We have built a Vauxhall Corsa B 1.6 16V C16XE grA rally car a year ago.
On this engine we now use a cams set with Inlet 12.00mm lift, 288deg,
3.7mm lift @TDC, Exhaust 10.5mm lift, 284 deg, 3.2mm lift @TDC. As the
car is grA we need to keep the OEM inlet manifold and throttle body and
the OEM exhaust manifold. The torque of the engine is 16.7Kg m @ 5200
rpm and the power 161bhp @7400 (rolling road test). Since the cams are
relatively big but the peak torque rpm point is low we wonder if there
is potential to this engine to move the peak torque point 1000rpm higher
(as is the case with the Peugeot 1.6 16V engines) and peak power at 7800-8000
rpm so as to improve high rpm range performance, or there is something
(for example the inlet manifold) that will not allow this.
Answer: We have not worked on Group A Corsa engines for more than 10 years, the
intake system is the biggest restriction & is one of the worst we
have seen. This is the part that holds the power back in comparison with
better intakes, such as the Peugeot. We did use much bigger cams than
you are using & although the engine produced slightly more peak bhp,
in our opinion the engine generally got worse meaning less driveable.
Unless you can replace the intake system, your options are limited.
replied: 'Thank you for your e-mail. The customer had said that he
would use this engine for one year just to get familiar with the car and
develop the chassis and then he would go for a kit car spec engine with
throttle bodies etc. so we did not spend time flow testing the head, intake
etc. But the data I had during mapping and then on the dyno showed that
something does not allow the torque to go at high revs and cam timing
changes affected low but not high rpm performance. We tried a bigger exhaust
but things were the same if not worse. So the only thing that was left
to blame (we had a bad feeling for the intake from the beginning) was
the intake which is very long with small chamber volume and many curves.
Your e-mail confirms this impression. Thank you for your feedback.'