Case Studies

1998 Nissan Skyline 

Specifications:

RB26DETT, RWD, Stock bottom end, ARP Studs, HKS 272 Cams, Trust Twin TD06 20G

Fuel and Ignition Control by Apexi Power FC

This vehicle is a great example of the case for high boost pressure and pump fuel. Thanks to a relatively low static compression ratio and generously sized turbochargers, we were able to extract very high horsepower from this engine without any knock or elevated exhaust gas temperatures.

Increased intake manifold pressure increases mechanical efficiency and improves piston ring sealing. Additionally, low static compression with high boost pressure can help provide a high BMEP(brake mean effective pressure) to generate high output without the high peak cylinder pressures that stress and fatigue mechanical components.

The result of this well matched setup is an excellent corrected specific output of 90hp/(L*Bar) at any boost pressure up to the mechanical limits of the factory shortbock.

1974 BMW 2002 turbo

Specifications:

M10 2L SOHC 4cyl, turbocharged, 6.9:1 Compression

Boost Control by Intake Pop-Off Relief 

   Initial testing of this vehicle showed the air-fuel ratio to be in the stoich range at full load on the as found tune.This is generally considered too lean for full load operation as combustion temperature and stability are far from optimal. Adjustment of the fine control on the mechanical fuel injection pump yielded a richer mixture but it still remained lean of our typical target. This change brought power up by 7%, and we exceeded factory rated power in the process.

   Further enrichment without significant rework was not practical in this application but for the sake of testing, the cold start enrichment was activated for the duration of a full throttle run to create a temporarily rich condition. This change provided fueling consistent with the range typically targeted for peak power on gasoline. An overlay of the test results showed no change in power or torque at any rpm to our previous run.

   While the enrichment provided a safer environment for full throttle operation without a doubt, this test shows that conventional tuning wisdom does not always prevail. The ideal air fuel ratio for the stock timing curve is likely somewhere in between the as found condition and the typically targeted value. More advanced equipment like our combustion pressure monitoring equipment, could have shown the ideal fuel mixture for an unusual application like this if time would have allowed. More on this in the future...