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MG MGB Technical - 25D4 V 45D4 why advance curves so different

I wonder if one of the Gurus can help me. As you read on you will understand why I've asked this question. I myself have a 71 B with an 18GG engine and apart from the Burgess Econotune head its bog standard. It has a 25D4 distibuter and goes very nicely. The problem is my friend who has a 68 B and his engine is a Bitzer. It has a 1975 18V bottom end but with an early 18V head with the bigger inlet valve. He stiil has the HS4 carbs from the original 68 engine. It also has a fast road exhaust system, including manifold. As I understand it engines with the larger inlet valves ran with a 25D4 distributer, but he has a 45D4. You can understand that we are struggling to achieve the optimum tune. I've done some digging using Paul Hunts Bee and Vee website and dicovered that the advance curves for the two disis are compleatly different which is really hard to understand for what is nominally the same engine. All I can do is advise hime to get hold of a 25D4 distributer to try. What do you all think ?
Please see below the advance curves from Paul Hunt.

Paul Hollingworth

Paul
The advance curves set by the factory were designed to make a car run under a wide range of environmental conditions - and from time to time (and by export region) may have changed to address emissions demands, fuel availability, mpg....not necessarily because of the engine spec.
unfortunately, a factory specified curve for these reasons would probably not have been optimal either. then, and certainly not now.

really, a rolling road session and a programmable unit is the only way to get the optimal curve - HOWEVER, in my opinion you can get pretty close without one and it would be questionable (assuming no major performance mods) as to whether your seat-of-the-pants meter can tell the difference between optimal and a close stock curve.

i guess that is the attraction of 123 as it offers a selection and you can pick which runs the best. but not a quick effortless solution ...

curves in the cheap (new) electronic/points distributors are a bit of a lottery. there is also of course something like aldon amethyst or accuspark blackbox. programmable from £100. start with a know curve and experiment.

G

Graham Moore

Paul
As I understand it----------
The 45D4 shown on Paul's graph is for a later 18v emissions type engine
They used this curve as a method of being able to have the throttle slightly more open for a given speed compared to the older curve to get more airflow
This was done to meet emission requirements for some markets and was also needed to generate enough heat in the exhaust to make the cat. converter operate efficiently
You would need to check what you have with a timing light
IF your dizzy has this curve it is unsuitable for what you are doing
They are easy to pull out---If he's a mate, you could poke your own good known one in there for a try maybe

willy
William Revit

You could just have some fun and experiment with the bob weights and their springs. It's quite interesting.
Allan Reeling

How do you know it is not optimum tune? How will you know what to do to improve it?

Lots of aftermarket bits are described as Fast Road. A standard big valve head and FR exhaust is not going to make a huge difference. In fact I doubt if seat -o-pants meter would tell the difference.

Replacing the 45 with a 25 might solve the problem but it will be luck! The bonus of the 25 is the adjustable vernier which allows you to "tweak" the setting. Tho note this simply moves the curve up /down the graph as a whole; not at individual rev points.

As said the simplest solution is a RR session with someone who can interpret the curve. However in the absence of a fully programmable set up the only quick adjustment is twisting the dizzyi. More involved/expense will be changing dizzy springs and bob weights.

If you hook up a dial back strobe and note the strobe readings at 500 rpm intervals you will be able to plot the current curve. When driving it where does it appear "flat" -what rpm?


Back home, you can twist the dizzy at that same rpm and note the new advance position using the dial back strobe. Try advance a degree or two, drive it and repeat with retard the same amount. If you can physically mark the dizzy and a datum point on the clamp/block this will aid accuracy.

Pinking is the goal-or rather just before pinking, so retard a little till it goes.

It is entirely possible that setting it for optimum at say 3k is not so good at 1k or 5k.

That's as far as you can go with simple adjustment. After that it is changing springs and bob weights -a black art- or the programmable type.

FWIW I did the above after fitting a supercharger. However I was unable to get rid of pinking at a key rev band. Not having the knowledge to change springs and weights, I opted for a programmable type. I finally wound up with a curve where the advance remained constant around 3-3.5 before rising, which I doubt can be replicated with springs. Changing a setting on a laptop is a 1 minute job: changing springs means removing and dismantling the dizzy (and it's hot!).

Michael Beswick

That advance curves look odd to me. The "25" advance being all in at 2000 rpm, is a bit early I would think, suggests weak springs. The 45 also looks all in at 6500 rpm, late!!!. How often do we reach those revs in normal driving?
Allan Reeling

When I rebuilt my B engine I used a 45D4 as I thought it was less worn than the 25D4 that had been fitted. It worked ok but there was way too much advance on a light throttle and it pinked like crazy when hot. Not being one to give up I downloaded all the advance curves from Paulís excellent site and having selected the one for my engine used a test jig I built to set the curve of the 25D4 back to factory default. Iím sure an RR session would be able tweak it a bit better but it seems to work quite well.

One observation I did make was you can set these up at tickover with a timing light and they appear to work reasonably well but the engine feels flat at higher RPMís. I believe this is down to worn/stretched springs, this means the curve is ok up to mid RPM but runs out of advance at the top end. My engine now pulls well all the way to the red line, I donít do this on a regular basis but if I need a bit of extra acceleration itís nice to know itís there.

Bob


R.A Davis

Bob, Weak springs would surely not affect max advance amount, only when it occurred. Weak springs restrain the weights less allowing them to swing out earlier. Another difference between the 25 and 45 is the vacuum unit and where they were designed to have their vacuum take off. 25, carb and 45 manifold. Obviously only significant at idle and on a mostly closed throttle (trailing). I don't suppose either dizzy takes too kindly to it's vacuum being take from the wrong source.
Allan Reeling

Allan

Intuitively youíd think that would be the case, but what happens is the springs stretch slightly. one of the springs, by design, has some slack in it when stationary the other should be under a small amount of tension at rest. If either or both stretch slightly the spring thatís intended to have slack now has a little more and the other spring now has less tension or possibly a little slack in extreme cases.

If you now set the timing at tickover with a timing light thereís enough centrifugal (or should that be centripetal) force to move the weights outwards and take up this slack. The problem is youíve now used up some of your advance taking up this slack. This only becomes a problem at higher RPM as you now run out of advance because the weights come up against the end stops sooner than they would otherwise have done.

With my test jig I can map a curve from tickover to 6000 RPM, I then tweak the springs and re-run the test. It takes a very small amount of adjustment to make a difference on the curve and is quite laborious.

Bob
R.A Davis

I should just add a few more detials of my friends engine.It has been bored to +0.060. It has the standard camshaft with the 252 x 9.25 lift. The head has had unleaded valve seat inserts and a 'smoothing' of the ports and combustion chamber, hopfully without changing the geometry too much. Hes runing No 5 needles with paper air filters. The engine is still running in and runs smoothly, but doesnt pull as well as it did with the head from the 75 engine (small valve) now it has the big valve head on it. My question is would it go better with a 25D4 disi which was contemporary with the head. Hes talking about having a rolling road tune up when its run in. If it ends up with a new disi, how will the tuner select the advance curve ?
Paul Hollingworth

Bob, at what rpm do you look to get maximum advance, presumably 30 - 34 degrees?
Al
Allan Reeling

Probably the person who would be most helpful is Jeff Schlemmer of Advance Distributors, USA.

Cheers

Gary
79 MGB
gary hansen

Alan

I'm running an 18V 780 with the correct big valve head, unleaded valve seats and a pair of HS4's, although they should be HIF's I prefer the HS4's, the rest is pretty stock. It was a while ago that I set the distributor up and I set it up as close as I could to the 18V 780 curve on Paul's website, so it would have been 30 degrees at 6K RPM.

http://www.mgb-stuff.org.uk/41234l.htm

Bob

R.A Davis

"I don't suppose either dizzy takes too kindly to it's vacuum being take from the wrong source."

Makes no odds, vacuum is vacuum, the dizzie doesn't know where it comes from. Apart from at idle where carb vacuum is zero and manifold vacuum is at max or close to it, under normal driving conditions the vacuum signal is exactly the same. As the throttle butterfly starts to open it uncovers the vacuum port, and after that it is 'seeing' the same level of vacuum as the manifold port. More info here http://www.mgb-stuff.org.uk/ignitiontext.htm#vac

The reason for the change was to reduce emissions at idle, North America changed in 1971, UK not until September 76. Whilst North America had many different distributors as MG were chasing continually tightening emissions regulations, other markets only had five for high-compression engines, and from I have been able to find the UK didn't change throughout rubber bumper production, including the change from carb to manifold.
With more advance the engine operates more efficiently, so the revs increase, so the throtle can be backed-off a little to bring them back down, which reduces fuel consumption (at idle) and hence idle emissions.

The curves were also originally chosen for the fuels available at the time, those available now are very different. So much so that even ignoring the fact that the curves may have changed due to weakening springs, for many cars the original timing specs are not achievable without pinking, even on high-octane fuel. A range of brand-new engines to a given spec also differed as to when they would experience pinking, and the timing was set to cater for a 'worst case' plus a safety-factor. Some could always have benefited from more advance than book to give better performance and economy. An A-series I had in 73 certainly did, but my 73 roadster only just managed it and that was on 4-star. With the change to unleaded it has to be retarded slightly, even on 98 octane. Haynes advises flooring the throttle in 4th from 30 to 50, and keep advancing bit by bit until you get pinking, then back it off until it is just perceptible.

When you bring spring wear into the equation pinking becomes more likely. But unless you are going to spend hours on a rolling road plotting the ideal timing for a series of points in the rev and throttle opening range, and try and tweak springs, weights and vacuum capsule to achieve that, the only other options are to splash-out on a programmable ignition module, or do what Haynes says and live with it.
paulh4

Paul-
To try and get at your question, a 25 can be made to have an identical curve as a 45, or the other way around. There is nothing "high performance" about one distributor or the other. They can both be made to do exactly the same thing, though one might possibly be preferred by some people over the other, for whatever reason.

You say you are looking for the best "tune." That makes sense. Keep in mind many cars built since the advent of smog emissions concerns and testing have odd advance curves because of those concerns, and not for the best "get up and go" performance.

The basic scheme for better mileage and lower emissions, that manufacturers started widely using in the late 60's, was to figure ways to safely allow "lean" air fuel mixtures to be used. Often that led to odd advance setups being used by the manufacturers.

D mckellar

PAUL, your last sentence in paragraph 3:
"Haynes advises flooring the throttle in 4th from 30 to 50, and keep advancing bit by bit until you get pinking, then back it off until it is just perceptible."

After my engine rebuild,this is pretty much the way I timed my 79 MGB. I wasn't pleased with the performance of the car until I went through the "set up" of timing and carburetor settings. The factory timing specs were nothing more than just a starting point to "dial-in performance". Mine worked out at 17 BTDC @ 1500 RPM.

Cheers

Gary
79 MGB
gary hansen

R.A Davis, Essex, United Kingdom

Alan

I'm running an 18V 780 with the correct big valve head, unleaded valve seats and a pair of HS4's, although they should be HIF's I prefer the HS4's, the rest is pretty stock. It was a while ago that I set the distributor up and I set it up as close as I could to the 18V 780 curve on Paul's website, so it would have been 30 degrees at 6K RPM.

http://www.mgb-stuff.org.uk/41234l.htm

Bob


Bob, of course you mean 30 degrees of advance at 6K RPM, so 36 degrees advance measured with a timing light at 6K RPM. Static is 6 degrees. The chart looks quite messed up, though, as it says "strobe" 11 degrees at 1K RPM, yet the chart clearly shows just about 2.5 degrees advance at 1K RPM which would be about "strobe" 8.5 degrees. How do you explain this?
D mckellar

Paul Holl-

You asked a question---

"How will the tuner know which curve to choose"

That's the whole idea of having it on the rollers,
The operator, seeing as he's gone to the trouble and expense of setting himself up with rollers 'should' know what he is doing--otherwise he wasted his outlay but then I've seen some funny things come out of tune shops
Simply saying-but there is a bit more involved--
What he will do -or should do is run the car up at a set of rev ranges like every 1000rpm for example and tune to that rev then move on to the next 1000rpm step and repeat
At the end he (or she) will have plotted the desired amount of advance at each step -Then he (or she) can build a dizzy with the curve to suit--or reprog. it if it's computerised
The biggest mistake people make is to do the old run it up till it pings and back it off thing---not good as pinging is well past the safe limit
The only time you would use the pinging method is for using it to select the correct vac. advance can for economy after the initial work had been locked into the distributor or selecting simulated vac advance electronicaly on a computerised dizzy

The correct and only way on the dyno is to find best power at the selected rev. and then retard timing back till power just starts to drop then sneak back til max power is just regained--this is the sweet spot -If this is done at each step it will be a good dizzy when finished
It's a bit time consuming but well worth the effort

Hope that answers your question
willy
William Revit

Does anyone have experience of the Moss part 12H2772 which is a 25D4 with 41155 advance curve especially with an early seventies 18V engine with a larger inlet valve.
(see orange curve below)

Paul Hollingworth

"The biggest mistake people make is to do the old run it up till it pings and back it off thing---not good as pinging is well past the safe limit "

Not according to Haynes, and I've been doing that for over 20 years. If pinging/pinking is well past the safe limit, you could be fractionally short of pinking, and so well past your 'safe limit', and never know it.

The WSM curves do have some oddities. If you look at the early distributors e.g. 40897 you get things like 'static 10, strobe 14 @ 600, and centrifugal 4 at 600. This ties in, as the centrifugal figures are ADDITIONAL advance, not total advance as seen at the timing marks, so 10 static plus 4 centrifugal equals 14 strobe.

This follows through for a while, then they start to go awry e.g. 41032 which has static 5, strobe 15 at 1000, and centrifugal 4.5 at 900. OK, not 1000, but it's not going to suddenly advance an additional 5.5 degrees over the next 100 rpm.

It's why, unless you spend a long time on a rolling road, or buy programmable, the only other option is the Haynes method. All of which basically ignore the published curve data.
paulh4

Not sure when exactly Haynes became the holy grail for engine tuning!! "A faintest trace of pinking" is too much for me! Anyway not sure how in a noisy MG environment you would hear a "faint trace! Pinking is detonation, i.e., explosion, not the steady burn of the mixture across the combustion chamber., and, in my opinion, is potentially damaging at any degree. As for "vacuum is vacuum", that might be so but the manifold take off gave the maximum advance at idle which might go some way to explain why those later B's were notorious for "running on". My '79 B would still be banging away as I walked across the car park!
Allan Reeling

Not tuning, which is a whole different ball-game, but setting-up.

My carb-vacuum roadster had a slight tendency to run-on even on 4-star, much worse on unleaded, so much so I had to install an anti-runon system. To my sensibilities
that was far worse than the occasional trace of pinking, and flooring the throttle at switch-off or stalling it on the clutch just as bad. Each to their own :o)
paulh4

Paul Hollingworth
If I was going out to buy a distributor,for your mate's car, like if he didn't have one, that orange curve would be a better choice out of the three there-
The trend is to have all the mechanical advance over and done with by around 3500rpm and that curve is close to that- it's not bad at all really
Maybe someone has one and will respond here
willy
William Revit

I'm just going back to a 25D4 for originality and the convenience of the vernier advance/retard.
The 45D4 went on the noughties when the bearings were worn in the old one and I couldn't get it rebuilt because of damage to the casting. I was given a good casting which solved that problem.
I got the 45D4 from Summit and just went through the advance curves you could buy off the shelf and got one for an early B with a curve close to the original. Its been on a rolling road twice and has been able to be set with a static timing that works well. Its going to Novatech next week and they are cool with running it up once with the existing one then swapping it out for the original to compare results. I'll let you know. FYI my car is bored out to 1938cc Piper HR270 cam Peter Burgess fast road head 2" LCB manifold and single box exhaust original carbs with suitable needle (sorry cant remember it was recommended by Burlen and after a slight file at station 2 by Aldon they have been perfect). I've just changed from shallow pancake K+Ns to a big Ramair filter, it was big enough to let me get some Moss 1 3/8" trumpets inside it.
Stan Best

For info, in the UK, contact the Distributor Doctor. They may be able to advise you on what you / they can do.

Or get a 123 tune.

http://www.123ignition.nl/

Herb
H J Adler

For 25D4s Dist Doc can supply the original springs from the reference number stamped into the body (repro dists may not have this) - or least he used to. For 45D4 you have to give him the spring characteristics you want.
paulh4

Back from Novatech, Andy was recommended by Gary Smith at Summit and I can see why. The 25D is back on but setting the static timing made a lot more difference than changing the curve. Its great to know the mixture is close enough at all points and is correct enough at idle to keep the plugs clean.
The needles recommended by Burlen could be improved on, Andy recommends going back onto no 6, which would put the car back where the late Tony at Redline put it in 2007 :-). The lambda looks OK across the rev range, but the air/fuel ratio wiggles above and below optimum, he set it it for best fit.
Stan Best

This thread was discussed between 12/04/2017 and 25/04/2017

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