My last 4 cars all ended up with modified manifolds due to the fact that the standard ones where restrictive - why do Bmw/Ford etc not make some unrestrictive manifolds and then we could all enjoy 10% more power and additional noise ? 10% of 367bhp is quite a lot more power !!
Because the bean counters always win. The cost/benefit ratio just isn't justified in mass production. This might change in the future as fuel efficiency becomes the driving goal.
The biggest single gain I've seen from one of my manifolds was on my HSCC Fiat X19
When I bought the car it was fitted with an Ansa aftermarket manifold and had been dynamometer tested with 96 BHP (ok for a non cross flow 1300cc engine on a single carburettor)
I ran some engine simulations and redesigned and fabricated a new exhaust manifold and system, tweaked the carburation and cam timing to suit, and was rewarded with 128 BHP (and an engine that would rev out to over 10,000 rpm)
That's a 33% gain and damn near a 100 BHP/ltr
The next iteration of engine I was in the process of building would have pushed about 150BHP out, sadly we decided to go 24hr racing with the Aero and that pretty much curtailed my racing exploits.
I sold the X19 in 2009 and it's now very successfully used for FIA historic rallying
Simon @ SiFab.co.uk
