The Hot topic of the 87 Starquest fuel cut has surfaced again with, a new
twist. Scott Lambson posted this asking for comments. I ask you to
consider it as a possibility and comment. Please also take a look at my
other message on fuel cut that went unmentioned.
SCOTT:
Clark posted a question as to why he was not experiencing the proverbial
fuel cut in his 87. I have conversed with him about this before to which I
had no idea why
he has not been experiencing fuel cut. But, it got me thinking. I have never
had it in my 87 either.
Now, I have been on these Starquest BBS about 4 years now and have had my 87
for longer than that. I had never heard about a fuel cut until I got Topends
catalog and I bought it about the ECU needing upgraded but I could not
afford to have my car down for 2 weeks. So I waited and asked some
questions. Everyone
on the boards kept telling me that the 87's had a fuel cut somewhere around
14.5psi and the 88-89's was set somewhere around 18psi. I believed them.
And, I
proliferated that information.
I have been searching the manuals for anything concerning something about
the fuel cuts. I have found NOTHING. I have manuals for the 84's and 87's.
(If you
know of something please direct me to the page.)
Which got me thinking even further. How would the ECU know when 14.5psi is
met?(retorical) Anyone who still believes the stock boost guages is actual
pressure
or is even accurate, I can produce the schematics to prove them wrong. The
boost guage is based on two sensors. Mainly the counts coming from the Air
Flow
Sensor(AFS). Secondary is the engine speed sensor. So as more air flows
through the measurement tube more boost is shown. In stock configuration
this can be
fairly accurate. Unfortunately most of us modify these cars.
You can get around the fuel cut by modifing the AFS but you also make the
car run leaner. According to what I understand of the ECU operation, fuel
cut is actually
determined by pulses coming from the AFS or more specifically frequency of
the pulses. At some set frequency the ECU initiates the fuel cut. Fuel cut
should occur
like this. Timing starts retarding, fuel trim stays the same and boost is
cut back. Unfortunately, the ECU has no control over boost pressure other
than to adjust the
timing. The DSM's use a boost control solenoid the drop the boost when
excessive knock or fuel cut is reached.
Everyone that claims to have reached fuel cut explains it as a sudden surge
of non-acceleration which quickly corrects itself. In order for this to
happen fuel would
have to be cut. Most likely done by the ECU by not opening the injectors or
cutting the frequency driving them. This seems like a really bad thing to
design into an
ECU. But it is possible.
The other thing that I would like to propose is that there is no fuel cut.
O.K. this may be a shocker but I am starting to think we have all been feed
a line of bull. My
hypothesis is this. Somewhere around 80% duty cycle, most injectors reach
saturation or they will not cycle accurately anymore. This is due to the
fact that there
simply is not enough time left to open and close. Our injectors are a
slightly different breed. My understanding is that they are run on a 50%duty
cycle continuously
but the actual frequency that they are driven at changes.
_________ ________
_| |_| |_ normal injector @ 80%
_ _ _ _ _ _ _
_| |_| |_| |_| |_| |_| |_| |_ Our injectors at saturation
Our injectors reach saturation when the frequency gets high enough that the
time on and off does not allow the injector to open and close. So, what I am
proposing
is that 'fuel cut' people are experiencing is not actually fuel cut but
rather injector failure. Contrary to popular belief the stock injectors will
not get you over 250
horsepower at stock fuel pressures.
According to everything I have gotten from the experts the combined total of
the stock injectors will flow around 1250cc/min of fuel. Go to
www.rceng.com
There is some good explainations of fuel injector theory. And even though
their site still says they sell injectors for the StarQuests they should
tell you they won't.
Maybe, a good pestering will bring them back.
Anyways, the formulas there will tell you the stock injectors are a little
weak even for the 250 horse range.
I personally am running two things to correct the fuel problem. 1. A BCFPR
which raises the stock fuel pressure at boost. This gives you some expanded
fuel
capacity when you need it. 2. Larger injectors from RC Engineering.
According to my calculations this should get me close to 275.
I have talked to Topend about their 87 ECU and have not been able to get any
concise information. My assumption is that what they are doing is to add the
88-89
fuel maps and ingition curves. What they do physically is to add a
daughterboard. If anyone has any proven info. let me know.
Maybe I am way off base here but I figured I might as well post my concerns
as I would like to see a disertation as the the reality of an actual ECU
fuel cut. I
welcome any corrections.
New Message:
: Sorry about the ASCII stuff. It did not come across to well. Just assume
the top of the pulses are line up with the next line. The nomal injector
should have big spaces on the on pulses and small spaces the off pulses.
: On Off
: _|-------------------------|_|------------------------|_
: _|--|_|--|_|--|_|--|_ Ours
: Heres another site at greddy.
New Message:
: At saturation
: : off.........on...........off
: : _|-------------------------|_|------------------------|_
: : _|--|_|--|_|--|_|--|_ Ours
: : Heres another site at greddy.
: http://www.greddy.com/injectionsizer.html
What Do you guys think? Is he way off base or could it happen?
Below is A Question I posed back:
: Scott,
: A quick thought, Lets say topend was putting in the 88/89 fuel map in the
87 ECU conversion. If your theory was correct, wouldn't the richer 88-89
fuel map driving the unchanged 87 injectors cause them to fail even sooner?
Thanks All,
John