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.
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.
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 ...
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 ...
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.
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.
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!
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!
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.
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 ...