Ignition timing vac advance

[Neil Potter]

So I have removed the dizzy on my carb turbo and put in a Nodiz electronic ignition. Works very nicely now but I haven’t yet got any vac advance programmed into the map, mainly as I don’t have a laptop with a decent battery.

Question:

If I cruise at 70 say, which previously would have had extra advance dialled in from the dizzy vac pod, do I risk causing damage longer term or just wasting a bit of fuel? I have an idea from somewhere that the extra advance preserves a cylinder bore oil film?

 

 

 

[910Esprit]

The inlet manifold will be at positive pressure at 70mph.  Therefore a true vac unit would only be in significant effect on overrun or idle.   i.e. it doesnt sound too risky to run without it.

However, double check the manual as some of these 'vac' units may act on positive/negative pressure and have other valves in circuit to modify the advance curve in certain circumstances, so you need to be sure for your specific model

 

 

[Neil Potter]

That’s really interesting, thanks.  I honestly thought there would be vacuum at cruise, but sounds like the vac capsule doesn’t really add much. It has a neutral effect at warm idle for sure as both sides of the capsule get manifold pressure unless the cold start valve applies vac pump vac to one side to increase advance. Maybe it has some effect on the overrun briefly, i thought it was there for emissions and engine longevity. No matter, i’ll stop worrying.

I have a very slight hesitation at cruising speeds when varying power slightly which wasn’t there before and I have been putting down to lack of vac sensitivity in my ignition map.

Really need a rolling road session...

[snowrx]

I think you will find that there is indeed some vacuum at cruise. I'm on the wrong laptop to build a larger map, but here's a plot of Manifold pressure (Kpa) vs rpm for a 910 turbo. Everything below the horizontal white line is vacuum, and the blobs around around 2800 rpm are typical cruise. I can't speak to what an '88 offers or needs for cruise timing, but typically timing is advanced for the less dense mixture running against partially closed throttles.

I never really found much info on OEM timing tables when I was building a map for my car. Hard to separate the OEM emissions related strategies from the best performance settings when you do find hard numbers. 

[910Esprit]

That graph is art....   For clarity, can the graph plot RPM/KPA only?  Is the data collected in true 'on the road' conditions including rolling resistance and wind resistance?    If so, what RPM do you have where the car transitions to positive manifold pressure?

Edited May 31, 2021 by 910Esprit

[snowrx]

910Esprit, you've dragged me way off topic,

Graph is a scatter plot output from MegaLogViewer HD, using data from my Megasquirt ECU generated as I drove. It can graph any data collected any which way, with calculated or filtered options, histograms, whatever. A real time sink.

Positive manifold pressure depends on multiple factors, such as throttle position, load, rpm, and gas flow though the  turbine wheel, there's no set RPM that goes positive. At my elevation ~93 KPa is static pressure, anything above that is the turbo starting to add MAP. So the steeply climbing tracks in the top left of the graph below are full throttle runs up to the boost cutoff. The manifold is seeing positive MAP starting at 16-1700 rpm with lots of throttle, but without it it generally holds some degree of vacuum.

This is not a stock motor however, I'm running a different exhaust, turbo, intake manifold, pistons etc.  A mild tune, but not exactly what you'll see on a OEM car.

To illustrate Megalogviewer, here's a different graph; turbo out temp vs manifold air temp after the  charge cooler. You can pick your axis and displayed values. 

 

 

[snowrx]

Back on topic, here's an general example of how I set up my timing, assuming full "mechanical" advance of 29-30 degrees at 3000rpm and trying to add some vacuum advance proportional to the capsule that came fitted to my car. Timing also pulled out with increasing boost, and an extra degree or two below idle speed to help idle recover from dips.

This is an old map, currently I'm running 10 degrees around idle rather than the 13 shown here, and have backed off some of the torque peak numbers due to some knock.  Don't use this map on your car! It likely won't apply and has not been developed on a rolling road! 

[drdoom]

I believe Thomas is correct in regard to this. Any small automobile well powered and in good fettle should cruise 70 with minimal throttle opening, as the HP required will be small. Boost and load ( i.e. power output ) are utterly correlated and an Esprit should be breezing along effortlessly on a level road at 70. Less dense cylinder charge burns more slowly, indicating earlier spark initiation for max combustion pressure at the correct point ATDC. 

Cheers

[910Esprit]

Yes, I know, as I own one.  Also modified with HC Head, increased compression, Alunox manifold with slightly tweeked valve throats and manifold   However, there still becomes a point, on level ground, with steady throttle when wind resistance and other forces take effect and you are into positive boost to overcome that resistance.   For my particular car, I can reach circa 80/85 before I need a visible 1/2psi boost to simply maintain that speed.   I therefore reasoned, apparently incorrectly, that leading upto that speed the boost/vacuum condition must be pretty neutral.

 

[snowrx]

I was thinking in terms of day to day legal cruise conditions for an ignition map. Next drive I'll have to see what the level road breakpoint speed is! 

[drdoom]

As speed increases wind drag becomes ever more the substantial component of engine load, and the power to offset is exponentially related. To maintain equilibrium one needs a significantly greater increase going to 85 from 70 than in going to 65 from 50. Minimal boost at 80-85 would seem reasonable.

[Neil Potter]

Interesting technical stuff, thanks.  I have seen info elsewhere suggesting max advance from the vac capsule was up to another 20 degrees on the 910 HC carb turbo, which seemed high to me.  
 

Looking at Thomas’ map in general terms it looks like vac or boost moves advance up 5-8 degrees/retards it around 5 respectively.  It hadn’t occurred to me that any adjustment would help under boost conditions, as this isn’t catered for in the stock setup.  I guess this may be the result of your injection settings though.  What

As long as cruising without extra vac advance doesn’t cause any actual damage I’m happy, but sounds like a wise move to dial a little in until i can get to a RR session.

[cammmy]

Does the carby engine have boost retard or is the base timing set for full boost with only vac advance? If there is a boost retard feature, you absolutely want that.

For vac advance I can't see why not having it would remove oil from the bores. The only detriment I can think of is reduced economy and more heat in the exhaust valves/turbo. For the latter I can't imagine it would even approach the temps seen under full load, so doubt it's an issue. Though I will note, when you really load things up, retarded timing is safer for detonation but it's still not good for the engine, for the reasons mentioned above.

[snowrx]

I think the main benefit of vacuum advance is going to be better fuel economy and maybe lower EGTs at cruise.

Lotus is all over the place on distributor advance settings, I've seen 18-28 degrees of vacuum advance stated for different 910 OEM tunes (those that used a vac capsule at all) and some used dual (advance/retard) capsules for boost retard or idle management.  I figure the USA settings have more to do with emissions than what the engine wants.  I don't think I've seen the maps from the later ECUs graphed out.  Or a full dyno optimized 910 map in a MAP/RPM format. 

 I could probably run more advance at low loads, given that I'm adding in 10* when the factory capsule was 18*.  My boost retard is just from some rule of thumb from the interwebs, x degrees retard per pound of boost. My idle timing is higher as my particular setup seems to idle most stably there right now, I'd like to set it lower in the future. 

 

 

 

[snowrx]

@910Esprit

Too much Friday traffic to get any good data, but can confirm that at about 21% throttle and 80mph I'm looking at almost .5psi boost. But 2% more TPS and it shoots up to 3psi.  Lotus Smiths gauge showed zero psi at 80mph. Need more road, or a different technique to try to hold a steady minimum boost speed.

[cammmy]

I have a dyno tuned 920, though I can't remember if I pulled the latest map to the laptop or not. I could post that if you're interested? It's not a stock engine though. Full exhaust incl. manifold, vernier pullies, GTX2863R turbo.

[snowrx]

5 hours ago, cammmy said:

I have a dyno tuned 920, though I can't remember if I pulled the latest map to the laptop or not. I could post that if you're interested? It's not a stock engine though. Full exhaust incl. manifold, vernier pullies, GTX2863R turbo.

Sure, I'd like to see a copy, I'm always curious. I saw another map but it did not use MAP as the load factor. None of my local dynos is a load holding type suitable for finding best timing, and every time I think it's time to go further afield to find a dyno, I change something on the engine that will change the tune anyway.

[snowrx]

I tried speed vs boost again, and came up with kind of the same results, and quit at 100mph when the MAP wasn't climbing very fast towards positive.  Upon reflection, I realize I'm fighting my electronic boost control which is currently set to give boost proportional to TPS, so at steady cruise and throttle below 25% or so, the ECU is not allowing the boost solenoid to close the waste gate, because I don't have "my foot in it".  So a normal car might well hit boost at cruise where mine knows I'm not accelerating and gives me naught. 

 

[cammmy]

Okay cool, I'm working on a different car at the moment but I'll  try pull that down soon. Mine was tuned on a hub dyno.