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MG MGB GT V8 Factory Originals Technical - Dual Exhausts

I know all about moving the fuel tank, but what about the fuel line, battery cable, brake pipe, & wiring loom ? These lie immediatly above where the RH exhaust pipe or muffler would be. Do I re-route them thru the trans tunnel or thru the cab or where ? It would seem a bit unwise to me to leave them where they are.This was the reason I didn't go for dual pipes the first time around. I have more time now & the engine is out of the car (broken piston), so I might reconsider a dual pipe setup. TIA Barrie E
Barrie Egerton

Barrie, I am looking at this issue now as well. I am thinking of routing them out of thier ressed (sp?) locations and routing them outboard into the chassis an equal amount. That will clear the Right side pathway back to the rear axle. At the rear axle I think I will follow MG exhaust rout under the axle then up and back to the rear valance.. I was going to go up and over, but that posed problems like you mentioned and extra bending..
Larry Embrey

Do dual pipe setups really yield material extra HP? It's such a dead giveaway to have two exhausts sticking out the back ... so much cooler IMHO to just have that one innocent-looking tailpipe! I admit the sound is marginally better though.

I made another semi circle notch in the fixed crossmember similar to the factory ones,on the right side just outside the chasis rail. To get the fuel line and battery cable through the chasis rail I welded two,pass-through steel pipes in the chasis rail. These pipes go through the rail at an angle of approx 45 degrees and also add strengh to the rail.Pipe inside dia. is 5/8". The fuel line and battery cable go through seperate pipes for saftey reasons.
The brake line passes through in the tunnel. The tail light harness passes through inside the car. That harness is low amperage and fused so I felt it would be safe to run it inside the car.
The right side exhaust pipe can now be run where the wiring/plumbing used to be.
Hope this helps Bill
bill jacobson

Going from memory, very dangerous, I thought in the Supersports article they mentioned that dual exhausts reduced hp. So down to looks and sound?


I have always been told by the musle car guys that duals help HP and breathability of the motor, it is THE reason to do it. Remember an engine is a big air pump..

Think about the volume aspect (numbers are guesses not based on data), your engine is pumping out 200cfm of air, a single 2.5" exhaust can only handle 150cfm, while a dual 2.5" can handle 300cfm. are you using all 300cfm NO, but your engine is not being strangled either by being limited to 150cfm..

This is the same reason everyone goes with RV8 headers rather than the much more "clean and simple" block hugger headers. Same reason high performance mucle car guys use tubulare headers rather than the stock cast iron.

The sound is just a (blessed) byproduct of that extra breathing.. That said, exhaust design can play a big part. A big fat exhaust with lots fo sharp kinks, or weak mufflers can flow less than a smaller tubing system with better design.. It is a balancing game..
Larry Embrey

I wonder whether anyone can help me with this query on exhaust systems. I may go down the dual route, but I am given to understand that a balance pipe between the two sides of the system is vital if power is not to be lost. Does anyone know whether the Supersports system is so equipped?
David Daw

I am not sure about the Supersports system, but you are correct in that a "cross-over" (AKA: H-pipe, X-pipe etc) is needed to get a proper system.

This allows the pressures and pulses of the exhaust to balance each other and smooth flow, if no so equiped it could be why they said the dual robbed them power... Even if not so equipped originaly it can be added to most systems if you are handy with a welder. Some people will cut tubes out of each side and then weld up a "H", some will do a X, and then re-install, some will take the "simple route and cut openeings in each side then splice the piece between them without removeing the system from the car..
Larry Embrey


Without rereading the articles the Supersports does not have a balance pipe, hence loss in hp, unless your handy with the welder as Larry suggests.


My 3.5 litre V8 ex FL roadster has a full dual system running from RV8 headers. Let me assure you that without a balance pipe it sounded bloody awful. Like Larry said. the addition of a balance pipe smooths things out and the car now sounds ok. The car gives just over 200 hp at the wheels on Sigma Engineering's rolling road by the way, - however the engine is far from standard!
Uncle Bob
bob pulleyblank

bob pulleyblank, can you tell us what you did to achieve 200hp in your 'far from standard' 3.5?

Can I ask a dumb question here. It's my understanding that while free-flow exhausts, such as dual setups, are great at high rpms, some back pressure is absolutely key to low-end torque. Is this so, and if so, why?

I just have a single 2-3/4" setup, block-hugger headers; seems to work OK but maybe there would be even more with a dual? Something tells me, good heavens, what a humongous undertaking just to get a few more HP!


From what I've read, and racers & engine builders I've spoken with over the years - diameter being equal - a shorter length exhaust will produce more HP up higher and one longer more low down torque. Thats one reason you see a lot of race cars with side exhausts. As for why backpressure is necessary, exhaust system theory is so complex it's way beyond me - there are lots of books written on the topic, but from my very basic understanding it is mainly to do with the scavenging effects & overlap period of the camshaft as to the best exhaust diameter and length to choose for what rpm band the engine will operate in. Essentially, an exhaust only works best within a narrow range. A street car requires some backpressure to give it low down torque aided by a street type cam.

There are some interesting gadgets, some manually adjusted and other automatically that can alter the internal length and diameter of the exhaust on the go, resulting in better hp across the range. Nology offers one such product, probably lots of others out there. Frankly, I don't think mucking around with exhausts is going to give much unless you're racing looking for that edge. Induction changes are the way to go there.

I wanted to go to duals myself (sound reasons only) and was considering the V8 Conversions Super Sports setup. It makes me sick <G> that a TVR can sound so 'vicious' but my Factory car sounds so sterile in comparison. What turned me off duals is the literature on the V8 conversions site doesn't mention a balance pipe and you also have to cut the rear apron to fit (also not to mention they never return any emails). As I have no welding equipment and don't want to cut holes in my car just to get a better sound (Factory cars are so quiet - which is nice too in a cruisy way). I think I'll just install a US type glass pack instead. They sound pretty good to me. :) Any recommendations along those lines? Demon Tweeks sells 'cherry bombs' or something like that. Gross looking things but any suggestions? :)

74 V8
Neil Cotty

Neil, I guarantee that your single-pipe factory V8 will sound pretty burly with one or two cherry bombs. Factory cars came with a "lot of muffler". I have a single big-bore system with two straight-through mufflers and I will tell you it is _quite_ vicious-sounding. It's just not in stereo! Of course the factory V8 motor is horribly de-tuned so it's not going to sound like some 250bhp pavement-pounder. And the SUs probably won't yield as throaty a sound as a 4-barrel.

Hi Niles,

My car isn't totally stock, Weber, offy dual port and a single box big bore system. Figure around 180bhp at the moment. Some PB heads in the future should see it close on 220 I hope! :) Thanks for the info on the cherry bombs, think I'll try one if I can get one that isn't bright bloody red!!! <VBG> That or a spray can eh.. :)

74 V8
Neil Cotty

Flow-Tec (Holley Inc) makes some glasspacks called Purple Hornies. These are a louvered internal baffle that is supposed to be a better sound and system. the Cherrybombs really are nothing but a straight tube with some holes in it, so I am told they sound really harsh at times..

Bad thing is they are purple, unless of course you like purple.. <a href=></a>
Larry Embrey

You can also use flowmaster, borla, walker etc. Yopu will pay for quality exhaust but it 's like what was said what is your need high end performance or low end grunt. Supertrapps make dics that can be interchanged to give either setting.Like Larry said H, X, are all good crossovers but in the last two articles written in sreet rod and msutang monthly ( Ithink where they were) the siamesede pipe produced the most Horses. Larry stated that too I think. I f you think duals won't help just look at the ws6 firebird/camaro pckge. They added special exhaust to those carsalong w/ ram air etc. and got alot more horses. This is just what I have read , not trynig pinch anyones toes or feelings- mike
mike childress

FWIW I got my V8 sounding a lot better. I responded in another thread, but WTH I'll reply here for those following this one. :)

I *thought* I had a single box system but in fact I had a two box. I had an exhaust guy remove the front box and wow what a difference. Really sounds like a V8 with the single setup and not annoying at all. :) Quiet-ish at cruise, lovely changing gears and wicked under WOT <G>. So if you've got a two box and want a cheap improvement in sound this isn't a bad way to go. YMMV!

74 V8
Neil Cotty

Neil, sounds like a simple and fruit-bearing mod. Taking out a box can't hurt the performance either. I wonder where one can get a piece of 2.5" SS pipe; I would like to try this too ... will substitute for the front box (also will not hurt ground clearance, I think the front box is the lowest thing on the car). I know what you mean, those dual V8s can really have a crackling, intense bark!

Is you system a true Stainless or is it Aluminized? I would say call your local PERFORMANCE exhaust shops. I tried all the normal brake/muffler places and they won't give you the time of day unless they do everything. Most refused to sell me tubing.

I bought 20ft of 2.5" aluminized for $22.00 per 10ft stick.. I know it is more than you need, but call around, someone should be willing to help..
Larry Embrey

Larry, I have SS but it probably doesn't matter a whole heck of a lot if one piece is aluminized.

Yeah, the regular exhaust places around here (Midas, etc.) basically stink too. It would truly be a cotton-mouthed, sweaty-palmed horror to entrust your V8 to them for two minutes. In college I had a German car and I made the mistake of having them put on a new exhaust system ... it was a system they cobbled together from an entirely different car, such a wretched job it was comical.

(Killer web site, Larry -- great pics. I'll send over a picture some time, though you will probably be bored to tears with my plain-vanilla GT in contrast to some of those zooty cars in yr pix.)

Send away David!!

Mine is pretty plain jane also. I went to our club meeting lastnight, has gotten the bloood boiling again. I got headers lastnight, and saw someones conversion for some ideas, so I will be getting back on it aggresively this week..
Larry Embrey

A single exhaust is probably OK if it is made of decent diameter pipe ...

To the Rice-Mobile! Atomic batteries to power, turbines to speed...
Mario Funk

What is a siamesed exhaust pipe setup? I understand, I think, the H and X type of crossover pipe. I actually have a midget so not as familiar with the B but find the V-8 B's very interesting cars.

Best regrds, Dvid Walworth
DAvid walworth

I'm still in the planning stages of my V8 project and had an opportunity to see a few V8's and get a few ideas at the meet in Sebring, FL. One car in particular caught my eye as it had side pipes like a Cobra. Talk about an easy instalation. No cutting the rear valance, no moving the fuel tank, no rerouting fuel lines...just run short curved pipes out from your block-huggers or RV8 style headers and weld on the pipes. All you need to do is make up some simple brackets to hang the pipes. As far as draw backs go the only things I noticed were...

1. Can't use original jacking point and factory jack.
2. A little less ground clearance than stock
(but this car was lowered)
3. Possibility of burning your legs unless you wear
asbestos socks.
4. Appearance gives away the V8 (may give the Honda
next to you a hint of what's under the hood)

Some purists may not like the looks but the sound sure was sweet. I didn't catch the guy's name but I think he said he got the pipes from JC Whitney for less than $200 for the pair. Not bad in my opinion. Hope this helps.


Chris Mowris

This thread was discussed between 18/04/2001 and 07/05/2001

MG MGB GT V8 Factory Originals Technical index

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