Ignition Timing

[chris52]

I'm a little confused on where to set the timing.
I have heard people quote 8deg BTDC 12deg BTDC at idle vac pipe disconected
Also 28deg or 30deg BTDC at full advance (over 3.5krpm).

So what is the best way to set it.

Any help would be great

Chris

[Bluebottle]

TVR default setting is 28 Deg. advance at 3/3,5k rpm with vac hose disconnected, which equates to about 12Deg. at tickover as the (tvr)dissy full advance is 16 Deg. from memory which it reaches around 3-3.5k rpm.

I should also point out that many TVR's have been known to have the TDC marker out of position, so you should check to make sure the marker is in the right position first unless you know it to be correct already.

[chris52]

Cheers Bluebottle.
Found true tdc (was 2deg out)
Set the timing at 28deg BTDC at 3500rpm with the vacuum advance disconnected. Idle is 12deg BTDC. All good I thought but then you connect the Vacuum advance pipe and it jumps up to 40deg BTDC at 3500rpm Idle still at roughly 12deg. Does this sound right? If I try set the full advance everything connected to 28deg BTDC the idle is very rough as its only got about 2deg of advance.

Any comments and advice would be great.

Chris

[Bluebottle]

chris52 said:

Cheers Bluebottle.
Found true tdc (was 2deg out)
Set the timing at 28deg BTDC at 3500rpm with the vacuum advance disconnected. Idle is 12deg BTDC. All good I thought but then you connect the Vacuum advance pipe and it jumps up to 40deg BTDC at 3500rpm Idle still at roughly 12deg. Does this sound right? If I try set the full advance everything connected to 28deg BTDC the idle is very rough as its only got about 2deg of advance.

Any comments and advice would be great.

Chris

Your timing with vac advance disconnected sounds about right, but I don't think the vacuum advance of 40Deg. is right. i'm sure someone more knowledgeable will confirm what it should be. In the mean time running the car with vac hose disconnected will not do any harm, just tape over the plenum outlet pipe. the vacuum advance is only there for fuel economy on cruise which is why many run their car with it disconnetced.
As a thought Have you had the rotor arm off recently? if so, take the cap off and twist the rotor arm. it should only turn about 1/4 inch and spring back to position, if it turns more and is not spring loaded then one of the advance springs may have come off.
(I've had to refit mine a couple of times under the rotor arm theres a plastic nut the holds the spindle down the nut on mine is split and unless you hold the spindle down when removing the rotor arm the spindle come up with it and the damn springs pop off the balance weights)

[chris52]

Cheers for the feedback. I decided to test the car with the vacuum pipe fitted and it drove better than ever practically no shunting very smooth acceleration dont think it has increased the power though. I listened for pinking and knock but couldn't hear anything out of the ordinary. Temps were good at a steady 80 (still a bit cold here). The fuel economy has definatley risen and I was giving it some on the country roads, did about 60 miles in total. I will see how it goes when the weather warms up and if I hear any odd noises I will try without the vacuum pipe fitted.

Thanks again for the help

Chris

[TVR Beaver]

Did you ever test it with the pipe on?.. Was 40 deg correct???

[chris52]

TVR Beaver said:

Did you ever test it with the pipe on?.. Was 40 deg correct???

From memory John I just left it at 28deg at 3500 12 Dec at idle Vac disconnected only when setting up.
Im not sure what I have now as I have the Omex on and that was set up by Mark Adams.
Chris

[Sardonicus]

It will jump up to around 40 at high engine speeds with vac connected you have no load on the motor so you are basically simulating cruise (no load) many of us with after market ECU's are dialing in 40/45 advance on light cruise or very small throttle openings even though we max out 28/30 total advance

[TVR Beaver]

Great... obviously it has to go further with the vac on, but not sure how much it added.... another 12 deg then from vac of fully advanced position ....
always worth checking all basis

[ChimpOnGas]

TIMING & THE DISTRIBUTOR

• Set the initial timing at idle with the vacuum advance disconnected to 12 degrees BTDC, actually the way the vac advance is plumbed on our cars means it has little or no effect at idle, but more on this later

• As the distributor spins faster it progressively adds timing, by the time the engine is spinning at 3,250rpm the distributor is all done having added roughly 18 degrees of mechanical advance

• 12 + 18 = 30 degrees of total timing if the vacuum advance element is taken out of the equation

• Reconnect the vacuum advance unit and the amount of timing it adds will vary dependent on the amount of pumping force created by the engine (engine speed) and the amount of restriction the throttle butterfly is imposing

• The function of engine speed (pumping force) and the amount it's being restricted by the throttle butterfly is to generate varying levels of vacuum

• At higher engine speeds (over 2,500rpm) with small throttle butterfly openings you are creating high levels of vacuum, but even at the very highest levels of vacuum the vac advance unit can only ever add a maximum of 10 degrees

• So under cruise conditions where the engine speed is high (80mph in 5th = 3,000rpm) you are getting pretty much maximum mechanical advance (18 degrees) and maximum vacuum advance (10 degrees) which obviously also needs to be added to your 12 degrees of initial timing to give you your total timing figure

• 12 degrees (initial) + 18 degrees (mechanical) = 30 degrees of total timing at 3,250rpm with no vacuum advance, add in the extra max 10 degrees of advance the vacuum unit gives under high vacuum conditions (cruise) and you get and absolute total timing figure of 30 + 10 = 40 degrees

• So when your buzzing along the motorway at 3,000rpm doing 80mph in 5th with a small throttle opening your Chimaera engine will be seeing 40 degrees of timing, and interesting point of note is the experiments I've conducted with my Canems system show you could actually get a bit more economy by running up to 45 degrees under these specific low load cruise conditions

VACUUM REFERENCE POINT

• Look closely and you'll see Land Rover chose not to plumb the vacuum advance unit to traditional full manifold vacuum, but connect it right on the edge of the throttle butterfly between manifold vacuum and atmosphere, this is commonly known as ported vacuum

• Unlike full manifold vacuum, ported vacuum is actually low at idle so you really don't get much if any contribution from the vacuum advance unit at all when the engine is idling

• Using ported vacuum rather than manifold vacuum means Land Rover could get the fuel economy benefits of the extra 10 degrees of advance when cruising at speed but allowed them to keep the idle timing low at 12 degrees, but why?

• Well the only reason Land Rover wanted to keep the timing low at idle was to reduce emissions, this is because a Rover V8 idling at 12 degrees creates a lot more heat than one idling at 22 degrees

• This low idle timing was a very common strategy in the early 1990's, the heat it creates helps light the catalytic converters and so reduces emissions

WHAT THE ENGINE REALLY WANTS

• Those of us running after market engine management systems have found running idle timing numbers substantially greater than 12 degrees delivers substantial improvements in idle quality while also delivering a significant (and welcome) reduction in heat

• On my Canems equipped 4.0HC with the cats removed I have found the optimum idle timing figure on petrol is in fact surprisingly high 22 degrees

• I now actually idle at 25 degrees simply because there is no difference in idle quality but the extra 3 degrees brings me even smoother pull aways and aids low speed manoeuvrability (reverse parking ect)

• I have found a flatter ignition map gives much better drivability, by this I mean the increase in timing advance should be as shallow and linear as possible while still achieving your ultimate maximum advance target, big jumps in timing from cell to cell should be avoided at all costs as they tend to introduce nasty stutters especially under lightly acceleration

• Starting with a much higher idle timing figure of 22-25 degrees puts you closer to the timing figures in the higher load sites so the gap is less, this definitely makes for a smoother drive not to mention a smoother idle and much lower engine temperatures at idle

BACK TO THE DISTRIBUTOR - SWITCHING FROM PORTED TO MANIFOLD VACUUM

• Armed with the knowledge our cars idle and drive better with a lot more timing at idle, is there way for those who still run a distributor to implement the strategy?

• Well you could simply raise your initial timing to 22 degrees but knowing the distributor will always blindly add up to 18 degrees of mechanical advance this would be a very dangerous approach, 22 + 18 = 40 degrees, and you definitely don't want to run 40 degrees of advance under load because that's likely to give you engine damaging detonation, you'll also be running 50 degrees of timing under cruise conditions which the engine will tolerate but I've fond best economy actually exists 5 degrees lower so would be pointless

• However, assuming you aren't concerned about emissions and catalytic converter function you could try switching from how Range Rover plumbed the vac advance unit to ported vacuum, and simply plum it to manifold vacuum which being much stronger at idle will likely take your 12 degrees of initial idle timing to roughly 18-20 degrees

• In theory using manifold vacuum instead of ported vacuum should give you the smoother idle and lower engine temperatures I am enjoying from my mappable engine management system, it should also be safe because as soon as the throttle is opened and the engine is subject to load manifold vacuum will drop so the vac advance unit will no longer be adding timing

• Before ever stricter emissions targets and the use of catalytic converters forced manufacturers to run unsuitably low idle timing numbers the practice of plumbing the vac advance unit to manifold vacuum was common place, for evidence of this look at any 1960's Yank V8

• I have to admit I've not tried the idea on a Chimaera but the evidence based on my Canems timing experiments seems to suggest that simply switching the vac advance unit from the current ported vacuum point to full manifold vacuum should work well?

• Obviously you'll need to screw your base idle screw in as the extra timing will add at least 500rpm at idle, but this is an advantage too as I've found the more air you put through the throttle butterfly the smoother the engine runs, at the end of the day the base idle screw is just a managed vacuum leak... and all vacuum leaks are bad for idle quality and drivability

[carsy]

Just out of interest Dave what afr are you running at that 25 degrees tick over.

[ChimpOnGas]

carsy said:

Just out of interest Dave what afr are you running at that 25 degrees tick over.

Closed loop 13.3:1 AFR at 1050rpm at 25 degrees BTDC

[blitzracing]

I can give that a try- I have a take of on the plenum from trying to get the Amethyst ignition to work, so I can just try the vac pipe on there.

[ChimpOnGas]

blitzracing said:

I can give that a try- I have a take of on the plenum from trying to get the Amethyst ignition to work, so I can just try the vac pipe on there.

Let us know how it goes Blitz

Leave your idle timing at 12 degrees and report back on how much timing is added by simply switching from ported vacuum to full manifold vacuum at idle. Depending on how much it advances the timing you'll need to use the base idle screw to bring your idle speed back down to where it was before recording your new idle timing figure.

Keep in mind that (unlike a distributor) my Canems system separates the effects of engine speed from timing, obviously with a distributor reducing idle speed will naturally retard your timing and increasing it will advance it, so it'll be interesting to see what the net timing increase will be (if any) when you correct your idle speed back to where it was when the vac advance was connected to ported vacuum.

It's also important to acknowledge the distributor and vac advance unit have been carefully designed to work together in a certain way, messing with this by switching from ported to manifold vacuum is to force the system to work outside of it's design parameters. The vac advance capsule size, the size of it's diaphragm and so how much vacuum it must see before it starts to pull on the distributor base plate is a carefully designed fixed value that cannot be adjusted so I wouldn't predict miracles from the experiment.

IMPORTANT

When conducting this experiment please be very careful you are not going to end up with overly advanced timing and detonation as you pull away from idle!

With all this in mind it's quite naive to assume simply switching from ported to manifold vacuum with the standard distributor & vac advance pairing will give you what I have found to be beneficial when experimenting with idle timing using my more adaptable Canems system. As such if you're determined to stick with a distributor you'd be much better off using something like a MSD distributor with an adjustable vac advance unit so it's influence can be fine tuned.


If it helps, here are some further observations I've made while experimenting with ignition timing & AFR using my Canems system:

1. The Rover V8 with our single plane inlet manifold design does not like to idle any leaner than 13.5:1, I have found high 12's give the very smoothest idle but in the interests of idle economy I've settled on a 13.3:1 closed loop idle target

2. The smoothest idle at 13.3:1 is found by initiating combustion at 22 degrees BTDC with my Canems closed loop idle maintaining a target idle speed of 1050rpm, it should be noted I'm running a lightened (18lb) flywheel and a V8 Developments Stealth camshaft

3. Running a slightly higher idle timing figure than the optimum 22 degrees (ie 25 degrees) has no adverse effect on idle quality but gives a smoother transition to he higher load sites improving off idle pull aways and low speed drivability (reverse parking ect)

4. All vacuum leaks are a very bad thing (even tiny ones) and should be ruthlessly tracked down & eliminated, as a minimum hose clips should be used on the crankcase breather hoses where Range Rover deemed push fits acceptable. Push fits may have been fine when new, but over time its very common for the hot acidic crankcase gasses to turn these push fit rubber hoses baggy where they connect at the 'T' joiner, vacuum leaks being the inevitable consequence.

5. The crankcase ventilation system itself is a vacuum leak, and a very poorly managed one at that, the plenum chamber being engine side of the throttle butterfly is subject to strong manifold vacuum at idle. This manifold vacuum draws crankcase gasses back into the engine very effectively but the pipework is also linked to atmosphere via a completely open and un-valved hose connected immediately in front of the throttle body, this crude unregulated arrangement does nothing to improve idle quality.

6. Fitting a carefully chosen PCV valve in the hose between the plenum (manifold vacuum) and where it connects immediately in front of the throttle butterfly (atmosphere) helps improve this situation and so delivers better idle quality

7. The base idle screw and the idle valve (stepper motor) are also just managed vacuum leaks, in both cases 'less is more', reducing the amount of air bypassing the throttle butterfly (base idle screw & idle valve) then bringing the idle speed back up by increasing the throttle butterfly resting gap helps deliver a smoother idle and better transitions when pulling away

8. AFRs on the Rover V8 with our single plane inlet manifold design should should be kept below 14:1 until the engine is above 2,000rpm

9. While the Rover V8 will tolerate very lean AFRs at light load cruise (as lean as 17.8:1 before stumble), best light load cruising fuel economy over 2,600rpm is actually found at maximum lean figure of 16:1, there's nothing to be gained in going any leaner, all that happens if you go leaner is EGTs rise

10. These leaner cruising mixtures take a lot longer to burn so must be lit earlier, at a 16:1 AFR peak torque and so peak economy is found by initiating combustion 45 degrees BTDC

LPG

While all the above relates to petrol when it comes to LPG much of it is equally relevant, however when attention is turned to ignition timing you do need to respect the slower burn rate and other unique behaviours of LPG if optimum results are to be achieved.

Interestingly it's been my experience that no matter what I do to improve my petrol idle quality (and I've had many successes) it will never be as good as my LPG idle quality! This is because at low air speeds a gaseous fuel delivers better cylinder filling and fuel distribution than a liquid fuel, the weight of liquid fuel droplets tends to mean it falls out of suspension (to some extent) with the air at idle air speeds, LPG on the other hand mixes much much better with air than petrol.

You can actually hear this when you switch from petrol to LPG, idle quality audibly smooths and idle speed will increase by almost 100rpm, even though I am running closed loop idle I've found to maintain exactly the same idle speed at the moment of switching between petrol & LPG it's advantageous to retard the LPG idle timing slightly, this is slightly ironic as what LPG actually wants is more advance not less.

The only reason I run a few degrees less timing at idle on LPG is in the interests of completely seamless and totally unnoticeable change overs, even with less idle timing delivering the same idle speed as petrol my LPG idle quality remains better than it is on the expensive liquid stuff.

However, from idle on my LPG timing ramps much more steeply than on petrol peaking at 48 degrees at cruise and at the same (displayed) 16.0:1 AFR, this is more like a true 16.8:1, but as I'm not tuning on Lambda 1 stoich is always displayed as 14.7:1 no matter which fuel type I'm burning, this also greatly simplifies closed loop tuning as I can use the same closed loop target table for both fuels.

Edited by ChimpOnGas on Sunday 19th March 11:03