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Single or twin front pipes on a V8.

Steve 90

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Just fitted another V8 in the truck. This one has single front pipes rather than the twins id seen before. The manifolds are identical but the front pipes are single approx 50mm tube right from the manifolds. Does anyone know how these effect the power of the V8. Are they, as I would imagine, similar to the 4-1 tubular manifold that give god high RPM power at the expence of some low and mid?? Or are they closer to the twin front pipe setup??



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Nige or Ian G are your men for this Steve

Hi Tony, Yep, Totally agree but ive been bothering Nige with Questions quite a lot recently so I thought I'd put it on the forum to see what people had to say about it, Would appear not a lot!

Why the y different engine?

I assume you mean the change from 4.2 to 3.9 rather than Diesel to V8.

The 4.2 has a multi pulley/belt setup on the front and I need to fit a higher output PAS pump. I got the pump in place but the bracing for the alternator was not in the right plane and without building some elaborate setup that weighs more than the engine itself I couldn't see any way of mounting the 12v alt, PAS pump and 24v alt that I was happy with. So, as im up against it with time I thought I'd get the truck up and running with the serp 3.9 which takes the same type PAS pump so that's a direct swap and 24v alt mounting is simple. once the truck is up and running and I have more time I'll give the 4.2 some love and attention for a few more BHP and address the ancillary mounting issues.

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There are several manifolds for the V8s, the best are the cast ones vs the horrible steel flexy 4.6 ones (they crack)

This means we have 2 cast ones to look at.

There is the early 3.5 single port exit (say 78-86) with a single down pipe, and the later 3.9 / 4.2 ones (86 onwards) with twin downpipes.

The twins are better for flowing mid and top end range, but many dismiss single pipes on a 3.9 as not enough, they can be fine unless your going to scream the engine, and there is even a view amongst some the single in a big enough bore = the twin, or comes close to it, its more also cubic capaciaty that comes into play, ie a single on a 3.5 is fine, a twin makes little difference, but the difference on a 3.9 or 4.2 iis greater

It would be easy to say that the twins are better, and they should be, however the benefit of having twin downpipes for

better gas flow / export from the system is often ruined IMHO by the appalling qulality of the twin downpipes, esp

with regards to where the 2 come into being 1 pipe, here this should be a smooth trasission from 2>1 but often seem to have

involved nothing more than a moron with a vice and a hammer sqeezing 2 pipes and jamming them into 1 bigger pipe just before welding said mess up

Both these cast manifolds have the same port to head sizes and fit, the only advantage is with the twin set up providing the dfownpipes are of a resonsable decent qulaity - even the genuine ones are variable, for my money I would buy a twin pipe downpipe, cut off the Y piece buy a decent Y piece and weld it in,



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Thanks Nige.

I think Ive just realised what I have. They are definitely identical manifolds to the 4.2 which has twin down pipes. Think they are the 3.9 CAT front pipes which have been cut off just in front of the CAT's.

Ive already got the twin down pipes fab'd and tacked together so I will stick with that. It was more a curiosity thing really.

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