Winter time is a great moment for any mods and upgrades as car in a garage is not really a wasted time. Already have some plans for my car, but I'm wondering if a list should not be extended by one more position - exhaust.

I have a tuned 5.0 s/c, with smaller upper pulley and tuned soft. It generates around 590HP. My current exhaust is modified to x-pipe and silencers in a mid section are gone. Cats and muffler are stock.

My tuner says - the thinnest exhaust for ~600HP car he has ever seen (stock).

Questions:
- is it worth going with bigger pipes (2 3/4" or even 3") - stock muffler would stay, but open valve would be on a bigger pipe (valves would be changed to match the size of pipes)?
- does it make sense to change exhaust only with higher flow cats (200-300 cells)?

Purpose - performance. Don't need a louder car. I will accept one, but definitely changes are performance oriented (600+). Already been disappointed with meth (hoping to get 20-30hp, gained maybe 5hp). Don't want to spend whatever amount of $$$ to have no gains, just a sound.

Last thought - I currently don't have any problems with cats (check engine lights and so on). So replacing them can only cause problems, and potentially give some more power.

What are your thoughts?

 

If you have fitted 50/50 methanol in the air intake then already you are ahead on thE 5 litre as this will entirely solve the GDI carbon build up problems. As for increasing power, depends on whether your tuner was aggressive enough with AFR and timing just keep in mind if something goes wrong with meth delivery it could be catastrophic for the engine unless some failsafes are in place such as changing the tune automatically if something goes wrong.

 

No major changes in soft after meth. It just lowers intake temp and increases octane value in cylinders.

They tried with more aggressive AFR and no result. Hypothesis - exhaust/cats are limiters...

The only failsafes related to meth are reputable producer, diode in a visible place, solenoid and filter. But no advanced logic like different softs automatically loading etc.

 

Yes a balanced free flowing intake/exhaust along with appropriate ECU adjustments will give you more power. There is someone in Europe who has managed a 700BHP/850TQ tune into the 5 litre...

 

700bhp -> 7.x 100-200kmh - sweet

So you say - exhaust modifications make sense and this is a limiter for this engine? What cats would you recommend and what diameter of pipes in exhaust?

 

I don't think the exhaust is a limiter but pumping more air through will help make more power and can be tuned for it. The majority of the limiters are in the PCM. I have the 4.2 which is not as modification friendly as the 5 litre but I have fabricated custom intakes and verified increased airflow and made a stepped exhaust with 200 cell cats, 2" to 2.5" to 3" with 2 X-overs and 2 resonators instead of mufflers.

 

I had thought that the 4.2 was infinitly more tuneable than the 5.0?

Also might be worth sharing with tmich how much louder your zaust is, as he's stated he doesnt want that.

Ive no backbox and a standard tune and think the sound is just about right, if I were to go with an x-pipe and 200 cell cars my Indy said it would increase the sound quite considerably.

As I'm end of cul-de-sac my old decatter x100 NA was considerably louder than my current, but respect for my neighbours would prevent me from doing further mods to my R

 

Its loud, but not obnoxiously so I say typically exotic car loud with modified exhaust.