928mac

Hey Mark, I'm glad you found the issues with the car. I guess I was close by suggesting to check the timing, but I wasn't thinking cam. Good catch Bill.

Mark, about the fuel pressure.
I have turned mine up as high as 60 and all it seems to do is increase the duty cycle so that you get more fuel across the entire range.
I am sitting around 50-53 right now.
good luck with your tune.

mark kibort

After 15 years, I think we finally figured it out. You were right . the knocking was due to lack of pressure. coming from 17psi, i ran it up to 35psi thiking it was so much more than the setting someone had used to "fix " the running issues. the knocking, expecially on a hot day, and just starting out was reallly getting annoying. driving a 928 like a VW bus, is not my idea of fun. So, i checked the ignition timing and it was set exactly at 10 degrees BTDC, just of idle , maybe 1500rpm to 2000rpm it jumps to 20 degrees.

so, it sounds like the timing is fine. so, i just though i would get the fuel pressure near mine, because the engines are not that differnent and should run the same, (6 liter vs 6.4liter). so, a turn on the fuel regulator adjuster and wala!!!!! no more pinging!!!!!!! fixed!!!

Good calll. should have tested this a long time ago.

Thanks for all the help. the problem now, is that it just doesnt seem that fast.
60 -100 mph is near 7 seconds. it should be near 6 seconds . the exhaust system still sounds bad, like a camaro! probably due to the S4 cams, as I have never had the S4 cams , even with the holbert car in original form. (always GT or 85 modified cams)

Speedtoys

I think it ws February 6th, that you were told to.

Speedtoys

...

Bill Ball

Mark:

I got a voicemail from your buddy that things still need some attention. I'm a bit tied up this week. Am I reading correctly that fuel pressure was 17 PSI?? 928s won't run at all at that pressure. 35 is barely enough to squirt anything as well, but I'm not recalling what injectors this car has. Also, if this car has stock brains, there is nothing to check and adjust for ignition timing.

Jeff, I don't understand your negative comments. This is not Mark's racer, which is running rather well, but about which there has been a variety of heated opinions expressed. It is a buddy's 6 liter early DEVEK stroker with limited tuning options. I reset the cams back close to stock and that seemed to fix it the day I was there. It's been quite a while since I looked at this thread, and I am surprised at the fuel pressure readings Mark is reporting. I'm not going to go back through all the posts to figure out how it got there, but I'm happy to take another look at the car when I have some time. You can join us.

dprantl

I believe that car has larger injectors hence the need to lower the idle fuel pressure, otherwise it would be too rich while running in open loop mode. I am assuming that it has stock chips, not SharkTuned chips, otherwise there would be no reason to mess with fuel pressure at all. You should be able to go as low as ~36 psi, which is what the early 32v 928's ran stock. I wouldn't go any lower though.

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

mark kibort

yeah, i cranked up the fuel pressure, after being in disbelief that it was running at all at 17psi, but it did run, but had issues. I thought my guage was leaking it was so weird anyway, the regulator allowed me to crank it up to 35psi, which was so much more than it was, i though it would be at least good enough to fix something. it did, but i was afraid to go all the way to 45psi where my car is, becuase his is only a 6 liter and mine is 6.4 anyway, raisng the fuel pressure sure fixed it up. its also funny, because now the injectors click, where before , they were pretty silent.

yes, all stock chips. its no where near as peppy as my car and is no where near as fast as the holbert 5 liter , which was 320-335rwhp. sure it had the 85 cam and a different exhaust. 60 to 80 mph for the holbert was around 3 seconds , my buddies car is around 3.6 seconds. feels very slow and i still cant stan the growling , low frequency exhaust sound. maybe its all the exhaust's fault?

Rob Edwards

Maybe it needs to be properly tuned?

Bill Ball

Yeah, we need to get a wideband into it at least. I have one I can spare. I'm tied up with serious domestic issues or I'd come down. It's going to be a couple of weeks.

Fastest928

Mark,

Built in 1996, the engine must now have over 120K miles, seen a few timing belts and likely needs much needed maintenance.

S4 cams are terrible, but check the cats..if it ran that lean that long, they are melted.

Headers and GT or S3 cams will bump the HP by about 30 rwhp to get it into the 350 rwhp range.

Best of luck to you and Bill ...

Cheers

Bill Ball

Thanks, Marc. Good suggestion about the cats.