Marelli S144C Distributor in High Compression 1608

[ScoopMan]

I am about to rebuild my 1608 with NOS European spec pistons, which I believe will increase my compression ratio from the US 8.5:1 to 9.8:1. My car came with a Marelli S144C distributor which is in OK shape so I was going to reinstall it. As far as my research goes, this unit is set with a static advance of 5 degrees and a centrifugal advance of 31 degrees for the 8.5:1 version. Should I worry? I have an old manual that states mechanical advance for the 9.8:1 1608 at 24 degrees. Seems like a big difference. Any thoughts? There is always the fallback of the 123 distributors with the assortment of different advance curves but those are kind of pricey.

[18Fiatsandcounting]

Scoop, no need to worry. From what I understand, the ideal ignition advance for a gas engine at several thousand rpm is 36 or 37 degrees TDC. It all comes down to how fast the fuel burns relative to the speed of the pistons. That being said, there are differences driven by fuel economy, emissions, intake/exhaust timing, compression, engine life, driveability, who's on first, etc. So, for my cars, I've always adjusted the ignition timing to be 37 degrees BTDC at 3000-4000 rpm, which is where it matters. Quite frankly, I don't really give a darn about the ignition timing at idle, because that is not where the engine spends most of its time.

-Bryan

[SteinOnkel]

Will 36 degrees of advance cause detonation on an engine with 9.8:1 compression ratio?

Hmmm this is a tough question to answer. It depends on many factors, including intake air temp, air/fuel ratio, engine load, piston swirl & tumble, intake/exhaust timing etc. You get the idea. It's hard to make blanket statements when it comes to timing. More is not always better, but generally speaking more results in better economy and more torque. MBT (mean best torque) is NOT achieved at the maximum timing before detonation, but usually 2-3 degrees before.

The good news is that the engineers at the factory figured all of this out for us by blowing up various test engines and determining what works. The bad news is that the engineers decided you only get 24 degrees (29 w/ idle advance?) with a different distributor.

The way I see it, you've got three options.

1) Acquire the stock distributor for that engine. That's easily your safest bet, but you may be leaving a few mpg and foot pounds on the table. 29 Degrees does not seem like a lot*

2) Say "Damn the torpedoes!" and run the distributor you have. Risky, but it will go like hell, I guarantee you that.

3) Buy a programmable ignition and make your own curve (don't get a 123. They suck and there are far better options out there). Ideally, you would then take it to a dyno tuner and have someone map the ideal curve for you. Almost nobody does this. it's usually pricey and who's to say the dyno operator doesn't blow your engine up? These guys don't work on old engines often, if ever and will be reluctant to do so. Making your own curve is possible and I'd encourage it, but your mileage may literally vary.

*Again, dependent on several factors. If you can only get crappy, low octane fuel (like here in CA, where we only get 91) it might be your only option. I've got a 11.5:1 cr engine in my other car w/ programmable everything. It's an eu-market-only engine and they have really nice fuels. Stock advance map is very aggressive and taps out at 38 degrees total advance. My own map dips set at 32 degrees most of the time and very carefully ventures into 35 degree territory during extremely light load cruising only. Full throttle is more like 29 degrees advance. Now, on the racetrack I can get 100 Octane leaded fuel. That's when I load up the stock advance map and the difference is very noticeable. I don't have actual data, but my butt dyno tells me it's somewhere in the region of +10% MBT/MBP.

I would carefully suggest Option 2) for now. An engine can handle a bit of knock and all engines will and do knock. It's audible knock that we concern ourselves with. That's what you should be listening to. Fill it up with premium, take it up a steep mountain in a high gear and very gingerly open up the taps at 2.000 rpm in your highest gear. This will be where your engine is most prone to knock. If it doesn't knock here you are likely in the clear. If it does, back off immediately and look at options 1 and 3.

Cheers
Steiny

P.S:

In this video, Jason talks about super knock: https://www.youtube.com/watch?v=LgNf5GaR73A&t=334s
Although it's about modern engines, a lot of what he explains applies to your engine as well.

[chrisg]

what was the base timing for the old manual to which you referred? I would expect it would be about 10-12 degrees BTDC which would put you at the same spot for max advance.

[18Fiatsandcounting]

what was the base timing for the old manual to which you referred? I would expect it would be about 10-12 degrees BTDC which would put you at the same spot for max advance.

My cars (engines) are admittedly older and with stock compression ratios, but I ended up pretty much at the point you noted: 10-12 BTDC at idle, which with 24-25 degrees of centrifugal advance, put me somewhere around 36 BTDC at full advance (at 3000 rpm and above).

My cars have only been for street use, so I defer to Steiny on the best setup for race track performance.

-Bryan

[ScoopMan]

what was the base timing for the old manual to which you referred? I would expect it would be about 10-12 degrees BTDC which would put you at the same spot for max advance.

The manual I have lists the 1969-70 1608 with a CR of 9.8:1, static timing 5 degrees and centrifugal 24 degrees. 1971 has a CR of 8.5:1 with static at 5 and centrifugal at 31. Therefore maximum total advance is 29 degrees for the higher compression earlier version and 36 for mine. The distributor number is not specified, only that it is Marelli. The manual was printed in 1971 in New York, and is pretty excellent in most regards. I also have the Brooklands manual that is commonly available and a Chiltons from 1974 that is kind of lame, as well as Brad Artigue's excellent book. And the interwebs of course!
Another pretty cheap option I am considering is a new electronic distributor for the FI cars, that has 22 degrees centrifugal advance. This 'modern' one has vacuum advance also, while mine does not. No biggie, I can drill a hole if I need to.
Not sure I want to go wild with programmable ignition, not even sure what is out there for our old cars. Want to keep it simple and analog as possible. Beside cost, I am nervous about the 123 as far as how well it might last, subjected as it will be to heat, moisture and whatever else.
Thinking I could also just set static at 0 instead of 5, then maximum total advance would be 31 degrees. I agree that the static setting is less important than where maximum total advance ends up at high engine speeds. I am not planning on driving this car in stop and go rush hour traffic, that is for sure.
I might also see if I can alter the maximum advance by limiting the rotation somehow, but have not taken it completely apart yet. Only looked in from above, and can't really see how it works from that viewpoint. Need to remove the gear and pull the shaft out. Maybe later....
I would love to know the advance specs for the various Marelli units that were fitted to our cars. I have looked but have not really found anything completely definitive. It would be cool if I could find out the model number of the older unit having less advance.
Thanks!

[ScoopMan]

Steiny, that was an awesome response, thanks. The 123 seems like a semi-high tech digital response to an old school analog problem. In other words, a compromise.
I am far more concerned with keeping things pretty bullet-proof rather than going Full Guy Croft for maximum power at the likely expense of longevity.

[18Fiatsandcounting]

Not to poke at this hornet's nest any more, but I'm figuring that each individual cylinder will require slightly different ignition timing for optimum performance, based on subtle differences between cylinders in compression, chamber temperature, A/F mixture, valve timing, etc.

My point is not to make you pull your hair out even more, but just that ignition timing is one big compromise based on a slew of factors that can differ between engines, and even within individual cylinders in a given engine. After all, how many of us correct for differences in spark plug wire length (resistance) between the different cylinders?

OK, time for me to get back under my shell...!

-Bryan

[ScoopMan]

I hear what you are saying 18fiats, and we are indeed talking about compromise. I am going on the assumption that this thing will be happy enough on e10 91 or 93, and that I will not have to fuel up at the local airport. My understanding is that 9.8:1 CR should be OK, as long as the carb is appropriately sized and set up properly, and ignition timing stays in a safe zone throughout the rev range.
I have read too many posts about the guy with the totally built killer engine that runs like crap and he can't figure out why. We do not have an OBD to plug into to read engine parameters in real time, instead we get to turn little brass screws this way and that until it sounds better, and look at our plugs now and then. We can do some crude calcs with assumptions that can be way off to try to get in the ballpark with some component like a cam, carb or a distributor, but in the end there is still trial and error, probably mostly the latter. Especially for someone at my level!

[SteinOnkel]

Not to poke at this hornet's nest any more, but I'm figuring that each individual cylinder will require slightly different ignition timing for optimum performance, based on subtle differences between cylinders in compression, chamber temperature, A/F mixture, valve timing, etc.

-Bryan

You are absolutely correct, which is why a modern engine will have coil on plug ignition. And a high-end one will have not only one knock sensor per cylinder bank, but rather one per cylinder. That way fuel and timing can be trimmed in accordance.

These are all features that my Derby's ecu has, but I opted not to use. I run wasted spark and one knock sensor. Fuel trimming is disabled, because I can't be bothered to add more wires, have more fine tuning etc etc.

[SteinOnkel]

Steiny, that was an awesome response, thanks. The 123 seems like a semi-high tech digital response to an old school analog problem. In other words, a compromise.
I am far more concerned with keeping things pretty bullet-proof rather than going Full Guy Croft for maximum power at the likely expense of longevity.

The 123 is the biggest detriment to your longevity. Not because of tuning, but because there seems to be a pretty major discrepancy between commanded timing and actual timing. The inaccuracy likes to increase proportional to the rpm :X

If you go programmable, I would check out megajolt. That way you can bin the dizzy completely (and all it's drawbacks) and run basically Ford EDIS. Which is an absolute bulletproof ignition system.

For now, I would source the distributor for the 9.8:1 engine and call it a day. Also, a richer mixture is a colder one and prevents knock. You could tune your carburetor slightly on the rich side. But not so much that you get carbon deposits, as these are a main source of knock.

Cheers
Steiny

[18Fiatsandcounting]

Awww, guys, put the top down on a nice day like today (at least where I am), drive your Fiat, and do not question the spark and its advance. Trust me, you'll feel better. Karma, that sort of thing.

-Bryan

[SteinOnkel]

Stock compression ratio for the win. Sure, it doesn't make any power, but it is indestructible

[geospider]

Bryan,
I do as you say as often as possible (including driving all around after You, me, Steiny ans Greenspider hung out Sat
The nest day at Ace hardware (of course I take the spider out on errands that dont require 10' lumber) and older guy asked "how fast does it go" I basically said, fast enough to be fun, esp around corners. The joy of our Spiders

Geo

[chrisg]

the questions about how fast they go indicate the inquirer does not understand what makes these cars wonderful.

interesting discussion, folks.

I'd say - sorry, without a whole list of detail - there are also a plethora of ignition system options from the European or Australian folks...try T124 or the like, perhaps? You could have a true Plex Marelli, a block mounted unit, a block mounted Marelli electronic (via Beta), a block mount w/ a Bosch 009-oriented cap triggering a modern unit, and of course all of the other options.

I'd presume 99% of our cars do not warrant the level of detail they *could* have to be perfect, but I'd also presume that putting all of the parts together does not have to be so hit-or-miss. Engine math and sharing info can go a long way. At the end of the day, just enjoy the car. The rest is just luxury problems.