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How to Get More Power from a
Turbo Engine
There are two clear ways to go to get power from a turbo
engine, or the third (and most common) that combines the two. Obviously, this
doesn’t state the individual, known components like ported heads with larger
valves, hi lift cams and air filters, it is an indication as to HOW you can
achieve the power figures you want.
Before we go any further however, if I can remind you of
the well known 'rule of thumb' for ignition advance on forced induction motors.
"For every 1psi of boost pressure over atmospheric that the engine sees, the
ignition should be retarded 1 degree over and above what the current advance
figure is at that transient engine state (ie load and RPM state)" – hence: a
boost pressure of 14 psi needs the ignition retarded by a further 9 degrees
compared to 5 psi of boost.
Now back to the options:
You can either use an engine more like a high performance nat-asp engine (high
CR, high overlap cams, etc) and a mere tickle of boost pressure to get another
few HP.
OR
You can use a low CR, a special low overlap cam and high boost to get quite a
lot of hp - pretty much like "Boosted45" has done.
Both of these have their pro's and cons....
The high CR option is pretty easy to set up (ign. retarded a few degrees, simple
low boost turbo, etc). This will have little lag from the turbo, and once 'on
cam' will be a flyer. A small, low inertia turbo is all that’s needed, although
it must not be so small as to be a restriction in the exhaust.
The second (low CR) option is easy to get near, but very difficult to optimize
(without large $$$ investment). A decent, large turbo that can provide
sufficient boost with little backpressure at high RPM will be required, as will
a suitable ignition system - although a heavily retarded, or locked up dizzy is
more often used than full electronic management. This will then wield problems
with lag, off boost power and a HUGE surge when the turbo finally spools up,
because the ignition is basically ‘wrong’ at every point other than full boost
at high RPM.
The third option is a combination of the two, and one which is used by the
majority of the 'turbo boys'. The use of a 'medium overlap' cam profile, and a
reasonable amount of boost (around 1 bar) allows the retention of a reasonably
high CR, and a distributor that can be modified and / or retarded by a bearable
amount to give the correct advance at higher boost / rpm, with an understanding
it isn’t optimized at lower boost levels.
So, the decision of what you want really has to be made on what you want to do
with your motor. You must also be honest with yourself when deciding.
As regards mapped ignition, I personally would never again run a boost level
over 12psi without a boost retard of some kind. Note I stated BOOST retard, not
RPM retard also... My personal reason for this is that it is all very well being
able to lock up, and retard your ignition by x degrees, so that the advance is
correct at your max RPM and boost level, but it will be wrong everywhere else.
This will mean that your engine will be slower to accelerate from tick-over, the
turbine will be slower to spin up (generating lag), the economy will be awful
and the general drivability will be compromised.
If you want to go distributorless, you need to budget for around
Fuel injection?
Sore point with me - I've tried it and won't be going back to carbs. Don't for
one minute believe that carbs are better than injection.
This info was originally written by Turbo Dave and was
modified to fit my purpose (I did not see any copyright notice on the page).
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