924Board.org

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

Hey all,
I like to think I know some things about how a motor works, but I can't reason out why the following is happening

I've been trying to get rid of a stumble/misfire under boost for a while. I'll feel a couple stumbles starting when the boost ramps up (only to stock ~7psi right now). The first stumble comes on under 3500rpm, then I'll feel one or two more on the way up to ~6500.

So I'm playing around tonight and decided to bump my entire ignition timing map up 2 degrees. Voila, stumble is gone! I made a run, full boost in 1st & 2nd gears before, then the same run after adding 2 degrees timing and now I get no stumble.

Another key, I didn't get the stumble before I had the motor rebuilt (again). The no-stumble motor had rings that weren't sealing well and the cranking compression was slightly less than it is now (155psi then compared to 160psi now). Compression under boost was likely much more different if compression was leaking past the rings. Even with similar air/fuel ratio's (around 11.5-12 under boost), the old motor wouldn't stumble where the new one does.

Putting the pieces together, it looks like more static compression lead to the stumble. And more advance is getting rid of it.

I can't imagine why this would be? Although I'm happy to have stumbled(!) upon it.

I bumped the timing again another 2 degrees and logged the data. I was hoping to be able to see a difference in the slope of rpm's (tracing acceleration), but I couldn't discern a difference in 1st & 2nd gear runs. The dyno may be the only way to really tune in timing changes - and I have a few more things to polish up before then (relocating the airfilter for iinstance).

Anyway, just some interesting info to pass on. Here is a copy of my timing map if anyone is interested. This one is +4 degrees over the map that was stumbling. It seems to be working fairly well.

_________________
1980 931S
15psi boost, MS-II, EDIS, 951 IC, custom intake, Ford 5.0L throttle body, Forge BOV, WB o2, G31 w/LSD, 964 wheels, 968 rear sway, Bilsteins, 200# Welt. springs. A laptop, and a partridge in a pear tree.
1991 964 C4 Cabriolet

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

That is an interesting phenomenon. Didn't make much sense till I took a look at your table. I think the boost section of your table is just a little to "chunky". Though it doesn't really explain why the old engine didn't miss and the new one does. I do know I have had some trouble with the interpolation on both fuel and spark tables when I try to force too much space between them. You might try spacing your NA portion of the Kpa tables by 15 rather than 10 so as to give you a couple more rows to spread your boosted section out on.

Todd
_________________
'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

Min · Posted: Fri Sep 07, 2007 2:27 pm Post subject:

Do you have a datalog of the stumbling behavior? Stumbling like you descibe sounds like misfires to me, but I'd have to see a full datalog to see whats going on. Advancing your timing could make it go away for a variety of reasons that I can see, post your MSQ and a datalog of the stumble. Also note your megasquirt code revision.

Min
_________________
Custom means it didn't come from a box.
1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting.

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

Todd, interesting thought. I'll try and spread out the MAP values so there isn't so much space between boost bins.

Min - sending an email with the info. I just loaded up the latest 2.872 code.
_________________
1980 931S
15psi boost, MS-II, EDIS, 951 IC, custom intake, Ford 5.0L throttle body, Forge BOV, WB o2, G31 w/LSD, 964 wheels, 968 rear sway, Bilsteins, 200# Welt. springs. A laptop, and a partridge in a pear tree.
1991 964 C4 Cabriolet

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

Wait, your car is idling @ 27 degrees?

The reason for stumbling is "the great" 2valve head of our cars. It needs loads of advance to overcome slow combustion.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold

'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

924RACR · Posted: Fri Sep 07, 2007 11:14 pm Post subject:

Slight digression here - anyone have any idea how much advance the NA motors really like and can tolerate???

We had my teammate's racecar on the dyno last night, and after fixing a massive misfire that's been plaguing him since day 1 (turned out to be a bad distributor), we ramped up the advance to, ultimately, 46 deg total at the top end (5000rpm). I have heard that generally 42-43 degrees is what the motors really want; now I'm not so sure?

Didn't experience any pinging...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Min · Posted: Sat Sep 08, 2007 3:05 am Post subject:

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

Good eye on the voltage! I hadn't noticed. I'll try and trace down where the trouble is coming from. There is a fairly large gauge +12v that goes to the unswitched pole in the MS relay board, so I can't imagine why I'd be loosing voltage. Although I did a quick check and I'm getting 13.6v at the battery while running.

This is EDIS and the spark gap is what comes out of the box for the Bosch copper plugs. I just check to make sure they're all the same, although I don't remember what that gap is. Same as stock I believe.

What kind of voltages are you recording Min?

Also, what's wrong with 27deg advance at idle under vacuum? What's stock? It can't be that far off.
_________________
1980 931S
15psi boost, MS-II, EDIS, 951 IC, custom intake, Ford 5.0L throttle body, Forge BOV, WB o2, G31 w/LSD, 964 wheels, 968 rear sway, Bilsteins, 200# Welt. springs. A laptop, and a partridge in a pear tree.
1991 964 C4 Cabriolet

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

Stock is like 10 degrees BTDC @ 900-950 rpm.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold

'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

Min · Posted: Sun Sep 09, 2007 1:41 am Post subject:

Min · Posted: Sun Sep 09, 2007 1:54 am Post subject:

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

I run 10-12 degrees @ idle and it idles very smoothly + emissions are well below our limits, can pass the emission test easily.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold

'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

Paul · Posted: Sun Sep 09, 2007 4:06 am Post subject:

GREAT STUFF GUYS!!!!

Keep it coming, I'm almost convinced to head down this path on my 80 931S....
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Min · Posted: Sun Sep 09, 2007 4:17 am Post subject:

Min · Posted: Sun Sep 09, 2007 4:36 am Post subject:

http://www.carprogrammer.com/Z28/PCM/Ignition/idle%20spark%20tuning.htm

Doing more searching, but his conclusion is "In conclusion, my hypothesis that more timing consumes more fuel, was incorrect to a point.
It appears that more timing burns fuel more efficiently, requiring less fuel and less air at idle."

Less fuel, less air, sounds like a good thing?

Min
_________________
Custom means it didn't come from a box.
1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting.