An important lesson in the way in which the different parts of the car are interconnected.

Posted on October 5th, 2003 in Opinion by Julian Edgar

As described in stories that we are running in AutoSpeed, over the last few months I have modified my 1988 Maxima V6 Turbo to have increased boost (and less wastegate creep), an intercooler, cold air intake and cat-back exhaust. Nothing wildly exciting or unique about that lot (although the prices for which the mods were achieved was pretty groundbreaking!), but even so I have noticed a few outcomes I never realised would take place. And that’s despite having in the past owned many modified turbo cars…

The Maxima is instrumented with a fast-response LCD temp probe placed in the intake system just before the throttle body. It’s a device that I have also talked about before, but it’s worthwhile stressing that seeing what is really happening beats all the theories in the world, hands-down. (When you’re reading web discussion groups, watch for the posters who write ‘I measured this’ or ‘The stopwatch showed that’ or ‘Here are the dyno curves’. The number of people who just theorise – often incorrectly – is bloody incredible. A few measurements and you know what is happening!)

The first thing that I noticed is that with the anti-wastegate creep boost control in place, the intake air temp gets higher, sooner. That is, the relatively small intercooler is pushed harder because the temp of the air feeding it is hotter, earlier.

As a turbo compresses air, it inevitably heats it. How hot it gets depends on the efficiency of the compression process – but even if the compressor were 100 per cent efficient, the air would still get much hotter than ambient. In the Maxima’s case – as with any other car running an anti-wastegate creep boost control – boost occurs earlier in the rev range. That’s exactly what you want – more boost sooner. But the corollary of that is that the intercooler load rises, even in normal point-and-squirt driving.

Click for larger image

In the hilly area in which I live, even urban 60 km/h driving can see the intake air temp rising. I’m not talking anything horrific here – the intercooler is still performing its function just fine – but close watching of the LCD display shows that it’s getting subjected to turbo heat flows far more often than it did before the anti-wastegate creep control was implemented.

Bottom line: if you install a device that brings on boost earlier (even if its peak isn’t any greater) the heat load being passed onto the intercooler will be higher. That has significant implications if you have a small intercooler – or none at all. In the past the temptation has been to say: “Well, I’m still only running 7 pounds,” without realising that if more of the time you are running even just 3 psi of boost, the thermal load will be greater.

The other characteristic that I have noticed is even more obscure – but equally significant.

By adjusting the throttle cable that runs to the auto trans I’ve been able to alter the transmission characteristics so that the car is much more inclined to hold a taller gear and torque its way forwards, rather than drop down a gear (or two). The result is that with the new-found mid-range torque (which for the same throttle position, feels something like 30 per cent greater), the Maxima drives like a much bigger engined car – say, like a good 3-litre V6 in the 2000 – 4000 rpm rev range.

But the way in which the 2-litre engine is developing that torque is by higher cylinder pressures, fuelled by a healthy dose of mid-range turbo boost.

Let’s say I am approaching a steepish urban hill at 75 km/h in fourth gear (overdrive). With the trans unmodified, the car would have required increased throttle as the hill was reached. The trans – which is pretty intelligent for 1988 – would have dropped out of lock-up torque converter mode and changed back to fourth. That wouldn’t have given enough urge and so a down-change to third would have followed. If decisive foot action wasn’t taken early enough and the speed kept bleeding away, a further downchange to second gear may even have been needed. But in second gear the torque multiplication would have been such that the engine would have almost freewheeled up the hill, no turbo boost at all being used.

Now contrast that with the modified car. The same hill at the same speed, and this time the trans doesn’t even flinch. Even with the extra throttle it holds fourth gear (although probably dropping out of lock-up) and just lets the turbo boost do the rest. The mid-range torque pulls the car up the hill easily, with the lack of wastegate creep giving the full 9 psi boost very early on the ascent.

And that’s the nub of it: again, the thermal load that the intercooler has to shed has been majorly increased. Because of the adjustment to the trans to take into account the far greater mid-range torque production, the trans in turn requires that the mid-range torque do the work. The result is that even on a relatively small hill, full turbo boost is being used. And it all can happen at slow forward speeds, so reducing the amount of outside air being forced through the intercooler.

Again, the heat outcome can be seen on the fast-response intake air temp gauge.

It’s an important lesson in the way in which the different parts of the car are interconnected in terms of their performance. Change one aspect and that affects other aspects.

After all, if I’d told you that I’d modified the trans and that had increased my intake air temps, would you have believed me?

Comments are closed.