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2.5 exhaust for 2.8 4jb1-t

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2.5 exhaust for 2.8 4jb1-t

Postby Waco1 » Tue Nov 27, 2018 4:13 pm

My exhaust is shot. I've read that a 2.5 exhaust is a good upgrade. Do you add a new exhaust manifold/header or just use the existing down pipe. I'm in the US so parts or a header would be hard to come by.

Thanks - Bill
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby geeves » Tue Nov 27, 2018 4:53 pm

It fits after the turbo. You get 80% of the benefit doing the pipe from the flex pipe back but ideally you need to make a custom dump pipe to bring the 2.5 inch exhaust all the way to the turbo
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby Waco1 » Wed Nov 28, 2018 3:34 am

Thank you sir.

Bill
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby Brinky » Fri Dec 14, 2018 10:17 am

Geeves have you ever seen the exhaust system on a 92 Mu with auto transmission. Its a crazy design as the pipe has to cross under the transmission pan. They even split the pan into two trays with two drain plugs to help get the exhaust slightly higher. I cannot go straight back from the flex pipe due to the very large 30-80LE transmission, transfer case, cross member and front torsion bar chassis mounts. the 2.5" pipe has to start after the transmission crossing but still makes a big difference. Maybe with a 2" body lift I could go straight back and around the back of the transfer case like the manual gearbox cars do. I can see why they changed the transmission on the 3.1 Mu's. The first exhaust change I did was replaced the original muffler with a straight through bullet and that made a big difference by itself. Damn the original mufflers are heavy.
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby tonkamu » Fri Dec 14, 2018 1:20 pm

Is there no room to go over crossmember on the side of transmission? Then wrap under rear driveshaft. I can't remember my 2.8 auto underbody but I remember the bottom pan hanged like sheep balls dragging on the ground
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby geeves » Fri Dec 14, 2018 1:45 pm

Dont know. Cant remember my Wizard being like that though when I had it
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby Brinky » Mon Dec 17, 2018 7:30 am

Nope. There is no room to go over cross member on the side of transmission, then wrap under rear driveshaft without a 2" lift. I will take a photo to show you. When I got the Mu in street form it only had 140mm of ground clearance, my Subaru Outback has more! With a lot of work I now have 280mm of clearance. To quote Tonkamu the transfer box was dragging like sheep balls, the lowest point right between the two axles. Not a great design. The 3.1 autos have much better clearance. One day it will get a manual conversion or body lift, then I can run the new dump pipe and 2.5" system all the way through.
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Auto Trans, Inter-cooled, 2" Raised, Standard dump pipe with 2.5" exhaust, K&N filter, Snorkel, Raised Trans and Transfer Box with modified cross brace, Not lifted, 32 muds, Runva 9500 winch, disconnected sway bar, double air lockers

Re: 2.5 exhaust for 2.8 4jb1-t

Postby geeves » Mon Dec 17, 2018 1:36 pm

So your Mu had been lowered? Standard clearance at the gearbox xmember was about 250 with 28 inch tyres.
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: 2.5 exhaust for 2.8 4jb1-t

Postby Brinky » Tue Dec 18, 2018 7:37 am

Nope. 31" tyres and standard suspension. Front drive shafts were level but the rear springs were getting tired. Next time you spot an auto 2.8 Mu in a car park have a look under it. The original trans cross member is factory packed down 15mm and that has a 50mm hoop in it to clear the front drive shaft UJ off the transfer box. The transfer box bash plate then hangs down another 25mm below this. I should have taken photos before I chopped it all up. Maybe I got the pre-production model!!!
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Brinky
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Posts: 41
Joined: Thu Dec 06, 2018 6:45 pm
Age: 56
Country: New Zealand
City: AUCKLAND
Vehicle: 1992 Isuzu Mu 2.8l UCS55
Auto Trans, Inter-cooled, 2" Raised, Standard dump pipe with 2.5" exhaust, K&N filter, Snorkel, Raised Trans and Transfer Box with modified cross brace, Not lifted, 32 muds, Runva 9500 winch, disconnected sway bar, double air lockers


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