ShawnSmith

Having just driven to lunch and back in the car, I can give immediate testimony that Greg is delivering what I asked for. Enormous piles of torque across the 2k to 5k region while retaining near stock refinement. I'm particularly impressed with how he managed to combine good breathing and a no-louder-than-stock-plus-rmb exhaust note.

Now I just need a proper LSD so I can widen the range of throttle positions that can reasonably be used at low speeds

dprantl

Interesting discussion. I haven't noticed any problems at 1,500 RPM WOT on my car with 30lb injectors and stock fuel pressure, even in 5th gear uphill. Then again, I only have a wideband down in the exhaust so have no idea what kind of mixture each cylinder sees separately.

Dan
'91 928GT S/C 475hp/460lb.ft

ShawnSmith

I think the dimension Greg is looking at is the civility of the motor at low rpm and _small_ throttle input - your stop-and-go-traffic kind of scenario.

dprantl

Yes, that too. My car is a daily driver and I typically like to shift early when in traffic. I usually keep the RPM below 2,000. I notice that at those lower RPMs I'm much more quickly into boost at part throttle. I still get lots of torque and the wideband says I'm dancing around 14.7:1 AFR and it's very smooth. I'm not saying Greg is wrong, I'm just giving my experience and hope it will help. Greg's theory seems sound IMO.

Dan
'91 928GT S/C 475hp/460lb.ft

Lizard928

Greg, I do understand your approach, and I didn't mean to imply that it was wrong, just that there is more than 1 way to skin a cat.

The main reason that I dont like to increase fuel pressure too much is mainly for heat. The more pressure that you run the more heat you generate in the fuel for this. I have as well as Louie experianced issues with the fuel getting too hot and causing a lean issue even on stock fuel pressures, there are ways to cool the fuel and running the stock fuel cooler and keeping the AC on can definetly help this issue.
I also dont really like to run more than an 80% duty cycle, as it generates abit more heat and can lead to a quicker failure of the injectors.

mark kibort

Fuel is incompressable. I have 100psi water pressure and the hose water is very cold! its not like air. How would warm fuel cause a lean effect anyway?
Better vaporization of the fuel can have cooling effects of the air/fuel charge.

mk

atb

Mark, don't be so "dense".

Lizard928

Adam that is one of the best posts of the year!!!!

Mark, whenever you pressurise something, you use energy to due so, when you transfer the energy it is not 100% efficent and heat is the byproduct. Not only that the fuel pump turning generates heat and uses the passing fuel to cool it. So yes the higher the pressure you run the more heat gets transfered into the fuel.

I am not even going to get into the details as to why it goes lean when the fuel is hot, but I can assure you that it DOES. Louie has a sensor in his passenger side fuel rail just to moniter the temp and increases the amount of fuel once the fuel exceeds 50 deg C. I also know that I see issues with the fuel getting hot and causing it to run lean (to the point of needing 15% more fuel).

Z

Just as a data point, Todd's car is currently using either 82lb or 83lb injectors. It's an automatic transmission equipped car, and I've never seen any other 928 anywhere with a better idle or off idle drivability. That's with an aftermarket engine management system on it, but even without the individual cylinder tuning that he's done on it now.

With stock engine management but SharkTuning, there are some automatic transmission equipped 928s out there with stock fuel pressure and 42lb injectors that have completely stock idle and off idle drivability quality. Since at least two of those cars have the 42lb injectors pretty much maxed out at their stock fuel pressure, there are plans to try something with bigger injectors than that with the stock but tuned engine management system.

Using smaller injectors but with higher fuel pressure is one way to increase the amount of fuel that can be delivered, but does have limitations. I'm not going to say what kind of fuel pressures have been experimented with when smaller injectors were used on a couple of cars around here, but they're pressures that I'm pretty sure nobody else is even going to come close to seeing on their 928.

Louie928

Hot fuel is less dense than cool fuel. It takes more (volume) of it or the AFR will lean out.

mark kibort

So, at 50degree C, fuel density goes down by 15%?

If this is true, it might explain why my car is running so well in the races, above its dynoed HP, as I need to be leaner and this could be happening if this theory is true.

So, you are saying that 50psi pressurization is raising fuel pressure to a little over 3 times ambient atomospheric pressure. And this creates a temperature of over 50degree C in the fluid (fuel)?




Mk

GregBBRD

You guys are kidding, right?

The heat put into the fuel by the pump, is insignificant, compared to the heat from the engine in the fuel rails. Go stick your hand on the fuel line from the pump, when the car is running. Now go grab a fuel rail.

Isn't it interesting that Porsche put the fuel cooler on the return fuel line and not on the feed line? If there was significant heat from running the fuel through the pump, wouldn't they have done this differently?

Colin:
I agree with you that there are different approaches to solving a problem. I just choose to do it this way, because it works, for me. The other way probably works fine, too.

I found out, many years ago, that if you ask 10 mechanics the same question, you get 11 different answers.

mark kibort

Exactly!

By the way folks, the density of gasoline (C8H18) is 0.694 g / cm^3 at 300 K and 0.622 g / cm^3 at 400 K. (300 K =80 F).

You probably are lucky if you see a 2% change in density for a 20 to 50degree C temp change.

The reasons you have the fuel circulating, is that you avoid the fuel boiling if it was sitting in the fuel rails over a hot engine. its density doesnt change enough to alter mixtures.

Most of the heat generated near the pump is caused by the electric motor in the pump! (and not the pressurization of the system). As Greg says, if heat was a problem, porsche would have put that cooler line after the pump, not after the rails!

Oh yeah, I forgot, I'm "dense".

mk

Louie928

Mark,
While it's true that the fuel density doesn't change "much" with the normal temperature rise you might expect in the engine compartment and also true the heat imparted to the fuel by the fuel pump isn't much either, it all adds up. Even the 2% change you cite is significant enough to do something about if you can. That 2% would change your mixture from 13:1 to about 13.27:1. When you are trying to tune and make sense of correlation between adjustments and results, that can drive you nuts. Having the fuel constantly circulate through a 180F engine compartment will raise the temperature of all the fuel in the tank a noticeable amount over a 3 or 4 hour drive. The A/C isn't running all the time. At least not here. On my engine, I use throttle position for the load signal into the ECU. I have no direct input to give air mass or fuel mass input to the ECU. Different conditions are handled by compensation tables for intake air temperature and atmospheric pressure with other adjustments for engine temperature. This for both fuel and spark timing. I was having a terrible time getting the engine to stay running after a hot start. The engine would start ok, but would lean out and die after about 15 seconds. The mixture would go to 18 or 19:1 and the engine would stall out. Normal idle mixture would be in the 14:1 range. I made temperature measurements and found that the fuel in the rails would get to 150F or so after only a few minutes stopped. If I ran the fuel pump to circulate fuel for 2 to 3 minutes before starting, the fuel temp in the rails would go down to near the tank temp. Then, the engine would run normally after a hot start. My fuel rails are large aluminum and acted as quite a thermal mass that the fuel had to cool down and fuel doesn't conduct heat very well. Insulating the rails didn't help significantly. It did give me a few minutes of hot engine off time before the hot start lean out problem happened. It didn't fix it. Since running the fuel pump for 2 minutes before a hot engine start wasn't practical, I added a fuel temp sensor to the rail and created a fuel temp compensation table. In practice, the fuel temp compensation is extremely non linear. At 40C (104F) I only richen it a couple % as compared to 0C. However at 50C, it's about 8% and up to 80C it needs 15% to 20% richer. This just to keep the mixture after a hot start close to the normal idle mixture. I know the amount of extra fuel required does not follow the fuel density formula so something else is affecting it. I surmise it is a property of the fuel injector where the hot fuel may vaporise before fully exiting the pintle orfice. Maybe it's something else. Whatever, the engine needs quite a dose of enrichment when the fuel is hot. I had always wondered why the 928 fuel rails were so thin. They can be easily damaged taking a fitting off. It didn't seem like making them a bit thicker and stronger would add significant weight. After seeing the effect of overly hot fuel on a heat soaked engine start, I now understand that the thin fuel rails are to reduce as much as possible the thermal mass. With low thermal mass, as soon as the fuel pump begins circulating fuel, they'll cool right down quickly and the hot fuel/lean after start mixture won't be a problem.

John Speake

I don't know what the injector size options are above 42# but with the stock electronics used with SharkTuner should be able to cope with up to 60#.