14 replies to this topic

[skaterava]

I've looked around a bit and it seems to me that the only twin exit exhausts you can buy come bored too large to be beneficial for a 998 engine.

I'm going for a muscly, road racer look and the twin centre exhaust is pretty vital, but only if it doesn't decrease my fuel consumption or overall performance.

ideally, i'd want a 998 Stage one kit except with a twin centre exit exhaust, not just an enlarged single side exit.

Are there any twin centre exit exhausts under 2" bore?

If not, what will i have to do to my 998 in order to make a large bore exhaust compatible?

Or, am i getting this all horribly wrong by imagining that backbox bore size matters?

thanks

[Cooperman]

Repeating what has been said on here many times, for a 998 the optimum exhaust pipe diameter is 1.625" internal diameter from the end of the exhaust manifold, where the pipes join into one right to the final exit point at the end of the tail pipe. That optimises performance and fuel consumption. For a 1275 to 1380 the optimum is 1.75" internal diameter.
Now, the scientific bit is known as 'Bernouili's Theorem'. This states that when a pipe or tube carrying a fluid (exhaust gas is a fluid) changes in diameter, if the diameter increases, then the velocity reduces and the pressure increases. So, if you increase the cross-sectional area of the final part of the exhaust pipe, then the velocity of the exhaust gas molecules will reduce and the pressure will increase at that point. So, the increased pressure and lower molecular velocity will cause the exhaust gases to exit less efficiently, losing power and thus increasing consumption.
If you look at any serious competition Mini, race or rally car, you'll see the constant pipe diameter from front to back. Take a look at an ex-works rally car with its centre exit pipe.
Enlarged or twin final exit tailpipes are just noise-boosting power-sappers and are avoided by serious Mini owners looking for best performance.

[skaterava]

right, it seems i'll have to choose one, performance or looks. Thanks for the info

[Sherlock]

I totally agree with Cooperman, there is no point in big or twin tailpipes, but there is an idea I've been kicking around for a while & this may answer two questions in one. The Mini Miglia racers use a twin box side by side system, very much like the Lotus Elan of the early 1970's & for people who like the look of twin exhausts, I wonder if a pair of smaller bore silencers fitted in the same way as the Miglia set up, would work on a 998.

[Cooperman]

That would be great if the Mini head had one exhaust port for each cylinder, as the Ford Twin-Cam engine as fitted to the Lotus Elan does.
However, for a Mini maybe if one pipe came from a merged exhaust ports 1 & 4 and the other as a single from the siamesed 2 & 3 it would work. To equate to a 1.75 ID pipe for a 1275, you would need twin 1.25" pipes. There would not be an exhaust manifold as such, rather pipes from 1 & 4 would merge into one pipe and for 1 & 3 it would be a single pipe from the head all the way back. I guess you would brace these together with pieces of steel plate to enable effective mounting and for strength. In theory, the pipes from 1 & 4 should be 0.81" dia, so 7/8" pipe would be good merging into a 1.25 via a specailly welded up adaptor pipe near the bottom of the gearbox. That would keep the exhaust gas velocity fairly constant from cylinder head outlet right to the tail-pipe end.
As for silencer boxes, it seems unlikely that two could be mounted in the 'tunnel', but a pair of 1.25" entry/exit bore silencers would fit inside the area of the rear sub-frame.
It would be well worth looking at as a gain in power would be almost certain a well as better low-down pick-up through more consistent breathing.
Who wants to make the prototype?

[Jay3178]

I totally agree with Cooperman, there is no point in big or twin tailpipes, but there is an idea I've been kicking around for a while & this may answer two questions in one. The Mini Miglia racers use a twin box side by side system, very much like the Lotus Elan of the early 1970's & for people who like the look of twin exhausts, I wonder if a pair of smaller bore silencers fitted in the same way as the Miglia set up, would work on a 998.

i have a miglia set up on my mpi i went to maniflow for lots of advice and they were very helpfull, i told them the spec of the mini and they put together an exhaust kit list for me of what i needed to achive optimal performance with the exhaust design and engine spec, this is from a stage 2 LCB so you dont need additional pipes in the middle. authough mine is 1340 i cant see why you couldnt do the same on a 998. if you speak to the chaps at maniflow i'm sure they can advise you, my exhast system was not cheap but it made a big difference from the run of the mill show purchased exhaust i had before! money well spent i think. here is a picture of my twin exhaust not the best picture but only one i have a the moment.

Edited by Jay3178, 09 December 2009 - 01:11 PM.

[skaterava]

Jay can i just say your Mini looks fantastic. Can't believe i haven't noticed it since i've been here

my mechanical expertise is pretty poor, but what you're saying is that unless i'm looking to spend a lot of money, or upgrade for a 1275 or higher, fitting a twin exit will decrease a 998's performance.

i suppose i'll just have to wait until i have a better income and lower insurance quotes before i can get precisely what i want.

[paul6266]

i used to have the playmini twin exit round tailpipe backbox, made all the right noises but by god it really did bog the car down, it was only a standard 998 but i could set off car alarms easy which is never a good thing at 11pm ish, and going to riv run was a nightmare with all the steep hills as i never had any back pressure, so its down to you......i have a playmini side exit now, sounds great and retains back pressure

[Cooperman]

An example of how an incorrectly designed exhaust can ruin a good performing car is to think about how much performance and fuel economy you lose when an exhaust box or pipe blows. My other rallly car, a rover 214Si, blew its back box a while ago whilst helping with the organisation of a rally. The fuel consumption dropped from about 37 mpg to about 24 and I actually ran out of petrol, luckily whilst with othe organisers in their car. It was so poor acelerating and lost all its performance.
It's so important to get the correct exhaust gas flow speed.
The guide sizes I've always worked to are:
1.625" diameter (internal) for a 998 road car bored rom standard to up to 1061 cc (that's +0.080").
1.750" dia for a 1275 to 1380 car using up to 6500 rpm at full throttle
1.875" dia for a 1275 or over revving up to 7000 rpm
2" dia for race cars pulling over 7000 rpm

There is some flexibility, but from what I've learned, the absolutely worse thing you can do is to fit a larger bore final outlet pipe as this just slows the gas velocity and loses power. It's meant to come out at speed to achieve the 'extractor' effect.

[steble]

this was one of the most useful threads i have read

made me think about replacing my exhaust and fixing that blow i have lol

but what exhaust size would be good for a supercharged mini running 122bhp???

[yeti21586]

I totally agree with Cooperman, there is no point in big or twin tailpipes, but there is an idea I've been kicking around for a while & this may answer two questions in one. The Mini Miglia racers use a twin box side by side system, very much like the Lotus Elan of the early 1970's & for people who like the look of twin exhausts, I wonder if a pair of smaller bore silencers fitted in the same way as the Miglia set up, would work on a 998.

i have a miglia set up on my mpi i went to maniflow for lots of advice and they were very helpfull, i told them the spec of the mini and they put together an exhaust kit list for me of what i needed to achive optimal performance with the exhaust design and engine spec, this is from a stage 2 LCB so you dont need additional pipes in the middle. authough mine is 1340 i cant see why you couldnt do the same on a 998. if you speak to the chaps at maniflow i'm sure they can advise you, my exhast system was not cheap but it made a big difference from the run of the mill show purchased exhaust i had before! money well spent i think. here is a picture of my twin exhaust not the best picture but only one i have a the moment.

i wont that exhaust!!!!!!!! can i have it

[Cooperman]

A forced induction Mini which is giving over 120 bhp will be using only up to about 6000 rpm, so take the capacity of 1275, multiply it by the boost percentage, say about +7.5 psi boost which is about 50% up from atmospheric, and you have an effective capacity and thus exhaust gas volume increase of, let's say, 50%. This means you need a 50% increase in pipe cross-sectional area. A 1.75 ID pipe has an area of 2.4 sq. ins, so a 1275 boosted by 50% needs 3.6 sq. ins. That means that a pipe internal dia (ID) of about 2.125 or 55 mm would be ideal. I guess if only 5500 rpm was to be used, then a 2" pipe would be OK and they are commercially available for the Mini. With 55 mm you may have to make one up to suit.
An oversize exhaust is, in many way, worse than one which is too small, as with an oversize pipe at low rpm's the gas velocity is so low that it 'bogs down' the engine as the gas velocity won't 'self-eject' properly.

Edited by Cooperman, 10 December 2009 - 03:09 PM.

[Sherlock]

That's a lovely car Jay, this has been a very interesting thread & thanks Cooperman for the information. I fear those words, "Who wants to make the prototype?" will come back to haunt me someday!

[steble]

your numbers seem pretty accurate and luckily for me my tailpipe is already 2" i think

[cooperdan]

I totally agree with Cooperman, there is no point in big or twin tailpipes, but there is an idea I've been kicking around for a while & this may answer two questions in one. The Mini Miglia racers use a twin box side by side system, very much like the Lotus Elan of the early 1970's & for people who like the look of twin exhausts, I wonder if a pair of smaller bore silencers fitted in the same way as the Miglia set up, would work on a 998.

i have a miglia set up on my mpi i went to maniflow for lots of advice and they were very helpfull, i told them the spec of the mini and they put together an exhaust kit list for me of what i needed to achive optimal performance with the exhaust design and engine spec, this is from a stage 2 LCB so you dont need additional pipes in the middle. authough mine is 1340 i cant see why you couldnt do the same on a 998. if you speak to the chaps at maniflow i'm sure they can advise you, my exhast system was not cheap but it made a big difference from the run of the mill show purchased exhaust i had before! money well spent i think. here is a picture of my twin exhaust not the best picture but only one i have a the moment.

liking your underside very nice...