Spark map

[TheRecklessEngineer]

Some of you may be aware I've been 'squirting a Sebring kit car. This has a Nissan L28 straight 6 engine, 2.8l with twin SUs and dizzy in standard form. So, I've been reading up on spark table tuning. There seems to be a lot of information on this forum about the physical aspects of fitting MS, and tuning the fuel table, but very little in relation to spark. I thought I'd post up what I think I've learned and let you lot shoot me down correct me.

So far, I've read the bit in the MSExtra manual, this thread in the msextra forums, and I've also looked at maps from people who have squirted these engines before.

All of them are (more or less) suggesting the following:

Start with a diagonal line bottom left->top right increasing advance with RPM. This is what you'd expect - engine runs faster, mixture burns at a constant speed so you need to ignite it earlier in the cycle so that peak pressure corresponds with TDC. From what I've read, it is usual to get your advance 'all in' by 2800/3000 rpm. So a rapid slope to begin with, then tailing off towards top RPM.

A richer mixture burns faster than a leaner one. Therefore you need to retard the timing slightly in places where you want to be running richer - i.e. high engine loads (towards the top of the map). Conversely, towards cruising sections of the map, you want more advance - this enables a weaker mixture to be used resulting in better economy.

On overrun, more advance yet again as the mixture will be leaner still.

There is also the suggestion that just below your idle speed, you should add a bit of advance to help the engine lift in RPM should it ever get there.

Symptoms of the engine being too advanced in any particular place are pinking/detonation/knock. If this occurs the mixture should be leaned, or the timing retarded. In the case that you are tuning cruise, you can lean the mixture a bit, then further advance the timing until the engine knocks again. Repeat until you find the limit of advance/lean mixture, then back off a bit.

Too much advance may also result in 'surging' where the engine tries to accelerate then drops back. To correct this, you could add more fuel (which may result in knocking) or retard the timing slightly.

With this in mind, I've guessed at a map for this Nissan engine. It isn't too different to what other people are using (albeit on highly tuned and turbo'd versions of the same engine). It looks like this:

Now this is what I'm using on my 3.5 RV8, courtesy of HFH/FF.

What is immediately noticeable is the direction of advance down the left hand edge of the map. The RV8 map retards the timing towards load. This has me stumped as it goes against what I've learned.

Can anyone shed some light on this, or add some more advice on tuning a spark table/correct me?

[Bowie69]

Is it likely to see high load at < 1000 rpm? TBH, those points of the map mean very very little, and wouldn't worry about it too much.

[TheRecklessEngineer]

Hmmm...I've got my explanation backwards. Knew I should have read it more closely!

The table I have created for the Nissan is loosely based upon tables I have seen from similar engines - albeit much more tricked up ones.

The direction of advance for the V8 and the Nissan is different. The Nissan map is advancing timing towards WOT, whereas the V8 table is retarding it. Why the difference?

[Hybrid_From_Hell]

I have created spark maps from engines I didn't know jack on by

disconnecting any vacumn advance, and reving with a dynamic timing light and getting base

spark at 500 rpm set points, sometimes its then easy to find out the max mech / vacumn advance, and then

build a table from that as a start point.

Go easy on spark, too much will hamper performance and damage engines fast, too little is sluggish and

flat, to be sure you then need a rolling road, and a knock sensor to help tune spark, which

is both time consuming tricky and costly

Hope the above helps some

Nige

[FridgeFreezer]

Bear in mind MS can use a knock sensor if one is available.

[Paul_VR6]

That Nissan table has some strange things going on, and without knowing that particular engine it's hard to say why.

In general you can start with an advance 'curve' or ramp in the 100kpa bin that will increase in timing until it peaks around 3-4krpm. Then as you go down in MAP you can add timing, with timing peaking in the mid rpm low load ranges. Sometimes there are 'funny' spots during actual driving using this logic and you need to add/reduce timing for knock or to make the motor more responsive.

Remember also that many tables that come from computer controlled stock ECUs have advance that is balancing emissions requirements with power and drive-ability. This ends up with some *very* strange timing numbers on some engines.

[zoltan]

Hmmm...I've got my explanation backwards. Knew I should have read it more closely!

The table I have created for the Nissan is loosely based upon tables I have seen from similar engines - albeit much more tricked up ones.

The direction of advance for the V8 and the Nissan is different. The Nissan map is advancing timing towards WOT, whereas the V8 table is retarding it. Why the difference?

Assuming you are mapping MAP against RPM as per RV8 tables, where are you taking your MAP vacuum from?

On my 3.5 RV8 I initally had the MAP sensor pipe connected to the takeoff by the throttle spindle and it effectively ran backwards as you describe. Error discovered and pipe connected into the plenum and the map ran the right way round again