THE MINI MAG. Volume 2 No.7
  July 2000

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Unleaded Motors.
With Paul Gulson

As the lead content in fuel decreases, so does the lubrication of your Mini engine. This burns out valves, wears out guides, glazes your cylinders and runs your motor hot. Usually the timing has to be retarded to accommodate the poor quality of fuel.

Early in the 1990’s I built a lot of 1275-1310 type engines, 1100’s, Metro A+ and found that compression was the limiting factor in producing power. All Mini motors respond to compression increases. What I had previously discovered was most larger engines had a very short life-span - caused mainly by overweight rods, pistons and crankshaft, which caused the engine car to harmonic. [I learned this from previously building and racing that type of engine and I knew the problems associated with them and the remedies for those problems.] Reciprocating weight and stroke are the two most important things in any engine. Take off the weight and you will take off the load and increase horsepower without high compression.

So I started working on the 1100 motor because of the large stroke-factor and found a suitable set of motorcycle pistons that were half the weight of the standard pistons. We all know that Mini specialists around the world hate the 1100 engine - mainly because they didn’t understand the reason behind piston failures. The main reason why the pistons failed was because of poor quality pistons and too much piston-to -bore clearance and too course a grit hone used on the bore. Because of the large stroke of the 1100 engine the pistons rocked too much at the top of the stroke, causing the top ring to wear out and gouge a huge lip in the bore. The overcome this problem was simple - lower the crown height of the pistons and put it closer to the gudgion pin, which takes out most of the rocking problem and by holding the piston tighter in the bore by using only 1 thou. , you also limit the rocking factor of the pistons. Also using a very fine grit stone on the bore, engines I have inspected after 4 years of running had virtually no wear to be found. No harmonics, no oil-leaks, no overheating problems – DOESN’T SOUND LIKE A MINI ENGINE, DOES IT ??

The reason the engine runs so cool is because of the shape of the piston doesn’t touch the bore, therefore, not transferring latent heat into the cylinder wall under compression. Also, chrome rings running on a cast iron bore is very friendly and good for unleaded fuel.

Stainless steel valves and hardened inserts in exhausts prevent valves burning out. Also, extractors and a suitable exhaust system, small cam and a decent timing chain (that doesn’t rattle it’s head off), a 1 SU (modified), ported manifold, standard air cleaner and a balanced engine with lightened flywheel and clutch, high volume oil pump and water pump, the bypass hose blocked-off and the oil pick-up tube magnetised. Naturally, the gearbox and differential have to be overhauled when the engine is overhauled, or the gearbox destroys the engine. I also found, because of the big stroke benefits, the engine would quite easily pull a 3.4 diff. ratio, which makes for a fuel-efficient engine.

 36 degrees total advance
 6 degrees static timing
 175 pound compression
 Runs on unleaded fuel
 “No pinging”
 “No harmonics”

1275 or larger engines all produced harmonics and are also very heavy internally. When you look at the weight differential between the 1100 engine when bored 1 mm. equals 1160 cc’s, the capacity difference is only 115 cc’s, but the 2 kg. lighter 1100 engine far outweighs the capacity loss. Without harmful harmonics you have a more reliable engine.

Also, I noticed the engine would not run up to temperature without a thermostat and even runs cool on a hot day without an oil cooler. I have built hundreds of these engines over the last four years with no failures or comebacks and I exhausted my supply of 1100 engines. So now I have started the development of the 998 engine (my favourite).

This little engine outlasts most body shells because of it’s short stroke, creating no wear. Boring to 1100 cc’s, and using the 1100 cylinder head to increase breathing, coupled with porting the cylinder head and suitable camshaft was a surprise package for me and also pulled a 3.4 diff. ratio quite easily, but not quite as powerful an engine as the 1160-1100, but extremely more fuel-efficient, cheaper to maintain, and much more reliable.

The only thing that will destroy a Mini engine is overheating, lack of oil, abusive treatment and stupidity. Apart from that - for an engine which was designed in the 1950’s - they are still an incredible small engine - and I am still learning and trying to improve them.

I still build 1275-1600 cc racing engines or street motors, but the limiting factor is the fuel. Modern engines specifically designed for unleaded fuel, coupled with good fuel management systems, makes it possible to run big horsepower motors. You have anti-knock sensors , auto-retard systems and now variable valve timing and a computer to run everything.

Paul Gulson.