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

Hey folks,
I'm trying to get the ignition dialed in on the 931. Looking at the ignition curves published in the Haynes book (p. 131 in my copy), I'm a little befuddled.

Just to get the two obvious hang-ups out of the way off the bat:

"distributor shaft speed" = 1/2 crank speed
"advance at distributor" = 1/2 crank advance

Even still, I can't see how the published specs are achieved:

'76-'77.5 total advance at 4500rpm = 42deg w/out vacuum hoses
'77.5-'80 total advance at 4500rpm = 41deg w/out vacuum hoses

Looking at the centrifugal advance only chart (no vacuum right?!), I'm reading off max advance of:

36deg at 4500rpm (aka 14deg at 2275rpm on chart) for '76-'77.5
29deg at 4500rpm (14.5deg at 2275rpm) for '77.5-'79

Am I reading these completely wrong?

nick

Lizard · Posted: Thu Jul 06, 2006 1:59 pm Post subject:

EDIS will work differently than the way the factory did, ask Min for a copy of his, or endwrench.
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-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

That's obvious

I want to get an idea what the stock curve looks like. I'm getting some detonation and trying to track down the root... Might as well start off with the factory ignition curve.

nick

Lizard · Posted: Thu Jul 06, 2006 2:09 pm Post subject:

ah ok,

and how is it that you seem to be the only one who gets that,

what fuel do you run, do you only fuel up at one service station?
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3 928s,

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

Ah... light bulb just came on. The total advance will be total centrifugal advance + initial advance.

For '76-'77.5 initial advance with no vacuum hoses installed should be 6 degrees.

For '77.5-'80 initial advance with no vacuum hoses installed should be 12 degrees.

Ok, that makes sense

Now how to add in the advance/retard from portion to the centrifugal graph to get a picture of total advance? I believe that the retard function gets ignored almost as soon as any throttle is given, then it goes all to the vacuum advance curve.

Lizard, there could be all sorts of reasons for it at the moment. I'm just working on narrowing the possibilities. I'm starting with a pretty conservative advance map, but getting some rattles at low load (high vacuum). My AFR's are still hitting pretty rich at ~11:1 in those areas of the map. I'm wondering if leaning it out might actually keep it from knocking in this situation.

nick

Lizard · Posted: Thu Jul 06, 2006 2:40 pm Post subject:

there is another fix for that, keep the RPMS up

although on a more serious note, how much boost are you running as well? what about your fan setup, still factory and what is your temp switch in the rad kick in temp?
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3 928s,

Min · Posted: Thu Jul 06, 2006 3:53 pm Post subject:

I would definately try leaning it out nick, its possible its not detonation that your hearing, it could be something else. Lean it out to 13ish and see what happens. My ignition table was built from scratch using the basic guidelines outlined on the megasquirt forum. What kind of spark advance are you getting when your noticing the detonation? have you datalogged the spots its detonating at? ... hows the rest of your tuning coming along?

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

Hey Min,
I can duplicate the sound at a high idle if I dial in a lot of advance. I can locate the rattle pretty easily with my ear over the engine as coming from within the motor, so pretty confident that I'm hearing knock. Unfortunately...

The datalogs don't show anything odd when it happens. I haven't let it happen too often, but when if does I'll glance at the laptop and see something like: 2200rpm, 28deg advance, and 20-40kpa. Should be well into the safe advance zone.

Fuel is premium (93 in the states). Coolant temp is rock solid at 180F (exactly where my fan switch is rated to kick in, imagine that!). Intake temps are anywhere from 111-130F (it's been HOT out here lately). Those should easily drop by 10+ degrees when I get around to shrouding the air filter from the hot radiator air, and shroud the intercooler to force more air through it. It still shouldn't be hot enough to cause this kind of trouble.

The only thing out of place are the really rich AFR's under low load. I found an article testing a 2-stroke bike engine which showed low-load detonation quickly spiking at very low and very high AFR's. I'm hoping that might be the issue.

Going to change out the straight 30 weight run-in oil for dyno 10w40 and pop a compression gauge on each cylinder over the weekend. I want to at least compare what I'm getting to stock turbo numbers. I'm curious what the flat-top pistons and my headwork combustion chamber work works out to be for compression - especially whether or not the volumes are as even as I measured them to be...

I found a dyno shop about 10minutes away too. I might get some expert ears and put varying loads to get the ignition advance sorted if I don't have any luck with leaning things out. Or even if I do

I haven't done too much with the VE tables yet.

nick

Min · Posted: Fri Jul 07, 2006 5:03 am Post subject:

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

OK Nick, here is my opinion. Just remember we all have one and some of us are one

Leave the AFR alone for now. It is safe and should actually cool the combustion reducing detonation.

I'm glad you can actually "hear" detonation this time

but you might have some old mechanic go for a ride with you and see if he identifies what you are hearing as detonation.

You shouldn't be getting any detonation at the loads you described. I ran over 30° at that load an higher for quite a while with my NA motor running 9.5 to 1 compression with no problems. Are you sure you have the trigger setup "exactly" 90° from the missing tooth? Have you checked the base timing to see if you have "exactly" 10° advance? If you are off on this a little you will be getting a false reading in your charts.

I know this isn't condusive to ultimate turbo performance but I would suggest you first setup a "centrifical advance only" with boost retard. This will keep it simple until you get things dialed in then you can start playing with the vacuum advance part in the no boost section. Keep your variable to a minimum while getting things sorted. A good centrifical only advance will perform very well so you'll still have fun while tuning!!

Todd

Min · Posted: Fri Jul 07, 2006 10:39 am Post subject:

Min · Posted: Fri Jul 07, 2006 10:45 am Post subject:

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

Sorry for hijacking here but I have to say a little on this. Min, I completly agree with the quote you posted but I think it too should not be taken as gosspil and "blindly followed". I think you can run too rich and I think 11:1 is too rich as far as power is concerned with this engine combo. 11:1 is safe though while tuning. Once it is mapped out and scaled he can start pulling fuel and timing until he reaches what he is after. In this load range he should actually be in the 14-16:1 area of AFR. There is something else wrong here other than too much fuel.

I have no way of proving it but I would suspect if you did some sort of study on engines that detonate due to too much fuel you would find they are doing so at high boost AND with high tight quench areas. Short rod ratio probably doesn't help things either. I doubt any of them do so at cruising speeds. I'm just guessing here though

Nick, it would be nice to see a shot of your advance curve or atleast the area you seem to be having some problems.

Todd

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

According to Jeff Hartman, author of the book "How to tune and modify engine management systems", at 11.1 :1 AFR, the flame speed is fastest, and falling dramatically on both sides of it, mor on the richer side. Rich best torque is is 11.7:1, and the best power is somewhere 12-12.2:1.
Note that all these values are a bit different on each engine.
But, nick, I reaaly suggest you to use stethoscope for listening knock as I already did on bass GT's thread because when you hear it already over the engine noise, it's very loud. With a stethoscope you hear the beginning of it VERY precisely and know when to step off the throttle and not damaging your engine (most likely head-gasket).
There's a saying regarding the turbocharged rotary engines : "They only knock once"

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