twin exhausts

Hi I am trying to fit two seperate janspeed exhausts to my 3.9 discovery, does anyone know if this will effect the power or the running of the engine as I have been told that a v8 won't run properly on two seperate exhausts and the manifolds have to meet.
thanks

hennas said:

.......................as I have been told that a v8 won't run properly on two seperate exhausts

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Yes this is true. Normal practice is to put a balance pipe between the two exhausts.

With na engines it's all about a thing called scavenging. As a V8 such as the RV8 has an unequal firing order you end up with 2 cylinders firing after the other on the same bank.

This means you need some kind of cross over to merge the two exhaust flows. The engine will run without it, but you'll be leaving a chunk of HP on the table.

The two main types of cross over are H and X pipes. A H pipes needs to ideally be a bore of similar dimensions to the the rest of the exhaust or as big as you can fit.

You can buy H pipes off the shelf or you could fab one up. Jetex sell some T pipes that would be ideal for doing this.

I like H pipes, they tend to make a more classic muscle car noise and promote lots or low/mid end grunt.

X pipes look likes X's. Same principle as a H although they to tend to produce better high rpm results (something not really a concern with an RV8!!). And they do sound slightly different, more racey and less bassy maybe.

Very interesting reply! I'd love to hear more about why the firing order makes the engine behave in a certain manner.

But ill try google it if you cba!

Mr Noisy said:

Very interesting reply! I'd love to hear more about why the firing order makes the engine behave in a certain manner.

But ill try google it if you cba!

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Think of most traditional V8's as more like two V4's with a common crank rather than two inline 4's.

V8's of this type cover almost all BMW, Merc, Yank and even Jap V8's. It's robust, easy to make and smooth.

Some V8's, TV8 AJP, Ferrari V8's and the Lotus 3.5TT V8 where flat plane crank V8's. These are essentially two inline 4 engines with a common crank and have an even firing order. This is much easier for exhaust tuning which is why it's normally done. But they aren't as smooth or as well balanced. They do give a totally different type of sound too, much more of a wail than the off beat rumble a Rover V8 makes.

think of the way yank V8s sound and you'll get the idea - I doubt you will really loose anything if you don't fit a balancer pipe though.

[nomedia="http://www.youtube.com/watch?v=pKPhT9gPz5Y"]YouTube - ‪Ford Mustang 66, V8 sound‬‏[/nomedia]

Hi guys

Cheers

I looked up exhaust manifolds on Wikipedia, I see it is to do with the exhaust pulse creating a low pressure at the manifold as it travels down the pipe and this helps to suck out exhaust and suck in fresh air and fuel.

I also see now that because the RV8 fires 18436572, when it fires 84 there is loads of pressure on one side and a gap in pressure/vacuum on the other side. Same when it fires 57.

The balance pipe thus helps to even out the pressure between the manifolds, and therefore helps to create more of an even scavenging effect on both banks of the engine.

That's the way I read it anyway.

What we're saying with the 2x inline 4 engine is that each bank behaves in a typical 1342 style and so it manages it's own scavenging quite easily. The 2x V4 style engine needs to help out the other side of the engine when it is firing two cylinders consecutively on one bank.

I think I geddit.

Cheers!

if you are into performance then fitting the best free flow single exit is the best for the rover v8
also consider the tail pipe depending on weather it is a rolled in or rolled out or slashed or straight cut and does effect the running and the note

They sound mean as **** with twin exits though, don't know if anyone heard v8-disco's car? That had bike cans as side exits, not sure whether they were balanced or not, but the sound was exquisite!!

I don't know whether it's in out or whatever but a Janspeed tail pipe looks like this and it's not to loud just right to pass the MOT.

A single might work better or not. Depends how it's designed. The RV8 is not a special case, it responds exactly the same as any other engine. Tailpipe design won't affect performance at all.

300bhp/ton said:

A single might work better or not. Depends how it's designed. The RV8 is not a special case, it responds exactly the same as any other engine. Tailpipe design won't affect performance at all.

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done plenty then have you?
i did a theseis for my mechanical course 20 years ago and flow and hymonic dynamics from the air filter to the tailpipe determin the engine note and can effect the engine performance using the wrong air filter and exhaust can either improve or restrict it.

classic example one of my customers has a 110 3.9 with headman headers one failed and he fitted a set of rimmer stainless headers not only were they a smaller pipe they changed the note of the exhaust and also power it was flat compared to the headman ones he has since replaced again back the headmans and has power and the right exhaust note.
it also droped 5 mpg because of the manifolds!!!!!

vougese39 said:

done plenty then have you?
i did a theseis for my mechanical course 20 years ago and flow and hymonic dynamics from the air filter to the tailpipe determin the engine note and can effect the engine performance using the wrong air filter and exhaust can either improve or restrict it.

classic example one of my customers has a 110 3.9 with headman headers one failed and he fitted a set of rimmer stainless headers not only were they a smaller pipe they changed the note of the exhaust and also power it was flat compared to the headman ones he has since replaced again back the headmans and has power and the right exhaust note.
it also droped 5 mpg because of the manifolds!!!!!

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Yes a few

BTW - headers are not a tailpipe :doh::doh:

Yep I agree headers are IMPORTANT, the most important bit really, but an exhaust is entire system.

Scavenging is what a good set of long tube manifolds do and that's what makes the biggest different on a n/a motor. Collector size makes a difference too as can primary pipe bore.

But I'm yet to see anything that says a chromed slashed tailpipe makes less HP than a rolled outer tip one. Do you have a link for something like this?