Ignition curves. How to judge..?

[otto in norway]

Hi

As some of you may have seen, I haven't paid much attention to the ignition curves for my rebuilt motor.
I rebuilt the engine, and in the process replaced the cams for 964's, and got higher CR. (9,8:1) But I left the diz alone, -no alterations at all.
No damage has occurred yet, and probably won't happen either, at least due to a too far advanced timing. ...If I understand this right...

I have attached some charts for my diz, (78-79 SC), and a 81- diz.
The reason for this, is that I want to show the difference between the curves of the 8,5:1 CR vs. 9,3:1. Again, I NOW have 9,8:1 CR, but used to have 8,5:1.

Can anyone tell me if I understand this right?
I think that, seen from a engine safe operations perspective, I need not worry...
To me, it seems I could get some more advance on lower RPM's, and get better performance.
What do you think?

[Paulporsche]

Otto,

Sorry I can't help you on this, but I'd like to know where you got those curves. I'm currently looking for ones for 79 and 80 SC 3L USA engines.

[otto in norway]

I got those from a workshop manual. I downloaded it online, can't remember where, though...
This manual, has listed a lot of older engines too... and If I'm not mistaking, these charts are for both european and USA versions. Other engine differences has been clearly indicated/stated. (Like california spec, etc...) But here, as you can see, they are only labeled "for 911 SC". Apparently no need for more specifics...?
My assumption may once again be wrong...?

[scarceller]

I'm not a SC expert but I do know the 84-89 Carrera cars push 25 degrees advance at WOT with no damage. My 3.2 car has custom ignition curves that push 30 deg advance at WOT with 93 octane fuel and Euro PCs. So my educated guess would be a 3.0 CIS car pushing less that 20 degrees advance should be safe.

[otto in norway]

Yeah, that's how I figured it too...
I'm running on 99 octane, so I could theoretically have more advance. I'll off course have to check how my diz behaves in real-life, then decide what to do...

[scarceller]

The area to be most careful is the WOT at max Torgue (5200RPM range) this is the most abusive area for the engine. If you push ign advance to far you will detonate or if the mixture leans out to much you may also detonate. I do think that any timing advance below 20 degrees would be safe.

The 84-89 Carrera cars can push the advance safely because the advance is computer contolled and not done via mechanical distributor advance. I think the engineers had to take into concideration that the mechanics in the distributor could get loose and cause more advance than desired. Just a guess.

[psalt]

Hello Otto,

You do realize that these charts are based on a distributor test machine. To get crankshaft degrees and rpm you must double the fiqures as the distributor turns at half crank speed. Are they ROW or US charts ? For ROW cars, the MFG fuel spec was changed, so your compression comparisons are not apples to apples. Porsche was conservative with advance, with temperature being the key factor. The 911 is a knock sensitive design, so the cooler the engine and air intake temperature, the more timing you can run for a given octane.

[scarceller]

Originally Posted by psalt

Hello Otto,

You do realize that these charts are based on a distributor test machine. To get crankshaft degrees and rpm you must double the fiqures as the distributor turns at half crank speed.

This makes more sense, I was thinking that advance less than 15 degrees seemed low but I'm not a SC owner. So, the 81 SC has ignition advance in the low 20s this sort of lines up with the early 84 Motronic cars which also are in the low 20s.

Thanks for tip on howto read the graphs.

[otto in norway]

Hey
Thanks, that was why I felt like asking... -In case I missed something! -And once again, I did!
OK, that means I really need to take this seriously. Funny you should mention the temperature thing, because it has run better and better along with the temperature going down! (It's soon full winter now) I just thought: "-Hey I guess it's just getting run-in! It's supposed to go better as the miles roll by at first..."

So -lesson learned!

[scarceller]

The air cooled cars are very diffrent from water cooled. I've just started monitoring Cylinder Head temps in my car and what an eye opener! Avg cyl temps are about 150C at part load but boy do they start to creep up under heavy load. At WOT I can see temps in the 160-170C. This is why you must be real careful with these motors. At low cyl temps they can tolerate tons of ign advance but as cyl temps go up the advance can't be very agressive.

And yes what fun these cars are in the Winter and cold temps!

[psalt]

Hello Otto,

If you look at the advance curves of the early 911's, before unleaded, low octane fuel, they had max advance in the 32-38 degree BTDC range. The later engines are all detuned based on the fuel specified for the market. ROW and US engines are different because of the fuel spec. A hemi head engine with domed pistons needs more advance to make best power. Think of the combustion chamber at TDC like an orange peel and light the fire on one side and you need a lot of time to burn the mixture on the other side. Peak advance also increases with bore size because of distance traveled. The ultimate two valve, hemi head, GP engines of the 1950's, like the Maserati 250F, used 14:1 compression, domed pistons, alcohol/benzene based fuels, to make max output at 50 degrees BTDC. Modern, four valve, pent roof engines have much smaller combustion chambers at TDC and need less advance to get all the energy out of the fuel.

[RoninLB]

Originally Posted by scarceller

monitoring Cylinder Head temps in my car and what an eye opener!

Avg cyl temps are about 150C at part load but boy do they start to creep up under heavy load. At WOT I can see temps in the 160-170C.

170C = 338F

that's hot

what do you consider WOT ?

[RWebb]

duh

[scarceller]

Originally Posted by RoninLB

170C = 338F

that's hot

what do you consider WOT ?

After a 2 or 3 2nd gear WOT runs I get temps about 165C! As I said it was an eye opener. Track like conditions will easily see 170C.

[RoninLB]

Thx Sal


I have no experience with factory 3.2's so I find in unusual. Member KTL posted that he sees 300F in his Carrera.

I have trouble reaching 300F. I could pull 2 & 3 red line all day and stay around 285F. I once had 5th gear at 5-5.5k rpm for 20 miles in Nevada and only saw 300F.

do you know if others see that 165-170C ?

[scarceller]

Originally Posted by RoninLB

Thx Sal


I have no experience with factory 3.2's so I find in unusual. Member KTL posted that he sees 300F in his Carrera.

I have trouble reaching 300F. I could pull 2 & 3 red line all day and stay around 285F. I once had 5th gear at 5-5.5k rpm for 20 miles in Nevada and only saw 300F.

do you know if others see that 165-170C ?

Just to be sure, this is not oil temp, we are talking Cyl Head Temps.

I also have a friend (Geoffrey Ring) who's a Motec cert tuner and tunes many 930 track turbos, he has seen CH Temps as high as 200C! Nearing 400F at the heads on turbo cars.

He also has seen 170C in track 911s (non-turbo).

He also said the hottest cyl is #5 because the center cyl (2&5) tend to hold more heat than the outers. But #5 has a longer intake runner than #2 and thus tends to run leaner and hotter. So the best place for a CHT sensor would be cyl #5. I'm using the stock 3.2L sensor and location on cyl #3 which is one of the coolest running.

One last thing my 3.2L is not stock, it's a Euro engine with Web-Cams and SSIs 2in2out M&K pipe, also has flowed and polished intake. Pushes about 260HP at the crank, the extra HP could be the cause for more heat. I also run WOT ignition curve more aggressive than stock.

[RoninLB]

Just to be sure, this is not oil temp, we are talking Cyl Head Temps.
-------- I monitor all 6 head's CHT. I started out only monitoring 2 & 5 as a heads up on what will happen to my oil temps. Even with a fender cooler when I saw CHTs start to rise I knew higher oil temps were right behind it.

I also have a friend (Geoffrey Ring) who's a Motec cert tuner and tunes many 930 track turbos, he has seen CH Temps as high as 200C! Nearing 400F at the heads on turbo cars.
-------- different animal but nice info.


He also has seen 170C in track 911s (non-turbo).
------- OK.. then it's not unusual. That's good news for me to know. I'm always wondering what CHT limits are when pushed. I read that aircraft air cooled gas engines routinely run 375F but they're built for it.


He also said the hottest cyl is #5
I'm using the stock 3.2L sensor and location on cyl #3 which is one of the coolest running.
------- TRE reports that #5 is usually the 1st one to blow when the engine is pushed. Local 911 shops say same thing. With my routine I find 2 & 5 pretty close to all others.

One last thing my 3.2L is not stock,
Pushes about 260HP at the crank, the extra HP could be the cause for more heat.
------- More HP definately produces more CHT heat.
That's what caught my attention. I thought you were running stock HP.

I also run WOT ignition curve more aggressive than stock.
------- Whatever your curve is part of the game is to prevent high CHTs from detonation and a noticably lower than usual CHT due to pre-ignition.

sounds like a fun engine.

[moneymanager]

I made a note of this question and answer which appeared somewhere last year.
Q... "I'm getting a VDO cylinder head temp gauge & kit. What kind of temperatures should I consider to be nominal &/or excessive after I install it? For street conditions? For race conditions?
A.... I asked the same question last year since I'm also adding a trick VDO cylinder head temp gauge set up to monitor all 6 cyls & Steve Weiner responded with the following: "Cylinder head temps will scare you at first. The redline for cylinder heads is about 450 degrees. I like to see about 325-350. Best power & fuel economy is in that range. The temps will change when you lug the motor. Lower RPMS & high loads will really push those up very quickly. This is one reason why the factory used a flat fan for best cooling. That provided the most even airflow over the heads. The other most overlooked factor regarding cylinder head temps are the rubber boots around the plug connectors. These seal cooling air into the hottest part of the engine, the heads. Its amazing how people don't inspect & replace those cheap items often enough. If I saw the temps going over 450, I might remove foot from the throttle & let it cool off with the engine running at no load." Of course, this info was for a 911..

[RoninLB]

Originally Posted by moneymanager

I made a note of this question and answer which appeared somewhere last year.
..

nice




I ran this dual gauge for awhile

[scarceller]

Originally Posted by moneymanager

I made a note of this question and answer which appeared somewhere last year.
Q... "I'm getting a VDO cylinder head temp gauge & kit. What kind of temperatures should I consider to be nominal &/or excessive after I install it? For street conditions? For race conditions?
A.... I asked the same question last year since I'm also adding a trick VDO cylinder head temp gauge set up to monitor all 6 cyls & Steve Weiner responded with the following: "Cylinder head temps will scare you at first. The redline for cylinder heads is about 450 degrees. I like to see about 325-350. Best power & fuel economy is in that range. The temps will change when you lug the motor. Lower RPMS & high loads will really push those up very quickly. This is one reason why the factory used a flat fan for best cooling. That provided the most even airflow over the heads. The other most overlooked factor regarding cylinder head temps are the rubber boots around the plug connectors. These seal cooling air into the hottest part of the engine, the heads. Its amazing how people don't inspect & replace those cheap items often enough. If I saw the temps going over 450, I might remove foot from the throttle & let it cool off with the engine running at no load." Of course, this info was for a 911..

Nice to get Steve Weiner view here, looks like 450F (230C) is the upper limit. Yes I was amazed how hot these temps can get and Steve is saying they can even get hotter.