How to get more power from a naturally aspirated engine?
There are many ways to get more power; it all comes down to budget and application.
Power is defined by the rate at which work is performed over a unit of time. Used in our context, power will represent the maximum kilowatts (kW) a vehicle produces as measured on the dyno. However, the key factor in the above equation to take into consideration here is 'time', which is essential for 'response'.
Being a naturally aspirated, the motor is neither turbocharged nor supercharged. This means that the motor relies on vacuum created by the cylinders and natural atmospheric pressures to draw in ambient air for combustion.
In order to get more power you will have to rely on the basic principal of: letting the motor breath/work better, so to speak. This is done by removing the restrictions within the system and thus creating more efficiency.
Here is a basic list of what can be done, we shall elaborate on them at a later stage:
Intake (pod filter, panel filter, larger throttle body, cold air feed)
Neat Cold Air Intake
Professional Looking Heatshield
Exhaust (extractors, cat converter, cat back exhaust)
Motor (shave the head and/or block to increase compression)
Port and polish (remove restrictions in the head)
Oversized valves (allow more air to flow)
Cams (regrind, intake and exhaust side to improve breathing)
Gasket (thinner gasket to increase compression ratio)
Pistons (increase compression, light weight)
Crank (light weight)
Rods (increase compression)
Electronic Control Unit (ECU)
Piggyback systems (SAFC, E-Manage, GTS-Link, allow modifications of engine parameters; air:fuel ratios, timing, etc.)
Flywheel (lightened to decrease unsprung mass)
Tailshaft (one piece, carbon fibre; lightened to decrease unsprung mass)
Rims (lightened, forged)
RB25/30DE and RB26/30DE Hybrids
A good and useful NA motor has larger power band, this means the car is able to put down significant amounts of power through a large rev range. This is achieved by an increase in cc's -as in an RB30DE and the compression ratio*. High performance NA motors run high compression ratios (CR) to give more torque and power through in the low rpm range, this will see the motor rev faster. In the RB30DE hybrid a significant amount of the extra power being made is gained via the head, so particular attention to this area is ideal.
Block: RB30E NA block (Found in R31s and Holden VLs)
Capacity: 2982 cc
Bore: 86.0 mm
Stroke: 85.0 mm
The RB30 crank is nitrited standard and is strong enough to withstand a mild increase in kW.
The RB30 rods are suitable for simple low - mid range and mild CR motors. However aftermarket items are required for motors looking to do beyond 6500RPM, as the standard items will snap and/or crack.
RB30 or RB25DE pistons are able to be used. CR using RB25DE pistons using the stock RB30E pistons will give you a CR of ~8.2:1. This can easily be increased to 9.5 - 10:1 (by shaving the head, ~2mm shaved) depending on fuel availability for regular street use. It is possible to run up to 12:1 CR before you need to switch to race fuel to prevent pre-detonation (aka pinging).
Cam belt tensioners needs to re-positioned. Using a tensioner and idler the timing belt tension to be greater than the factory recommended spec of 20kg's. You can use a second tensioner in place of the idler bearing in order to bring the belt tension down to the factory recommended spec of 20kg's. It is possible to use the factory tensioner and idler locations however, you will be required to use two tensioners to get the correct belt tension. Some do not recommend this method as the belt comes too close together.
A 152 tooth 20kg timing belt must be used.
All RB26, RB20, 25 and RB30 water pumps and thermostats are interchangeable.
It is best to use an oil pump from a twin cam motor as these provide more flow and pressure. All RB oil pumps are interchangeable. The RB pumps do have a reliability issue and are prone to cracking if used at revs over 6500rpm without a crank collar. A good supply of oil is required for high rpm applications.
Whilst the RB30 does have a strong bottom end it may need a freshen up as many of these engines have done over 200,000km. If you're chasing big power out of an RB30DE you will also need to think about:
Bearings: ACL race bearings
Rings: Molly ring sets or stock alternatives
Balancing: Best done with the harmonic balance, clutch and flywheel attached.
An oil cooler, oil pressure and oil temperature gauge may be required for high rpm applications.
R32 RB25DE, R33 RB25DE, RB25DET, RB26DETT (NVCS and Non-NVCS)
Rb20DE/T heads will work with modification.
RB26DETT heads have the best flow. They also run multiple throttle bodies which are ideal for NA motors. An adaptor plate is required to attach Rb26DETT throttle bodies to the RB25 head.
RB25/26 heads have cc capacity of around 62-64cc's, in comparison to the RB30 55-58cc's
All coolant/oil galleries line up between the block and the head without any issues.
An oil feed from the block to the head using a T-piece is required to maintain NVCS.
Upgraded valve springs may be required for high power applications.
Standard camshafts, reground or aftermarket items are available.
The max regrindable standard cam profile is 8.3mm & 255º. Remembering that Lift = torque and duration = power.
GTR camshafts can also be reground to use hydraulic lifters. They are a recommended budget upgrade.
Adjustable cam gears/pulleys are recommended if using modified/aftermarket camshafts.
Port and polishing is recommended if looking for both power and response.
All of the RB20 and Rb25 flywheels are interchangeable.
The RB30DE block is approximately 38mm taller than the other RB blocks.
Engine mounts will need to be modified to lower the motor by 15mm on the driver's side and 12mm on the passenger's side to allow for bonnet clearance if maintaining the standard RB25 intake plenum.
The lower lip of the radiator may need modification to allow for fan clearance.
Gearbox and center bearing mounts may also need modification to reduce driveline angles.
RB25DE stock ECUs may restrictively be used, they are not mappable
If utilizing NVCS an appropriate ECU is required.
RB20 ECU is able to be chipped for RB30 use. *to confirm?
NA Z32 (300ZX 3L V6) ECUs are mappable and can be found with VCT variants and are more appropriate. *confirm?
Apexi SAFC can be used to tune air/fuel mixtures
Aftermarket stand alone ECU's (MoTec, GTS Link, Apexi's Power FC etc.) will be needed for high power applications
Forced Induction (not considered naturally aspirated, although nitrous is debated)
Nitrous Oxide (NOS or N2O, injected into the intake to increase oxygen content during combustion.
Turbocharging (exhaust gases used to create intake vacuum to compress and increase oxygen content for combustion)
Supercharging (crankshaft energy used to create intake vacuum to compress and increase oxygen content for combustion)