ptuomov

A small update. Today, I spent some time playing with the rpm specific boost control features.

Recall that the 617 dynojets of torque at 4250 rpm and 572 dynojets of power at 5750 rpm were done with the eboost2 controller set point equal to 50, which translates to approximately 14 psi boost. The boost was constant over the rev range. At 4320 rpm, the superMAF signal was 224 and, at 5782, it was 219.

Today, I did some experiments with the wifey riding shotgun. We set eboost2 with a rpm-specific controller set points. We then kicked it up a couple of notches:

Under 5000 rpm sp=40 (12 psi)
5000 rpm sp=45 (13 psi)
5500 rpm sp=55 (15 psi)
6000 rpm sp=65 (16 psi)
6500 rpm sp=70 (16.5 psi)

The boost gauge recall gave 16.9 psi max during the run, so those approximate boost numbers look about right. At 4641 rpm, superMAF only read 204 now. However, at 6244 rpm, superMAF signal stayed high at 221. There were two knocks in the log above 5000 rpm, so it seems to be pretty safe.

I am guesstimating based on the MAF signals that the peak power increased by about 9% over the flat-14-psi dyno run, while the peak torque came down about 9%. The air density computations also say that those are reasonable guesses. So, in addition to reducing mid-range knocks, the change will likely bring the peak torque to a safer level of about 560 dynojets.

With the new settings, the car seems comfortable with traction. There's noticeable wheel spin starting from 4000 rpm on the first three gears, but it seems the good kind of wheel spin since we couldn't feel the flat spot in acceleration and the car wasn't whale tailing at all.

Gregg K

Holy cow. That really is testament to the stoutness of the 928 engine. Not insignificant boost levels, those.

IcemanG17

17psi.....are you using 93 octane gas?? Do you have any type of octane booster? Thats an effective compression of close to 20-1...VERY high on street gas...but you weren't getting any knocks...so thats good...but still risky!! Assuming your last dyno was 572 at 14psi......you could be around 650+whp at 17psi

blown 87

From what i have seen the 928 folks on average are not exactly kids.

ptuomov

- I am getting some knocks, but only a couple at high rpm.
- East coast pump gas, just filled her up this afternoon. There's nothing special in it.
- The dynamic compression isn't that high since I only turn up the boost at the high rpm. The normally aspirated volumetric efficiency of stock S4 falls on it's face after 5500 rpm. I am simply compensating for that.
- It was 572 dnojets, not hp. A reasonable guess would be the current spec making 620 dynojets, not 650. How any of those numbers relate to the actual horsepower figure that the car makes is for now unknown.

Iwanna928

Sounds like you had fun with the wife, that is always cool!

Sounds like things are working really well. Are you planning on turning up the boost higher at higher RPM's or have you reached a number you are happy with? Could get risky if you go higher, but like they say, you won't know what she will take until she blows!

She must be an AWESOME RIDE!

ptuomov

Yes. The car is nice, too! ;-)

I don't think I can go hugely higher than this with what's in the car now. The main reason is that with 17 psi and 60 lb/h injectors, I am momentarily at 92% injector duty cycle between 6000 and 7000 rpm. I think the next injector size up is 83 lb/h, which is likely to be an idle adventure. The other alternative is to up the fuel pressure with either load or rpm, which the fuel pump would be up for without any problems.

I don't want to burn this engine because of detonation, but it's welcome to grenade for any other reason. So far, the signs that I am getting from the '87 engine is that it's just waking up and realizing it's true calling. It's Morse coding thru the exhaust pulses ".. / .-- .- ... / -... ..- .. .-.. - / ..-. --- .-. / - .... .. ... / ... .... .. -"

John Speake

Are the injectors switching to opening once only for every engine 2 rpm at high injector duty cycles ?

If so, you can probably push the duty cycle somewaht higher, before having to raise the top end fuel pressure.

entropy_engineering

Hey I don't know how small you can go on the pulse width, but I've got a set of 1000cc injectors in a 4 cylinder here that idles like bone stock. Perhaps the bigger injectors won't be problematic.

I'd like to see what the stock motor would hold on pump gas with water/methanol injection or running on E85. A car at my shop now is running 27 psi on e85 with no knock issues, but it takes a BIG fuel supply (we had to run two 044's and -10 line etc.).

Iwanna928

Sounds like a plan! No reason to push the fuel issue. Looks to be dialed, and she is making a boat load of power!

Time to sit back, drive and enjoy your labor! What is next, any big plans?

ptuomov

I am still running at the single mode high up, so there's some extra headroom there. Not a huge amount, since the injectors still need to be closed a large enough fraction of the time to cool down, but one definitely gains half the injector opening/closing and fuel acceleration times.

Here's another question that I am trying to figure out. With the superMAF, how is the EZK load variable related to the LH MAF signal? The "EZK Load [%]" pegs at 100 before 3450 rpm and stays there for the duration up to the rev limit. Is that supposed to happen when the "LH MAF signal" is still showing a lot of variation in the range 160-225?

[EDIT: Some extra info here on superMAF EZK behavior: https://rennlist.com/forums/5308293-post12.html. Do I need the special EZK maps, too?]

ptuomov

I think it has at least as much to do with the ECU's resolution than with the injectors themselves. We're hot rodding a 23 year old car, I bet yours isn't as old.

I've got a pump that can match two 044s at 100 psi and -10AN hard line. This thing is not going to run out of fuel before the rail (as long as I don't boil the fuel).

928autobahndreamer

Too funny.

Translation for those who didn't look this up






I was built for this ****

9x8

hacker-pschorr

Once you get much past 50-60lb injectors you are dealing with low impedeance injectors.
Low impedance injectors can be drilled down to a much lower pulse width due to a faster reaction time, which is a direct result of how much current it takes to drive them.
The 80lb (or larger, cannot remember) injectors Todd is using with Autronic are low impedance.

Some claim you can install a simple resistor pack to use low impedance injectors with a high impedance driver like the LH. Even if this were true, the stock system isn't going control them properly. A true low impedance system is "peak and hold", high impedance systems are known as a "saturated" system.

There are secondary controller packs designed to go between a stock high impedance system to properly drive low impedance injectors. I have yet to find someone that's actually attempted this on a street car.

Has very little to do with age since all 80-84 US L-Jet 928's have low impedance injectors.