I am planning on lowering the fuel pressure in my 1981 to take full advantage of the high flow injectors that were installed by Callaway back in 1983. Anyone have experience with going to the high flow injectors and then cutting back the fuel pressure to get a good match?

I'd like to know what fuel pressures people are running.

I plan on installing a gauge to verify where the pressure is set, and then start coming down slowly and reading the O2 sensor under various engine loads.

My goal is to get more power and save some money on fuel costs. Right now in open loop I am running quite rich (0.8 volts @ idle). You can smell the richness of the exhaust at times.

I figure I might gain 20 or 30 hp on the top end by getting the mixture closer to where it should be ideally. If I was smart, I'd go to a smart injection system, but the cost is much too high.

 

John,

If you have the high flow injectors and lower your fuel pressure you would end up with poor fuel atomization and burn wouldn't you?

Just a thought <img src="graemlins/wave.gif" border="0" alt="[byebye]" />

 

[quote]Originally posted by John Kuhn2:
<strong>I am planning on lowering the fuel pressure in my 1981 to take full advantage of the high flow injectors that were installed by Callaway back in 1983. Anyone have experience with going to the high flow injectors and then cutting back the fuel pressure to get a good match?

I'd like to know what fuel pressures people are running.

I plan on installing a gauge to verify where the pressure is set, and then start coming down slowly and reading the O2 sensor under various engine loads.

My goal is to get more power and save some money on fuel costs. Right now in open loop I am running quite rich (0.8 volts @ idle). You can smell the richness of the exhaust at times.

I figure I might gain 20 or 30 hp on the top end by getting the mixture closer to where it should be ideally. If I was smart, I'd go to a smart injection system, but the cost is much too high.</strong><hr></blockquote>


The biggest concern with this will be leanout under boost. Fuel pressure is critical to maintaining fuel flow when your manifold is under pressure from the turbos. In fact, most factory turbo cars run higher rail pressures to reduce the effect of having the pressurized manifold.

Remember that Callaway, like many others, richened the mixture a lot, trying to use raw fuel for charge cooling. All in the interest of preventing detonation of course. High-speed detonation under boost is not audible, and is the kiss of death for any engine, and especially damaging to the 928 because of the relatively soft cylinder walls, low-tension rings, etc.

You would probably do well with a more adaptable computer, but unless you are willing to commit the dyno time to getting it tuned well, you might be better off with it the way it is now. A more capable computer will need knock sensors installed in the valley between the cylinder banks to protect the engine, and will also probably want to have a controllable bypass valve to limit boost when it hears detonation.


I built a fuel computer and boost controller for my L-jet Saab 900 Turbo, a pre-APS (auto pressure control) car. Also put in the 9:1 pistons from the APS car, replacing the 7.2:1 originals. I had to put in knock sensors, thermocouples under the plugs, and even then ended up with spark retard and some additional enrichment under load to keep the engine happy with 20psi of boost. It made well over 200 ponies, but took a lot of care to get to that level without melting anything.


Don't melt anything!

 

John, without knowing what size injectors your running now, and at what PSI\Bar, it would be hard to know if you are over pressurizing the injectors or not getting enough fuel burn at WOT.

Years ago even the big guys like Callaway hadn't come to the full realization that when you modify one part of a system, the other sub-systems need to be brought up to the same level. So while checking your fuel pressure, you may want to take a look at your ignition system to see if it is up to snuff.

I would have to suggest reading up on this one before doing any fiddling. Here is a links that may help you in your quest.

<a href="http://www.rceng.com/technical.htm" target="_blank">http://www.rceng.com/technical.htm</a>

Cheers

Max

Edited in-&gt; Here is a link to a nice FMS that is (IMO) perfect for the 928 and even more so for a TT 928 (P8PRO).

<a href="http://www.dtafast.co.uk/index.htm" target="_blank">http://www.dtafast.co.uk/index.htm</a>

 

Thanks for all of the feedback on the situation. I actually had already contacted RC for some information. Here is what I know thus far....

My car runs a stock LH brain with stock fuel pressure.

Callaway fitted the car with 280 150 009 Bosch injectors. These were used on the 1972-1973 914 1.7L and nowhere else. These are supposed to floe 330-350 cc/min.

The stock injectors 280 150 154 are supposed to flow 235 cc/min, while the other injectors are supposed to flow 330-350 cc/min.

Since my car runs a stock brain, under normally aspirated conditions, I am probably getting 30% to 40% more fuel than I really need.

Now, on the boost side. Callaway fitted a microfueler which dumps in raw fuel into the intake stream under boost only. There are two more injectors (914 units) which are installed in the intake pipe. These are controlled by the microfueler and the amount of fuel increases with the boost pressure (i.e. longer "open" signal).

By reading the O2 sensor I know I rich across the board, and do get some black smoke out the back under full throttle (i.e. unburned fuel).

My thought is to install the adjustable regulator, lower the pressure (slowly, perhaps in 2 psi increments) and monitor the results by reading the O2 sensor.

The hope is to bring the HP up a little and gain some economy at the same time.

I determined that my Microfueler had been changed at one time, and one injector was not wired at all (this must have been replaced way back with a unit for a single injector only). So, for a long time, I was running with only 9 injectors instead of 10. I never damaged the engine then, so my hope is that with all 10 injectors, I can find a happy place somewhere off of the stock fuel map. Now, both additional injectors are operational.

This is a twisted maze, and a lot of stuff isn't up to modern standards, but it is all they knew back then. For example, the air flow meter is measuring compressed air, not atmospheric air. Not sure how to compensate for that one?!!!

The formulas on the RC webiste are a lottle cook-booky, but they are a start. My problem is I have no way to alter the fuel map in the brain. The brain is set to run the lower flow injectors under NA conditions only.

I'll still be rich on the NA side, and there isn't a thing I can do about that. My goal is to come up the middle, lean out a bit, yet still run on the rich side to be safe. Believe me I don't want to detonate at full boost.

I just got off the phone with Bosch and they won't give out any technical information on the injectors. The plot thickens.... I do believe I am on my own.

 

John, like I suggested in my previous post, I would seriously consider installing an aftermarket engine management system like the Pro8. All LH\EZK based 928 cars use batch injection and wasted spark ignition systems (firing four each at a time), this is just fine for the stock or lightly modified 928 but not for a TT like yours or any other heavily modified 928. You need a system that can tailor the injection pulse per injector (including the 9th and 10th injectors) and fire each spark plug at a precise moment. One of the things I like about the Pro8 over some of the others is that it can fire eight independent coils sequentially without being in a wasted spark mode. MoTech, HalTech and others only do four or six sequentially and with an added box will fire eight coils but in a wasted spark mode.

As far as getting injection specs, you will most likely need to find a friendly rebuilder willing to help you in your quest.

I don’t know which “Microfueler” you have, but I know they have been around for many years and have been used on just about every car worth modifying.

Is your Microfueler digital or analog? You may want to update this unit as well.

Can you step the 9th and 10th injector so they open up at different RPM’s or levels of demand?

Are your 9th and 10th injectors the same or is one smaller than the other?

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Max