From digest.v6.n541 Wed Apr 16 17:38:47 1997
From: "Brendon Whateley" <brendon_at_tibco.com>
Date: Wed, 16 Apr 1997 12:20:03 -0700
Subject: Re: <M42> Sebring Supercharger & How it works!
This is my nth retry to post this to the digest. Thanks to Richard Welty's
help I finally got it into the digest, it kept disappearing without trace...
Don't make posts over 10000 characters!
In case you think Jim (I included his some of his response at the end) is under
stating the problems with the design of the Sebring Supercharger "engine
management" system, let me relate a little tale...
I installed a supercharger in my Miata several years ago and experienced a lot
of problems, culminating in the supercharger being removed and returned. I am
not going to say which organization I bought it from (Miata's have many
options, I was well treated and got my money back after 3 months of trying to
sort out the kit.) But the kit had many similarities with the Sebring Kit used
Before you start, the ground rules are:
=> Be sure that you can get your money back if it doesn't work out.
=> You really get what you pay for.
=> Anyone can buy a turbo or supercharger unit from a manufacturer, it's the
bits that stick it to the engine that make all the difference.
So here is the wisdom gained from having done it...
My experience: I installed a XXXXXXX kit on my Miata. The installation
took a little over 2 days, not the claimed install time (5 - 8 hours if I
remember correctly) This was mostly because of some quality control problems
with the kit that resulted in me having to fabricate a few fittings. I had
problems once it was installed, I had to replace 2 defective fuel pressure
regulators - they were shipped to me broken. It took endless phone calls
before I could convince them that the kit was at fault, they kept telling me
that it was a problem unique to my car - essentially my car was supposed to be
"out of spec." The end result was that I gave up, uninstalled it and sent it
The car performed fantastically with the Supercharger on, even when it was
half-working, it was great. The same power as the stock motor produced at its
peak was now available at about 3800rpm with much more available above that.
The same effect as putting a 3 liter engine in a Miata. My problem was that I
experienced lean conditions at high revs, a very dangerous condition.
Now many people installed it and it worked, so they do have many happy
customers! As aftermarket kits go, the quality of the kit was very good! Just
don't go into this blind and you could be very happy.
If you are still interested in buying one, the things to check: How is the fuel
enrichment done? Unless it's done electronically (i.e. a new chip or add-on
computer) I wouldn't install the kit unless you enjoy messing with engines and
relish the possibility of using the pistons as ashtrays/paperweights. The
cheap and simple "raise the fuel pressure" mechanical regulators are NOT the
correct way to go. It is not very accurate and not completely reliable. That's
not to say they can't be made to work, but if anything goes wrong, you can
damage the engine... The ignition control is a secondary issue. Many kits
retard the static timing or rely on the existing engine management system to
retard the ignition using knock sensors. In the Miata case they also rely on
going from regular to super.
A short explanation of how the rising rate fuel pressure regulator attempts to
provide the correct mixture under boost is in order. The fuel injection system
uses a computer system to determine the exact amount of fuel to inject by
evaluating a variety of inputs such as RPM, throttle position, air flow (or
mass) air temperature, engine temperature, knock sensor input and exhaust gas
composition (O2 sensor). The number of parameters varies from system to system
and in some cases with operating conditions. This information is used to
calculate both ignition timing and fuel quantity. The calculated quantity of
fuel is delivered by opening a valve (the fuel injector) for the length of time
it takes for that amount of fuel to flow through the injector. In other words,
this is like measuring a cup of water by opening a tap for 10 seconds. If you
want two cups of water, open the tap for 20 seconds, etc. This all works
because the fuel supply to the injectors is maintained at some constant value
in relation to the intake manifold. It needs to be maintained relative to the
intake manifold because the manifold pressure changes with throttle opening
(and boost) from a big vacuum at 6000rpm with the throttle closed to the boost
pressure under full throttle. In a supercharged engine this can span a large
range of psi's. The fuel supply is usually maintained at some pressure about
35psi above the manifold pressure by a regulator that returns fuel fuel to the
tank when the rail pressure exceeds the set amount. (BTW this return flow has
an additional benefit of keeping the fuel cool and preventing vaporization
Now we finally get to a "down-stream" fuel pressure regulator. This is hooked
up to the return line of the normal regulator and designed to cut off the
return fuel flow to raise the rail pressure as the manifold goes into boost
conditions. This device is designed so that the pressure will be raised enough
to supply the correct mixture under boost. Remember the computer is calculated
an opening time of say 2ms with a fuel pressure of 35ms. We have bumped the
rail pressure up to say 50psi because of the boost. If we got everything just
right, the extra fuel will be the correct amount and we are all happy.
Sadly, the world is not perfect... As an example, the sudden rain storm cools
the air. Suddenly the same boost pressure results in a higher air mass, which
requires a whole bunch of extra fuel. Sadly, the simple "down-stream"
regulator does not know this and we start running lean which just happens to be
one of the worst ways to run under full boost. You could explore all the many
ways this method fails to keep the mixture correct, everything from not
providing the correct enrichment curve to getting old and stuck.
Two other areas of concern with this picture, First the injectors are designed
to operate at 35psi not 50psi. If the pressure gets too high you run into two
problems, the injectors can leak when closed and they can fail to open against
the pressure. The second is the stock fuel pump, these are usually designed to
provide a certain volume of fuel at a particular pressure. When you raise the
pressure they are required to produce, the volume drops until they provide
insufficient pressure or flow. The kit I fitted to the Miata relied on the
stock fuel pump operating above its maximum specified pressure, which will
usually work but not always. If the pump fails to keep up, you will run lean.
Some kits address this second problem by supplying a second high-pressure fuel
pump. None supply special injectors...
The sceptics cry out, why doesn't the computer adjust for the lean mixture when
it detects it using the lambda (O2) sensor? Two reasons, the computer doesn't
always use the O2 signal - full throttle being one of those cases (why? another
day, OK?) and even if it did, it would run out of duty cycle time on the
injectors (the maximum time they can be held open, which you can calculate as
an exercise!) (Could this be why Dinan et al use bigger injectors?)
Why is running lean such a problem? Two words, temperature and knock. The lean
mixture raises cylinder temperatures and you get knock. (You will loose all
your power if the knock sensor backs off the ignition timing.) Unfortunately
the Miata does not have a knock sensor, so you go on to burn holes in your
pistons - giving you the interesting ashtrays/paperweights that I promised up
On my Miata, because of a series of broken pressure regulators, my poor car
would run out of just over 4000rpm at full throttle and experience a symphony
of knock (and loss of power.) I fortunately knew what was happening and didn't
let this go on - a dozen seconds or so would have trashed the motor! So I
still have the same motor in my normally aspirated Miata. (They build them
strong - the 1.6 engine was used in the Mazda GTX 4wd rally car in a turbo
form, producing about 250bhp...)
How do they get this "thing" past the smog check? I'll spare you reading
another several pages! Lets just say that the smog test (the real one, not the
joke used on your car) is very well defined - conditions, speeds,
accelerations, etc. and since the car does not see much in the way of stress,
it is relatively easy to ensure that the car never goes into boost during the
test. No boost, thus no rising fuel rate stuff, thus the car is STOCK. The
factory could pass a stock car, so can the kit maker. All the dirty stuff is
This is enough, we could spend all day on the ignition side of the puzzle.
I hope you enjoyed this, if not: This is all just my opinion without any facts
to back it up. If you find any factual errors, assume they were included as a
test. (Forward them to me for extra credit.)
Look after your cars.
Land Shark wrote (edited for size):
>>Neat URL eh? There is a great deal more info here than I have see at
>>other tuners WWW sites for the same application. Will T., perhaps you
>>will want to read this and rebut.
>No.. I'LL REBUT THIS... <web page quote>
>>The stock fuel injection system is used unchanged save for a Sebring-designed
>>"downstream" fuel pressure regulator that is boost sensitive. No chip, EPROM,
>>or other electronic changes are necessary - an important reliability point
>>engineers were careful to adhere to.
>It's not just "plug and pray"...
[ Help ]
End of Land Shark quote.