CPR R35 GTR Stage 4-5 Tuning - Engineer's Notes

For our STAGE 4 tune the working brief was to achieve a strong and repeatable 600 hp 600 which from initial study of the stock turbo spec, would possibly be the most we could reliably find from them. Well this is when the tuning of the GTR became both interesting and more involved than basic breathing add ons. The HKS panel filters were finding 10 WHP (additional power at the wheels) in the lower tuning stages. It was an obvious thought to us that a good improvement of air intake design would result in reasonable gains over the stock air box, as peak power was influenced relatively well by the basic introduction these HKS panel filters we thought it may be a restrictive area and worth a rework.

From the advance of gaining the Cobb Tuning PRO-ACCESS TUNER Software we are now able to 'live access' the GTR ECU during tune on our Dyno Dynamics AWD dyno and therefore gain valuable data logs of critical input signals from our previous stage tuning packages that work so well repeatedly exceeding target performance claims. More specifically data logs from output 'injector duty' and input 'air flow meter' signal on our STAGE 3 (using cobb generic tune) offered concerns as to the 'head room' left in these components to enable safe power increase. With this STAGE 3 generic tune running out quite rich in A/F ratio to mid 10:1 there would be room in the injector to find 30 -35 hp over our 580hp STAGE 3 with a more realistic and leaner target A/F of low 11:1. Unfortunately the same could not be said for the air flow meter signal as they would already reach their 'full scale deflection' of 4.9-5v in the 580hp STAGE 3. Therefore ANY increase in air flow over them would NOT be seen by the ECU and an accurate tune would be severely compromised at high load and high rpm!!

To gain headroom we chose to remount the stock air flow meter cartridge sensors into a larger body, therefore lowering the sensor voltage for a given total volume of air measured (Grammes/sec). Obviously correctly sizing this housing to allow the higher air flow and still be 'future proof' in further package development of higher power was critical. As the stock housing is the actual air box moulding we combined the development of our larger body housing into a full replacement induction kit, drawing air from the preferred forward area front panel so as not to pull ANY air from the warmer engine bay itself. To filter the air thoroughly, a cone shaped filter element was chosen to optimise flow in that given are which is in fact is capable of more than twice the flow of the panel / flat stock alternative!

With both these component limitations addressed we wanted to drop the total exhaust back pressure to a minimum and so the development of our replacement 3”front turbo outlet elbows allowed the removal of the more restrictive forward catalysts.

Time to see what increase in performance would come from these mods on the dyno, but first the critical rescaling of the airflow meter bodies was necessary to correct the ecu's interpretation of the now respectively lower voltage. This became a VERY technical necessity as side effects of getting straight into the tune would be extended to gearbox clutch control and calculated torque changes!

In our opinion there is only ONE way to skin this cat and it took time to build the file to support these hardware changes but the results were absolutely perfect, offering scaling to modify to an estimated 800hp!

Alternatively to this it is possible to buy the HKS 570 / 600 R kit which comes with NO ECU TUNE but chooses to interrupt the input signal and corrupt or clip its appropriate value. In our view and with our understanding of the side effects this technique causes at the transmission control unit (TCU) the age of corrupter and interrupter boxes have gone and should be left in the 'Nauties'. We would NEVER fit a Fuel Cut Defender (F.C.D.) to an '97 evo 4 , so why are their equivalent appearing on GTR's???

Back to the tune and it soon became obvious that the induction kit and turbine elbows combined had released far more power than expected, achieving over 530 whp but leading us to severe leaning out above 6000rpm at 15psi which we could not support with the stock fuel delivery! Injectors were flat out after 5500rpm with 17psi boost held up to this rpm offering 630+ torque.

It became clear that this combination of hardware would need more fuel delivery on tap, so the introduction of larger 750cc/min RC injectors made safe this power level again with a comfortable amount of headroom in the injector for 800hp. Again the scaling of these injectors within the ECU is critical for its closed loop and target A/F ratio control to function accurately, amongst other side effects caused if this is not done correctly.

Alternatively, it is possible to use the same thought process as HKS used in their 600R kit and change the stock fuel reg to an adjustable unit to raise the fuel rail pressure and force more fuel through the stock injectors , which in turn, loads the stock pumps to the point they too must be changed for stronger units!! Again a very unprofessional approach in honest opinion!

This inadvertently became our STAGE 5 package but we found it necessary to add after market stronger sprung actuators to keep stable boost control. The lower exhaust back pressure limited the repeatable boost level at peak power and on back-to-back dyno runs the boost would fall dropping expected power at higher temperature. The after market actuators held consistent boost on repeated runs and deemed well worth the labour intensive fitments of them and earnt their place in this 630 hp / 630 CPR STAGE 5 package.

Our STAGE 4 tune saw the emittance of the turbine elbows and the forward cats replaced. This corrected the boost control inconsistency and dropped power to the desired level that could be SAFELY supported by the stock fuel injectors. So we found our 600hp/ torque package by essentially detuning the power found using hardware that is now our STAGE 5 630hp package!

Strange how things can work out!


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