Passing the MoT

Last issue I mentioned some testing I was doing about passing the MoT test. To pass a basic MoT test any modern engine will have to be fitted with a working catalytic converter. The mixture will be set to 14.7 to 1 AFR and either stay there for the duration of the test or "swing" across 14.7 to 1, under a closed loop setting. That's all you can do but it helps to have the throttle open a little on idle by setting the timing to zero degrees BTDC on idle. You can also employ a few tricks to help stabilise the rpm at the test point. For example: at 2500rpm (which is the fast idle test area) you can advance the timing at that point and retard it significantly either side of 2500rpm. The timing tends to "suck" the engine into staying at 2500rpm as the power drops away either side of that point. This is a very good reason for having map-switching, as the engine will not want to drive very nicely set up like that.

On the fuel map you will find the closed loop or adaptive map. You can set it to closed loop around the MoT points only for an MoT map. You can have every load site on a different strategy if you can think of a reason to!

Not all engines will pass an MoT test no matter what you do with them. The worst possible scenario is an engine fitted with full race cams, big throttle bodies where the butterfly is positioned for maximum power, and big injectors with a poor spray pattern.

Tail pipe problems

Ideally the injectors should be selected to give maximum fuelling at about 80 to 90 per cent of the available time and have a good "fog" type spray pattern. The throttle butterfly wants to be as close to the inlet port as you can get it, to kill any pulse reversion on idle. If you have an engine with big cams and a plenum type intake you have no hope at all of passing the MoT or getting a smooth idle and light throttle drive. The injector position needs to be as close to the inlet valve, or valves, as you can get it with a good fog type nozzle. The nozzle spray pattern is more important than the siting of the injector if you want to keep the hydrocarbon emissions low.

I did some quick tests with the gas analyser using our mule 2.3 Duratec engine. I had two sets of injectors, one set with a fog type spray and another with a twin pencil spray. The injectors were tested in two positions; up in the head where Ford put them and out in the Jenvey throttle body. I did power runs with both injectors in both positions and the power was pretty much exactly the same regardless of injector type and position. You have to bear in mind that this is a standard engine, no porting or cams of any description.

When it came to the gas tests it was a very different story. The twin pencil spray injector was really poor on hydrocarbons when fitted to the Jenveys and not a lot better when put into the head. The fog type injectors were nothing less than brilliant in the head and not a lot worse when fitted into the throttle bodies. The HC readings went from the hundreds with the twin pencil injectors down to just 6ppm with the fog type injectors. All the modern injectors that we see have this multi-hole approach to break up the fuel. Everything these days is driven by emissions and economy but it is nice to know that this was not costing us any power on the test engine.

If all goes according to plan (does it ever?) next month I will be playing with a VVC K-series engine and trying out a few different exhaust designs.

BMW Mini

First BMW Mini

We did our first BMW Mini here at Emerald, basically a competition car fitted with throttle bodies. The reason we have not done these before is the fact that they run a drive-by-wire throttle and also there simply hasn’t been the demand. The throttle isn’t such a problem for our K6 because we now have the drivers in the ECU but I am not sure about the demand. We have a Mini specialist opposite us here in Norfolk and he reckons that the gearbox is a weak point so maybe that limits people’s enthusiasm for tuning?

Frozen 7

Frozen 7
Frozen 7 2

It is not uncommon for people to make a rolling road booking and not turn up. What is unusual is for a Caterham Seven owner to not only turn up but to drive here across country in a snow blizzard. What’s more the car was only running on a base map so it wasn’t the easiest of things to drive. We have done zillions of K engines here but this one was interesting in that the trumpets had been extended with home-made spacers. They did what they always do; make better low speed torque.

After mapping the car was driven back out into the snow and off to Northern Ireland.


Frozen F5000 F5000 on rollers F5000 stand off

Now this one really was a challenge as it also turned up in the middle of a snow storm albeit not being driven on the road! The car is a historic F5000 fitted with a V8, sporting mechanical injection. I only normally use the rolling road for mapping our ECUs but this one was just too interesting to turn down. The team had some shims and cams to alter the fuelling and basically all I had to do was drive the car and report on the fuelling.

F5000 tyre clearance

Problem number one was getting the car to fit the rollers. The back end is so wide we had to take the end plates off the chassis section of the dyno to make clearance. Problem number two was getting me into the car. The regular driver uses a foam seat insert and there was no way to squeeze my aging and expanding body into the seat. I ended up sitting on the bare floor with some padding material.

Traction was problem number three because every time we tried to do a power run the power came in so suddenly that it broke free. Eventually I worked out that we could maintain traction if I set the rpm to steady state and applied the load gently down to full throttle. It was like a step test but done manually. Check out the picture of the intake trumpets, you may be able to see the fuel stand-off in the picture. There’s nothing you can do about this, it’s just a fact of life with carbs or mechanical injection. It tends to suggest that the trumpets are the wrong length, the wrong shape or a combination of both.

We did get the full-throttle fuelling somewhere near right but it is so frustrating when you know that with an ECU you could have it perfect and so much more driveable. But then it would not be historic so poor carburetion is all part of the game when you are re-living the past. Somebody pass the rose tinted spectacles!


I started this month talking about MoT testing because I get so many calls on how to map engines I thought it might help if I put some of this stuff down in writing. Then I started to think that maybe some sort of training might be in order. I have had people here before but when you are mapping an engine it isn’t always possible to keep stopping and explaining why you are doing something or how you are going about it.

For that reason I am putting together some briefing notes to include in a classroom training day seminar on how to map engines. This isn’t just a software lesson, I wanted to pass on some of the practical experience I have gained in the last 20 years (I have now mapped over 3000 engines), in a more formal environment. No firm idea on cost but I am guessing something like £140 for the day, it depends on how much a buffet lunch is going to cost and the hire of the room. I wanted to keep it to no more than 12 people, assuming there might be 12 people out there that are interested in what I have to say on the subject. If I get enough interest then I will organise it. I already have nearly 6000 words of briefing notes to accompany the seminar and am sorting out example pictures and demo software. If any readers are up for it they can register their interest with Emerald on 01953 889110.