Colouring-in Team

Rolling-road tuning session

Had a great time yesterday morning watching someone clever tune my car on a rolling road. I spent quite a lot of time last year building a new engine for my 1973 MGBGT. Obviously not 'new' new, but rebored, reground, ported, balanced, timed and generally built to be better than new in every way I could reasonably manage. With it finally back in the car I was naturally hoping for about 2000bhp from my little, cast-iron, pushrod one-point-eight. Unfortunately what I ended up with felt like somewhere south of 50bhp, and no amount of tweaking seemed to help. As always, my Dad came to the rescue, and offered to treat me to a couple of hours with Alan Jeffery in Plymouth, trying to work out why my new engine wasn't as powerful as the knackered one I'd replaced.

The first thing Alan spotted as soon as the initial power run had been done (48bhp at the wheels!) was that the distributor was behaving very oddly indeed. It turned out that the "extremely good value" electronic distributor I had purchased a while ago from Powerspark as in fact just cheap, and not very well made, having actually disassembled itself internally. With the connecting rod that links the vacuum advance to the timing adjuster tapped back into place with a bit of Loctite, he was able to do some proper power runs and set the timing to be correct at 4000rpm. He then moved on to the mixture, and the needles. One thing I learned was that the mixture adjustment on the carbs is not actually that important in terms of getting everything right when compared to the needles themselves. Having discovered that the AAA profile needles I was using were far too rich, he dug out some replacements in a different profile, and did another run. These too were deemed not quite right, and were 'adjusted' with a fine file to alter the profile to better suit my engine. I suspect this is the kind of thing you can only do with a great many years of experience, and it was impressive that it seemed to do the trick, taking the final figure just over the 100bhp mark.

I was told that standard MGBs tend to make about 87bhp, so although I was probably hoping for more, that's not a bad improvement when you consider that the engine is only a rebore bigger than normal. More impressive was the torque, coming in at 125lbft at 3100rpm. That's more than you get from a modern Ford Focus of the same engine size, which bearing in mind that this is an engine first built in the 1950s, and of a fairly agricultural design at that, isn't too shabby at all. Here is the graph from the final power run.

I've only actually done about 3000 miles on this engine so far, so I think a return visit may be in order once things have properly loosened up a bit! I'm sure the 2000bhp I was hoping for is in there somewhere...

Categories: Cars, Projects

Tags: engineering, mgb