(Bit off topic) turbo lag...

Axel Rietschin axel_rietschin at compuserve.com
Wed Jan 10 03:56:29 GMT 2001


>   I know it's a bit of topic and  I know it's been discussed many times
> before (read the archives!) but I would appreciate some advice.
>   I'm planing on turbocharging a Hillman Imp engine (a 930cc SOHC) with a
T3
> Garrett, which is bit on the large side for the engine.

More than a bit on the large side, IMHO, at this unit is often found on
engines with twice the displacement.

> An intercooler would
> also be used. The boost will be electronically controlled to slightly
> increase and flatten the torque curve. This is to keep the head gasket in
> one piece, allow a high static CR to be used and increase the toque at
high
> rpm to make as much power as possible. I'm hoping to allow the engine to
> usefully rev to 8,500 rpm.

What kind of high CR are you planning to use? 9 - 9.5 to 1 is about as high
as you can get if you use 102 octane race fuel. With 98 unleaded, 8.5 - 8.8
is probably very high. If you compress more, you'll reach the knock limit
really early under boost and you'll need to set your spark advance to a
(much) less than optimal point, negating most of the benefits of
turbocharging.

>   There seems to be two options to keep the turbo spinning between
> gearchanges:
> 1) Re-circulate the air from the outlet back into the inlet. This would
> occur during gearchanges and, perhaps, below a given RPM so the turbo
would
> spool up faster. However I am concerned that the turbo *could* be over
> revved.

This won't really cut it. Recirculating the air will make it slow down a
little slower, maybe. However, this won't be really noticeable. In my
experience, dump valves and the like do absolutely nothing to the turbo lag,
just noise and some 'peace of mind'. The turbo will certainly not over-rev
with the throttle closed :)

> 2) Restrict the air into the turbo like a carburettored suck-thought
system.
> Could cause problems with oil seals and would increase the throttled
volume
> and so reduce throttle response. Perhaps two throttle bodies could be used
> in series?

This will most likely over-rev the turbo as you'll create a vaacum condition
at the turbo inlet.

Anti-lag systems basically work by bypassing a *lot* of air around the
throttle butterfly, either by using a solenoid air bypass valve or by
keeping the throttle open (say 25-30 degree) using some kind of actuator. As
a rule of thumb, you need to bypass enough air to make the engine rev up to
its max rpm speed in neutral. Ignition is retarded according to throttle
position, starting around mid-throttle and ending at 40 to 50 degrees of
retard (from the normal spark advance) when the throttle is fully released.
Also, a certain amount (say 0 to 50%, again starting around mid-throttle) of
fuel pulses are cut to partially stall the engine so you still have some
engine brake and the vehicle slows down normally when the throttle is
closed. Older anti-lag systems used to cut part of the ignition but this
resulted in gratuitous bangs and flames in the exhaust with big surges and
spikes in the boost - not a really smooth ride. Cutting fuel is just as
effective as slowing down the engine despite the bypassed air and is less
destructive (and still bangs enough because of the retard). Of course, your
ECU must support all those retard and cut functions. This system is
extremely effective, the turbo lag is reduced to zero when shifting, exiting
tight corners etc. However, you have to *seriously* heat-shield everything
around the turbo and manifold and make sure there is absolutely no oil leak
in that area to avoid engine fires. The turbo is orange-yellow when driving
hard with ALS active.

>   As the car is going to be fundamentally a road car but will also be used
> for hill climbs and sprints, keeping the turbo in one piece is of the
utmost
> importance! i.e. I don't want in inject raw fuel into the exhaust
manifold!
> ;) I have asked a Garrett main dealer for advice but they basically told
me
> they weren't interested (didn't want to be held responsible if it went
> wrong) and I should go to a tuning company (i.e. with a blank cheque).
> Many thanks, Toby

What you really need to do to your T3 if you want to make it ALS-proof is to
use a 360deg thrust bearing and also replace the turbine with a stronger
one. The standard turbine has Inconel blades and a steel shaft and tend to
break at the junction when heated above 950 degrees C or so, temp which is
more than easily reached when ALS is activated. What you want is a type 247
'Maram' turbine, which is made out of a single piece of Inconel. The turbine
blades can machined ("cut back") in a way that will reduce backpressure and
limit the max turbo speed to a safe value. This part is, however, *very,
very* expensive. Check www.turbotechnics.com they are extremely experienced
as they developped the T3's and T35's used by Ford for World rallying, where
anti-lag systems are born and heavily used. They can supply all parts and do
the necessary machining or supply complete, ready to race ALS-capable units.
They usually are happy to give advice as long as you ask the right questions
:)

>
>
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HTH,
Axel


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