Exhausts - RC40, Millennium and others tested
The recent rolling road re-test of the 1275 engine build I did saw an opportunity to do some exhaust systems testing. I had noticed a few years back that the RC40 was being manufactured by a different process than the originals - giving rise to a change in the shape of the bends applied to the system. I had wondered if they would be causing a restriction at all - but hadn't had the opportunity to test it. The lack of expected power from the aforementioned engine lead me to investigate all possible angles from the tyre contact patch to the carb mouth. I found nothing that explained the 14 or so bhp I was 'missing'. After some application of the old grey cells I mentally narrowed cause down to one of two things - either the piston oil control rings were still causing excessive drag (75% of all internal friction of an engine is generated by the piston rings) or perhaps these new bends in the venerable RC40 were indeed causing a power loss.
Since I needed to gauge the RC40's performance I needed something to compare it to. The engine dyno results for this specific type of engine build were given with no exhaust system attached - so that would give a comparison for how much the RC40 was - if at all - restricting power. This was also an opportunity to test the latest 'Millennium' range of systems, compare side exit against centre exit, medium against large bore and single against twin box. Before visiting the rolling road I spent an afternoon trial fitting all the exhaust systems and their cross-fit combinations to minimise hassles and time on the day. This alone highlighted some fitment issues that would allow those who would listen a chance to better their product package. Each system was also tested for 'on the road' noise by driving around a particular route set to use the engine in different situations.
The first test was done with the whole engine and set-up as was taken off the rollers last time with the exception of having removed the vac unit from the Aldon Yellow dizzy and having covered some 1200 miles instead of the original 400 when the last test was done. Amazingly the missing power was suddenly there! This pretty much confirmed my other suspicion for the missing power - those AE oil control rings. On the original engine build when fitted they were much tighter in comparison to just about anything else I've ever used. Clearly they need a long time to bed them in to relieve the tension. The good thing is that they do settle in, and provide excellent oil control with no power loss.
The mixture was showing a tad lean at the top end, so a needle change was made and re-checked - no more power but a safer mixture reading. The results gained when computed compared practically identically to those recorded on an engine dyno. This meant the RC40 was obviously not causing any power loss at all despite those differently formed bends. I suspect they may cause a slight restriction on engines producing over 115-120bhp though.
Road Test Test Test Test Test Test Test RPM
speed RC40 1 2 3 4 5 6 7
30 47.74 47.60 47.74 46.90 41.05 44.94 43.65 41.71 2,600
40 67.04 66.40 66.70 66.03 60.07 65.00 62.98 61.97 3,500
50 80.16 80.30 81.60 58.44 74.32 77.66 76.25 74.33 4,400
60 90.30 90.00 90.30 89.40 84.30 87.90 87.10 85.01 5,200
70 92.00 91.30 92.03 91.26 88.46 89.06 90.01 86.80 6,200
80 87.00 86.40 86.30 85.03 83.20 83.95 85.03 82.07 7,000
I have omitted torque values for clarity. For those of you who want to compare torque values simply multiply the bhp figure by 5252 then divide by the rpm for the road speed indicated.
Test 1 -the first exhaust test was a direct swap for the Millennium 'RC40' in stainless steel - the twin box set-up. The power figures were near identical to the RC40 - so it performs at least as well as the original RC40. Great news as now you can have RC40 performance with the longevity given by stainless steel. The stainless steel system didn't markedly increase the 'on road' noise either.
Test 2 - the Millennium large-bore tail pipe rear section (again twin box). Again power was practically identical to the previous two tests. Noise was markedly up on the 'standard' stainless steel Millennium RC40 - but not too bad except when cruising at around 60-70mph.
Test 3 - the centre-exit rear box was fitted, again run as a twin box set-up - and showed a slight power loss by 1-2bhp, but quite a surprising increase in noise level. It would be very aggravating on a long run.
Test 4 - I then fitted the single box side exit Millennium RC40 rear box plus down-pipe with no centre box. Power was down by some 6-7bhp at the bottom end (up to 3,700rpm) and 3-4bhp at the top end (from 3,700rpm upwards). The noise was great from outside the car - horrendous inside the car. I didn't bother with the centre-exit single box test - the foregoing made the results pretty predictable.
Test 5 - for comparison one of the RC40 'copies' was tested. Although performance was quite good, it was still 2-3bhp down on the original. Noise levels were OK too.
Test 6 - for the 'boy racers' I wanted to test one of the 'budget' 2" systems that abound on the market. The twin box version caused an almost blanket power loss of some 3-4bhp and the single box (Test 7) some 5-6bhp. The noise was very intrusive in all but very light throttle conditions.
All tests were carried out in third gear at 30, 40, 50, 60, 70 and 80 mph to more realistically represent on the road use. Mixture and ignition timing were also checked for maximum performance. The 2" bore systems caused weirdness in the mixture readings - being inconsistent from one run to the next and across the tested power band/road speeds. Idle was lumpier, idle mixture went up and was erratic and hydrocarbons were up some 300 points. Increasing idle speed to 1,200rpm helped this - but is way too high for a road-going car.
The Millennium systems fitment was not quite as good as the original RC40. The rear box sections were fine; the problems were with the two centre sections. They both pushed the rear sections over to the nearside because the rear exit point is slightly off the centerline of the exhaust tunnel (see drawing for clarification). Apart from having to juggle the position of the system components to get it to fit without hitting anything, it also causes the hangers/rubber mountings to be slightly under stress. Both sections are also a little short - they need to be a little longer at both ends to get a better engagement with the mating piece to ensure no gas leaks; care when assembling the current systems will overcome this small niggle. These fitment issues are being addressed by Mini Spares with the manufacturers to ensure even better fitment in future.
The Millennium fitment problems were miniscule when compared to trying to get the 2" systems fitted in a reasonable fashion. Largely because they simply do not have any corrective bends in the down-pipe from manifold collector to rear box. This means the mountings/hangers are placed under considerable stress - practically guaranteeing accelerated mounting failure. It is also very likely to cause premature breakage of the manifold - either at the primary collector or possibly causing leakage at the manifold to head joint/gasket - and excessive on the road noise by transferring harmonic noise from the system through the stressed rubber mounts into the subframe/bodyshell.
The original RC40 fitted perfectly - although I didn't try fitting the RC40 centre section with the Millennium rear sections, as the problem was very clearly the Millennium centre sections. A Maniflow LCB was used, the collector position being absolutely spot- on - i.e. pointing straight back down the tunnel parallel to the gear change linkage.
In future I will be using the Millennium stainless steel systems now I have carried out a decent back-to-back test. The mild steel systems are identical in design to the stainless steel ones so the power results should be all but the same.