Originally Posted by Karl

One very final point about RR's and I will give an example of something I tell my customers all the time, yet when a RR day comes round it beggers belief the questions I get about why the power was lower than expected.

T34 car example 1:

I map the car to the maximum safe reliable output of the turbo and engine spec. This is usually around 380bhp mark, and on a typical car will peak at say 30psi and tail to 24psi when held in 4th and 5th gear. This will be running the turbo very near its limit.

Car goes on RR. Car only makes 360bhp. HOWEVER, lets look at what has really happened.

1. Lack of load has now meant the car has only held 21psi instead of the 24psi it held in 5th gear on the road during its top speed run. That is 35psi absolute V 38psi absolute. This equates to a mass air flow reduction of at least 8% even if ACT was good on the RR which it wont be!

8% of 360bhp is 28bhp. Hence if the car had been able to replicate the real top speed condition and held its real 24psi it would have shown 388bhp on the RR, which is of course the safe limit of a T34 and exactly the conditions it was actually mapped for.

T34 Example 2:

Customer has run said T34 car up on RR and is unhappy it only made 360bhp. RR operator suggest they can do better. Boost is increased to hold 24psi on the RR and ignition timing optimsed to suit. Power now shows 390bhp on the RR. Customer thinks new tuner has done a great job!

HOWEVER, now lets take said car for its top speed run again in 5th gear. Now instead of running its 24psi held, turbo is holding 28psi! This is actually resulting in turbine overspeed, excessive back pressure, lean running and excessive ignition timing. Engine will likely sh!t itself.


Moral of the story: DO NOT USE RR'S FOR FINAL MAPPING OF A CAR!