Top end performance.

[Steve Webb]

Something about my Q4 has bugged me for quite a while now, infact since I had the majority of work done on it and had the chip written on a rolling road.

Peak power arrives really quite low down, 5400rpm or there abouts. Now the car was running without Cats, and with a different (larger) turbo. Works well around town, but lacks top end power and grunt on track etc.

Anyone got any ideas about how to address this and open up the top end a bit more.

Thanks.

[jNk]

Is this still an issue or you already solved it ?

I've got no personal experience but as an armchair engineer I think 5400rpm is pretty top end for our engines :tongue:

[Steve Webb]

I've just checked as I posted up the dyno plots for my car ages ago This Thread (http://forum.berlinasportivo.com/showthread.php?1778-Dyno-Plots) and max power was at 4800 rpm.

[jNk]

Based on that thread I share corriedw's view on the issue and either the turbo not supplying enough air, exhaust manifold limiting turbo, exhaust too restrictive or there is boost leak which prevents your engine to reach higher numbers at the top end rpm range.
Did you check the AFR numbers during that live road mapping ?

[Steve Webb]

I actually think its the injectors.

If you look at this calculator fuel-injector-calculator (http://www.deatschwerks.com/fuel-calculators/fuel-injector-calculator) you can see that with the standard injectors at 3 bar fuel pressure, they are going to be running at 85% duty cycle even to make 250 BHP.

The turbo is a T3/T4 hybrid so should supply more air than standard, the exhaust manifold is a superspirnt so less restrictive than standard, and it runs up to 1.3 bar of boost.

From the Air side of things it should be fine, hovever, the injectors, 7 years old at that point so far from perfect, are already theoretically struggling.

The AFR looked fine on the mapping, just refused to make power at the top of the rev range.

[jNk]

You are probably right, our injectors are at the safety limit 90% of duty cycle at roughtly 260bhp so it seems those are the limiting factors in your cars.

[#84]

Interesting problem. I make the same torque, but about 325 hp at 6600-6800 rpm with rev limit at 7050 rpm with standard internals, intercooler and injectors. However I have fitted a Wallbro 255 l/h fuel pump and the Delta Integrale fuel pressure regulator, turned up to ?? (don't remember, but as much as possible). I would check the fuel pressure, or just fit a proper fuel pump if you haven't already done that. It is one of the cheaper things to fit. If that is not the problem, I would assume something with the boost regulation. You manage to get a second peak in the dyne plot, which comes from the intake tuning and obviously there is fuel to get power from it, that confuses me.

[Steve Webb]

I'm ahead of you, a new Walbro 255L pump has been recently installed along with a Deatchwerks adjustable fuel pressure regulator. Once I get a few more things ironed out, I'll be looking at upping the fuel pressure as well, currently the car was setup running 2.5Bar.
I've got a Kappa\Integrale inlet manifold to install at some point as well.