123 Tune which Coil?

[Tom W]

The instructions on the 123 are a bit vague regarding setting the distributor statically to TDC or 9deg. The examples they use just show how to copy the factory VW curves, so the static advance would be set when the distributor is mounted, rather than in the app. However, I think you’d be better setting at TDC, and the 9 degrees at idle be obtained via the software. This would than allow any point on the curve to be directly adjusted from the app, with its value equating to the advance the engine sees, rather than having to add on the 9 degrees. It’s easy to see what you’ve got at the moment by checking with a timing light, as you would with a conventional distributor.

Regarding the RPM value, I highly doubt the 123 software/app is out by 500rpm. More likely that the original tacho is inaccurate. Easy to check with an independent diagnostic tacho.

[mgcjag]

Hi Tom.....on my friends set up the app was definatly reading 500rpm high....apart from trying other rpm guages you could actlally hear it.....123 technical guys were aware of the problem

[Tom W]

That’s a bit worrying. Was that 500rpm out at idle, and consistently out across the rev range? The distributor must be giving 6 ignition pulses per RPM, or else the engine wouldn’t run for long, if at all. Have the 123 engineers solved this yet?

[andrewh]

My 123 App revs are spot on with the instrument. No problem there. I shall have to static time it again at tdc and then put the advance curve in via the app

[abowie]

46 degrees of advance at 3000 rpm is too much. From memory stock would be around 32 to 34. Could this reflect that as you say the 123 thinks it's at TDC when it's set to 10BTDC?

Here are the numbers for the DMBZ6A fitted to earlier 3.8s and the 22D6 fitted to late 3.8s and the 4.2. I have no experience with the 123 but were it me I'd probably start off with the stock curve and then experiment from there.

[abowie]

This is a plot of actual advance vs RPM from the DMBZ6A fitted to my 3.8 FHC (NOT a 40617A as I didn't have one). Note that for some reason the X axis scale isn't linear; stupid on line graph program.

It tops out at 36. Once I have the car on the road, if it is a bit too advanced I'll grind a bit off the cam in the dizzy to bring it back down to 34.

[andrewh]

thanks very much Andrew. Yes I agree the 46 degree of advance recorded was way too high and I cannot work out how it got that high! . What I fail to understand is that if it is adding the 9 degree of advance at static timing to the curve I have input , how does the 123 measure that? It must use the point you set the distributor as the zero and can only reference back to that as a measure of advance, unless I am missing something. There is no way in my mind that it can see what actual advance is being recorded is there? All this would be so much easier if the 123 people wrote their instructions more clearly . I am now assuming that the distributor must be put in at TDC. Then you have to put your first point as 9 degree BTDC at 500 rpm ( rough starting speed?) and then the rest of your curve there after. Any 123 experts out there who can tell us whether the 123 is able to assess actual degrees of advance on its own would be welcome to here. The actual curve I loaded is here.

[andrewh]

thanks very much Andrew. Yes I agree the 46 degree of advance recorded was way too high and I cannot work out how it got that high! . What I fail to understand is that if it is adding the 9 degree of advance at static timing to the curve I have input , how does the 123 measure that? It must use the point you set the distributor as the zero and can only reference back to that as a measure of advance, unless I am missing something. There is no way in my mind that it can see what actual advance is being recorded is there? All this would be so much easier if the 123 people wrote their instructions more clearly . I am now assuming that the distributor must be put in at TDC. Then you have to put your first point as 9 degree BTDC at 500 rpm ( rough starting speed?) and then the rest of your curve there after. Any 123 experts out there who can tell us whether the 123 is able to assess actual degrees of advance on its own would be welcome to here. The actual curve I loaded is here. edit: on reflection I am not sure why it has so much advance low down, this is a curve supplied by a leading XK race company to me . Maybe they got that wrong?

[abowie]

It must use the point you set the distributor as the zero and can only reference back to that as a measure of advance, unless I am missing something.

I agree this is the only logical approach.

All this would be so much easier if the 123 people wrote their instructions more clearly

"There are only two things I hate; people who are intolerant of other people's cultures, and the Dutch."

[andrewh]

I like the Dutch as it goes, always been a good ally to the UK, and I like the 123 , its a good product. I just don't like the inability to speak with a technical department at the manufacturer and their ambiguous instructions about fitment. Their recommendation is to speak with the supplier who will have a good working knowledge of the product . Yeah right!!

[Tom W]

The 123 only measures advance beyond the point it knows. You have 2 possible approaches:

1, install it at TDC. The advance values in the graph will be the actual advance the engine has then. You’ll have to add the static advance in on the app. I.e. the first point should be 9 degrees, not 0 degrees.

2. Install the distributor at 9 deg BTDC. Then draw the factory curve in the app (except you have to double every value as the app works on crankshaft degrees. This means the distributor now mimics the original factory distributor.

I’d go for option 1 as it gives you control entirely from the app, and the numbers relate to what the engine is actually doing.

[andrewh]

Thanks Tom. This is precisely where I had arrived at as my plan of attack from here. If i can summon up the energy I shall shuffle some cars around and get it on the ramp for the TDC set up and go from there. I am not sure quite why you say you have to double every value, surely you are only adding the advance , 9 degree in this case, to the curve you wish if you follow option 2?

[Tom W]

The distributor rotates at half engine speed, so you double both RPM and advance from the workshop manual figures to get crankshaft advance. The app works on crankshaft degrees/rpm. Vacuum shouldn’t be doubled, but the advance value for if should. The figures in the workshop manual are in distributor degrees/rpm to allow the testing of the distributor on a test rig remote from the engine.

[MarekH]

Why does the crank turn at twice the speed of the distributor?

Because the instructions are in double-Dutch !!

Seriously though, do the instructions at any point not have a line that says "Check the timing with a timing light" or do people just leave this to chance?

kind regards
Marek

[andrewh]

The distributor rotates at half engine speed, so you double both RPM and advance from the workshop manual figures to get crankshaft advance. The app works on crankshaft degrees/rpm. Vacuum shouldn’t be doubled, but the advance value for if should. The figures in the workshop manual are in distributor degrees/rpm to allow the testing of the distributor on a test rig remote from the engine.

Ah ok, that makes sense so its working on crankshaft advance not distributor advance as per the manual then? Seems to me that the option one timing in your earlier post is the easiest to use.

[andrewh]

Why does the crank turn at twice the speed of the distributor?

Because the instructions are in double-Dutch !!

Seriously though, do the instructions at any point not have a line that says "Check the timing with a timing light" or do people just leave this to chance?

kind regards
Marek

not sure Marek, I will read the booklet over the next couple of days prior to me setting this up again.

[MarekH]

Here's what I think happens:-

123 looks up a number in the table and advances the rotor that amount.

The first line in the table is 500rpm and 9' advance.

123 doesn't care whether you are at 0rpm or anywhere up to 499rpm - it advances the rotor 9' because that's the first number in the table and you haven't got onto the second line of the table yet.

At 500rpm it looks up the table and advances the rotor 9' because it is a special case (the rpm appears in the table).

At any higher rpm, it interpolates between two lines in the table or choses an exact number if it appears in the table.

The instructions probably tell you set up the distributor at 0' static advance as anything at 500rpm or below just advances the rotor 9'.

If you static timed it at 9' advance, that that explains how you added 9' onto the whole table and Andrew in Australia's common sense ~35' advance reconciles as your 46' advance.

The table lookup might also explain why your app has a 500rpm shift reported.

kind regards
Marek

[Tom W]

Having a better look at the curve you’re loading, it has a lot more advance at idle than the original factory curve. I suspect this curve is for an engine that’s some distance from standard. Possibly one with Webers, they like a lot more advance low down. Unless your engine is substantially different from standard spec, then you’re probably better drawing a curve that matches the original engine advance curve, and go from there.

If you’re following option 1 from my post above, it’s easy to work out what the original engine advance curve should be. The first point should be the static advance (9 degrees). For the next RPM point, take the “no advance below” value from the workshop manual and double it. The advance for this RPM should be the set to static advance too. For every subsequent RPM value, double the distributor RPM from the workshop manual. For the corresponding degree value, double the angle from the workshop manual, then add the 9 degrees static. When you get to the RPM point by which the original distributor had reached maximum advance, add a further RPM point beyond the rev limit, and set the advance to the same maximum value.

To draw the vacuum curve, match the vacuum value from workshop manual, then double the advance value to get crankshaft degrees.

This gets you something that mimics the original curve, but there are further tweaks you can do beyond what’s possible with a mechanical distributor. This includes lowering the timing below idle speed, to make starting easier. Useful if you’re running a curve like the one you’ve been supplied by race outfit. You can also lower the timing slightly at red line to discourage the engine racing beyond the rev limit.

[MarekH]

From post #28, the clue as to the reason why "that curve" has so much advance low down is that you say it comes from a race car company. It probably relates to an engine with a high lift camshaft. High lift cams typically have a lumpy idle at low rpm, so to make idle smoothly, the advance is bumped up. This increases the rpm so then the throttle plate is probably restricted a bit to compensate.

If you have a standard camshaft, look to use a standard advance curve.

kind regards
Marek

EDIT:- Tom I think I've just repeated what you've said - posts crossed.

[andrewh]

OK , well I have now timed the 123 statically to TDC and will put in the curve on the APP. I am assuming that we start at 500 rpm 10 degree advance and go from there. What I am unsure of is what the vacuum advance should be in inches of Mercury. I should have listened more in Physics The 123 not so helpfully is graduated in kilopascals ( ?)
I had heard that curve 7 on the standard 123 works very well, does anyone have the profile for that, at least before I go off to the rolling road. Sorry this thread has developed into a 123 Distributor thread but I am sure it will be of use to others. Anybody got the book on power tuning XK engines? That may have some useful stuff in it, but I have not been able to purchase a copy.