LT Texan

I started scratching my head about my ignition timing on my '78. I have the stock distributor and set the timing stock at 31 degrees advance at 3,000 rpm.

But, what I have is a 4.7L short block from an '83 Euro S and '86 Euro S heads.

Everything else is stock, '78 cams, distributor, CIS, intake runners, etc, etc.

The resulting compression ratio is just over 10:1. (I have the numbers somewhere, but not at hand for the actual number.)

So, I'm thinking, 31 degrees advance is crazy for a 10:1 compression ratio engine with a very good design on combustion chamber.

So, I figure I should see where the '83S and '86S are timed. Well, the '83S Euro is timed at 26 (23 for Austrailia) degrees advanced at 3,000 rpm, and the '86S is not published due to the engine management system.

So.....I figure it makes much more sense to time mine more like an '83S.

But the first thing I did was figure out how the stock motor is timed. I was pretty surprised that the vacuum advance is actually a vacuum retard! I was shocked when the RPM decreased when i reconnected the vacuum line.

Here is the timing advance stock (I had it off a couple degrees):




I went ahead and set the advance without vacuum retard as follows for now:



It seems to run better.

I'll need to look into a distributor for an '83 Euro S soon.

123quattro

It does that so when you are at WOT it's retarding timing. My car sounds pretty similar to yours. I have a 84 US 4.7 with Euro heads/cams and a US distributor. I think my timing is set at 33* static. It's never knocked in 100F weather. The wedge heads aren't fast burning. I'd leave the timing and listen for knock at 4000rpm on a hot day to make sure you are OK.

LT Texan

Not really, at wide open throttle, it STAYS at 31 degrees advance.

Only with vacuum, does it retard the ignition and that only happens under 1,500 rpm.

LT Texan

And the heads combustion chambers on my '86 ero s looks just like the compact design of the Chevy fast burn small block heads.

upstate bob

those are pretty good numbers with the vac hooked up. I'd leave it operational and set at 28BTC @ 3000. Knowing your CHT would be nice.

LT Texan

CHT?

Ok I found it. Cylinder Head Temp. Beats me how to measure it.

Bottom line is I need dyno runs to confirm this is a good change.

I'm leaving it timed like this for a while and then consider sneaking it up to 26 degrees.

LT Texan

I corrected the timing stated in the original post for 83 Euro S'.

The original number I used was for the US 83.

V2Rocket

Not completely apples to apples, but in my 944 I am running 29* at WOT from 2800rpm to redline, with 9.5 CR on 87 octane (AKI) gas. No pinging, WOT fuel target is ~12.7:1.

A few months ago I was sometimes pinging at 35* on 91 AKI.

The 944 head should be "pretty close" to the ignition requirements of the 928 Euro S head.
Although with the smaller chamber of the 928 Euro S head and higher CR you should be able to get away with a little less advance.

LT Texan

Thanks for the data point Spencer.

I did get some pinging at 33 degrees advance at full throttle on premium.

So, backing it off a bit was necessary.

And I am running about 12.5:1 A/F at full load, so not lean at all.

What I am most concerned about is the amount of pressure I'm making before TDC. With the higher compression ratio and compact '86 Euro S heads, it just doesn't make sense to me that it needs 31 degrees BTDC.

At 33 degrees advance, the engine felt a bit burdened. It feels better at 21 degrees. I know that is way too subjective, but that's all I got for now.

So maybe in a few days, I'll move the advance to 26 degrees and see how that goes.

V2Rocket

You have higher CR, smaller chamber, and a smaller bore, so you should need less advance than my engine.

When I was getting pinging at 35*, on the dyno I took out I think 4-5 degrees in the offending areas and re-ran the test...same peak power/torque numbers across the board but the curve was smoother (before the pinging had made the lines jiggly). And the car drove just as well, but quieter

I think Porsche got the ignition timing right for the ROW stuff, because they didn't have to deal with emissions that killed the US-spec stuff.
And remember the Aussie stuff was de-tuned ROW stuff, because Porsche was anticipating poor-quality gasoline there. So taking 3 degrees advance out of it was probably the right answer there.

Since the gas is good now, and you're in the US, 26 or maybe a smidge more (if you can get 93/94 octane locally) is probably as good as it gets.