GU 4.5 what size exhaust???

[4by]

My muffler needs replacing and I'm thinking I might replace the full exhaust.

It already has extractors, but what size exhaust should I fit to get the best performance without incar droun.

Anyone recommend someone in Melb??

Thanks in advance....... ;)

[BearGUST]

What size is the outlet of the extractors? That will probably give you a good idea of what the optimum size exhaust size is...

[Sir Roofy]

for a 4.5 stick with a 2, 7/8th'' exhaust for economy then again depends on what size
extractors you have

[4by]

for a 4.5 stick with a 2, 7/8th'' exhaust for economy then again depends on what size
extractors you have

The extractors are just over 2.5in. Roofy I've heard most people suggest 2 1/4 or 2 1/2, you recommend 2 7/8. Sounds like a particular size you've had experience with...:wink: Does it work well?

Had mandrel bend on my td42, do you recommend this with the 4.5? Would like some more power.....

[BearGUST]

When the extractors were developed they would have tried all different sizes to see what worked best so 2.5" is a good indication.
A lot/most people get bigger than what they need, this generally sacrifices low down torque (which is what you want in a 4WD) but feels like its making power because it might make power high in the rev range.
The main key to it all is - hot gases flow faster, allow gases to cool down in a pipe that is too large and they will slow down which loses performance and can cause drone.
I've worked on quite a few race cars that would stun most people to see that the exhaust pipe diameter is not large at all.
I've never worked with your particular engine so I don't want to name a particular size but do your research, see if you can find any dyno sheets online... And don't just look at top end figure! Look for torque low in the rev range.

[4by]

Cool, so it sounds like 2.5 is the go and a good free flowing muffler.

[Winnie]

As a general rule of thumb with 6 cyl engines most people go 2.5"
The 4.5L is a big engine though so it could be different. 2.5" system on my TB42E made a massive difference.

[4by]

As a general rule of thumb with 6 cyl engines most people go 2.5"
The 4.5L is a big engine though so it could be different. 2.5" system on my TB42E made a massive difference.

Thanks for the first hand info. I was only thinking of changing the muffler which is stuffed as I wasn't sure if changing the whole exhaust would help with performance. But it sounds like it made a difference in your vehicle so I'll be doing the hole thing...... Awesome... :)

[Winnie]

I got extractors done first and they didn't seem to make a difference to performance at all, but they did make a 10% difference to fuel consumption (in my favour). The exhaust didn't do anything to fuel consumption but made a rather big difference in power. If you're after a bit more power some other things that made a difference was getting the injectors tuned and also doing the valve clearances (yours may not need doing)

[4by]

I got extractors done first and they didn't seem to make a difference to performance at all, but they did make a 10% difference to fuel consumption (in my favour). The exhaust didn't do anything to fuel consumption but made a rather big difference in power. If you're after a bit more power some other things that made a difference was getting the injectors tuned and also doing the valve clearances (yours may not need doing)

Thanks mate, I imagine you just went pressed bend corners instead of mandrel.

[Winnie]

Yeah I did, nobody around has a mandrel bender that would do it at a reasonable price. I don't know if mandrel bending would make a considerable difference or not, I had a press bent system on my old Falcon and "upgraded" to mandrel bent and was very disappointed, couldn't tell a difference.