Jaguar XJR X308 Customer exhaust power gains

[l80ous]

15 years of english salty roads have finally taken their toll on my exhaust and it has over 7 holes, so its time for a new system, as Jagaur want more money for a standard one I thought I'd go custom.

Question is if I get a cat back system what sort of BHP gains can I expect if any. Ive been reading a few other american forums and I set up like mine with panel filter and said custom exhaust system other people are pulling between 400-412bhp on the dyno, any truth behind this? (Link below)

http://www.jaguarforums.com/forum/xj-x308-xj8-xjr-...

The system Im looking at is a PowerFlow, what gains if any do you think I can expect......

[Sedox]

Powerflop? probably minus 10% of what you have now

[Monty Python]

The only way you'll get more power is by remapping - changing the exhaust does nothing (and can sometimes make things worse), and neither does changing the filter (http://www.nicoclub.com/archives/kn-vs-oem-filter.html)

Exhaust myths:

1) The catalyst sucks a lot of power. In reality if you completly removed the catalyst you would see a power gain of around 4%. Fitting a new high flow sports catalyst will also give the same power gain. By taking out the catalyst you are breaking the law in many countries and risk fines penalties and car siezure so with this risk in mind I would urge you to fit a sports cat instead.

2) Bigger exhausts are better. The larger the exhaust pipe the lower the flow rate will be. You want the exhaust gases to exit at a high flow rate as this improve engine scavenging and as TorqueCars.com is always stating bigger really is not better. An exhaust size depends on the displacement of the engine and the amount of gases to expell. There is an optimum size and many manufacturers exhausts are already pretty much spot on.

The power generated by an engine is a function of the combustion process, which is governed by the ECU. It alone decides how much air and fuel are needed.

[Prof Prolapse]

Can't you just get it touched up?

400-412bhp? They make 370bhp standard, I find it very difficult to believe you'll get 42bhp out of an exhaust mate.

Maybe if you get the pulley changed at the same time with a remap.

Or if you're flush, consider importing a twin screw charger if you want decent power returns. Then you just need an LSD to keep the power down.

[Krikkit]

Monty Python said:

The only way you'll get more power is by remapping - changing the exhaust does nothing (and can sometimes make things worse), and neither does changing the filter (http://www.nicoclub.com/archives/kn-vs-oem-filter.html)

Exhaust myths:

1) *snip*

2) *snip*

Exhausts are much more complex than that Monty - having a cat in increases restriction in the flow, obviously this changes the scavenging characteristics, but removing it isn't always about increasing power, sometimes it's about improving the high-rev performance whilst sacrificing a little low-rev drivability.

Exhaust sizing is a complex issue, but often the OEM exhaust systems are sub-optimal for performance because they're restricted by the noise profile - either by regulation or to make sure the character of the car is as the manufacturer decides. A good example is a tubular exhaust manifold, on a 4-cylinder there are decent increases to be found by fitting either a 4-1 or 4-2-1 depending on your preference for peak power or increased torque range. They're also sized according to the gas flow characteristics of the engine you're fitting it to. Another good example I stumbled across is here, fitting aftermarket manifolds to a C63, increasing power and torque throughout by 3-5% throughout the rev range without even touching the cat or later system parts.

That's before you get onto anything like owner preference - some of us would sacrifice a couple of hp for better noise.

[l80ous]

Nice thanks for the help