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V8 exhaust headers.

Steve 90

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As most of you will know from my recent posts im gathering bits together for a recently acquired 4.2 V8 auto to be fitted in my challenge truck.

Was going to run the standard manifolds and get some non cat front pipe's which I will cut and shut to fit better/much more tucked in. Ordering front pipes seemed easiest to get flanges, Collectors and a selection of bends but ive seen a few sets now and they are all horrible, The bends are poor and the collectors are even worse.

So now im thinking of going for some tubular manifolds. What works and what doesn't? Is a 4-2-1 better for bottom/mid than a 4-1 on a RV8? Or is the V8 not really that fussy?

Whatever it is, it will have to be tucked in neatly, above the chassis bottom and over the crossmember. If possible in mild steel as I will almost certainly have to alter it and add Lambda sensors (if they dont already have them)., Although if it has to be, stainless would be OK.

Any recommendations or advise from all those that are more familiar with V8's than myself (that will be everyone) gratefully received.



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I have (lucky for you :lol::P) done a lot a research and spoken to those who know a bit about exhausts , and particularly around V8s :)

4 into 1 a la janspeed moves the power UP the rev range, and also for anything over 3.5 the bore of the pipe is too small as well, so the janspeed systems fail on 3 counts (mine also didn't fit properly).

4 into 2 into 1 moves the power DOWN to give low end grunt, this then needs to be coupled with the right OD but the 2x twin pipes off std 2 exit port 3.9 manifolds daoesn't do as bad a job as you might think, often the tubework is poor with more a crunching of the pipes into 1 than a blend :(. Also the pipes are hugely different lengths each side which doesn't help, but this could be easily modified. a 300 TDi rear over axle and box section mated to a V8 Y piece would make for a decent base system as the pipework is reasonable size.

If you are MS it (think you are) then weld a lambda boss into the Y piece and there you go, 3.9 twin port manifolds, plus std twin downpipes (modded a bit to be more equal in length if you can) into a 3.9 Y piece and join to a 300 TDi rear section, loads of low end grunt and reasonable flow for a few £s

I sadly do have a load of pipe specs per V8 sizes from my research but these will all mean loads a money spending :(

Hope this helps :)


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Cheers Nige.

Yes re MS, There's a man you all know on the case as we speak!

Im not looking for perfection just something decent for an otherwise std 4.2. The 3.9 front pipes I have here are poor looking, Not overly big diameter, Even smaller on the bends and as you say, More crunched into one than a blend.

not keen on moving the power up, Hence this thread. the only "well tucked up" headers I could find were 4-1.

So I might well use the 3.9 front pipes I have and mod them to fit. Any idea what sort of bore they are supposed to be at each stage? Just so I can check these arn't a very poor set, Made well under size. In which case I'll order another set.

The rest of the system will be a straight through silencer and 65mm tube exiting behind the rear wheel just above the chassis, the same as it is now really. Will that be OK?

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For a 3.9 / 4.2 the std 3.9 twin port exit manifolds are fine. These really then need to go into twin downpipes, around 36-38mm OD without the deformation / crushing as they go into 1 end pipe as so many of them do. For low end grunt these twin pipes should be around 16-20 inches long, not essential but somewhere in this regions, and also similar lengths (RR isn;t as std), this drives bottom end grunt and helps stabilise the effects of 5 and 7 firing consecutive and gives what is called an extrator effect as to the lengths of the 2 pipes of 16-20 they will be different as the exhaust drops are different so build this in, with say 2-3 inches will be fine.

Then the twin pipes need to go to 1 pipe, here this can be less than the finished bore (yoursd being 65mm) or the same, again I would say smaller is better so the steps up are uniform. what you then need is a Y piece where the exit is 65mm as you need for the rear end and the twin pipes of a bore to match the single exit from the downpipes, bigger than the individual pipes at 36-38 but less than 65, its what you can find, suggest you 1st look at 3.9 then 4.2 50th Anniversary and 4.6 are all poss options maybe ?



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  • 1 month later...

Right, Think I'm sorted as for headers, I'm looking for some 4.2 or 4.6's still but if I cant find any I have a 3.9 set that will do but the Y's and bends are very poor and will need re-making along with a bit of messing to keep them tucked up a bit more out of harms way.

Been looking at the rest of the exhaust and the system I have for the TDi is going to be difficult to fit a silencer to and due to engine etc repositioning its going to be easier to start from scratch with an assortment of bends and pipe.

So question being, As I'm starting from nothing what size pipe is best to go for. Stick with the 65mm as per the 300TDi system mentioned above? Go for 3"? Or something in between? Is 3" too big for a 4.2?



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  • 1 year later...

I'm in the same boat I think. I have the 4.2 Y piece but the 3.5 standard box. I think it's a bit small and it doesn't line up on 2 of the mounts so I'm worried the manifolds are carrying too much of the exhaust.

Has any body done the 300 tdi rear section ? I just have visions of them not meeting in the middle .......

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I've currently got a completely home made (read bodged) system on my 4.6 90".

I've got four branch tubular headers that join up to a home made Y piece (couldn't find an off the shelf one to fit the original 300tdi cross member)

which then join a cherry bomb :blush: (bit chavvy i know) which in turn joins a LATE 300 tdi 90" over axle pipe with a td5 90" back box grafted on to it. It sounds plenty growly enough without pxxxing me off on the motorway.

If i can get some time to take pics of it i will.


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I'm rapidly heading toward this situation. I have a stainless exhaust, that was from Rimmers IIRC, currently with a bit of flexi-tube instead of the middle box. Sounds pant-wettingly gorgeous, but sets off my mate's house alarms if I start it too near his garage doors :lol: . The headers and Y are tubular stainless, and came on the 4.2 when I got it.

I'm about to put in a 4.6, so will the standard 4.6 headers be better than these tubular ones (I think that they will...) and are they 2-port? If they're not, will the 3.9 two-port ones do? I seem to remember that HFH's 4.5 loveliness has them on, so I guess that they would work.

Where can the 3.9 twin-jobbies be found?

For low end grunt these twin pipes should be around 16-20 inches long,

How long should the bits after them be? I take it that the above refers to the twin pipes for each bank so how long should the pipes between them and the single back section be? Obviously this'll be limited by the route they have to take, but is there a vague distance?

Enough questions me-thinks, I'll shut up and eat my doughnut.

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