Ignition advance question

[aus9000T16]

I have re-built an engine for an old Renault which I hope to use for hillclimbs and motokhanas.
I have a few different distributors with fairly different advance/vacuum compensation curves.
Is there a formula for deciding which advance curve is better?

I have graphs for where the advance kicks in and finishes and what vacuum is required for initial advance.

[jmac]

Loose guidelines

Things which have to be taken into account (not a complete list)

a.fuel
b.typical temps
c.comp ratio
d. combustion chamber design
e. cam specs
f. rpm range (as raced)
g. weight of car and length of race (the greater either of them is the more chance for chamber hot spots to develop so more conservative timing is needed)
h. specific gear ratios
i. if you will be on & off the throttle a lot
j. induction type
k. a/f ratio (though I make the assumption it will be tailored to max power output and to prevent detonation, rather than being a little leaner to max fuel economy on an endurance race where 1 less fuel stop is vital)

a. Obviously the higher the octane the more timing it will handle. This only makes a difference up to a certain point, and if the compression is particularly low, the difference in advance you end up using between average pump fuel and hi octane is not huge

b. the hotter the temps, presumably the hotter the intake temps (as well as the harder it is for the cooling system to remove heat to the atmosphere) so the overal timing would be reduced (intial/total both reduced)

c&d are linked. The higher the compression ratio generally the less timing it will like (both initial/total) It also generally tends to like the centrifugal advance to come in slower (i.e. reach max tot adv 500 or more rpm higher.)

Some chambers provide a quicker more complete (but still controlled) burn, and will make more power with less timing. Others (open chamber, and or if there is a piston crown shape that interferes with flame path propogation) have trouble achieving a complete burn in a short time, and you need to end up with more advance so that it can still make peak cylinder pressure before the piston has moved too far down the bore (the further down it is, the more volume the cylinder will have, so maximum potential cylinder pressure can be cut to pieces by virtue of more space for it to expand into). It also means that since you intiate the spark much earlier before tdc (relatively) it will be burning and expanding whilst the piston is still rising, and you end up doing a little bit of negative work. The timing there is then a compromise between how much is traded off as negative work vs how much would be bled off if ignited too late

Sharp edges or hidden pockets can lead to parts of the mixture not burning initially, and then when cylinder pressure (and temp, from the burn) rises, that last pocket instead of burning can then violently explode (hence the term detonation) . Generally that would mean that less timing can be used safely.

An alloy head dissipates heat better so generally will be less likely to lead to detonation (on an otherwise identical head) so a little more timing can be used. However it's typically more valuable to opt not to go for more timing, but instead to run higher compression with the alloy head and similar timing.

Due to poor flame front propogation, sometimes compression ratio is sacrificed to improve flame path, and since it's compressed less, it can burn a little slower & can require more timing.

If the port and chamber allow decent swirl it will tend to like less timing (everywhere). In this case it's great - the burn is so quick that it can be intiated much closer to tdc and practically no negative work is done, yet it still is able to push on the piston when it's at a more advantageoush point. Additionally those chambers that tend to shunt the compressed mixture toward the exhaust valve(s) & have a spark plug angled toward the exhaust valve side see a quicker burn and want less timing.


e. Generally the shorter the cam duration, the less intial timing is be run, but since it will run out of it's optimal rpm range sooner, it won't be filling the cylinders nearly as well at higher rpm it can handle more total timing.

A longer duration cam will end up with later intake valve closing (well after the piston has started to move back up from bottom dead centre) and so at full throttle but lower rpm, it won't be as effective at filling the cylinder, so it can handle more intial timing (it's also the case since some of the intake can end up being bled off due to the increased in/ex overlap - the time when the exh hasn't yet closed fully, and the intake is starting to open). And as such it's possible to also (to hark back a section) to run slightly more compression safely. SO the intial timing has to take all of that into account. The longer duration cam will be more effective at filling the cylinders at higher rpm so it will tend to like less total timing than a 'stock' motor.

f. If it's a stock rpm range, it all holds true. with a serious race piece with practically zero use & power below 3500-4000rpm it's possible to run a locked distributor in a lot of cases and whilst it's a touch harder to start, it's actually able to produce identical power (without having to stuff around with different springs etc) in it's intended rpm range.

after around 4000rpm (it varies 2500-3000 somtimes) the timing tends to want to remain the same. Even though at 8000rpm (hypothetically) 30 degrees total timing would mean the flame has half the time to develop max pressure than at 4000, due to the heat from the higher rate the mixture is being compressed in the cyl, it ends up balancing out and not needing more timing. this varies from engine to engine, and for ultra high rpm stuff it would differ (it's even possible, if rare to need less timing at higher rpms on some engines)

It can also occur if the engine for some reason has to be run well past the peak power rpm it has. One e.g. would be certain car racing categories. HQ racing uses a nearly stock 202 (stock carb inlet manifold, head,mild control cam, std air filter, pts dizzy) where one of the only allowed tweaks - extractors are free. They have to run the 3.55:1 diff and std ratio 3 sp man. On many tracks the cars have to be held in a gear through certain corners (and I suspect at somewhere like thunderdome in top) so they need to rev the motor well outside of it's powerband. The VE would drop off so far after a certain rpm that they'd likely get noticable gains in that upper rpm range from more ignition timing. I'd suggest they'd all be running a curve that has one amt of advance to around 3000rpm and then a slower curve that continued up to (possibly even above) 5000-5500. Just including it for trivia sake

g&h are linked. The weight of the car. Presumably the heavier the car, the harder it is to accelerate (compared to an identical engine in a lighter car) so the longer the engine is at full throttle in each gear, and the less airflow through the radiator (due to lower speed all else being equal) etc, so typically it'd like a touch less timing. It also means localised hot spots have more opportunity to develop under load. In drag racing, chamber surface temps can vary enough to cause a problem by the end of the strip and they actually run a different advance curve for each gear - typically the hotter it gets and the higher the gear, the less timing. Another way to look at it is that a race car that does short sprints would be fine with a given timing curve, but that same setup towing a trailer up a long hill would be far more likely to see detonation (and hence would like less timing, and more beneficially should be put together with a lower compression ratio as well.

i. If it's a pure drag race setup, you don't have to actually come off the accelerator at all (in an auto, or a flat shifted manual) so it's a moot point, but it you do have to, then having vacuum advance retained (some old timers tend to think of vac advance as purely a fuel economy aid, I don't and I'm not alone on this one) may not make a big difference. But if you are on and off the throttle a lot (I'd even push it for drag racing if you run a manual, hell I'd even run it for auto backed drag cars) I'd suggest giving serious consideration to retaining it.

Furthermore there's two options, ported or manifold vacuum. Manifold vac is as the name implies connected to the inlet manifold below the carb somewhere. It will see vacuum any time the throttle plates are closed. Ported vacuum is tapped into a point on the carb just above where the throttle plates sit when at idle. AT idle it gets 0 vac. With the throttle plates open any further, the plate will rise above that point, and the ported vac port will then see vacuum. It's more often than not used to achieve a given timing at idle which might help emissions (there's a few reasons).

Anyway, the reason for it's use at all? Simple. At idle, if there's a higher amt of vac (from vac advance, and we aren't talking about a 40 degrees extra or something silly) then in that transition from idle to full throttle the vac cannister takes a fraction of a second to spring back to 0 when you floor the throttle. This slight 'residual' advance can end up giving that fraction more advance whilst the fuel circuits catch up with the newly opened throttle and higher airflow. The accelerator pump is there for this purpose (to add fuel till the main feul circuit catches up) . But the little bit of residual advance from the vac cannister can add it's two cents to the throttle response equation. Obviously since this can be of dramatic effect (if there's 10 corners and it makes a difference of 0.05 second to how you come out of the corner it's half a second, not too shabby for something that is more or less 'overlooked' on some setups.) then to get maximal effect from it would require manifold vacuum in preference to ported vacuum.

j. with a big single carb it will (to some minor extent, and assuming it's a performance oriented carb and inlet manifold, not some poorly flowing stocker single carb that the factory used on low po models, I can show example pics of various single carb setups for inline engines to give a basic idea) tend to like a touch less total timing (due to slightly better cylinder filling possible from a larger single carb and largish plenum) and a little more intial timing since at low rpm and flow the larger single carb and plenum dissipate the carb signal and general cylinder filling. With multi carb setups (like one carb throat per cylinder or per port) you can actually get (even with a very hefty cam) a particularly good idle (due to the port pulling on the carb in one sharp shunt rather than a series of waves all dissipated through one another) and low-mid range output. SO they can tend to like a little less initial timing. They also tend to like the centrifugal advance to come in a bit slower. They also tend to like a fraction less total timing. (It should be pointed out and multiple webers often outpower a large single carb on many engines, but when they do, speaking abt max rpm peak hp only, it's a sign of shortcomings with the single carb arrangement). They typically have a much stronger mid range (all else being equal) a wider powerband and better throttle response.

You also have to sometimes alter the timing curve (a fraction less, but not always) for certain points in the rpm range with multiple carbs. Since they allow the inlet path length to be tailored for a ram effect at a particular rpm range (that is often chosen to be lower in the rpm range than peak power for better track performance, where mid range, and power out of a corner is more valuable than peak power) and you might need less timing there for optimal power. In practice it's dead simple to do with programmable igntion. With a conventional distributor it typically takes the form of two advance springs, one with an elongated hook. This means the weaker spring acts from idle to 3000rpm (for arguments sake) and then the second spring hook finally hits the prong it attaches to and slows the curve from there so that it doesn't then get full centrifugal advance till say 4000 (or wherever the other side of the inlet VE 'boosted' peak starts to drop off)

k. a/f ratio. Differences in a/f ratio have an effect on both burn speed (sometimes in burn quality too) and likelihood to detonate. I could say 'the richer you go the more or less timing you need' but frankly it's just not that easy. It's trends rather than absolutes. What I _WILL_ say is that after you get a baseline (very very conservative) timing curve (enough that the engine will run without stalling etc) you _must_ dial in the a/f ratio across the rpm range. Then and only then can you test to find the optimal timing curve as the a/f ratio affects what it requires probably more than all the above.

--
the most important thing of all the above is that it takes experience with the engine in question to find out just how these trends actually manifest themselves. You need to talk to people with experience with those engines.

it'll require some fine tuning based on that experience and taking into account how the specific engine differs from other engines of it's type (i.e. all the specific application idiosynchrasies)

The least advance you have to run to get max power, the better, as it means the chamber is producing a better burn. So when someone thinks such n such head must be great since it runs more advance, it probably means that the engine designers ended up running lower compression and having to run more advance than ideally would be the case.

Another thing to take to heart is that the max timing before detonation is not the best timing. Generally optimal power is a number of degrees short of that. I'd go as far as to say that if 36 degrees was the onset of detonation, and 30 degrees gave max power, and 28 degrees gave identical power, I'd personally opt for 28 degrees every time.


spark plug heat range can alter timing needs. If too hot, they end up causing detonation. And related to that, ignition type can affect timing needs. Generally the better the spark the better it initiates the burn and the more complete the burn can be. And as a result it sometimes means that you end up needing a little less timing with HEI (and you'll make more power than the points ignition and a couple of degrees more timing). As a general rule, I'd personally run HEI on anything that had an accessable factory system (or one that could be adapted cheaply) over points on everything I owned (and I do more or less too) and I'd combine it with colder plugs (you might end up needing to run a stage hotter without hei to help keep the plugs from fouling, hei gives a stronger spark and resists fouling)


If you can post back with details on all of the above, and (if it's not too much trouble) a list of any factory advance curves that were used, and the exact spec of the engines that they ran on, I can possibly give you a better 'educated guess' but it'd of necessity need to be a little on the conservative side since I don't have specific experience with renault engines (except the P/R/V v6)

John McKenzie

[aus9000T16]

Thanks Jmac.
Glad you said loose guidelines. Just out of interest, what car/s were the PRV V6 engines in.
I didn't anticipate such an involved reply. I am not building anything full on at this stage either, just a fun driver, as I haven't driven in a hillclimb or any other competitive motorsport yet. I haven't actually had thiscar on the road, but hope to before mid July this year, with aims of a few hillclimbs in the near future.
There are so many more things than I could have imagined would be a factor. I am only a beginner at this stuff.
I can supply cam duartions and lift, combustion chamber design, total capicity, comp ratio I am not certain on, but I'll give you bore/stroke and comb chamber capacity. I will photograph the advance curve pics for you too.
The car is a 1969 R10.
Engine type.....Renault 'Sierra' 4cyl. Aluminium head
Bore = 71.9mm
Stroke = 77mm (1250 cc of pure French power )
Combustion chamber capacity 32.18 cc
(Does the equation for compression ratio include the combustion chambers in the total capacity?) If so that gives me 10.7:1 comp ratio. If not, about 9.7:1.
Cam timimg...Ex Opens..65*btdc, closes 27*Atdc In Opens 27*Bbdc, closes 65*Abdc....Not over the top, and lift is .248"
I can tell you two of the advance curves..one gives 11*@1375RPM and full16*30'' @ 2300RPM and has a vacuum compensating curve topping out with 10* from 324mm of hg.
The other has 7*@800RPM and full adv of18* @2250RPM. Vacuum on this one is 5.5* from 334mm of hg.
Induction will be initially just a weber 32 DIR4 carbie.
I'll add gear ratios, and vehicle details leter, when I can be borthered to get it all out and look at it.

Thanks so much so far.
Cheers, Chris.

[tortfeaser]

So for cars with ECUs controlling timing, the bit above about ported vacuum providing a little advance before the centrifugal advance might be useful in terms of acceleration advance.

Is that true? When you stomp on the throttle, a little accel advance is going to be useful prior to the advance from the MAP/RPM table 'catching up'?