IanM

I'd just like to share my personal experience with setting up a modified 951 for proper air:fuel mixtures under boost.

Following installation of my Huntley Racing Stage 2 Garrett-hybrid turbo, Stage 3 MAF, Rev.2 ARC2 controller and all other usual bolt-ons, I took my car in for some dyno tuning. While I wasn't too disappointed with 305hp to the wheels at 14psi of boost, the car was running lean up top, and was bucking under deceleration. I had the MAF sensor moved to a new position directly after the air filter, which cured the bucking problem immediately. I also installed an electric fuel pressure gauge inside the car to figure out if my adjustable fuel pressure regulator and fuel pump were up to snuff. After monitoring fuel pressure under boost, and varying the fuel pressure at idle (vacuum line disconnected), I concluded the following: the stock fuel pump has no problem delivering all the fuel you need, and the fuel pressure regulator will not allow more than 65psi of total fuel pressure. Therefore, at 15psi of boost, the most static fuel pressure you can run is 50psi.

After talking with the various tuners out there, I decided to get some bigger injectors. I was thinking that my Autothority Stg 2 chips might need upgrading (they're designed for K26 turbos), but I was hoping I could sort things out with the ARC2 fuel controller. I bought a set of Bosch 52 lb/hr injectors from Marren Motorsport, who seem to have the best prices out there. Marren says you can use up to 90psi+ of fuel pressure through these injectors if you want, but recommends running them at a minimum of 36psi (2.5 bar, stock fuel pressure on a 951). I installed them in the car, hooked up a Motec air:fuel meter and wideband O2 sensor (Note: can't get an accurate reading with cat in place, I ran a testpipe during the tuning process), and took the car out for some runs. I soon realized that 52 lb/hr injectors are pretty much the MAXIMUM size that will work properly in a 2.5L car. I ended up setting fuel pressure to 36psi, and the "high" **** on the ARC2 to the lean end of the scale, and just managed to get proper air:fuel ratios.

I ended up with fuel pressure at 36psi, and my Rev.2 ARC2 settings of Low 11:00, Mid 3:00, High 8:30, Accel 1:00. Under normal cruising, the air/fuel sits steady at 14.7:1, which is confirmed by a dithering of the ARM1 fuel monitor. Under boost, the mixture is flat at 12.5:1 right up to redline. Perfect!!! Set the boost controller to 16psi, the car feels very strong, and I feel better knowing that it's not leaning out up top.

Lemon Yellow 87 n/a

Oh yeah? I have a stock 8v n/a engine baby! Holy moly Ian, you car is NUTS!

Perry 951

Seems that everyone is using the 52lb/hr injectors. My concern is running too rich while not in boost.

Anyone thought about the 48lb/hr injectors from Marren? I spoke with them already. I might try them. I hope they flow enough.

IanM

It seems like if you want Bosch injectors, there's only a 42 lb/hr unit between the stock 37 lb/hr and the 52 lb/hr one. The 42 seemed a bit too close to 37, and if you do the math they're a bit undersized. Actually, the equations tell me that the 52's are just right, but my experience tells me that they are on the big side. Who makes the 48 lb/hr injectors? Are they peak-and-hold injectors (low impedence)? I'm sure they'll work fine. You'll probably end up with your fuel pressure set somewhere around 40-45psi.

OZ951

Great post Ianm, just the sort of reference that will help out other folks who are tinkering with their systems.

I know when I hook my TO4E & stage 4 MAF into my car I will be looking closely at the fuel system.

Wayne

Perry 951

I think he said they are TRW or Nippondenso injectors.. but could be wrong. That was a while back.

They are peak and hold, and about $74 each.. cheaper than the Bosch.

I will probably go with the Bosch becuse of the OEM quality.

David Salama

Let me tell you about my injector woes. I also have a 307 RWHP 951, #2 HR turbo, #4 MAF, ARCII, ARM1, etc. I recently upgraded to 55# Siemens injectors (peak and hold) also from injector.com (Marren) I am still trying to find a perfect setting with my ARCII, Autothority II chips. My biggest problem is that my car runs rich at idle. While the ARM1 dithers, the idle wants to surge to 1300 RPM, which triggers the computer to cut out the injectors briefly, then on again. With the fuel pressure turned real low (30-32psi) I can stop the idle bouncing, but it still idles about 1200 rpm. If I sit at a stop light for more than a couple minutes, I get white fuel smoke out the tail pipe (BIG TIME) I have ordered a heated O2 sensor to see if this helps. As a last resort, I may have to switch injectors to perhaps the Bosch 52#ers or ??? When I drive aggressive, the car performs awesome with these injectors, and I can get it to bounce between one and two blue lights on the ARM1 (rich). Any similar experiences ??

Perry 951

I think your fuel pressure is too low. You are dumping the fuel into it rather than a good spray. What happens when you get the pressure up to 40psi and lean out the ARC 2?

I would say that your idle problems should be solved, but you could be very rich in the mid. Try it and let us know.

Danno

David, sounds like you might have problems with your idle settings and your idle-stabilizer. Disable your idle-stabilizer with the test-port and see what happes to your idle.

David Salama

How do you disable the idle stabilizer with the test port? Also, I bought my car with the top half of the screw on top of the throttle body broken off. Does this make any difference? I'll try 40 psi fuel pressure and let you know. Thanks.

IanM

David,

Try upping your fuel pressure to 36psi. Your injectors are slightly larger than mine, and you've got a Stg4 MAF, but other than that our mods are pretty much identical. Like I said, the 52#'ers are pretty much at the limit size-wise, so your 55's might just be too big. When I turned my fuel pressure above 36psi, I ran too rich under boost, even with the "High" **** on full-lean. If you do need to replace your injectors, you might consider the Bosch 42 lb/hr units. I'm sure they'll be big enough, as long as you don't plan on making over 350hp to the wheels. Your fuel pressure will probably end up around 45psi. Those 48#'ers that Perry mentioned might also be a good choice, that size would be right on the money. I can't say this enough though, I highly recommend tuning with a wideband O2 sensor, NOT with the ARM1. On the street, I found that one click of the mid and high ***** was making a SIGNIFICANT change to the A/F ratio. Using a wideband O2 sensor on the street also has advantages over tuning your A/F on a dyno, as heat-soak is not an issue. I recommend tuning on the street, and fine-tuning on the dyno.

David Salama

Well no good news today. I tried turning my fuel pressure to 36 and 40 psi. The car screams and I am able to turn down my ARCII settings. I cannot get the car to idle however. The car still revs to 1300 RPM then cuts out every few seconds. Still if I idle for a few minutes, major white smoke. Still open for suggestions. I turned back to 32 psi fuel pressure and have got the idle to stay at 1200 rpm without bouncing. To do this the ARCII settings seem odd compared to others I've read. Low = 9:30 Mid 2:30 High 1:30 Accel 1:00 I still have to turn the low setting very low to stop the idle problem. I am unfamiliar with the idle control mechanism. Can anyone help me? Also, does anyone else have the problem with white fuel smoke while idling with big injectors?

IanM

Your ARC2 and fuel pressure settings are consistent with a car running overly large injectors. I know that mine are right at the limit, I kinda wish I'd gone a bit smaller myself. Unfortunately, I think you may be over the limit with yours. Your injectors probably don't produce an optimal spray pattern running 32psi of f.p., but you don't have much choice, unless you get smaller injectors. Turning your f.p. down even lower will probably just make things worse. It makes sense that your Low **** is slightly leaner than mine. It sounds like your High **** is too rich, but that's not the problem you're trying to remedy right now. Do you still have your stock 37 lb/hr injectors? Did you buy the big ones because you were leaning out on the top end like I was? Perhaps someone on the board who's thinking about boring out to a 2.8L or 3.0L would be interested in buying yours. I should put my car on a gas analyzer, and see how my emmissions look when my car sits idling. I haven't noticed white smoke, but I might be loading up slightly at idle myself. I'll probably have to put the 37#'ers back in as well as my cat for "Aircare" (gov't emmissions testing).

David Salama

Ian,

Yes I still have my 37 # ers. The car was leaning out at the top end like you say. The seat of the pants power is much better with the big injectors. I did place my old injectors back in as a test and everthing works perfectly. The car will idle at 900 rpm, no smoke etc. Perhaps the answer is changing injectors, but I would like to get more feedback before I decide. I have heard that some 951s are running 72 # injectors. So how do they manage the idle ??

John Anderson

I don't get it, THere seems to be a real issue here with these mods you men have, I should be seeing 340hp at the rears, not 307 and lean issues. YOU can get 307 with a stcok turbo, 20lbs boost and stock injectors with a maf kit. I've been through the entire mod process.

Back when I had the MAF kit, 52lb injectors I was able to run 24psi boost on a 200+k motoe and lay down 370HP, I do have the charts to show this. This was with a prototype HR stage III turbo, non ballbearing back in the days when I worked at huntley.

We have another customer that made 340HP with a MAF kit, stock injectors and HR stage II turbo. With stock cat in place...his motor also had 250+K miles on it.

THere seems to be something wrong here. I'll tell you what, I know you guys don't all buy this stuff from SFR, but I will gladly take your calls at tyhe shop to try and troubleshoot your issues. Its for the love of Porsche :-) Lets all get this worked out!

Take Care All!