And you thought XECUTE had alot to say! Jmac is someone I turn to when stuck for an answer, and he provided this gem on the Yahoo Torana Mailing List recently (yes, I pillaged it).
It covers dizzy locking, vacuum advance, dialling in advance curves (theory), track vs street setups, fuel curve choices - a lot of stuff we all ponder. I tried to hack it into a little more form than the "stream of consciousness" original post.
It's assuming you're prepared to lay down the beans for some dyno time, but the benefit is at least you'll know where things are leading, and can even steer the mechanics somewhat.
It covers dizzy locking, vacuum advance, dialling in advance curves (theory), track vs street setups, fuel curve choices - a lot of stuff we all ponder. I tried to hack it into a little more form than the "stream of consciousness" original post.
It's assuming you're prepared to lay down the beans for some dyno time, but the benefit is at least you'll know where things are leading, and can even steer the mechanics somewhat.
[b:3fm3sdgu said:jmac[/b]":3fm3sdgu]Don't set maximum timing by fastest quartermile time - use the time to work out the ignition advance _curve_ but use the mph to find the total ignition timing -
OK - realise this isn't written in stone, but it's to the best of my knowledge.
it'll take a little bit of background stuff - again to the best of my knowledge.
It's compounded by the fact that I type as I think things - the posts as a result aren't really well organised.
If you run too much total timing, you'll get detonation and it can and will destroy the engine (too much boost, or comrpession ratio for a given fuel will do it too).
Sometimes you can hear pinging at lower rpms, but at higher rpms it can be masked by the normal engine noises of an engine at full throttle above 5000rpm.
In my own experience, when I was very young, I tried to 'cheat'. I had a hot cammed engine, and instead of recurving it, I just increased the initial timing. This gave it a hell of a gain from idle to around 2500-2800rpm, but it _felt_ identical above that point. I ended up melting a piston. Didn't feel or hear it, I was thrashing it the previous night, got home late, and when I drove it in the morning it was doing a James Bond 007 smoke screen coming from the rocker cover breather....
When you get an engine dyno'd they can either keep the engine steadied at each rpm segment for a number of seconds to get a steady state reading do an acceleration run - where they allow the dyno to let th engine gain some amount of rpm per second - sometimes around 500rpm per second - they can probably vary the rate. It can be useful because it helps to tune the fuelling for an increasing rpm rather than
steady one.
There's two ways you can get a power increase with the acceleration dyno run - if an engine is kept at any rpm for enough time, the piston and the chamber heat up. The cooling system can be perfectly adequate in dealing with this heat, but the actual chamb er temp can go up more than is reflected in the coolant temp - because a hell ofa lot more heat energy is being dealt with. That means if you are just zipping up through the rpm range, there isn't as much of this happening, so a little more timing can be run safely - and hence a power increase.
The other reason is that steady state fuelling and timing requirement woud be a little different that what would be optimal for an engine going up through th rpm range. It's not just about the heat, a little more or less fuel (probably more) would allow the engine to gain rpm quicker - even if it actually made less power at any fixed point, the speed it can rev up against a set load will be quickened - and that's what racing is - accelerating a given load, not pushing against a resistance that prevents rpm gain....
So on the surface the optimal setting would be whatever gives the best acceleration dyno result.
Now we enter the real world. engines done on an acceleration dyno in street driven cars can be ok, but they can alsio be in trouble. If you drive around, underbonnet temps can be far in excess of anything you would see on the strip if you've just got the car going, warmed the engine and then rolled into the stagin area - so if the temps start off higher, the intake temp (even via cold air induction, the inlet manifold and so forth are heated a lot more) is higher, and you can cross the detonation threshold.
Its also a big issue in a car used for towing - you can have extended periods of high or full throttle operation. As a general rule, if the combustion chamber shape and engine combination can run 10:1 compression without detonation worries at the drags, you are probably better off with somewhere between 9-9.5:1 (still retaining the same basic chamber shape, and quench and so forth) to avoid detonation.
For a non race car, you'd probably want to do some acceleration runs and some steady state dyno runs. As completely hack effort at it, I'd try to retain the majority of the transient response gains while avoiding the steady state detopnation or power loss. A very simplistic approach (all I can offer that would be in any way practical for most people to implement - including myself) would perhaps be to use the fuelling setup that provided the best acceleration dyno runs, but teh timing settings that delivered the best steady state dyno readings.
Keep in mind, detonation can be difficult to hear, and you don't lose a lot of power - it doesn't drop off massively instantly - you can easily be unaware of it.
•OK but back to the track. The ignition advance curve generally shoudl be worked out via the acceleration dyno runs or the et. generally on any setup you'll be likely to see, the total mechanical advance is in by 3000rpm, 3500rpm at the highest. from then on it's locked in - although engine rpm goes up the optimal timing doesnt - well at least not to a degree worth any concern - like 1 degree per 1000rpm.
If you have the frontal area of teh car and the drag coefficient (you can calculate it even simpler) you can believe it or not calculate the hp the car is making by the mph at the end of the strip. Even if you lower the gearing and grab 0.5 second better et, the actual mph doesn't change dramatically. the extent most cars accelerate gets less and less the further down the strip the car gets. Whilst acceleration is initially power to weight, it begins and dramatically increases to the point where by the end it's bhp versus aerodynamic drag.
Whilst it's not a real issue at the start line, the further down teh strip you go, the longer the motor has been running at full throttle, so the hotter and hotter the piston crown and combustion chamber get. So the start isn't a huge deal but further down the track you are beginning to get into the territory where it is more prone to detonation. By setting teh total timing by the et, you wil get a faster time, but this is probably some combination of the extra advance providing a boost in acceleration at around 2/3 of the way down the strip, and it actually crossing into the beginning of detonation as by teh end of the strip that same ignition advance was now too much.
OK so bhp is indicated reasonably well by mph. The higher the mph at the end of the strip the more power you are making. If the engine is detonating - even just at the onset, tehn it will drop power. It's not always audible - and at it's onset, it's practically impossible to notice a power drop. but there will be one. the power drop itself isn't a concern, but the fact it's crossing into dangerous territory is of some concern. The trick here is that while you won't likely notice any difference whatsoever if you dropped 5bhp in the last 1/3rd of the run, it _will_ show up in teh mph. Of course you can't mix up the reading by swapping other things like diff
ratio etc. You basically have to do everything else the same until you find the optimum point, I'd actually do it early in the buildup - to ensure it's not
detonating etc, and then as a bunch of other stuff is changed, take a second look at it.. You won't notice the power drop, but it will definitely show up in the mph. If the engine is detonating or even close, it will be detectable in teh mph..
So basically it's an engine longevity thing to some degree, but it's also capable of providing a better 1/4mile performance in the long run. Believe it or not, some people employ a high speed retard, where there is a given timing curve but toward the end of the strip in top gear, they actually retard the timing a few degrees. They get the best et _and_ the best mph.
If you aren't a sponsored professional race team, that can afford to test and if need be deliberately destroy parts to find that fine line between optimal performance and component failure, then there is no room whatsoever for 'crocodile wrestling'.
FWIW - lets say 28 degrees timing is found to be the best mph, but 32 - the best et (and it's probably less than that) then the actual et difference isn't likely to be more than 1 or 2 tenths of a second.
Just because it's not detonating, doesn't mean the advance is optimal either. It's often a number of degrees less timing than the detonation threshold that returns the best peak power.
In terms of getting it right, personally I would start at around 22-24 degrees total, then do a few runs to get the average mph of them, then increase the timing 2 degrees and repeat. Basically do a bunch of tests. the mph will increase, then will level out, an finally it wil drop a fraction. I'd go to the step where the mph just begins to drop off. You really need to go to that point, since you might find it levels off at 28 and 30 but at 32 it gets a fraction better. Basically go to the point it's just dropped off (you will be on the veerge of detonation, but not into it, or its' borderline so it isn't life threatening to the engine).
Once you have established the drop off point, and the level off point, I'd drop the total timing back to the figure where it first levelled off. Lets say it peaks at 28 deg, and at 30 it's the same, and 32 it's the same and 34 it starts dropping off - in that case I'd run 28 total. Apart from the fact you aren't losing any bhp, it as far away from the detonation limit as you can get without dropping power. If nothing else, for street operated cars in particular, you are a hell of a lot better off if you accidentally cop a tank of dodgy fuel, or on an unusually hot day, or even both at the same time.
•If the cam is hot enough, you could potentially get away with running a locked distributor to sort out teh best total timing. It depends on the whole combination, but if the motor doesn't spend much time below 2500 (and definitely it doesnt drop below 3000rpm) then whilst it will be too much timing down low, the motor doesn't see it so it's safe to test. Then you tailor the initial timing, the amount of mech advance to achieve the total timing figure you have found is best. The initial timing increase that's appropriate for lower rpms with a bigger cam is predominantly
in the area below where the engine would rev to in a race (depending on how you launch - heaps of rpm and clutch slipping, or on an auto convertor stall rpm) - with the exception of the launch. The difference is it's performance and manners, idle quality etc - are dramatically improved - but worse case scenario on a race only engine you can probably get by locking the dizzy all teh time. It was apparently not
uncommon on nascars a few decades ago. their rules call for 'stock appearing' or stock parts - i.e. distributor, no cpu controlled coil packs, a carb - no efi, mechanical fuel pump, etc.... Instead of mucking around with a curve that had no advantage the cars never saw below the rpm where total advance would be in by - except coming out of the pits - where you have initially have a speed limit, so you would be nowhere near full throttle - and the rpms would be high enough that it would handle full throttle - so the fact it had too much advance at lower rpm wasn't
an issue.
•On a streeter there is absolutely no reason not to recurve it - the engine
will spend in those lower rpms on teh street is significant and the best output here will make for a much more pleasant streeter - lets say you need to acelerate and get around a car etc, but don't want to drop the clutch jam another gear and rip forward at massive speed with heaps of noise - you just need to move a bit quicker - the better setup the lower rpm ignition and fuelling the better off you are.
•Vac advance is on engines generally for one reason and one reason alone - economy. At part throttle there is less a/f in there so it's not as highy compressed as a cylinder full would be at the same rpm. This means the engine can tolerate and delivers more at part throttle with more advance than would be right for full throttle at that rpm....whilst it's not worth any power at all, I'd still suggest keeping the vac advance in there.
Fuel economy is a good thing - might as well make the most of things. there is another reason. Remember the bit about transient response - where it's not producing more power on a steady state dyno, but picks up rpm better against a fixed load, and how this is important and can potentially provide better acceleration than a setup that provides the better steady state fixed rpm bhp. Well the vac advance comes in at part throttle. Depending on setup they are either routed the the manifold - where whenever there is inlet manifold vacuum the vac advance will be in effect. Others are 'ported' that's where the vac line is hooked to a port on the carburettor that is above the throttle plate slightly. At idle the orifice is above the throttle plates, so at idle it gets no vacuum at all.
As you open the throttle partially, the throttle plate lifts on that side and the orifice is now situated a little bit below the throttle plate - so then it sees vacuum. I'm not sure what all the reasons for such a setup are, but possibly there is an emissions related factor. At higher ignition advance NOx emissions go up. in the UK the epa testing is mostly concerned with CO2 percentage at idle, but quite possibly there are other standards in effect in aus built/sold cars...
•Personally I would - given a choice - run teh vac line to the inlet manifold. The reason? this extra part throttle advance will drop away when you nail the throttle, but that split second where it lingers - the extra advance will actually allow the engine to pick up rpms a little bit quicker - so it will have a sharper throttle response. This will benefit a street driven car massively, a circuit racer enough that I'd run it for sure - even if it was worth 1/10th of a second per lap, it's got no downside or risk.
This is by far the most 'hack' idea I have had yet, but for those who can't afford a programmable ignition module - like an msd one which can provide ignition retard tied in with boost level, a nitrous switch etc....there is an option.
I am a big fan of the blue motor dizzy, but for a dirt cheap racer, you could make your own twin point dizzy. The aftermarket ones actually have one set of points aligned about 2-4 degrees differently from the perfect 180 degree separation if they both opened and closed identically they would have no advantage. They are offset so that as one is not yet closed, the other is, and that one opens before the other. It enables you to get a decent dwell angle (time where the primary ignition circuit is charging - i.e. when one of the points is closed) but still both points can be open to a wide enough amount to get the coil secondary winding to discharge the current. To try and get this much gap limits the amount you can have the points closed for, or it requiress a more agressive distributor 'cam' that opens the points. The twin point can provide more dwell without points float or bounce.
On most streeters - particularly holden 6s - a single point dizzy is enough to adequately provide spark to 6000rpm. An electronic ignition will be better here, makes for easier starting and generally lifts power slightly. but if you didn't have one, or access to one, you could make your own twin point plate for $1 or so of sheet metal and a little time. Instead of having both wired in parallel like standard, you have the second set wired to a switch - so still in paralell with the other set points - but has a switch in it's circuit- that can be operated when you hit top gear. Basically, you could have the switch activate the later opening set of points - that way even when the earlier opening ones open, the primary circuit is still complete running through the second set of points. The circuit isn't opened
till the second set of points are open so it retards the timing by whatever number of degrees teh second set of points are offset by. Just an idea that would be if nothing else a bit of trivia that someone might like to play with.
