Keep the single pipes you have and replace the 2.5" with some 1.75" or 2" , then replace the collector with some 2.5" pipe.
The header you have now has no tuning effect to the engine, thats why you lost hp and torque. I bet the ET dropped as well as mph, 60 ft times are probably way off.
You keep talking about equalized air flow, I assume you had the head ported and the ports all flow about the same. This doesn't change the header design. If you didn't increase the rpm range of the engine or the intake system, the air flow quantity hasn't change all that much. Better heads only create more flow if you run the engine harder.
And talking about heads and porting, if the ports were opened up just to get maximum air flow, they you may never get your speed back where it was. You can trash a head pretty fast by just opening the ports. They have to be tuned and sized for the intended use and driving style. You can kill the exit speed of the exhaust with big ports just the same as with big pipe. You can also kill the intake side the same way.
The intake mixture needs to cram itself into the port. If the speed is high enough, it will force it self into the cylinder as the valve is shutting even though the piston has reached the bottom of the stroke. This is why small runner intakes work better then large ones, they keep the velocity high.
A head that flows 300cfm will not work as good as a 200cfm head on a street driven 4000 max rpm engine, no matter what the cubic inch is. Turn the rpm up to 7000 rpm, now the 300cfm head will work and the 200 will choke it down.
Joe
