Ok,
This isnt going to be the easiest thing to translate into words, so I hope the pictures help.
(one is 2wd and ones 4wd but same engine specs. Ive graphed road speed in 3rd so only one "curve" to look at)

Lets look at a typically standard Amal valve controlled boost curve for a standard YB with Siemens 55s and T34 turbo.



Ok, so note that we have a nice big slug of midrange boost at 31psi and then the boost tails down. Now, for those of you whom have never considered why this is, its because as the turbine housing exhaust pressure increases as the engine pumps more and more air through it per second, it starts to overcome the spring pressure in the wastegate and opens it a little.

When this is allied to the fact that the turbocharger is well out of its efficiency range we have only one result - the boost starts to tail down... its an inevitable curve caused by the bleed off method. As a matter of fact worth noting, when using amal boost control, the boost spike is largely determined by the fuel we have available at 6500rpm and as such, how much boost we can have at 6500rpm. Essentially, we up the boost in the midramnge according to how much we have left of it at high revs when we are running out of injector opening time. (Induction stroke time base)

So this engine will have been checked and calibrated when mapped as follows:

• 4600rpm - 31psi - fuel ok - no detonation
• 5000rpm - 31psi - fuel ok - no detonation
• 5400rpm - 30psi - fuel ok - no detonation
• 5800rpm - 28psi - fuel ok - no detonation
• 6200rpm - 25psi - fuel ok - no detonation
• 6600rpm - 24psi - fuel ok - no detonation
• 7000rpm - 23psi - fuel ok - no detonation

That map would be perfectly safe to mail out to anyone with a YB and that engine spec controlled by an amal valve and couldnt achieve over 25psi after 6000rpm. Happy days.

However, lets get that same car back in and fit a cheap electronic boost controller with no overboost facility and see what happens to the boost curve to keep the same engine safe.



The very same engine is now running....

• 4600rpm - 25psi - fuel ok - no detonation
• 5000rpm - 25psi - fuel ok - no detonation
• 5400rpm - 25psi - fuel ok - no detonation
• 5800rpm - 25psi - fuel ok - no detonation
• 6200rpm - 25psi - fuel ok - no detonation
• 6600rpm - 25psi - fuel ok - no detonation
• 7000rpm - 25psi - fuel ok - no detonation

As you can plainly see, the boost curves generated by the 2 boost control systems are very different indeed, and since that controller cant have an overboost function, the boost is set to the max those injectors can deal with at 6500rpm, which on this particular engine turned out to be 24psi.

Now Imagine if the owner had just thought, well, it used to run 31psi, so I will set it to that. This same engine would have had 31psi from 4600 - 7000rpm as the boost doesnt tail down with a good controller because good boost controller monitor the boost pressure and opens/close the wastegate accordingly to ensure the set boost pressure is maintained.

With a jap controller set to the same boost as it used to "peak" at on the Amal system, that same engine would now run:

• 4600rpm - 31psi - fuel ok - no detonation
• 5000rpm - 31psi - fuel ok - no detonation
• 5400rpm - 31psi - fuel ok - no detonation
• 5800rpm - 31psi - fuel lean - probably no detonation
• 6200rpm - 31psi - fuel lean - probable detonation
• 6600rpm - 31psi - fuel lean - probable detonation
• 7000rpm - 31psi - fuel lean - probable detonation

Happy days, lets roll, here comes the boost!

• 5000rpm, yeah!
• 5500rpm. feeling good still, might even be faster! What an awesome investment!
• 6000rpm, god, feel how much faster it is now...jees!
• 6500rpm... cant believe this, wish id bought one years ago... wait, what was that missfire?
• 6750.... Still pulling hard... god, im quite scared... (rattle, tinkle...)
• 7000... BOOM.