Variable Compression, Variable Displacement you decide

[Roger Heflin]

> 
> Roger - as promised, now that I am back from the dead I shall reply to
> you. DO NOT think that because I used single word replies in a previous
> corresponence, that that was all I understood on the subject. The reason
> single word replies were used is because it physically HURT me to type -
> so get off your high horse, calm down (Have a large drink) and read
> wwhat I think.
>

I can buy that.
> 
> Ok, what exactly are you saying in this passage? - Are you saying that
> the super-charger is producing too much pressure? - or is compressing
> the charge too much? - this passage just doesn't work - please clarify.
> As to why anyone should believe my 'false' - well I will explain, just
> as soon as you do.
> 
Yes they supers have a bad habit of wasting power by overcompressing 
from what all the books say.  It may be bad design on the intakes part,
but it seems to be very common.  I think it compresses the charge 
excessively and then it expands while still in the intake.   I guess
this is a design problem, it may not be a problem in the 
mass production super cars (ie the perviously made Fort Thunderbird SC,
and I believe the pontic bonniville).  It is probably a tuning issue
and the wastegate on the turbocharger fixes most of the issue with 
the turbo.  That is part of why I claim an electric would help,
it would be able to now regulate boost without wasting power compressing
air excessively.



> Many problems here - I will go through each in turn.....
> 1.
> 	"This allows a turbo to have high boost at lower rpm where a
> superchargers boost curve goes up with rpm until at close to redline it
> is at close to the detonation limit.". 
> 
> What is being said here???? A turbo's boost is directly proportional
> (inertia/heat not included) to exhaust gas flow - therefore to achieve
> higher boost with a turbo, the engine must first be doing some of its
> own work. The exhaust gas pressure rises, and so the turbo spins faster
> - etc etc. A supercharger will supply boost from tickover. The only
> reason it doesn't produce boost all the time is the fact that you are
> stopping it by keeping the throttle shut. Open the throttle, and it will
> produce as much boost as it can NOW. The amount of boost it will produce
> is directly proportional to its drive ratio (And of course how much air
> it is trying to move against what sort of restirction etc etc ). Just
> what has the detonation limit got to do with any of this I don't rightly
> know - maybe you can explain.
> i
If you set at a fixed rpm with a supercharger (lower rpm) it provides
a lower boost.  I am guessing if you set at a fixed rpm on a turbo (fully
spooled up, wot) it will have a higher psi.  The only problem with the
turbos seems to be the time it takes to spool up, which is not so much
a low rpm issue as a just started going issue.  A superchargers boost
rises pretty much straight line from low rpm to high rpm (pressure in
the intake) hitting high boost at high rpm.  I don't think the turbos
do this, they have a wastegate to bypass excessive exhaust to keep
the pressure down at high rpms so you don't blow the engine, which would
lead me to belive that they hit that max boost sometime before redline,
I guess if you tuned it right quite a bit before redline, where a super
will do say 5psi at say 4000 and 10psi at 6000.  If you raise the lower
number of 4000 your will blow the engine at 6000, because of the 
characteristics of the boost curve.

> 2.
> 	"With real world examples if you look both a turbo and super engines,
> same displacement, and same boost makes close to the same peak power. 
> The curves are somewhat different because of the boost characteristics
> of each, but fuel efficiency is very close."
> 
> Wrong. From what you are saying is that both the supercharged engine and
> the turbo engine must therefore have the same losses??? - But arn't you
> turning the supercharger round with some energy from the engine????
> This fact alone could account for the supercharged engine being down by
> 10% at peak power. (Not peak torque through). The torque curves will be
> radically different (Different charge delivery = different curve) -
> true.
> 
The turbo is getting power from the engine also.  It is rasing backpressure.
Raising backpressure is going to cost your hp.  It just a question of whether
it costs you less than the supercharger.  The energy is not free, especally
if you already have a free flowing exhast.  Adding a free flowing exhast
can get you 10-15 hp, now you are adding a turbo on the car that restricts
the flow more than the original exhaust, so you have kissed the 10-15hp
plus some more goodbye.  The energy for the turbo does not come free.


> > > > The power the turbo uses is not free.  If a super
> > > > engine dumps its exaust at x psi into the exhaust, and the turbo
> > > > dumps it at the same x psi into the exhaust (after the turbo), they
> > > > can have the same efficiency if they are both designed correctly.
> > > >
> > >
> > > Sorry, but that last bit is pure rubbish as well...
> 
> The second half of this paragraph doesn't quite make sense, so I refer
> to the bit that says "The power the turbo uses is not free". Well this
> statement is true. It is not free - you had to pay for it by putting
> fuel in first, then burning it, however, what were you going to do with
> it then? Let me guess - make some noise, heat the atmos up a bit with
> it????? - It may not be free but the fact that you are now going to use
> some of that energy to turn a pump round can only be a good thing!!!
> 
Read my above statement.  Raising the backpressure makes the engine have
to go to more work to push how the bured gases, this is where the energy
for the turbo comes from, then engine works harder to push out the burned
gases, it does drag on the engine and removes some horsepower from it.
It is not free.  With a free flowing exhaust the energy going out the
exhaust does not have enough pressure to be usefull you have to raise
the backpressure.


> Expert? Me?, No I am no expert - far from it. Its just my life's work
> thats all. I do not believe in experts - (A drip under pressure etc etc
> etc) - However I do seem to know more about the subject than you do, so
> if you have any further questions, please put on your cone shaped hat
> (Yunno - the one with the 'D' on it), sit in the corner, put your hand
> in the air, and I will see if I can help you.
> 
> Me - I just luv a bit of forced induction...

I amr really not sure you do.  You seem to not realize where the energy
from the turbo is coming from.  If the energy was being truely wasted,
then turbo vehicles should get better gas mileage than any NA vehicle
in existence, becasue more of the energy from the fuel is being effectively
used, but turbo cars don't get any better than cars with na engines
of similar weight and hp.

				Roger