Catalytic Converters and their alternatives

Words: Paul Sharpe, Pictures: Rob Bell

Introduction

The catalyst is an essential part of your car's exhaust emission control equipment. As its name suggests, it catalyses the conversion of potentially harmful gases (carbon monoxide, super oxides, hydrocarbons, nitrogen oxides etc.) into substances that are less damaging to the environment. In order to attain this goal, the exhaust gases are passed though a cylindrical tube containing a large surface area matrix containing a number of precious metals that enables the oxidation of the noxious gases. Whilst this is undoubtedly a 'good thing', some might argue that the obstruction to free gas out flow will impede engine efficiency (power output), plus some prefer the increased volume of sound produced by the catalyst's removal... But, environmental concerns to one side, is this really a good idea?

The Options available

If you are in the position to want and/or need to change your car's catalytic converter, there are four options available to you: replace your cat with another standard item, a cat- bypass tube, a sports catalyst or a generic catalyst. One other option not mentioned here is an alternative to the straight through cat- bypass tube. Motobuild Racing retail a 'silenced' cat- bypass tube for owners who wish to have the claimed power advantage but without the excess noise this can entail. We shall see exactly how much power can be gained by this route later on. The four options that we tested in March 2000 are pictured and described below.

1.

The Standard Catalyst

Purchased from your local MG dealership (or from Unipart). Cost 255 GBP + VAT.

Pros: it passes all current exhaust emissions tests

Cons: very few - other than the fact that it appears quite pricy.

2.

The Cat- bypass tube

Effectively a length of tubing with two flanges welded on! Very simple replacement of standard catalyst.

Pros: Makes the exhaust sound very loud, and should be very cheap...

Cons: ...but usually isn't. It will also fail all current emissions tests.

3.

The Sports Catalytic converter

As found fitted to high performance MGF derivatives (MGF Cup cars etc.), promising power advantages over the standard catalytic converter.

Pros: There is a theoretical performance advantage over standard cat, whilst still passing emissions tests

Cons: It is rather expensive! Est. cost 400 GBP.

4.

A Generic catalytic converter

Mass produced catalyst supplied without flanges to enable fitment to any make of car.

Pros: It should be cheaper than the standard catalyst from MG. Usually cost approximately 200 GBP.

Cons: It is inconvenient to fit - you have to weld on new flanges to fit to exhaust system. Probably is no cheaper than the standard MG cat once fettling work is figured in to the overall cost.

 

The Catalyst Power tests

Aims of the tests

To demonstrate whether the standard catalytic converter represents a significant impairment to engine power and torque output (or, to put it another way, whether a cat- bypass tube releases significant gains in power). Secondary end-points being recorded sound output (dB) and subjective driving impressions.

The cars employed in the investigation:

We chose to employ both versions of the 1.8 K-series engine, the multipoint injection and the VVC. The cars used and their specifications are shown below:

Car #1

1.8i MGF belonging to Andrew "Scarlet Fever" Phillips pictured left on the Aldon Automotive rolling road.

Engine Specs:

  • 1.8 litre K-series. Standard 4 valve head and multi-point fuel injection
  • RoverSport/ITG foam air filter mated to a non-standard VVC inlet manifold.
  • Mike Satur Daytona Quad pipe sports exhaust

Car #2

VVC MGF belonging to Andrew Gilhooley, again, pictured on Aldon Automotive's rolling road.
 

Engine Specs:

  • 1.8 litre VVC K-series. Standard large 4 valve cylinder head with variable valve timing.
  • K&N 57i air filter induction kit on standard VVC inlet manifold.
  • Mike Satur Daytona Quad pipe sports exhaust system.

Methods

The Rolling Road

How the tests were conducted

We arrived at Aldon's rolling road early in the morning with two cars and a single objective - to determine whether alternatives to catalysts were worth while. To do this we needed two cars, because to swap the catalysts takes time - so while one car was being tested on the rolling road, the other could be cooling down before removing the hot exhaust components. Once the catalyst had been swapped, the car headed outside for the sound test outside the front entrance of Aldon. Following this, and to ensure that the engine was thoroughly warmed through, the car was taken out onto the local roads so as to gain a driving impression, and to allow some time to allow the MEMS engine management to come to terms with the new exhaust set up. The car was then taken onto the rollers for the power run - and all the data recorded, as can be seen at the bottom of this page.

The rolling road employed was Aldon Automotive's impressive facility on the Breener Industrial Estate in Brierley Hill (West Midlands, DY5 3JZ). Aldon has a long association with MG performance modifications, race car preparation and fast road car development. They can be contacted by phone: 01384 572553, Fax: 01384 480418, e-mail: aldon@yesit.co.uk and have a web site: http://www.aldonauto.co.uk

Parameters measured:
  • power (bhp),
  • torque (lb.ft),
  • ambient temperature (Celsius),
  • barometric pressure.

Just how much power and torque does a Catalyst steal, and is it significant? To determine the answer to this question, we used Aldon's rolling road to measure these parameters over the engine's rev-range. Power was measured from 2,500rpm on both 1.8i and VVC cars, to 6000 rpm for the 1.8i and 6,500rpm for the VVC, 750 rpm below the red line of both of the vehicles being tested. Each test was repeated three times to verify the results.

Noise tests

One of the side effects of a catalyst fitted is that exhaust noise is reduced. Depending on your point of view, this is either a good or a bad thing. Therefore we set out to determine whether there were significant differences in the exterior noise level with each of the catalyst/replacement options. To measure noise, we used a calibrated noise meter to record the dB output, measured one metre from the exhaust, and at an angle of 45 degrees. We performed this under three separate conditions:

  1. at idle
  2. 4000 revs
  3. 90% of peak revs (5750rpm for the 1.8i and 6750 for the VVC).

Emissions tests

As mentioned in the introduction, the role of the catalyst is to remove noxious gases from the exhaust. By removing the Cat, will your car fail a road side or MOT emissions test? Therefore, we measured exhaust gas emissions for carbon dioxide and hydrocarbons. It should be pointed out that the MOT also requires a third emissions check called a Lambda reading, however this reading was unavailable while we were doing our tests because of technical problems. Bah humbug!

Observation

The size and structure of the catalytic matrix varied between the catalysts studied. The generic cat shared matrix construction pattern with the standard catalyst- a criss-cross kind of pattern (catalyst on the left in the picture below). The length of the catalytic material was however some 2-4 cm shorter than the standard catalyst, suggesting that its service life will be significantly less than for the original equipment (O.E.) item. The generic cat also suffered from a marked stepping from the inlet into the matrix whereas there was a 'gas flowed' gradation in diameter for both the sports cat and the O.E item. The Sports cat had the best gas flow design characteristics in this respect. In addition, the Sport cat's matrix pattern appeared more 'spiral' in construction with larger gas channels between the layers of the catalytic surfaces. See picture below: O.E cat on the left, sports cat on the right. The cat- bypass tube looked like a bit of scaffold tube with two flanges welded on- which is essentially what it is!

O.E catalyst (Left)'Sports' catalyst (Right)

The Catalyst test Results

Table 1 – Rolling Road Results

Car / Catalyst

Power (bhp) at 3000rpm

Power (bhp) at peak rpm

Change from standard

Torque (lb./ft) @ 2,500rpm

Torque (lb./ft) @ 5,500rpm

 

 

 

 

 

 

1.8i / Rover Standard

72

124

 

129

118

1.8i / Rover Motorsport

73

126

+2

128

120

1.8i / Aftermarket

74

125

+1

129

120

1.8i / Bypass tube

73

127

+3

128

120

 

 

 

 

 

 

VVC / Rover Standard

72

147

 

121

128

VVC / Rover Motorsport

72

148

+1

122

130

VVC / Aftermarket

67

141

-6

116

122

VVC / Bypass tube

67

144

-3

117

126

Table 2 – Noise Tests

Car / Catalyst

Noise (dB) at Idle

Noise (dB) at 4000rpm

Noise (dB) at 90% max revs

 

 

 

 

1.8i / Rover Standard

61.7

81.4

89.6

1.8i / Rover Motorsport

63.5

83.6

89.3

1.8i / Aftermarket

61.5

81.5

89.2

1.8i / Bypass tube

62.7

84.7

90.2

 

 

 

 

VVC / Rover Standard

67.0

86.5

92.5

VVC / Rover Motorsport

65.5

86.5

93.4

VVC / Aftermarket

65.4

86.2

92.4

VVC / Bypass tube

66.8

89.0

93.0

Table 3 – Exhaust Emissions

Car / Catalyst

Carbon Monoxide
(percent %)

Hydrocarbon
(parts per million)

UK MoT Limits

0.30

200

 

 

 

1.8i / Rover Standard

0

45

1.8i / Rover Motorsport

0

30

1.8i / Aftermarket

0

47

1.8i / Bypass tube

0.69 - FAIL

107

 

 

 

VVC / Rover Standard

0

0

VVC / Rover Motorsport

0.02

41 

VVC / Aftermarket

0.42 - FAIL

43 

VVC / Bypass tube

0.46 - FAIL

68 

 

  Conclusions

The Cat Bypass Tube

its illegal for road use and gave no power or torque advantage on the 1.8i or VVC – so not much to recommend here

This failed the emissions test for both cars miserably. Although it produced a 3bhp gain over the standard catalyst on the 1.8i, this would hardly be noticeable in day to day driving, and, surprisingly, it caused a power decrease on the VVC – possibly because the VVC may need a degree of back pressure which is lost by having not Catalytic honeycomb. It did produce more noise, particularly in the mid range, although it would be down to individual drivers to decide whether this was an advantage or not. So, its illegal for road use and gave no power or torque advantage on the 1.8i or VVC – so not much to recommend here.

The Aftermarket Catalyst

it would appear that the higher volume of gas through the catalyst is either too much for the honeycomb to cope with, is too turbulent to provide adequate operation, or maybe even both

This performed well on the 1.8i car, passing the emissions tests and releasing 1bhp. Okay, so this is hardly remarkable, but at least it wasn’t a power loss. However it failed to make the mark on the VVC by failing to pass the emissions test and robbing the engine of power. Examination of the unit produces a possible explanation for this power loss: the unit has very sharp tapers to the inlet and outlet, which would produce extra gas flow turbulence within the catalyst and hence resistance to flow. This, coupled with the honeycomb which is much smaller than either of the Rover units, may help explain the emissions failure; on the VVC it would appear that the higher volume of gas flowing through the catalyst is either too much for the honeycomb to cope with, is too turbulent to provide adequate operation, or even maybe both. The fact that the honeycomb is small raises worrying questions regarding the operating life of the unit, which would also may make this a doubtful purchase for the 1.8i owner. It does have a price saving of 55 plus VAT, although it should be remembered that currently the unit has not Type Approval for later cars and comes without fixing flanges.

The Motorsport Catalyst

this unit worked very well, although the power increases were hardly impressive

As would be expected from a unit that has been developed to cope with the demands of Motorsport applications this unit worked very well, although the power increases were hardly impressive. The fact that the heat shield needs to be removed to enable it to fit on a road car is a concern although its absence did not effect the operation of the unit. However the heat shield would prevent heat seeping into areas of the car local to the Cat, so its absence may cause long term problems. Added to that, the extra cost of in excess of 100 over the standard unit hardly seems worthwhile.

The Standard Catalyst

The overall impression at the end of the day was how surprisingly good the standard catalyst actually proved to be

The overall impression at the end of the day was how surprisingly good the standard catalyst actually proved to be. This unit worked very well on the VVC car, along with the Motorsport unit being the only one to pass the emissions test. While the motorsport unit yielded a negligible 1bhp at peak revs over the Standard Cat, the Standard Cat does not have the concerns over the heat shield – it is cheaper. An obvious choice for VVC owners. On the 1.8i car the power is slightly down over the other options, but it doesn’t have any of the legislation or installation problems associated with the others, so has to be the recommendation for 1.8i owners as well.

Thanks to:
Paul Sharpe
for organising and orchestrating the tests that you see written here. Aldon Automotive in Brierley Hill for the use of the rolling road and ramp. Fuel Parts in Bromsgrove for providing the aftermarket Cat. Tech-speed Motorsport in Leamington Spa for proving the Motorsport Cat and welding the mounting flanges onto the aftermarket Cat. Andy Gilhooley for the use of his VVC MGF and Andy Phillips for the use of his 1.8I MGF. Tom Randell for the supply and operation of the noise meter. Roger Parker who got his hands dirty doing the work to change the units over and Rob Bell, who along with Roger provided the technical insight.