A known problem
when using an AFPR is the fact that fuel pressure always runs behind on turbo
boost. The reason for this is because you have to insert a little
restrictor in the vacuum line to the AFPR. This way the signal to the AFPR
gets limited a bit. Downside is that this way, fuel pressure raises too
slow when boost comes on. Most of the time you actually run lean this
way. Only when applying throttle slowly or when rpm's get high enough
(above 6000 rpm in my case) the mixture will become correct (when the AFPR has
had time enough to raise fuel pressure). When you leave out the little
restrictor, fuel pressure follows boost perfectly, so no more lean mixture when
boost rises. Of course you'll have a new problem, fuel pressure will be
way too high! In my car fuel pressure goes up so high, that the needle of
my fuel pressure gauge goes completely around and then hits against the stop at
0psi ... about 140psi I think, the meter only goes up to 100psi. I
tried to use a needle valve on the input, to be able to control the boost signal
without restrictor. This didn't change much though, the more the needle
valve is open, the quicker fuel pressure rises, but the higher max fuel pressure
gets. I also tried to drill out the needle valve, but that didn't help
either. It's just too small to get rid of unwanted pressure. As a
side note, I also think that the little one way valve would be too narrow too,
even though you would use a bigger needle valve. You should use a bigger
one too I guess.
The only
solution I see is to make the Pierburg turn slower. I'm going to do some
experimenting with a DC-DC converter to lower the 14.4V the pump sees with
running engine. I would lower the voltage to the point (10V for example)
where the pump puts out just the fuel pressure I need (about 100psi). The
exact voltage is to be determined experimentally of course.
A second
possibility would be to limit the signal by leaving out the restrictor and to
use a second needle valve on the output (with its own one way valve of
course). This means of course that you would have to drill a hole in the
AFPR. I prefer to try out the electrical way though, because, from a
performance point of view, I don't think bleeding off al this boost pressure is
an improvement.
June 2000: Well, the first results are not that promising...
I made a converter which turns 14.4V into a variable 1.5 tot 11V. That way
I can make the fuelpump turn slower. At 14.4V, it consumes about 7A.
At 11V, it's about 5A, and yet fuel pressure remained the same, +140psi!
One thing's for sure, that Pierburg sure can pump some juice!
So, I turned down the voltage to the point where I couldn't here the pump's
whining noise anymore. I didn't measure it, but my guess is that it must
have been about 5 to 6V, cause at that point it consumed only 2.5A. With
this setting, everything appeared to be working just fine. In the first 3
gears that is ...
It ran a bit rich in first, but that's no problem since it's only for a few
seconds. In 2nd and 3rd, the mixture is perfect, 0.84V at 100psi fuel
pressure! Marvelous if it weren't that it begins to run lean in 4th.
In 5th, I only have 0.75V at 90psi fuel pressure. Apparently, the Pierburg
now turns so slow that it can't deliver enough flow. I have to find a way
to increase the flow at higher boost levels. I've thought about placing
the original pump in parallel with the Pierburg, but since it sits inside the fuel
tank I doubt that this will be possible.
If not, there's always the mechanical way ... thus a second needle valve and an
additional hole in the AFPR.
July 2000: I've
replaced my new style AFPR (6 bolt, part n° 20008) with my old style
AFPR (4 bolt part n° 20005). I don't know what the
exact difference is between the two, because at first sight their internals are
similar. The new one should allow fuel pressure to rise even before
there's boost! It's a mystery to me how that is accomplished, I've never
managed to do it anyway...
I only used the new style AFPR because the old one couldn't deliver enough fuel
pressure with my original injectors and restrictor. With the 1.6 injectors
I needed 120psi fuel pressure at 9.5psi boost. For some odd reason, my old
AFPR could only deliver 90psi. With the new AFPR fuel pressure was way too
high of course (140psi).
In a last attempt to limit max fuel pressure I put the old AFPR back in without
restrictor and ... it works! With the needle valve completely closed I now
have about 100-110psi, which is just enough to get 11psi boost with my 250cc
injectors.
The only problem that I now have is that my mixture goes lean for a few seconds
as soon as I go WOT. The mixture goes so lean that all LEDs on my A/F
meter go out! That's under 0.58V ... very dangerous!
Only when I keep the revs above 5000 rpm after shifting, my mixture stays
ok. Well, always shifting in the red zone is not so advisable of course
... bugger ... still not right ...
August 2000: Upped base pressure from 46 to 56psi. WooHoo!! Problem
solved! No more lean mixture, always nicely 0.84 - 0.88V! I can
floor it in any gear at any rpm without my mixture going too lean. It's
even a tad on the rich side, the orange LED on my
A/F meter lights up a bit too often (0.90V). Lowering the base
pressure a little or opening the needle valve a bit will solve this.
What I did find out about is the famous 'lean spot'. By design, the miata
goes a bit lean between 4K-5K rpm. I hadn't noticed it before, because
I've always been running a bit lean and my mixture was only good above 5K
rpm. Now that my mixture is ok, I can clearly see that at 4000 rpm my A/F
meter starts to oscillate between 0.7 and 0.78V. At 5000 rpm, it's as if a
switch is being activated, because the mixture instantly gets ok again (0.84 -
0.88V). This phenomena is there in each gear and at exactly those rpms.
Since this problem is inherent to the original ECU of the car, it's very hard to
solve this. The only way to do this is by adjusting the mixture for the 4K
- 5K range, but then of course it goes rich outside that range. The
resistor trick (splicing a 1.8K resistor in the thermistor line to richen the
mixture) might be a possible solution here, but it isn't rpm dependant, so
that's only half a solution too. I'm going to try it anyway ... results as
soon as possible.
Actually, the resistor trick should be both pressure dependant and rpm
dependant, thus only activate when it's necessary ... Hmm, anyone fancy making
such a thing? Anyone? Anyone? Bueler?
Turbo pressure
back to 12.5psi. Fuel pressure has gone up to 120psi by doing this,
causing the mixture to be far too rich. The needle valve is completely
open, so the only way to lower it is to add a second needle valve. I guess
I'll have to drill a hole in the afpr after all ...
September
2000 Found out what the actual difference between the 2 afprs really is.
The 20008 acts exactly the same as the 20005 but has a bigger rate of
gain. This means that at any boost pressure, the fuel pressure will be
higher with the 20008 than with the 20005. Not exactly what I'm looking
for since I have more than enough fuel pressure. Max fuel pressure will
therefore also be higher, of course limited by the fuel pump. With my
Pierburg, I have 140psi without any problem. Conclusion: I'm keeping the
20005.
October 2000 Drilled a hole in the afpr to release some pressure. Instead of a
second needle valve, I just installed an extra nipple. This has the same
result as a fully opened needle valve. Even with the extra nipple and the
existing needle valve completely opened, fuel pressure at 12.5psi is still more
than 110psi. More than what I thought it would be, but my mixture at least is ok
now. Think I'm going leave it alone now ... although 14psi must be
possible again without having to run insane fuel pressures.
The 2nd needle
valve is installed and is completely open. I'm running 14psi with a fuel
pressure of 110 to 120psi. The car drives unbelievably well, perfect
throttle respons and very smooth. The mixture stays nicely around 0.84V
without going lean for a second. Only the off boost mixture is a bit too
rich because of the bigger injectors. This is not a point though as I
can't feel it when I'm driving, I only now because the A/F goes towards 0.9V and
more. The only noticeable effect of this is that mileage has gone down a
bit, currently 23.5mpg, but even this is very good. The installation of
the RX7 AFM will solve this last minor thing and a mileage off about 26mpg must
be possible.
And then ...
it's all over, I've reached the mechanical limits of the fuel system, the car
runs perfect and ... I don't now what to do to it anymore.
Jan 2001
Haha, the game has started again! I ran the car for 2 months without any
major modifications. I think that's long enough so I just bought myself a
Link mk2, boost controller, knock sensor, keypad and the new style airbox, which
turns my system IV into a full FM2. Play time!
Apr 2001
Shiv Pathack (Vishnu Racing) has released a new fuel rail which supposedly gives you up to 30hp
extra! I ordered one of course. Results will be online as soon as
it's installed!
May 2001
The new fuel rail is a bunch of crap. It did exactly nothing. I
should have known better. The reason people are finding up to 30hp extra
is because they never tuned their timing. Now that they have the fuel
rail, they obviously start tweaking their timing and suddenly find extra
power. Well, I already tweaked my timing before I got the fuel rail and
found about 25-30 already. So, the gain is not from the fuel rail, but
from the timing. Somewhere I knew this was too good to be true, but I
figured that if I could get 10hp or so that it was worth the money ($300 +
shipment + taxes etc, over $400 when it arrived).
I must add that I am running pretty aggressive timing already (maybe our
European fuel is better than that in the US) so adding timing didn't do anything
for me.
June 2001 I drilled a hole in the rear end of the fuel rail and put an extra fitting
in it. This way I sort of duplicated the Vishnu fuel rail. Of
course, nothing changed, but I figure that the #4 cylinder gets more fuel this
way. I look at it as an extra insurance. Cost me almost nothing and
can't hurt.
The Link is pretty well tuned by now. I never thought it would be that
difficult, but I had to go through the entire learning process all over
again. Actually, the 'fiddle factor' of the Link is just as high as that
of my old afpr based setup, only I can do it with the keypad now. Before I
had to stop the car each time, pop the hood and turned a few knobs.
I gained well over 30hp over my old setup. This is because of the ability
to tune the igintion in 500rpm zones with the Link. Well worth the money.
I also found out that my boost gauge is reading low by about 1.5 psi. At
11psi on the gauge, the Link says that I'm running 12-12.5psi ... to think that
I ran over 14psi on the gauge with my old setup. Sjees ...
I
tried to use a needle valve on the input, to be able to control the boost signal
without restrictor. This didn't change much though, the more the needle
valve is open, the quicker fuel pressure rises, but the higher max fuel pressure
gets. I also tried to drill out the needle valve, but that didn't help
either. It's just too small to get rid of unwanted pressure. As a
side note, I also think that the little one way valve would be too narrow too,
even though you would use a bigger needle valve. You should use a bigger
one too I guess.
