Project Small Bore Pt3

In the last article we chose a manifold based on flow volumes, and a carbie based on personal choice, but which combination is best on this engine?

With the engine finished, it is time to look at a variety of carburettors, inlet manifolds and exhaust extractors, to find the most suitable combination. There are a lot of figures, but they are necessary, so bare with me. But, before we do, we need to make sure the valve timing is set up properly. As I said in the last issue, even a very small difference can have a significant effect on the torque across the rev range. I also mentioned the standard Cooper S rockers have a tendency to be quite a bit inconsistent. This was perfectly illustrated when I came to put the rockers on this engine – boy what a mess! After changing the rockers around, I was able to get the four inlets somewhere near each other.

Table 1


Tappet                                   BTDC              ABDC                Duration               Lift

Inlet No 1 @ 0.016”              20                     60                        262                   0.332”
Inlet No 2 @ 0.016”              22                     56                        258                   0.328”

Inlet No 2 @ 0.014”              23                     59                        262                   0.330”
Inlet No 3 @ 0.016”              23                     60                        263                   0.332”

Inlet No 4 @ 0.016”              22                     61                         263                   0.332”
Exst No 1 @ 0.016”               55                     34                         264                   0.330”

Exst No 2 @ 0.016”               59                     40                        279                   0.345”
Exst No 2 @ 0.025”               52                     32                        264                   0.333”

Exst No 3 @ 0.016”                58                    38                        276                    0.340”
Exst No 3 @ 0.023”                53                    31                        264                    0.332”

Exst No 4 @ 0.016”                56                    37                        273                     0.336”
Exst No 4 @ 0.021”                51                    32                        263                     0.329”

As you will see from Table 1, all the inlet valve timings are pretty close to each other, at a setting of 0.016”. Only the inlet on No 2 cylinder needed adjusting, to 0.014”. But, the exhaust timings were still all over the place. This shows why some standard Cooper S were very quick, and some were only ‘not bad’. I decided to aim for a duration of around 263 at 0.016”, with 0.332” lift. To achieve this I had to adjust the rockers for the exhaust valves to some fairly radical settings. When they are this far out, you would be advised to look around for some better rockers. But, I decided to carry on with these ones to prove the point, that when they are adjusted properly they start to come back to where they should be. This motor has one of my RE13 cams in it, which should give a duration of 276 @ 0.016”, off the lobe, and a lift of 0.290” on the lobe. Because of the poor rocker ratios we were a little bit short on both duration and lift, but we will have to make do with what we’ve got. I have found that with setting up the cam on the small bore engines, they like to run slightly retarded – not split overlap or advanced. So, retard the cam 3 to 4 and you will get a bit more torque and horsepower. For this sort of testing, you’ve got to make sure all other variables are standardised.

I ran all these tests with BP 96 RON unleaded petrol, and used my favourite oil – K-Mart’s own brand, KMX 20W50. I use this oil in all my motors, for running-in and tuning, whether it is a high-revving Ford BDA, or a 500hp V8, and I have never had any problem with this oil.

The distributor is a Lucas 29D, with the advance stop welded up to give 7 to 8 at 3000-3200 rpm. The total advance for this engine is 26 to 27 , which means it had a lot of static timing. I find this a lot better because it allows the motor to idle off the advance, which means you can close off the carburettor butterfly further. This in turn helps the motor to shut down and not run-on, when the ignition is switched off. There is one drawback, though – you do need a good starter motor, as the old Lucas standard one doesn’t have enough horse•power for the job.

OK. Moving onto the dyno. First, we tested some of the more popular types of extractors and exhaust pipes. With the motor run-in, I had to choose an induction setup that I knew would make good torque and horsepower. I selected my 5.25” Weber manifold, and a 45mm Weber with 34mm chokes. (I know we’re mixing metrics and imperial here, but that’s just the way they are.)

Project Small Bore 3
45mm Weber on RE manifold

After sorting out the jetting, with the water temperature set at 80 C, oil pressure 80psi hot, air temp 22 C, humidity 50%, and barometric pressure at 29.85, the correction factor was 1.003. After a bit of testing of different exhaust pipe and muffler setups, we settled on a pipe with 1 ” Outside Diameter and 1 ” Inside Diameter, a 16”-long muffler with 2” ID, and a full-length back pipe – to simulate a full system on a Mini. The five sets of extractors we tested were, in order, Big-bore LCB (long-centre-branch), Maniflow Free Flow, Medium-bore LCB, Big-bore 3 into 1, and Small-bore Supa Pipes 3 into 1. See Table 2 for all the figures.

Table 2

               Big LCB             Free Flow           Medium LCB      Big 3 into 1           Supa Pipes

RPM            lb/ft hp            lb/ft hp              lb/ft hp               lb/ft hp                lb/ft hp

3000            79 45               70 40                  79 45                   78 44.5                82 47

3500             80 53              71 47                   81 54                   80 53                   82 54.6

4000             80 61              75 57                   80 61                  81 62                    84 64

4500              82 70             80 68.5               80 68.5              81 69.4                 83 71

5000              82 78              79 75                   80 76                 80 76                    84 80

5500              82 86              79 83                   81 85                 80 84                    85 89

6000              81 92.5            75 86                  76 87                 75 86                     81 92.6

6500              70 86.6            68 84                 70 86.6             68 84                    73 90.5

Project Small Bore 3_3

 Project Small Bore 3-4
Maniflow Free Flow

Project Small Bore 3-5
Medium LCB

Project Small Bore 3-6
Big 3 into 1

Project Small Bore 3-7
Supa Pipes


Being tested first, and a popular choice for Minis, the big-bore LCB set a standard to measure against. The Maniflow extractors were very interesting, dropping a lot of torque down low, but starting to pull back good figures at the top end. The medium-bore LCB, took a big dip in power at 6,000rpm, compared to all the other manifolds, which surprised me a bit. No matter how many times we tried it at that rpm, the result was always the same. I originally wasn’t going to test the big-bore 3 into 1, because I thought they were far too big for this engine. To my surprise, they made very good torque down low. The small-bore Supa Pipes are Australian-made, have a very small diameter pipe, with a special collector, are all Mandrel-bent, and produced the best results. The figures were all quite good, but in one way I am a little disappointed with the results, as they did not really show up the advantage of the smaller pipes on the small-bore engine, which I expected. I should have built the engine with only around 60-70hp, as the medium-bore LCB do perform better than the big-bore pipes, from my experience, on the smaller horsepower engines, but I have found the Super Pipes superior on small engines. As the Supa Pipes gave the best results, we will use those to standardise the results for the induction tests (Table 3).

TABLE 3 – All tests done with the Supa Pipes extractors. Note: hp figures not corrected 

                   40mm Weber           40mm Weber              45mm Weber          45mm Weber
34mm chokes            34mm chokes             34mm chokes         34mm chokes
Redline manifold          Swan Neck             Redline manifold    RE 5 ” manifold
RPM            lb/ft hp                       lb/ft hp                         lb/ft hp                     lb/ft hp

3000             77 44                           78 44.5                         77 44                          82 47
3500              80 53                          80 53                           80 53                          82 54.6
4000              79 60                          80 61                           79 60                          84 64
4500              80 68.5                       81 69                           80 68.5                      83 71
5000              80 76                          80 76                           80 76                          84 80
5500               77 80.6                      76 79.6                        80 84                          87 89
6000               72 82                         70 80                           75 86                          81 92.6
6500               65 80.5                      65 80.5                        68 84                         73 90.5

Project Small Bore 3-9
45mm Weber on Swan Neck manifold

Project Small Bore 3-10
Twin 1.25” SUs on std manifold

Project Small Bore 3-11
Twin 1.5” SUs on std manifold

Project Small Bore 3-12
1.75 ” SU on RE 7” manifold

 RPM             Twin 1.25 ”              SU Twin 1.5”             SU Twin 1.5 ”        SU Single 1.75 ” SU
Std manifold             Std manifold               6” manifold              RE 7” manifold
lb/ft hp                       lb/ft hp                        lb/ft hp                         lb/ft hp

3000               81 46                           81 46                             81 46                           83 47
3500                80 53                          81 54                             82 54.6                       85 56.6
4000                76 58                          76 58                             84 64                          86 65.5
4500                 78 67                          79 68                            83 71                           87 74.5
5000                 77 73                          79 75                             86 82                          90 86
5500                  75 78.5                      76 79.6                         85 89                          87 91
6000                 69 79                          78 89                            77 88                          83 95
6500                  65 80.5                      67 83                            72 89                          76 94

Project Small Bore 3-2
Some of the carb/inlet manifold setups that were tested (not to scale)

Starting with the Weber carbs, I tried four different combinations: 40mm with 34mm chokes on the Redline 4.5” manifold, and on the Lynx Swan Neck (these aren’t made any•more, but there are still quite a few around, and there are similar manifolds from other overseas brands available). Then I tried the 45mm with 34mm chokes on the Redline 4 ”, and on my own RE 5.25”. Although the RE gave good results, I think this manifold is too big for the small-bore engines. It has too much volume for a road engine, in so far as part-throttle driveability goes. The Swan Neck is a manifold that a lot of people like to run down, but for the small bore road engines, making up to 90hp, it works very well. It has excellent throttle response, because of the small volume and high air speed, and is a very good manifold for road car use, with the 40mm Weber and 34mm chokes. However, it is not really suited to the 45mm Weber. The Redline manifold, with 45mm Weber, was very good. With a bit of grinding down on the manifold face, this manifold would have matched my RE manifold on this engine. A fully-ported one of these will stop at around 125hp, and has excellent driveability on the road. The 40mm on the Redline gives excellent driveability on a road car. You will notice the photo of the ram tube and the machined venturi, which both gave the same results. However, with the ram tube I had to go one set richer to match the machined venturi.

Now, looking at the SU carburettors.

Project Small Bore 3-14
My gyzmo for checking the SU slide. Note the large radius machined on the mouth of the carb. I find this as good as, or better than, ram tubes.

Before we start, you will see the little gizmo I made up, to measure how far the slide is lifting, when you can’t see the slide itself. All you need is a bar or tube of steel alloy, or wooden dowel. I’ve marked rings at 1/8” apart. Strap a pointer to the bell of the carb, and line it up with the top ring on the bar. As the slide rises, you can simply count off the number of rings to see what part of the needle your engine is running on. This is excellent for part-throttle openings, and I use this setup on engine dynos and drive-on dynos. It’s bit hard to use when driving on the road though! To set the standard this time, I used the standard Cooper S twin 1.25” SU setup. These proved to be a lot better than I expected, considering how short the standard manifold is. What was most interesting, though, was that we couldn’t improve on the standard air-box arrangement – BMC knew what they were doing here, for sure. With the twin 1.25” SUs on the standard manifold, we had to do a bit of fiddling around, trying various needles, to get the mixture right. We ended up with No 7 needles for the best results on this engine. These carbs did not have ram tubes or a standard airbox, but a radius had been machined on the front of the carbs. We then tried the twin 1.5” SUs on a 6” manifold, which was a setup we used on the Mini 1000 racing cars about ten years ago. These worked really well once again, and I thought they would come out on top. They really don’t fit on a road car, without a lot of body modifications, but I thought I’d throw them into the mix, to illustrate what I was saying in the last article about longer manifolds being better.

The next setup was with a single 1.75” SU on my long (7”) manifold, which has a very large volume at the carb face and leads the carburettor into a long runner of the right shape and size. We had a lot of trouble getting the right needle for this setup, as the slide was fully up by 3,500rpm, and stayed there until we shut-off at 6,500rpm. A single 1.5” SU was a waste of time on this engine, as it was just too small and could not supply enough air for this setup. But, it will work fine on smaller horsepower engines. Two other manifolds I didn’t try were the standard cast manifold, cut off the exhaust (it simply didn’t fit over the exhaust pipes), and the Mini Spares 1.75” competition manifold. I’ve tested the Mini Spares manifold in the past and found that it works very well, is easy to fit, makes good horsepower, has good volume and have no problems fitting air-cleaners. However, I didn’t have one available for this test.

Summing Up I think I made one mistake with this engine, in so far as it made too much horsepower. This meant it didn’t show up the differences between the big-bore and small-bore extractors, as I think would have been much more obvious on a motor that gave 60-70hp. When choosing extractors, you can’t go past the small-bore pipes, medium-bore LCB or 3 into 1 small-bore. On the dyno, running the motor at and throttle, by putting a stop on it, the small pipes do make more power and have better throttle response, which makes for better driveability. With inlet manifolds; if you want to run a Weber carb, my choice would be the Redline 4.5” or the Swan Neck. These manifolds on the small motors have very good throttle response, are easy to fit an air-cleaner when in the car, make good power and are good value for money.

Project Small Bore 3-13
Swan Neck manifold – a good compromise

If, like me, you prefer SU carburettors, then I reckon there is nothing quite like lifting the bonnet and seeing a well set up pair of SUs. If you are going to build a motor around 60 to 70hp, I would stick with the 1.25” twin SUs. I would rather see a better manifold, a little bit longer. If you are going to run a single SU manifold, you have a few choices. Firstly, you can cut the original cast one off the exhaust and use this. If it is a late model one, then you can use any SU carb, from 1.25 ” to 1.75 ”, with good results. If you use a Mini Spares, Maniflow or similar, then stick with the big-bore one. There should be no problem fitting any of them, even with an air-cleaner.To fit the Russell Engineering manifold, you will probably need to do some body modifications, depending on which carburettor you use – HS6 or HIF – and you will only be able to use an offset-type air-cleaner.

I would like to make special thanks to the following people, for their help with this testing. Greg Brown from Karcraft Australia, for supplying the extractors for testing. Greig Malaure from Mini Classic, for all his work in changing very hot exhaust systems, while I had to spend time answering the phone and making the cups of tea! And a big thanks to all the people who loaned me carburettors and inlet manifolds.