Take a look at the pics above before you read on. This Jaguar XJ started out as a full-on race conversion but got sold on when the builder ran out of money. Now it is a really amazing track day/road car. It was driven to us for mapping by keeping the revs and the load down to a minimum to avoid any possibility of damage to the supercharged engine. In a way it is a pity that the owner was so careful; part way through the mapping I seemed to be getting wheel spin. Power at the wheels was only 90 bhp and the boost was minimal. Straight away that rings some warning bells. If the Speedo is working and driven from the rear wheels then the road speed on the clock should go up as the wheels spin. In this case it didn’t.
The problem was that the clutch was slipping. I checked the fluid which was a tad over-filled but pulling some fluid out made no difference. Sadly on this occasion the car went home just as slowly as it arrived. Having said that, at least it will live to fight another day.
People often ask me how many engines blow up on the rollers. To be honest we have had very few failures in the last 16 years that I have been mapping engines. The Elise I was doing was fitted with a 2.0 Duratec conversion. Basically the Ford engine is mated to the original Rover/Lotus gearbox with an adapter plate. I have seen 275 bhp from a good engine in these cars and they are super fast on the track.
This example had been trailered from Germany for mapping. That’s some 1200 Kilometres door-to-door. Everything was going swimmingly and we had good power up to 7300rpm where I had set the rev limiter. The owner reckoned 7500rpm would be no problem and since the power was still going up I increased the rev limit to 7500rpm. You are probably ahead of me...the first run to 7500rpm resulted in a crunching noise and a sick engine.
When I tried to turn the motor over it jammed up around TDC. Out came number one plug and it was a mess, really beaten up with mechanical damage. The other three were fine. I had a fish around down the plug hole with a magnet-on-a-stick and came up with two curious bits of debris. Obviously magnetic, I had two fragments about 5mm deep and slightly circular. If I put them end to end they looked to form a circle about the size of an inlet valve.
Then the penny dropped, it was the inlet valve seat insert I was looking at. When the head was lifted (back in Germany) this was confirmed. The head seems to have been ported and the said insert left so thin that at high rpm it simply fell out of the head. As you can imagine, it was a mess. The fact that the other three cylinders were spot-on tells you that it wasn’t a case of the valves hitting anything first, otherwise all of the cylinders would have suffered.
I had a Capri V8 to do and it was a nice one – full-on throttle body conversion. I was mapping only for a couple of minutes and I ran into a problem. If I had the mixture right you only had to crack the throttle open slightly to get a weak-mixture misfire back up the throttle body. Usually this is a question of the throttle pot lagging behind the butterfly (free play in the drive) but this wasn’t the case here. I have to admit I was scratching my head for a bit. Then I spotted the throttle linkage. The central tower was offset and that meant that the linkage had a different ratio on each bank. The bank with the throttle pot opened slower than the second bank. Hence the second bank went weak as the throttle opened unless you made the first bank with the pot attached too rich.
We managed to tweak the linkage to get it much closer to even opening and that pretty much solved the problem. I see dodgy linkages all the time, especially on bike body conversion and it can give some nightmare mapping problems. If you are not going to fork out for a decent linkage kit please take some trouble over making your own – it’s more important than you might at first think.
Another head scratcher.
On low boost the engine ran fine but when you flipped the switch to the maximum attack boost map the engine surged forward, then slowed down and surged forward again. It went up the road in a series of leaps – not the ideal situation. The guys tried in vain to find the fault and with a car like this you can’t really test it in the middle of town. Jim asked me to take a look at it on the rollers.
Looking at the log of the boost it was obvious what was happening. The boost was surging so the first port of call was to try to eliminate anything mechanical. It was fine on low boost but as you adjusted it upwards it started to take on a mind of its own. I put a mechanical boost bleed valve in place of the electronic N75 valve and tried that. It worked fine. We tried a new 12volt feed to the valve, we tried a new valve, we ran wires directly back to the ECU, but nothing would do the trick. I spent all day messing about trying everything I could think of and all to no avail. I said to Jim that when we did find it, it was going to be something simple, it always is.
For want of something better to do I pulled the spark plugs out. Hello, hello, what this then? No resistor plugs, just plain grade 8 competition plugs. You MUST always run resistor plugs with any engine management. But, they argued, they were the same plugs as before. How about the HT leads? The leads looked the part, really thick and colour coded to the hoses, they looked as fast as a fast thing. But when I did a resistance check they only showed 0.7 K Ohms. Proper resistor leads should be about 12.0 K Ohms at that length of lead. That was the answer; the problem only appeared after the new leads had been fitted.
We put in a set of grade 7 resistor plugs and the problem disappeared! Some days are like that, all day messing about for the want of six spark plugs. The leads were basically crap for all their thick insulation and bright blue colouring. I knew it would be a simple fix when we found it; it always is, but that one was a bit of a choker.
Adjustable Length Intakes (ALI)
I will not bore you with it here because Will is publishing a bigger story elsewhere in the magazine; but I have done even more testing on intakes using 42mm ALI sets for the K Series engine. Results are pretty impressive but you have to look at the overall picture.
The power graph shows the same length ALI set up but one with foam filters and the other with a closed air box. The numbers speak for themselves!
However, on the Datsun 260Z featured recently we had the opposite result, with the air box we gained power. It’s very much a case of suck-it-and see. Llike people, every engine is different.