I think what I’m about to say might cause disagreement amongst some of the Mini fraternity, but this is my ‘forum’ so there you go!
In a previous post referring to my engines performance, I wrote:
“There is a great big flat spot when accelerating of idle. It seems fine if I floor it, but hesitates and has a lot of induction noise, but not much go if I tread lightly on the gas. I’ll check the mixture as I still have the wideband O2 sensor installed, and maybe thicken up the oil in the SU damper, to give a richer mixture when accelerating. I currently have ATF in there, prior to that I had 3-in-1 oil which I believe is 20 SAE, the same as original specced ‘SU oil’. I’ll also re-visit the ignition advance curve, not that I can do much for it at the moment.”
Well I started to go down the air fuel ratio route as well as the type (viscosity) of damping oil in the SU carburettor, but it dawned on me last night that maybe it might be the 1.5:1 ratio rockers that I fitted well over 18 months ago. The other clue is the ‘reversion’ noise that it has in the lower rev range. When I had the ECU on I had a very unusual hump in the Volumetric Efficiency (VE) table.
If you where to research VE tables, then you would note that this one is far from ideal, in fact it’s plain wrong. The following table demonstrates what it should look like (this is not my VE table).
So what would cause the VE table go like this, that is increasing the VE which results in a richer mixture at the hump? My theory is that it is intake reversion i.e. where the intake valve is opening and some of the exhaust gases get pushed out of the intake diluting the intake charge and causing ‘standing waves’. It’s these standing waves that I believe cause the growling sound from the intake. People with a Weber or Dellorto carburettor would know what I mean!
So what caused this sudden increase in noise from the intake? From what I can remember, I put on a K&N air filter along with a stub stack at the same time as some Mini Spares 1.5:1 ratio rockers. I have blamed the smaller air filter all along for the increased noise, which is understandable, but why the loss of torque down low?
So getting back to reality, it slowly dawned upon me (I know I’m a bit old/slow/stupid) that maybe it was the 1.5:1 ratio rockers. Since I never throw out anything, I put the old rockers back on, set the clearances and started her up.
The first thing I noticed was that she started a lot easier, required less choke and once warm idled smoother. I adjusted the mixture (slightly leaner) and then took it for a brief fang around the neighbourhood.
Chalk and cheese, 90% gone was the ‘reversion’ noise’ a heap more torque in the lower to middle RPM band, and would easily rev out to 6000 RPM. The car would easily accelerate away from 20 KPH in third gear with no pinging or hysterics. I later went for a drive down to the beach and surrounds with a friend, who remarked that it was a lot smoother and quieter.
So in summary, it appears my engine, intake, carburettor, camshaft (standard Mini Metro) and cylinder head combination just does not like the extra lift and increased overlap that these rockers give. It also could be a gas velocity problem, due to the valves opening ~ 20% wider and quicker. Who knows? I’m not a mechanical engineer, but my seat pants tell me that something has changed for the better! Regretfully I don’t have the ECU on, so I can’t see what effect it would have made to the VE table…. but you never know I might revisit it again!