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Exhaust

The Imp sport exhaust is a good alternative for the standard system. the drawbacks are that it is tuned for maximum power at 6100 rpm and the primary pipes are on the short side. For a 998 engine the diameters should be bigger. As an example the exhaust dimensions suitable for a 998 sport :
headers :length = 38 cm , internal diameter = 28 mm
downpipes : length = 37 cm , internal diameter = 37 mm
tailpipe : length = 82 cm , internal diameter = 40 mm

Sport manifolds are stronger than standard.
An Imp Sport exhaust system shows a remarkable improvement, even on an otherwise standard engine, giving better performance, economy and a more suitable sound.
If you are fitting a Sport exhaust with a Solex carb and standard inlet manifold, you should remove the drain tube and plug the hole, as it fouls the Sport manifold. If your fuel pump and carb are in good condition, you should not experience flooding anyway.
Imp tuning: tuning on a budget / Richard Freeman. - Impressions 1983, Dec.

There are two types of Hartwell inlet/exhaust manifolds: 1 3/4" bore and 2". The larger is used with the rear-mounted works silencer.

Impressions Apr./May 1986: an alternative for the standard Imp Sport exhaust system, neater, cheaper and slightly more powerful. Remove the silencer and the (prone to crack ?) manifold. Invest in a Janspeed (or similar) 4-2-1 exhaust manifold, with a terminating bore of 1½". Purchase an Imp Sport manifold gasket - not the bright silver one, which is for the standard Imp. The Imp Sport gasket seems more flimsy, but it is the right one for the job, and cheaper. Then purchase a front silencer box for a Vauxhall Viva HA/B.
All that is needed is a bolt welded to the front of the silencer and hacksaw the amount of pipe at either end that is not needed (keep 2" at the back).
This system shows hardly any noise increase - the note is slightly rortier. It has a power increase of just over 1 bhp. The carbs were slightly weakened by about one eigth of a turn upwards. Maybe put a chrome tail-pipe on it.

Manufacturers

Mamba

Motor Books and Accessories, of 33 St. Martins Court, St. Martins Lane, London, London W.C.2. also supply a range of special equipment for the Imp, including their own MamBa exhaust manifold and silencer, a range of American Iskenderina camshafts, Appropriate Formula steering wheels and their new MamBa car seat.
   MamBa exhaust system
Mamba exhaust manifold and silencer £14 19s 9d. from Motor Books & Accessories Ltd.
     

RJV

R.J.V. Automobile Engineering Co. Ltd.
Racing Car Show 1965, R.J.V. had stand nr. 71 (Paul Emery had 73). They developed and made performance inlet and exhaust manifolds in a very wide range. For the Imp a Weber carberettor conversion was available.

165a Westbourne Grove
London, W.1.
(Park) 4802

An R.J.V. inlet, exhaust manifold and silencer for the Hillman Imp

  R.J.V.
     
Roger Nathan's stage 1 tuning
Click on image to show it larger
Roger Nathan Stage One conversion

Nathan Racing

Nathan Racing Ltd.
Imp specialist Roger Nathan's stage one tuning includes:

Hot Car 1968, April. - p. 47:

Roger Nathan's Stage One conversion is simple but very effective. On paper it seems to be the best value for money at a cost of £38

Neal Davis

Neal Davis used to be located at: 106 Main Road, Sidcup, Kent

A good free-flow manifold is an essential part of the exhaust system. This is the Neal Davis Racing set-up. In April 1968, this cost £15 15s complete.

 
Neal Davis Racing set


Stainless
The Clan Club do nice shiny Stainless Steel silencers to go with their nice shiny (expensive) Stainless Steel manifolds. These are available separately from the supplier for about £60. These are repackable, fit onto the Janspeed manifold (as they are basically a better quality copy of the Janspeed box) and are guaranteed for life.

Janspeed

Janspeed will make a silencer to your design. A Mr. Ted Bailey is noted more than once in Impressions for 'knowing his stuff'.

Janspeed boxes were perhaps not designed with muffling as a major criterium. They are just about straight through. The packing burns out fast. Janspeed silencers are not universally loved. You might decide to use the Janspeed manifold with a different box. Viva centre boxes, Escort centre boxes or even Metro centre boxes have been used with good effect.

loud exhaust saves lives ;-)

This is mine:
a Janspeed big bore.
It got prettier...
 

PECO

Peco units

mm     Diep en sonoor. - Friese koerier, Heerenveen, 15-juni-1968.
secundary source: Delpher.nl op 11-03-2016, http://resolver.kb.nl/resolve?urn=ddd:010691034:mpeg21:a0248
   

J. Howe

Jeff Howe made parts for Imps, and exhausts amongst them.

Across the back

See:


Check also Rienk's spreadsheet file on Imp exhausts
Imp Anatomy
The Imp Site
Franka

 Edward Newman (15 Jun 1999):

Pulse tuning was the way they used to make all formula one exhausts. The way it was done was to keep cutting the exhaust tubes shorter until the power output peaked. You then bend the pipes to fit and then put the silencer on the end. The bends in the pipe don't make any real difference.

Apparently most decent modern motorbike exhausts have power valves in them. These can be set to come in at different speeds. As they open they allow the exhaust gases to bypass the main part of the silencer. This is why if you listen to something like a Yamaha R1 the exhaust note changes as it accelerates.
My local motorbike shop are fairly certain that you could get eg. an R1 exhaust to fit on an Imp. They only problem is that they cost about £160. Other ones are a lot cheaper, it's just that the R1s is all carbon fibre and very nice.

 

just mentioning, no recommendation intended:
Leo Vince exhaust systems