Turbo throttles and Speed sensors

Fri Mar 10 04:30:26 GMT 2000

Howdy,

I have responded to several items in the digest, in this one message,
since the topics seem to cross one another.  Hope no one minds.

SIMULATED CRANK TRIGGER:

> If you're in a hurry, check out National Semiconductor's site at
> http://www.national.com/pf/LM/LM555.html

This is good.  The 555 timer not only can be configured to adjust
frequency, but pulse width.  It puts out a fairly nice square wave too,
which can be changed using smallish capacitors.  The link Charles posted
is an excellent starting point.

TURBO SPEED:

There are many ways of doing this, and optical is probably the least
intrusive IMHO.  We tried several ideas, one being a small slotted disk
attached to the turbo shaft, which worked well.  Of course, this is
fragile hanging off a hot turbo, vibrating inside the engine compartment
when you slap the throttle shut :)  We even got real clever (har har)
and used reflective paint, painting the intake side of the turbo wheel
with little dots, then using an IR emitter/detector pair we were able to
count dots, i.e. speed.  Worked fine until the device loosened up and
was sucked into the turbo.  Also, this arrangement has the IR pair
sitting in your airstream.

Another option is if your turbo's shaft hangs out of the housing
slightly. Some do, some don't.  Our T03's do.  We cross drilled a small
hole side to side on the shaft, and put the same IR pair (pre ingestion
into turbo) on either side of the shaft ends, using little aluminum
brackets painted black we were able to shield stray IR light, and
everytime the hole through the shaft lines up, the detector emits a
trigger into a device which conditions it and then it can be counted. 
Two light beams per rotation (hole on both sides of shaft, all the way
through).

PRE/POST TURBO THROTTLES:

I probably should have said pre/post plenum. :)  Pre plenum you can get
away with one or two throttle bodies, which is much easier to
obtain/fabricate/tape together, where as post plenum throttles you need
one per cylinder, aka weber.  Bitch to set up.  The disadvantage of the
pre-plenum throttle, is if your plenum is sized appropriately (cid of
engine or larger), you have this giant air cavity containing air to be
sucked into the engine, forming a vaccuum.  Thus, when you close the
throttle after high boost runs, you have plenty of air to go into the
engine for a period of time.  Small amounts of time, but its noticable
when dropping off a 8000 RPM "yank" to idle.  The reverse is also true -
the larger the plenum, the longer it takes to build pressure, as the
plenum acts as a resevior, much like the vaccuum booster "stores"
vaccuum for your brake system.  So, turbo lag is slightly more
pronounced.  Also, having a 80,000 RPM turbo suddenly have its output
plugged might be rude to the turbo bearings.  Worn ones have made a
slight "clunk" kinda sound when this occurs.  At least in our testing,
so since we are not scientists, anything could be wrong.

With individual throttles per cylinder, post plenum, I see advantages. 
If you back off the throttle abruptly, the air in throttle body
underneath the throttle plate is all the air your engine gets, so its
decelerates much faster.  Also, if the plenum has any pressure left in
it when you open the plates (i.e the turbos are still pressurizing the
plenum), you have a higher level of boost right off the line.  This
assumes the throttles in both examples do not leak at all.  Obviously,
they do, or the car wouldn't idle at all - no air.  Using the plenum as
a cushion to the turbo also seems logical.

Which is better?  Jeeez, Not sure.  Just sharing some testing we have
done on a Buick V6.  The individual throttle bodies were an absolute
BITCH to make, even with fancy CNC machining equipment.

RUNNER LENGTH:

As with the TPI engines, the longer the runners, the more torque you
generally get in the low-end.  This is why camaros/vettes have about 17"
of runner length (curved, but long), because off the line, you want the
torque.  Shorter runners with a larger plenum also achieve this same
goal, however there is a lot of math involved.  We determined our runner
length by doing 5 dyno pulls, then changing the runner length, and
repeating.  The longer-runner-more-torque-down-low theory does in fact
apply to shorter runners with a larger plenum (throttles pre plenum).

However, the longer runners do more than provide an "air battery" on the
low end, it also directs the airflow.  If you were to inhale some cigar
smoke, and blow into the air, you get a cloud (or a ring if you truly
have a talent).  If you were to blow the same smoke through a paper
towel cardboard tube, the smoke coming out is more directed.  Longer
runners do this for your engine.  At higher RPMs however, its not as
necessary because there is enough flow/pressure that the air tends to go
somewhat more straight than at lower RPMs.  More kinetic energy in the
air molecules, and they continue to flow in one directions.  Turbos
enhance this even more.

The ideal runner length (without displaying all the engine specs) on our
twin turbo V6 (252 + .030 overbore engine) turned out to be 12.7 inches
for each runner, and a plenum equal in size to the cid of the engine
(ours was 260cid, close enough).

The last comment I will make about turbos before I turn in for the
evening (just got home from a 15 hour work day) is it was our experience
that the most significant gain (for the dollars) is with the
intercooling.  Longer/shorter runners, port work, runner shape, valve
size, etc, all are factors in increasing your volumetric efficiency,
however the most % gains were with intercoolers.  The more you can cool
the air post turbo, the more air you can fit into the cylinder.  We had
significantly higher power gains by using a water-based intercooler
attached to a garden hose (which attaches to a well, so the water is
prolly 40-45 degrees tops) over an air to air intercooler, which has
ambient air flowing through it at best.  Of course, the latter is much
easier to install, less to break, and easier to fix when it does.  two
hoses and an intercooler and a few clamps.  Easy.

Another comment... while I don't have facts in front of me to support
this... we found through experimentation that smaller displacements with
higher boost to produce more power, generally, keeping everything else
equal (stroke, etc).  I'm unsure if this is because there is less
surface area in the chamber for detonation so the timing doesn't retard
so much, or the piston top is much narrower in diameter its stronger
since there is less pressure being exerted on surface of the piston
(because the skirts of the piston are closer together they can absorb
more load?).  I dunno, one of those funky things I guess.  Though I have
violated most of my suggestions by building a twin-turbo large
displacement mopar stroker motor with a reasonable amount of boost
(10lbs max).  Idles nice and revs nice to 4000 rpm.  Stump puller.

As far as cams go... my advice is this - get as much air into the engine
as possible.  You can do this two ways... low lift with long open time,
or high lift and short open time.  Both will do approximately the same
thing, and a slew of math can tell you which is right for you based on
where your normal rev band will be.  OVERLAP is not your friend. 
Anytime we added overlap, more and more air/fuel blew by the exhaust
valve. With turbo, it flew out faster.  The more overlap you have the
less boost pressure you will realize in your engine because more of it
flows out the tailpipe.

Roller cams would be a good choice because you can have high lift and
long open times and have the lifters follow the cam lobes.

Just some random thoughts, feel free to thump upon.

-- 

Frederic Breitwieser
Xephic Technology
769 Sylvan Ave #9
Bridgeport CT 06606

Tele: (203) 372-2707
 Fax: (603) 372-1147
Web: http://xephic.dynip.com/
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