There's the interplay between equations, real world data, and estimating. Modding is certainly not an exact science or we would be done already. To answer your specific question, this would need to be determined in a R&D setting. No doubt Porsche has the answer since they haved measured crank HP and fuel consumed. For us lay people, it's a guesstimate, except for VE which is derived from all measureable input parameters which is why it's nice.

 

Thank you very much!It makes sense now...


As for the link to the Gerrett site, it's very usefull, thanks...


I need you to comment a bit on this one though:



Where:

· MAPreq = Manifold Absolute Pressure (psia) required to meet the horsepower target
· Wa = Airflowactual(lb/min)
· R = Gas Constant = 639.6
· Tm = Intake Manifold Temperature (degrees F)
· VE = Volumetric Efficiency
· N = Engine speed (RPM)
· Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61.02, ex. 2.0 liters * 61.02 = 122 CI)



In this formula, the required manifold pressure to obtain a given HP figure, it's only related to the engine's features and Intake manifold Temp. (affected by compressor efficiency and compression ratio, given a fixed intercooling efficiency).
I get the sense of it perfectly, BUT, why do we see (in real world) big turbos making more power than smaller ones at a given boost level?
May be that bigger turbos are often coupled with other "supporting" mods that affect volumetric efficiency and that they have better efficiency than smaller turbos at the same comp. ratio?


Sorry if my questions seem stupid, but I'm not used to learn formulas without knowing where they come from.


PS:any book suggestion on the subject would be MUCH appreciated..

 

Uhmmmm.......... I was hoping some one might have said that Durametric can determine the injector rate with a given pressure and injector cc size the squirt of fuel could be calculated.
Frank

 

Nothing at 700whp with the VTG's, here is our 997TT P800 dyno:

 

If you know how long your injectors are open and fuel pressure ( and of course fuel's density ), you can calculate the fuel consumption rate. For power produced you'll need an engine dyno.

 

How does the race gas dyno look for this kit?

 

Wouldn't it be simpler using AFR and MAF+air temp+baro pressure readings?On a dyno, you can measure power, then take MAF reading from the datalog, ambient air temp and barometric pressure (=air density), AFR with a wideband or with datalog lambda reading, and there you go.

 

So I got the values for my car when standard. I have plot these VE figures against two other cars that I have data for to hand. The question is whether there can be a correlation applied to known tq for standard 996 turbo for the VE figures, that could then be applied to show the tq of the other cars?

 

Yes, you can do that very easily. Just measure your duty cycle, you know the flow, you can calculate amount. But you need to somehow correlate that with bHP.

If you have access to a dyno, you get it all.

In theory, yes. You can calibrate to a known dyno and over lay the VE curve. And then extrapolate as you say. It would be interesting to do this but I don't have a bunch of dyno data hanging around (none for that matter) to validate the method.

 

So as I showed on the previous image, I have the VE for std 996, x50 stage1, and my car.
If I correlate the VE for the std 996 to actual torque values supplied by Porsche, and then use the same correlation factor to the other VE, this is what happens. I have plotted HP as well from the TQ calculated.

So what do we think?

I can tell you the x50 is definitely from performance showing around 540hp on testing up to 300kph. The x50 figures don't look too bad in terms of what you would expect from stage 1 map, dvs, exhaust, filter etc.

 

This example seems right on. From performance data, you estimate 540bhp and the graph show this. That's one data point. I'd say we need about 10 more examples to make this a statistically relevant exercise. The data is only as accurate as the dyno from which it is calibrated. Once you have a good calibrated VE curve, you can probably use that to test the quality of the dyno--lol.

 

The thing is that Todd K told me at 1.5 bar I could expect with my set up somewhere around 660-680whp. I believe that on a tip that the tranny loss is around 28% - anyone have any good figures? So if that is the case, taking 840hp at the crank, and take off the 28% gives around 656 at the wheels. It gets interesting.

Going to map some more cars into the graph.

 

Try Forced Induction Performance Tuning by A. Graham Bell. Lot's of good stuff. As for your big/small turbo question, the simple answer has to do with flow. You can make high boost on a little turbo but that will restrict flow and exhaust gases from exiting thus reducing your VE.

 

I'm beginning reading it right now....It shipped 15 days ago from US and arrived a couple days ago..

 

Brilliant. Like I said, it's a poor man's dyno...