Ignition Timing - unleaded fuels (Marquis_Rex please!)
Ignition Timing - unleaded fuels (Marquis_Rex please!)
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pentoman

Original Poster:

4,831 posts

280 months

Wednesday 8th November 2006
quotequote all
Hi all,

My Mercedes 2.5-16 has an adjustment knob for what type of fuel you are using, which I think adjusts the ignition timing via the ignition ECU. I think it may become a source of extra power for me .

The settings and information from the instruction manual are thus:

1 <- ultra conservative setting?
2
3
4
5 "N" <- for 'Normal' unleaded 91 RON (can you even buy this any more?)
6
7 "S" <- for 'Premium' unleaded 95 RON (everyday premium unleaded).

There is no setting for 97/98 RON Super unleaded.

Presumably then, this means the engine will not make any more power on Super unleaded over premium unleaded, because the ignition timing is not adjusted for it? The knob only goes up to 7 which is stated in the owner's manual as setting timing for mere premium unleaded. So if I could adjust timing to take advantage of super unleaded fuels, that should get me some free BHP, right?

The car has no knock sensor and I don't believe it has an adjustable distributor or anything for adjusting timing (the distributor can be adjusted but I am told it does not affect the timing). Is there any other way to change my timing? The car has K(E)-Jetronic injection. All I can think of is measuring the resistances of the timing adjustment knob in each position and coming up with a new position 8 just for super unleaded, and hoping that it works to retard the ignition even further than normal to get me some extra power.


Thanks for any suggestions or help,

Russell

eliot

11,927 posts

271 months

Wednesday 8th November 2006
quotequote all
if moving the dizzy doesn't effect timing then its done via a crank sensor presumably - maybe you can move the pickup point a few degrees.

pentoman

Original Poster:

4,831 posts

280 months

Thursday 9th November 2006
quotequote all
Good point, yes it does have a crank sensor which goes straight to the ignition computer. I don't think it's easily moveable though .

These are the settings for the switch position:

Engine 4000 rpm =
Point S = 18 - 22 degrees BTDC
Point N = 13 - 17 degrees BTDC
Engine idling =
Point S = 14 - 18 degrees BTDC
Point N = 14 - 18 degrees BTDC



Edited by pentoman on Thursday 9th November 12:06

Marquis_Rex

7,377 posts

256 months

Thursday 9th November 2006
quotequote all
The following is an excerpt that I posted in another thread that is useful background to my post:
“There's no rule of thumb.
It depends on the engine and how steep the "ignition loop" is.
An ignition loop is a plot of data on a graph with measured torque along the Y axis and ignition advance along the X-axis.
This will produce a curve- where the gradient = 0 or dy/dx = 0 is the point of best torque or MBT. If the ignition loop is steep you'll see alot of change in torque for a small change in ignition.
The ignition loop will be steeper for full load/WOT than at part load.
The ignition loop will be steeper at peak torque WOT than at nearer 1000 rpm WOT.
An ignition loop will be significantly steeper for a boosted engine than a naturally aspircated engine.
An engine with a high CR will tend to have a steep ignition loop at WOT.
An engine that breathes well with high volumetric efficiency will tend to have a steep ignition loop.

Those are some of the variables that effect this phenomemon"
Russel, I’m not sure how much you’d be likely to gain:
Here are some examples from measured engine dyno data to illustrate my example:
[I won’t quote any HP figures directly just deltas]
Rover K series 1.8 litre:
At 6500 rpm is slightly knock limited and seems to gain only 0.58 bhp when ignition is advanced by two degrees and a higher octane fuel is used. Hardly worth it, and would certainly be difficult to measure on a chasis dyno. Moreover it falls within the noise band of engine to engine tolerance, so you may not notice this on a particular version of this engine.
It’s interesting to note that at 4500 rpm the engine gained 0.2 bar BMEP or 1.3 Bhp for the same 2 degrees of ignition with a higher octane fuel

Honda S2000
Seems is not very knock limited- so it’s ignition loops are actually quite flat. This engine has a very fast burn and high volumetric efficiency at high rpm (115%!!)
If the ignition is advanced 2 degrees on higher octane fuel at 8000 rpm it gains about 1.2 Bhp [again it’s not something you could measure on some chasis dyno due to measurement noise and you certainly wouldn’t feel it]. It’s interesting that this engine seems to gain nothing anywhere else at the other engine speeds as it’s ignition loop response is flat again. Even at 8300 rpm nothing happens for an extra 2 degrees igniton.
BMW S54 M3 3,2 straight six:
Again this engine has been finely optimized toward MBT throughout most of its rev range- it has very flat ignition loops and so doesn’t gain or loose at low speeds.
With higher octane fuel and 2 degrees more ignition at 7500 rpm it seems to gain about 2 Bhp. There is a little more to be had at the 7900 rpm peak power speed but it’s hard to tell. Again nothing to Write home about. This engine and the Honda engine are unusual in that they’re NOT knock limited at low speeds and have very flat ignition loops. This is the opposite to most run of the mill modern VVT engines which are typically more knock limited at low speed- where there’s more time for knock to occur. My guess is that this has been done on purpose by design.
Mercedes 2.3 litre supercharged inline 4 (used in the SLK?)
I’ve thrown this in as an example of a boosted engine:
It gains 4 Bhp at 5500 rpm with an extra 2 degrees and higher octane. However it gains very little – infact loses torque for extra ignition at any engine speed up to 2500 rpm, is neutral to ignition at 3000 rpm and then gains a lot more as the Roots blower boosts more efficiently and gets into it’s stride!
These are just some examples of proper measured data under controlled conditions. Hopefully it will dispel all the bull shite churned out by hopeful "internet experts" and back street specialists.
I have no idea what the ignition loop looks like on the old Merc engine. If it’s EZK of EZL like a lot of that era Mercs were it’s likely the ignition was quite close to optimal from the factory, but if you did gain my guess is that it would barely be noticeable. My second guess is that because the engine does NOT have knock sensing the engine parameters [compression ratio, cam timing etc] are set conservative so that these ignition loops are very flat- to avoid potential damage. That was the way they did it in the 80s and before and my E21 was no exception.
Also, bear in mind that the ignition switch setting may not only be to do with ignition values but be there to compensate for poor volatility fuels- such as those sold in Arizona and New Mexico- where by the fueling may be set to rich on transients. Nowadays OEM ECUs usually have fuel volatility sensing functionality.

pentoman

Original Poster:

4,831 posts

280 months

Thursday 9th November 2006
quotequote all
Marquis_Rex said:



You are a genius, and that's the sort of precise yet descriptive and interesting reply I expected from you! Thanks .

So it's not as simple as I believed, but I now understand a lot more about ignition timing.

Did you read the fuel test in Evo magazine? The Golf GTI Mk 5 2.0 turbo had around 10-15bhp peak difference between high octane and poor "normal" octane fuels. But the new 507bhp BMW M5 showed almost no difference in torque curve between any fuel that they tested. Perhaps the M5 has a quite flat "ignition loop".

I have a plan that might tell me how much the timing switch affects performance. I have a GPS/accelerometer vehicle data logger designed for use on our race car. I will attach it to my car and record in-gear acceleration traces with the timing switch in all 7 adjustment positions. If there appears to be little difference, I imagine that I have a quite flat ignition loop too. Obviously this is quite unscientific and doesn't account for other changes the ECU makes when the switch position is moved, as you mentioned earlier.

Marquis_Rex

7,377 posts

256 months

Thursday 9th November 2006
quotequote all
pentoman said:
Did you read the fuel test in Evo magazine? The Golf GTI Mk 5 2.0 turbo had around 10-15bhp peak difference between high octane and poor "normal" octane fuels. But the new 507bhp BMW M5 showed almost no difference in torque curve between any fuel that they tested. Perhaps the M5 has a quite flat "ignition loop".

I have a plan that might tell me how much the timing switch affects performance. I have a GPS/accelerometer vehicle data logger designed for use on our race car. I will attach it to my car and record in-gear acceleration traces with the timing switch in all 7 adjustment positions. If there appears to be little difference, I imagine that I have a quite flat ignition loop too. Obviously this is quite unscientific and doesn't account for other changes the ECU makes when the switch position is moved, as you mentioned earlier.

Thanks
I haven't read that evo- it's difficult to get hold of here in Kraut land- but the results don't suprise me that much.
I would imagine the 507 bhp BMW V10 has similar characteristics to the S54 M3, and being peaky(It certainly is peaky compared to the old V8) the engine probably doesn't have very high Volumetric Efficiency at low revs- hence the ignition loops are quit flat there, and may be it's SLIGHTLY knock limited at high revs like the S54. I believe to get alot of HP/litre out of a naturally aspirated engine (amongst other things) you must specify the compression ratio such that it is slightly knock limited at high rpms.

I think you're wise to use an accelerometer, I've done alot of my BMW "Cal" optimisation using one of these.

pentoman

Original Poster:

4,831 posts

280 months

Tuesday 21st November 2006
quotequote all
The car is going on a dyno this weekend (not for any reason, just because some friends are going).

Would it be possible to get an idea of the ignition loop on that? If I set the switch to the different positions mentioned above and did a run in both those positions (or all 7 positions if I had the time and the ignition values for each position!) I could at least see the difference in power?

I've started another thread about the dyno day..

It is a Volkswagen VR6 owners day and I am expecting the little old 2.5 four-cylinder to show up some of those big V6s!!

Marquis_Rex

7,377 posts

256 months

Tuesday 21st November 2006
quotequote all
pentoman said:
The car is going on a dyno this weekend (not for any reason, just because some friends are going).

Would it be possible to get an idea of the ignition loop on that? If I set the switch to the different positions mentioned above and did a run in both those positions (or all 7 positions if I had the time and the ignition values for each position!) I could at least see the difference in power?

I've started another thread about the dyno day..

It is a Volkswagen VR6 owners day and I am expecting the little old 2.5 four-cylinder to show up some of those big V6s!!


I don't know if you'd notice a difference on a chasis dyno, possibly.
I guess you could try it, but you would have to know what the ignition value was against the recorded torque