand.

Do you expect to show up and blast off some BS from the keyboard and expect it to be taken as gospel? I call foul anyone that posts that the fuel does not need to be changed when using a larger compressor and that, some how, if a car is tuned correctly for a K26, no changes are required for a K27 or larger.

 

There isn't too much of a difference between the 951 and my 240's workings. The 951 just runs a slightly older version of the EFI.

 

What does your 951 have on it?

Good deal. be sure to post pics, specs, and dynos. Everyone loves a good build.

 

Uh, 9:1 compression, 16 valves, 2 liters.. But it does have 4 cylinders so yeah, basicly the same..

 

MAF Workings

If the MAF has 2v of resistance then the amount of airflow coming into the engine is the same as 2v of resistance with a GT40R. The airflow will be heated differently based upon the compressor efficiency, but that difference will typically be negated in large part by the intercooler's efficiency assuming you aren't exceeding common sense logic. The duty cycle at the injector will be set for the measured amount of airflow, as will be the ignition timing.

Turbo Workings

Expansivity principles aside.

The engine is rated at 2.5L. Meaning at 100% VE the engine will consume 2.5L of air per cycle. The volumetric efficiency is set via complex fluid flows within the motor (cylinder head, manifolds, etc.), you could say it is a mechanically limited efficiency. When you introduce a turbocharger you are introducing two things.

1. Altered atmospheric conditions
2. Reduced pumping efficiency

Essentially you are raising the pressure in the intake manifold to facilitate greater mass flow into the combustion chamber. From there the exhaust is mechanically pumped out into the exhaust. Which from there is processed via the wastegate and/or turbocharger creating a restrictive scenario (exhaust manifold pressures are much higher than N/a).

Mass flow is essentially based in large part to deltaP (change in pressure). So in order to increase mass flow you will need to increase pressure, or increase the flow choke area (ie. putting in larger valves) while maintaining a positive deltaP ratio. There are misc. efficiencies that can make alterations to how pressure affects mass flow. Helmholtz resonance within an intake manifold can increase momentary pressure at the valve upon opening. Resonance is based upon port design and manifold design, so we can assume the basis is infact static.

Engine mechanics are essentially static unless you have variables (ie. VTEC head, twin flow ports, variable cam phasing, etc.). In the case of the 951 we have no major variable systems.

Mass flow is then "static", and the only way to increase mass flow is to increase pressure via the turbocharger.

This is why compressor size is not a large variable in tuning, and it's why compressors of varying sizes will flow the same amount of airflow at the same boost pressure on the same engine.

HOWEVER...

There are limiting factors. One being in the instant that one turbocharger cannot supply the airflow to supply the engine with the target pressure. Ie. The K26/6 on the 951 at 15 PSI.

The larger K27 can then superceed the flow amount of the smaller K26 at all RPM where the K26 cannot supply peak boost pressure (ie. 5000 - 6500 RPM). This is where you will see increased duty cycles over the K26.

However. This is the kicker. Assuming the chips are tuned for all capable MAF voltages 0-5v, per RPM and load. Under the conditions I stated above you will have a greater MAF voltage than the K26. If the tuner skimped out and did not tune the entire map on the K26 chips then you will need to update to the K27 program. If they did tune the entire map, as long as the MAF voltage is within the boundries of the chip, you should not have to change out the chip.

So the key is, did the tuner tune the entire map? If you are getting running problems, detonation, or sluggish response then my assumption is no, or the car that it was tuned on had variations.

Power Difference

Most people incorrectly assume power gains are the result of more intake side airflow when comparing two turbos at the same boost. According to above this is incorrect. The gain is from the reduction in exhaust side restriction and turbine efficiency (turbine efficiency is almost ALWAYS overlooked). However don't forget that if the smaller turbo is dropping boost pressure then the larger compressor is responsible for the gain if it can hold the target pressure.

If you don't think exhaust side restriction can be that large of a factor, then I suggest measuring exhaust side pressure before the turbo. It will make the common talk of camshaft related exhaust reversion look more realistic.

 

LOL...

The physics and basis behind the engines operations remain the same. Obviously there are mechanical differences, but those can be related if you understand how they affect the system.

 

Why are you talking about MAF calibration when the 951 uses AFM?

 

Because it seems the hot-wire MAF is a common modification over the ancient potentiometer crap Porsche decided to put on the 951.

 

Correct me if I am wrong but if you run a MAF and raised boost on a 951 (even with a chip for the MAF) you will very soon run outside of the calibration area of the stock Motronics even with a K26/6.. This is unless you run a piggyback to massage the signal or the Vitesse chip board which has completely rewritten code.

 

Massage the signal? LOL, thats a pretty interesting way of putting it.

I haven't started going through the Motronics in depth yet, but I will as this 951 project starts rolling.

Porschephile has a habit of not ever making up his mind, so you might get 10 different setups haha.

 

Depends on the calibration of the MAF.

 

To tell you the truth, I really should participate on the 911/930 boards more. I still browse those boards and have done plenty of research but, I'm a lurker at the moment. Part of the reason is you guys over here are enough of a handful!

To back up my claims, here you go:




That's a dyno of my 911. It's a 7.5:1 3.4l built on the original 3.2l motor. It has 964 cams, a Turbonetics 60-1 T4 with a .81 exhaust, and tuned Protomotive chips. The dyno was at 1bar. I couldn't find the 431rwhp dyno sheet, but that one was from the second and third run, so close enough. The powerband is crappy for a 3.4l due to the way the timing was done (tons of retard). A/F's are dangerously lean on the top-end, which I'm currently fixing. Eventually, if I can ever get done with my 951, I will most likely be going with an AEM and a Garrett gt40r (4088r to be exact) as you can never have too much power. BTW, I bought this car from an old listing on Pelican Parts. The car was up for nearly an entire year for some reason, although it really wasn't in bad shape. The front main seal leaked, which was a 30min job and a few dollars of parts. The Bilstien RSR coilovers could use a rebuild and stiffer springs/suspension bushings. Otherwise I think it was a pretty damn good deal at $22k.

 

Here's some of the car:





Here's the thread where it was posted: Classified

I can take more pics and hell, video if you guys don't believe me.

 

V is not a measurment of Resistance. I quit reading there.

Alright, I decided to scan the rest of your post. I don't think it was English....

How does adding a turbo REDUCE pumping efficiency?

I have no idea why you are relating induced heat to compressor efficiency.

So why are power gains achieved with a larger compressor while maintaining the stock exhaust?