Distributors

[pat lockyer]

Like most engines there is nothing exceptional about the jowett engines to make it unique in respect to Tuning.

In reply to Keith A.

Best you run through the practicle procedure, to correct me, with the Jav and Jup engine of course! not the Bradford etc!

[pat lockyer]

Tension is not relivent to a car model but relivant to the model and geometry the points of the dizzy used.


Reply to Keith A.
So what is the required tension for the lucas distributor DM 2P4 contacts? when used on Jav and Jup.

[pat lockyer]

Total vac degs could be anywhere between 5 and 15 degs. (total advance 32 to 52 degs

Reply to keith A.

In all my days i have never had such a total advance variation given for a standard Lucas DM2P4 distributor.

[Keith Andrews]

The timing of the spark is like shooting a deer on the run...
One has to shoot in front..to alow the time of the flash across the compression chamber and max effect at at the appropiate time for max explosion and pressure to push the piston down..
This Piont has several variables, such as how well the the chambers transfer the explosion, the flow rating of the inlet and exhuast runners, the rev range of the cam, the dynamic compression ratio (pressure at piont of explosion, the type of fuel and efficiency of the carberation systems at different rpms...Some of these variables such as fuel types have changed quite dramitically over the yrs with very differeant properties.

As with the deer, the brand of gun used doesnt matter, the accuracy does.
Also as with the killing power a gun, which may vairy to a big or small caliber caliber, shots per sec etc... the same applies to the spark...ie high voltage (HEI), low voltage pionts and MSD (Multiple Spark Discharge)

These are all common on internal combustion engines (not so much desiel) be they a chainsaw, a Briggs and Stratton lawn mower with a magnito or a top top fuel dragster.

As I mentioned in passing above, one can have to identical engines, both to be used in the same applications, but when fine tuned they will have slightly differant tuning parameters for optimal performance.

There are differnces in a track engine to a street engine..
A track engine is held up in the power band and doesnt drop below full advance, and doesnt need vac advance as it is run at engine vaccums below cruise...Hence the advance is fixed at max performace with no curve at all.
A Street engine uses cruise, off cruise, WoT and full range of rpm therefore needs a curve otherwise it will ping at low rpm

Becuse of no requirements of a curve on a track engine it is very quick (1/2 hr) and easy to establish max efficency of the advance on a dyno.
A street engine takes a long time on a dyno (4 to 5 hrs) at many points of throttle, rpm pionts, to set centrifical and vac advance efficiecy.

[pat lockyer]

Keith A, yes know all that and more.
Pray do tell us the practicle procedure in setting up and tuning the Jav and Jupiter engines, i may have forgot somthing or can tell you a thing or two!

[Keith Andrews]

I sence an underlaying resentment, I have no intention of going down that track..
But if u wish to share how u curve a dizzy and why fine, There is more than 1 way to skin a cat...As I mentioned above there are 2 methods that Im where of, both have there good pionts, I have just presented the method I prefer...please share/explain how u fine tune a custom curve.
If there is an easier way I would like to know because it is a very time consuming job, like profiling a billet cam.

[Tony Fearn]

yes know all that and more

This is a worldwide forum, and I'll bet the majority of members and guests really don't know everything and more about Jowetts, and are actually grateful to be able to read posts by such knowledgeable Jowetteers. Explanations are necessary, as most of us perhaps haven't had a lifetime of working with motor vehicles, so keep it coming in a light-hearted way. Tony.

[pat lockyer]

I sence an underlaying resentment, I have no intention of going down that track..

In reply to The above.
Keith A no resentment i assure you, just would like to know the practical way[ procedure] of which you would go about setting up and tuning a Jav and Jup engine.
Distributor included of course! leading to the ;;;;;;;;;;;;;;
A clue to help you on your way pintle valve spring pressure!

[Forumadmin]

I copied this from a website, most of it is relevant. I have highlighted some points.

Maybe you've just built up a brand new engine, or upgraded to new heads and a cam, perhaps you're simply trying to dial-in an existing combination. In either scenario, one area of tuning that is highly overlooked and greatly misunderstood is timing. All too often we see people dropping in their distributor, making a quick adjustment with their timing light, and setting off to make another pass.

Timing is everything, and without a proper timing curve, every thing else goes out the window. Jetting changes, fuel pressure adjustments, are all useless if first the timing is not set correctly.


So what is timing? In a nutshell, timing or 'ignition timing' relates to when the sparkplug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to melted pistons.

In reality, ignition timing, is a complex physical process, dealing with multiple variable, including compression ratio, volumetric efficiency, combustion chamber shape, cylinder temperature, etc. Very interesting stuff indeed, but we wont get into it here.

In this article we're going to focus primarily on carburated, non-computer controlled, engines which have fully adjustable distributors. The EEC-IV computer controlled Fords allow for setting initial timing, but the rest is adjusted by the computer. The newer modular engine Fords have distributor-less ignitions which offer no adjustability from the factory, although companies like Steeda have recently developed timing adjusters for these engines. Some Fords, particularly in the 70's and early 80's, had distributors where timing was fixed due to emissions reasons.

When it comes to timing the most common myth is that adjusting the timing simply means moving the distributor clockwise or counterclockwise. While this does affect the timing, it is not the correct way to adjust the timing curve. To explain why, we first we need to define some terms.

Advancing and retarding timing refers to increasing or decreasing the 'time' at which spark is delivered to the cylinders. This 'time' is measured in crankshaft degrees, signified by marks on the harmonic balancer, and a reference pointer on the block or timing chain cover. When the piston is at Top Dead Center (TDC), this is synonymous with zero degrees on the balancer. Ten degrees before that point would mean the piston is ten degrees of rotation from being at TDC.

So how does the crank position relate to the distributor?
The distributor shaft on Ford engines is driven by the camshaft gear, which is turned at half-crank speed by the timing chain connected to the crankshaft. Thus there is a direct correlation between the position of the crank and the position of the distributor. Remember, the distributor is a switch. Regardless of the type of distributor you have, there is a fundamental design common to all of them; the shaft is in a fixed position, spinning in direct relation to the crankshaft. On the shaft sits the trigger which activates the switch. On electronic distributors the trigger may be a magnetic sleeve with eight openings, or in the case of points, its simply an arm that open and closes the points. The distributor housing does not spin and it contains the actual switch, such as the Pertronix box, which is mounted on a breaker plate. By rotating the housing you in effect move the position of the switch, changing when it triggers a spark. When you rotate the distributor to "adjust the timing" you are moving the switch on the housing side in relation to the trigger on the shaft.

Rotating the distributor housing clockwise on a Ford advances the timing (i.e. spark is being fired a greater number of degrees before the piston reaches TDC), and counterclockwise decreases the timing.

When referring to timing, there are really four terms that must be considered; initial timing, mechanical (or centrifugal) timing, total timing, and vacuum advance. There is also cam timing which is more appropriately termed valve timing, since it deals with when the valves open and close in relation to crank position. We won't talk about this since it has no dynamic bearing on ignition timing.

Initial: This is the most common adjustment that people associate with timing. At idle, with the vacuum advance hose disconnected and plugged, this is the timing that you would see if you flashed timing light on the timing marks. On typical stock engines you'd see as low as 0 to as high as 15 degrees. Most Ford shop manuals specify around 6-8 degrees initial timing advance for the 289-351 motors.

Mechanical/Centrifugal: Most V8 distributors contain an internal advance mechanism consisting of two each of weights, springs, and slotted 'reluctor' arms. There is also a stop tab for the arms. On Fords this assembly can only be seen by removing the cap, rotor, and breaker plate; we'll get to removal a bit later. As the distributor shaft spins with increasing rpms, the centrifugal force acts on the weights, which begin to force outwards against the springs. This movement rotates the shaft and thus advances the timing. The slotted arm controls how much the weights can move the assembly, and the springs control how fast the assembly reaches that limit. The reluctor arm on a Ford has two slotted sides, only one side contributes to the timing, the arm can be flipped around if more advance is needed (see pictures.) On Fords each side is stamped with a number, usually 10L and 13L; or some have 15L and 18L. These numbers refer to 1/2 of the total degrees of timing that will be obtained when using that arm. So for example a 15L arm would contribute 15 x 2= 30 degrees of timing when full against the stop.

Total Advance: So far we have looked at initial advance and mechanical advance. Both of these combined gives total advance. Say for example initial was found to be 6 degrees, and we visually verified that the reluctor arm was on the 15L side. Total timing, theoretically, is then the initial + mechanical. In this case 6 + (15 x 2) = 36 degrees. If we shined a timing light on the marks (with vacuum hose disconnected and plugged), at idle we'd see 6 degrees, then as we increased the engine speed, we'd see more and more advance, until at some point the total centrifugal advance would be reached, and we would see 36 degrees. When exactly the total advance occurs is of great importance when it comes to performance, and we discuss this in the section below on "curving."

Vacuum Advance: Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpms, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

In short, vacuum advance was developed to optimize fuel economy and reduce emissions. It is not a bad thing to have on a car which sees a lot of street driving, and in such conditions the engine will perform better with it properly adjusted. However many factory and aftermarket performance distributors do not even come with a vacuum advance. The reason is simply because race cars do not spend much time at part throttle.

Curving for Performance
A timing curve is simply a plot of how much ignition advance takes place over the rpm range. In other words, when the timing advances is just as critical as how much it advances.

When it comes to performance there are many different engine combinations, buildups, components, and uses….Each requiring slightly different timing curves. On the other hand if you have a stock motor, and do not care for every extra horsepower, you really do not need to do more than follow the shop manual procedures. However even a stock or mild daily driver motor can be made to accelerate faster with a five minute timing curve adjustment.

The rule of thumb is that the higher the compression ratio, the less total timing it can handle before detonation, and also the higher octane rating it needs to control detonation. Low octane fuels ignite faster, thus require less timing advance. Conversely high octane fuel can handle slightly more advance. Dyno testing has shown that most small block Fords with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance. Engines with 9.5:1 - 10.5:1 run best with 35-38 degrees total, and above 11:1, should not go higher than 35 deg. total. When using power adders such as nitrous, super or turbo chargers, the timing should be advanced accordingly.


The first step in curving a distributor is to set you initial and total advance. As detailed above and in the picture captions, the total is determined by the reluctor arm setting plus the initial advance. Ideally you should keep the initial between 10 and 20 degrees, and the total in the ranges listed above for your compression ratio. For example, if you are shooting for 40 degrees total, and your reluctor arm is on the 15L slot, you would have 30 degrees mechanical advance, requiring the initial to be set at 10 degrees.

The second step is to dial-in how fast the distributor achieves the total advance. This is controlled by the springs which hold back the weights, under the breaker plate. A stock distributor usually has one light and one heavy spring, and brings the timing in really slow, such that the distributor may only reach the total timing at very high engine speeds, 4500+ for example. For performance driving, the best acceleration comes when total advance is achieved before 2500 rpm. To adjust this rate, you can replace the stock springs with lighter tension springs. You can also bend the tabs on which the springs connect to change their tension.


Once you've set the initial and mechanical timing, and adjusted the curve, you should be very very close, if not right at, the optimum timing curve for wide-open throttle performance. You should use timing light at this point to confirm that the initial timing is where you set it, and steady, and then check the timing from idle to 3500 in 500rpm increments. The curve should increase a few degrees at every checkpoint until 2500, where it hits the maximum. After 2500 it should not go beyond the total advance.


Final Thoughts
Hopefully we taken some of the mystery out of properly curving a distributor. Keep in mind these are ballpark ranges, and every engine responds differently. Aluminum heads, large overlap cams, differences in cylinder pressures, all affect timing. Optimum timing can really only be determined on a dyno, or under very controlled and repeatable track conditions. When we dynoed Project 11.99 recently on a chassis dyno, we saw first hand a difference of 30 rear wheel horsepower from timing at 30 degrees total and 42 degrees total! We've also seen gains of up to eight tenths due to improper timing. It is a cheap and relatively quick modification that can be worth significant power.

[Forumadmin]

And this is for a VW engine, another flat four.

What you want is to set your ignition timing about 4 degrees retarded from the point where detonation begins. This setting means the flame front will meet the piston at the top, maximizing cylinder pressure and the time (crank degrees) to push the piston down, making the most power. If you ignite the charge too late, the pressure doesn't build until the crank has rotated some, and you lose precious crank degrees (HORSEPOWER) of work. If the charge is ignited too early, the flame front will hit the piston ON THE WAY UP, and this is a sure-fire way to destroy your engine very quickly.

What is needed is to find the timing point at EVERY RPM point where you are around 4 degrees retarded from detonation under full throttle, since this is the "ideal" time for our spark to occur. Obviously, you would leave yourself a safety zone of 3-4 degrees of timing at all points to CYA in case of bad gas, clogged main jet, vacuum leak, abnormally hot engine, etc. The power difference between detonation and 3-4 degrees of retarded timing from this point is negligible, so riding the ragged edge of timing is not worth the risk, IMO. Detonation does NOT have to be audible for it to turn your expensive mechanical marvel into junk in short order.

Finding the Advance Curve

Ideally, you would remove all advance from the distributor, and run a locked timing. You then put the engine on a dyno, and play with timing across the RPM band from idle to redline at full throttle (finding the timing where you are 4 degrees from detonation) and then chart it. Then, you remove the locked timing from the distributor, set your initial timing to match what you found was best at idle. Next, match the distributor's curve to match what you found to be best on the dyno by changing weights and springs in the distributor.

If you have the patience and time to set up this custom curve, and have a heavily modified engine, your patience will be rewarded! Get yourself a Mallory/MSD distributor. You will find they are VERY adjustable. The easiest method to follow is very straightforward. Find the MAX advance point your engine tolerates (between 30-40 degrees BTDC) above 3000 RPM. Now find the place where it responds best at idle. It may be 15-24 degrees BTDC. You may find that this much advance causes starting difficulties when the engine is hot, so be sure to check this out! Ignition before top dead center causes "negative torque" and that's what suddenly STOPS your engine from cranking. Avoid this, or use a retard that is activated during cranking.

Now, you know the two critical points in your advance curve. You only need to figure out how FAST you want the advance to come in. I recommend being conservative, and kick the advance in SLOWLY and see how the engine responds. Keep adjusting it (it requires distributor disassembly) so it comes in quicker and quicker, until you detect detonation (problem). Back it to the previous setting, and you are DONE with the centrifugal advance. Now, dial in 10 degrees of vacuum advance, and adjust the advance so the carb(s) will actually activate it and you are done! PHEW!

Frankly, though, finding the advance curve using this method isn't practical for most of us.

[Keith Andrews]

Both very good articules...
One thing to watch with remarks like

The rule of thumb is that the higher the compression ratio, the less total timing it can handle before detonation, and also the higher octane rating it needs to control detonation.

Yes that is correct, but calculated compression ratio is little morethan a advertsing ploy and means nothing...Due to overlap on a cam, compression starts after the piston rises, therefor REAL or dynamic compession ratio is what should be looked at...
When I was looking at the Bradford cam/tappet clearances, it would be the only vechile engine I know of that basically the valves close TDC and BTC!!!
A SB chevy/Ford/Dodge whatever modded up for performance with a 10.5:1 or 11:1 static compession ratio is quite street drivable and economic if a high duration cam is put in...reducing the dynamic ratio down as low as 9 or 9.5 :1
Most modern cars run around the 8 to 9.5 :1 Static, but calculate the dynamic one gets 8 to 8.5.
This is pleasing because it doent take much to bring low compression jowetts up to modern stds for modern pump fuels with a little maching and billet custom cam profiles

Add a good timing curve, bit of modern therory and /I recon these things will literially fly like they where never designed to do....Bigger brakes? lol

[pat lockyer]

bit of modern therory and /I recon these things will literially fly like they where never designed to do....Bigger brakes? lol.


Reply to Keith A.
You will need to do more than just brakes.
Doing those sort of mods will hit the relability factor in many engine parts to zero, hardly the way forward for the many standard cars with club and other owners of this most interesting car.
Before you carry out the above r-----h prehaps you better warn of all possible costly failures.

Now in my mind keeping to the standard practicle procedure of setting up is the way past and present.
Still waiting for your methods on a Jav Jup standard engines.
Another clue!
Not hard! with the cent adv and intermediates then your vac adv %in hg start/ends and totals?
Assuming your dwell is correct! HO Hum.
But of course much testing and setting up procedure to go through as you
may well know?
We have only got to the first bit in real terms ie checking the dist drive bevel gears and the shaft o/s drive for wear!
Await your method with interest!

[Forumadmin]

The whole point of this discussion to to answer the question ' Is the distributor curve correct for modern fuels?' with a supplementary question ' If you have performed the recommendations in the 'Jowett Tuning Notes' or any other mods ' 'Is the distributor curve correct for my engine in its current state?'

It is not all about power; but also about effecient burning and reducing polution. One day the cars may be subject to polution tests. Keeping the timing on the stochiometric curve or just on the lean side is key to economy AND polution.

Any comment guys?

[pat lockyer]

Chris Cole Wrote. like DM2 of D45 etc. etc. And has anyone the vaumun advance curves for this at various RPM's? Is it a diect replacement for my worn DM2?


Yes Keith C, before we go down the route of messing with vac advance certain practical procedure is first! vac advance is later.
We certainly do not want to mess with cam profiles and dist lobe profiles on standard motors.
Do we agree on this first. If so prehaps Keith A will proceed with his practical procedure, from where i left of for starters remember the pintle valve of course!

[Keith Clements]

No, we do not agree. Getting the curve correct generically for modern conditions and the way in which many Jav/Jups have now been modified since 1947, when they were designed, is important. Showing how to finely tune that generic curve for a particular engine is also important.
Knowing what the curve should be will also help choose or modify a replacement.
It may well be the modifications such as higher compression, better gas flow, and cleaner fuel cancel out the difference in fuel and driving conditions. Please let us discuss logically, technically and scientifically with some anecdotal experience as well.

Those Greens among us may also want to run leaner, cleaner engines, so how do we do that?

Please let us apply our massive collective knowledge to the problem and share it on the forum. Perhaps the timing was not correct on that modified engine of yours, Pat?