Ignition timing Z-head

[Lexveen]

I have a question on ignition timing with the Z-head.
I installed a Z-head on my ‘26 T. The most important feature is not the higher compression, but the squish zone which creates strong turbulence of the air-fuel mixture as the piston approaches TDC. This turbulence garantees a much completer combustion than with the original head.
I also installed a lambda sensor to monitor the mixture.
I set the timing as usual with the gauge, which matches with 15 degrees after TDC. The engine runs smooth and powerful at a mixture of 11.5-13.5. However, I experience spark knock if I advance the ignition according to the book.
I measured the timing at the lever setting where spark knock occurs and the maximum allowable advance without spark knock is not more than 10 degrees after TDC at 30 mph. This is about half as much as given for a stock engine.
When I investigated the ignition timing on engines with comparable cilinderheads, like the later Ford flathead V8, the ignition advance at engine speeds of 1500 rpm is just 8 degrees before TDC. That confirms my experience.
The reason seems to be that the turbulent mixture burns much faster and therefore needs less advance.
What do other users of this Z-head experience with advancing the ignition and spark knock?

[Kerry]

What we use in my part of the world is better octane fuel, your highest is the same as our lowest, 95 is our mid range which works fine but if we need to 98 octane is available as well.

[TXGOAT2]

With higher compression you might need less advance than otherwise. If the engine is in good condition and cooling properly, advance just short of detonation is correct at normal operating speeds and loads. For maximum performance, timing must be adjusted to accommodate the operating conditions of the moment. There are no hard and fast rules for T timing, and no fixed timing will be correct for all conditions. A lean mixture will promote detonation, and poorly adjusted coils might cause one or more cylinders to detonate under some conditions while the others did not.
If you hear "spark knock", back off on the advance just enough to eliminate it. If you'd rather not ride the spark lever for best performance, a modern distributor or an E-timer will do it for you.

[Lexveen]

What we use in my part of the world is better octane fuel, your highest is the same as our lowest, 95 is our mid range which works fine but if we need to 98 octane is available as well.

Luckily I live in Europe where we have 98 and in some towns even 102 octane. I will use the 98 in stead of the 95.

[TRDxB2]

Countries have different methods of for octane ratings - so its not a number to number comparison
--
In the United States, Canada and Mexico, octane ratings are in AKI, commonly shown as "(R+M)/2". All states require gas pumps to be labeled with the correct octane . There is some minimal variation by State
Regular Unleaded 85
Mid-grade Unleaded 87
Premium Unleaded 91
--
There are three standard grades for octane fuels in Australia and Eutrope, measured using the Research Octane Number (RON) rating.
Regular (unleaded) octane fuels (91 RON)
Mid-grade premium octane fuels (95 RON)
High-grade premium octane fuels (98 RON)
--

us - eu.png (88.95 KiB) Viewed 7659 times

[TXGOAT2]

A T with a Z head should not require high octane fuel. 85 octane (US) ought to be more than adequate.

[Oldav8tor]

I run regular gas (87 Octane) with my Z head and have never noticed any spark knock, regardless of spark setting. With no instrumentation, I adjust the spark to the point the engine runs smoothly.

[TRDxB2]

A T with a Z head should not require high octane fuel. 85 octane (US) ought to be more than adequate.

EXACTLY - Z-Head is advertised as having a 6 to 1 compression ratio. Modern Car manufacturers indicate that there is no advantage in using a higher than the recommended Octane rating other than to the petroleum manufacturers & service station owners.

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compression ratios.png (105.39 KiB) Viewed 7559 times

[Kerry]

One can run the maths, recommend, speculate, assume and guess what should or shouldn't need or work, but at the end of the day it is how the engine runs. I've fitted Z heads on what sound very healthy as stock, but after fitting the head for what sounds like pre- ignition turns out to be piston slap under load.

[MichaelPawelek]

I must be missing something but how is it possible to get spark knock at 10-15 degrees “after” TDC?

[TXGOAT2]

I wondered about that.... if that's not a typo, 15 ATDC is way too late for anything but starting the engine. Running the timing that late under operating conditions could result in hot carbon or very hot exhaust valves causing detonation under load.

[Craig Leach]

I'm going to assume he meant before TDC. 30mph @ 15 degrees after TDC may melt the engine down after a while. I once did a valve job & had
the head shaved ( stock a lot ) & developed a knock turned out to be a rod that didn't knock with much lower compression. Maybe this is a good
time to use a bore scope & check for shiny spots on the pistons if none found maybe a inspection cover removal & inspection is in order.
Craig.

[mbowen]

I run regular gas (87 Octane) with my Z head and have never noticed any spark knock, regardless of spark setting. With no instrumentation, I adjust the spark to the point the engine runs smoothly.

Same here, I’ve put 2400+ miles on my Z-head in the last year using regular 87-octane E10 pump gas, timing set by the book, and have never had any detonation. I can’t recall how many turns out the mixture is, but I leave it alone and regularly get 20-21 mpg and haven’t had to clean the plugs in the last 1000+ miles.

[Chris Barker]

Lex,
that timing diagram in your original post - much reproduced - is actually useless and wrong if you're running on coils and magneto.
With the magneto, you get just three basic choices of timing.
15.5 ATDC (just for starting)
18.5 BTDC
41 BTDC

If the lever allows it, you could have 63.5 BTDC but it's no practical use!

See here:
https://www.fordmodelt.net/downloads/Mo ... nition.pdf

There is just a slight variation around those numbers because as rpm increases, so does magneto voltage output, and coil charge time falls and there's a little 'auto-advance'. And there are certain lever positions where the timer and magneto combination don't give enough charge time - 'dead bits'.

If you run on 12v DC and/or have a distributor, you do get an infinite choice. With a 12v battery you could experiment and find the knock threshold.

[Lexveen]

I must be missing something but how is it possible to get spark knock at 10-15 degrees “after” TDC?

You are quite right: I meant before TDC. Sorry for the typo.

Another thing that I did not take into consideration: the time the coil needs to produce a spark. That takes about 1.5 to 2.0 ms, which matches 13 to 18 degrees crank angle at 1500 rpm. If the spark has to arrive at 10 degrees before TDC to get the full effect of the combustion, the timer should be set at 23 to 28 degrees before TDC.

[Lexveen]

Lex,
that timing diagram in your original post - much reproduced - is actually useless and wrong if you're running on coils and magneto.
With the magneto, you get just three basic choices of timing.
15.5 ATDC (just for starting)
18.5 BTDC
41 BTDC

If the lever allows it, you could have 63.5 BTDC but it's no practical use!

See here:
https://www.fordmodelt.net/downloads/Mo ... nition.pdf

There is just a slight variation around those numbers because as rpm increases, so does magneto voltage output, and coil charge time falls and there's a little 'auto-advance'. And there are certain lever positions where the timer and magneto combination don't give enough charge time - 'dead bits'.

If you run on 12v DC and/or have a distributor, you do get an infinite choice. With a 12v battery you could experiment and find the knock threshold.

I am not running on magneto (it simple is not there) but on 12 V battery. I made a trip last Saterday and experimented with the lever. At a constant speed of 25 mph and the lever at 9 o’clock, it ran smooth. If I advance, spark knock starts to occur when I set the lever at 8 o’clock.

[Lexveen]

A T with a Z head should not require high octane fuel. 85 octane (US) ought to be more than adequate.

EXACTLY - Z-Head is advertised as having a 6 to 1 compression ratio. Modern Car manufacturers indicate that there is no advantage in using a higher than the recommended Octane rating other than to the petroleum manufacturers & service station owners.
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compression ratios.png

Compression ratio is not the only determining factor, the size of the combustion chamber is also important. If it takes too long for the flame front from the spark plug to reach the other side of the combustion chamber, the pressure will rise high enough to cause detonation. In the T, the spark plug is above the valves, which means that the distance is large. The extreme point is the squish zone. The temperature and pressure there rise high at the end of the compression stroke because the mixture is compressed extra strongly there. Could that be a reason why a Z-head tolerates less advance with 85 octane?

[Moxie26]

Some Z heads were built with extra metal that would give a bit greater volume, but lesser compression ratio. ..... if you do have a Z head, you should really take compression tests on all four cylinders to come out with the true value . . . checking your piston rings and valves....

[TXGOAT2]

Later flatheads typically locate the spark plug over the valves with the squish/quench zone being most distant from it. With that arrangement, some quench effect exists when the flame front reaches the more distant parts of the chamber. Squish takes place as the compression stroke reaches TDC, and quench occurs as the flame front sweeps across the chamber into the squish/quench area and before the piston has moved very much off TDC.

[Lexveen]

When installing the head I checked if piston and head do not touch each other without the gasket and they don’t. Same check after laying the headgasket between block and head. No interference. So the distance between the top of the piston and the squish edge in the head is a fraction more than the thickness of the copper head gasket.
I just measured the compression on a cold engine and it is:
cylinder 1: 6.0 bar (87.0 psi)
cylinder 2: 6.05 bar (87.7 psi)
cylinder 3: 6.1 bar (88.4 psi)
cylinder 4: 5.9 bar (85.6 psi)
I have a wide band lambda sensor mounted and the mixture is 12-13.5 all the time. All plugs are light brown after a trip of about 60 miles.

BTW: crank (SCAT), bearings, rods, pistons, cam (standard), valvetrain, coils (adjusted and matched) and timer are new, cilinders are lined standard bore.

[Moxie26]

Try something simple first before you start tearing the engine apart... Use higher octane gasoline as an experiment to see if you can advance your spark lever past the 8:00 position and do not hear any knocking...... Really, what are your expectations of performance out of the 20 horsepower engine?

[Allan]

Might I suggest you contact an experienced T owner/driver and have him take you out for a ride in your car. That way you get a second opinion on its performance. I did this for a friend in Spokane in his newly restored depot hack. He was disappointed with its performance but really didn't know what to expect of his T. Even allowing for the ice cream we had taken on board at the last stop, the performance was doughy at best. It turned out that he was not getting full throttle. A tweak of the linkages and away she went.

Allan from down under.

[TXGOAT2]

I don't think high compression is causing detonation. As I understand it, compression with a Z head is not going to be much over 5.5:1 and volumetric efficiency is low, even at full throttle at sea level at moderate engine speeds.
The spark control linkage on Model Ts is not very precise at best, and using the position of the lever is a rough guide at best.
With 87 octane gasoline, a performance cam, high volume intake, .060 over bore, aluminum pistons and a stock head, I've never observed any indication of detonation or dieseling with my T, even with indicated engine temperature over 200F and ambient temperature over 105F at about 1200 ft above sea level. I usually start on Battery and run on Magneto.

[Lexveen]

I did not suggest to tear something apart. The engine is well built and mechanically running as it should. Neither do I have a complaint about performance. That is OK too. It runs smooth and strong from 500 to 2000 rpm. I only hear spark knock and that is why my question in this topic is: what do other users of this Z-head experience with advancing the ignition and spark knock?
I am greatful for your reactions. What I have read is that some of you use or have used a Z-head and did not experience spark knock. What I also have read is that advancing the lever too much does not bring anything but lower performance and detonation. I fully agree with that.
Is the right conclusion that with a combination of 12 V on a standard buzzer coil ignition and a high compression head with squish zone (which gives a strong turbulance and fast burning mixture), a lesser bit of advancing is needed to get a spark at the right moment?

[speedytinc]

Have you tuned your coils with an ECCT? Have a timer timing issue? Possibly you have a cylinder firing early or inconsistently.

Where your spark lever sits for best performance will depend on where your linkage is set & the play there in. A blanket "2/3" setting dont mean much.
I set mine with quite a bit more retard for better engine braking.

Your Z head doesnt have as much compression as advertised. I have run Z heads, Prus heads, a Lizard head(7:1) & a low head with 95# compression(6.5:1). I have not experienced knock from regular fuel, compression still is not high enough. In the 60's & 70's, for example, regular fuel was recommended in motors of 9:1 compression & less.
One can over advance the timing & get a knock with noticeable less performance. Set your spark lever by feel, where the motor is strongest/happiest.
It's another matter if you were running on magneto. With the 3 or 4 specific timing nodes, I could see the possibility of over advancing in that last node.
You would still feel it in the performance.

[Lexveen]

Does anyone have figures about the compression pressure? As I wrote earlier, I measure between 86 and 88 psi on a cold engine. What is the pressure with a standard head and pistons?

[speedytinc]

Stockish, Montana legal motor is 60#
55-60# with a stock head.

[Lexveen]

Stockish, Montana legal motor is 60#
55-60# with a stock head.

Whatever the real ratio of the Z is, 86-88 for a Z is about 1.5 times higher then 55-60 with a 4:1 ratio….

[speedytinc]

IF you are @ sea level, your compression calculates to 5.85. Thats higher than I have ever measured with a Z head.
Given the typical casting variations, I say you got lucky. Who's to say your block hasn't been decked?
Thats still a very low compression ratio by modern standards, but sure hops up a T.

[Moxie26]

Lexveen ...... Atmospheric pressure is 14.7 lb per square inch. Divide your compression pressure by 14.7 and you will get the ratio . Example .... 88 divided by 14.7 equals . .. 5.986 to 1 compression ratio !

[Art M]

Speedy,
Calculating the compression pressure involves thermodynamic considerations. The heat of compression must be taken into consideration. Simply put, compression pressure/atmospheric pressure ratio is higher than the compression ratio.

I have a std. 3.8 compression head and have 62 psi compression on each cylinder. Using a direct ratio, the compression pressure would be about 55 psi.

Art Mirtes

[speedytinc]

Speedy,
Calculating the compression pressure involves thermodynamic considerations. The heat of compression must be taken into consideration. Simply put, compression pressure/atmospheric pressure ratio is higher than the compression ratio.

I have a std. 3.8 compression head and have 62 psi compression on each cylinder. Using a direct ratio, the compression pressure would be about 55 psi.

Art Mirtes

Thats above me. I use combustion volume measures & compression readings for comparisons between various heads.
Using simpler formulas works for me. The #'s you quote dont fit my ignorant mind.
Feel free to attempt to educate me on thermodynamic considerations.

[TXGOAT2]

Compression pressure will vary at different engine speeds and throttle openings. Some engines will develop passive supercharging at some speeds and throttle settings, and volumetric (pumping) efficiency usually falls off a higher RPM and will fall off sharply as the throttle is closed. Detonation usually occurs at lower RPM and at or near WOT.

[TRDxB2]

Compression Ratios can be a false premise to actual performance. So what is the increase in horsepower.

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[Lexveen]

[/quote]
Feel free to attempt to educate me on thermodynamic considerations.
[/quote]

I am not in a competition on math, nor do I try to be smarter than others. I try to learn from others on a subject that puzzles me. I am searching for reasons why I hear spark knock. My simple statement is, that the end pressure of my Z is 1,5 times higher than a stock head -with clothed throttle- which could be one of those reasons.
And yes, I cannot exclude the deck has been surfaced, in a 100 years maybe more than once. However, the piston to head clearance didn’t give problems or was there a need to remove material from the head.

[Lexveen]

Compression pressure will vary at different engine speeds and throttle openings. Some engines will develop passive supercharging at some speeds and throttle settings, and volumetric (pumping) efficiency usually falls off a higher RPM and will fall off sharply as the throttle is closed. Detonation usually occurs at lower RPM and at or near WOT.

Compression measurement are usually done at the speed of the starter motor, so very low engine speed and with closed throttle. There are more things that influence the compression pressure: leakage along the rings and valves, carbon deposits (not uncommon ), camshaft timing. You only can compare the measurements if the engines are mechanically comparable. My engine is newly built so leakage is probably minimal, it has a stock cam which gives less blowback at low rpm and the combustion chamber is quite clean on the inside. However, an objectively measured 50% higher compression pressures doesn’t exactly reduce the chance of detonation.

[Lexveen]

Another thing that affects the compression pressure under load: the carburetor. I am a big fan of the British SU carburetor and have fitted one. A 1 1/8 inch model from a 1926 Morris Cowley. A "period correct" modification I would say. The SU carburetors can be adjusted very well over the entire load range, which is proven by the lambda values. The atomization of the fuel is also better than with the original carburetors. The spark plugs are all the same color.
Relevant for this topic is that they give little flow resistance, because the passage is straight. The filling degree of the cylinders is therefore very good, which results in a high working pressure. But.... also in a greater chance of....

I’ll fill up the tank (its nearly empty) with 98 octane and add a bottle of octane booster that increases the octane by a few points. Then the fuel will be as detonation resistant as it can get. I will make a trip this afternoon then lets see if that prevents spark knock.

[Moxie26]

Your lack of power could be directly related to using the standard iron intake manifold. Why don't you put on a high volume intake?.. your engine has the dual exhaust manifold, not Ford issue, but available through vendors.

[Lexveen]

Your lack of power could be directly related to using the standard iron intake manifold. Why don't you put on a high volume intake?.. your engine has the dual exhaust manifold, not Ford issue, but available through vendors.

Lack of power? I am very satisfied with the power.

[Lexveen]

As I wrote, I filled up the tank yesterday with 30 liters of 98 octane and a bottle of octanebooster. I drove about 60 miles with speeds between 15 and 35 mph. I could pull down the lever up to 8 o’clock without spark knock at constant speeds. When accelerating I had to retard a little to avoid spark knock. However, I experienced that pulling the lever further down than 9 o’clock at constant speed did not make better running. If I leave the lever between 9 and 10 o’clock I have a satisfactory performance over a wide range of speeds and only have to retard a little when accelerating. I am starting to understand my T better. Next thing I’ll do is measure the crank angle timing in degrees before TDC with the lever at 9 o’clock.

[Allan]

Be aware that there is a multitude of different needles to fit SU carburetors, each with its own profile to provide different fuel metering as the piston rises.

Also, a Morris Cowley engine has considerably less cubic capacity than a T and was also meant to run at higher revs. A needle that suits the Cowley is likely not going to suit a slow revving bigger displacement T motor. You will need access to a range of needles, and details of their profile, to better suit the carb to its different role.

By any means, get the timing right, and then work on finding a needle to deliver the fuel as required.

Allan from down under.

[TXGOAT2]

May be too much advance plus a very lean mixture. Both will contribute to detonation.

[Lexveen]

Be aware that there is a multitude of different needles to fit SU carburetors, each with its own profile to provide different fuel metering as the piston rises.

Also, a Morris Cowley engine has considerably less cubic capacity than a T and was also meant to run at higher revs. A needle that suits the Cowley is likely not going to suit a slow revving bigger displacement T motor. You will need access to a range of needles, and details of their profile, to better suit the carb to its different role.

By any means, get the timing right, and then work on finding a needle to deliver the fuel as required.

Allan from down under.

The SU is the same size as the original NH: 1 1/8 inch. I have been working with SU for over 20 years on Austin Healeys and yes, there are a lot of different needles and a few springs too. This historical bronze bodied carburator (1926, as my Touring) has been rebuilt by the Burlen Fuel Company who owns the brand SU and manufactures the carburetors and parts new. They als selected the needle and spring. So I think they can be trusted. They did a good job because according to the wide band lambda sensor, this combination is spot on. From idle to full load the lambda sensor shows a mixture of 1:12,5 and 1:13,5 which is perfect and not too rich or too lean. Lean would be if I saw 1:14,7 or higher. The plugs are brown. And I was supprsed by the fuel comsumption. I made several trips now and it runs 10 to 10,8 km on a liter, which is approx 25 mpg.

[Lexveen]

I was not happy with only the practical experience and impression, so the last week I read a lot of technical/scientific publications like the book of Harry Ricardo on the internal combustion engine. Some of this I would like to share with you.

The flame that is started with the spark, takes some time to burn the complete mixture in the combustion chamber. Meanwhile the pressure rises. The peak pressure must be reached about 15 degrees after TDC.
The burning rate of the mixture depends on many things like mixture, compression and turbulence of the mixture in the combustion chamber.

The standard head of the T has a combustion chamber that is common for pre 1914. It simply is a space above the piston and valves. The mixture in the chamber is slow moving and the corresponding burning rate is low.
The Z-head and the Prus head have designs that became common for side valve engines in the period after 1914. You can find this style of head on many side valve engines like the flathead Ford V8 and the competition after market heads from Edelbrock. These heads are designed in such a way, that the mixture gets turbulent during compression. This combustion chamber is located partly above the piston and above the valves. A part of the piston is so close to the head that it is practically blanked of, the “squish zone”. At TDC there is no mixture is that area. Because the mixture is very turbulent, this head design is known as “turbulent heads”. Due to the turbulence, the burning rate is much faster than with the standard head.

Another thing is the dimension of the chamber. The flame needs to reach the outer edges of the chamber. The more compact the chamber is, the faster everything is burnt. For a standard head the largest distance from the spark to the most outer edge of the chamber is 4 1/2 inches. In the turbulent head designs the chamber is much more compact, because the area above the piston is partly blanked off. The largest distance from the spark to the edge of the chamber is only 2 inches.

The result of both is, that in the turbulent head the complete mixture is burnt in a shorter time and peak pressure is reached earlier. This also means that when the highest pressure must be reached at approx 15 degrees after TDC, the advance of the ignition in the turbulent head must be less, otherwise the peak pressure is reached before TDC. Then you loose power and you get spark knock.

How much less advance? To get a feeling for that, I did some calculation on both the standard head and the turbulent head. For the turbulent head I chose the PRUS head, because that is the most turbulent one. My math is done primarily to illustrate the differences. Some figures and assumptions, which are open for discussion.

Distance spark plug to outer edge of the combustion chamber: 4 1/2 inch for the standard head, 2 inches for the Prus head.
Burning rate according to the literature is approx 1200 inches/sec for the turbulent head. For the standard non-turbulent head no figure could be found, but my assumption is 800 inches/sec.
Time to get a spark varies, but figures I found are 1,5 to 2 msec after timer starts conducting. Lets put that figure at 1,5 ms (on 12 V and magneto that will be about right).

First calculation at 800 rpm which is 800 x 360 / 60 = 4800 crankshaft degrees/sec

Standard head at 800 rpm
Burning time 4,25/800 = 5,3 ms
Time to spark 1,5 ms
Total time from timer is conducting to peak pressure 6,8 ms
Crank angle = 0,0068 x 4800 = 33 degrees
Peak pressure at 15 degrees after TDC, so timer must be set at 18 degrees before TDC

Prus head at 800 rpm
Burning time 2/1200 = 1,6 ms
Time to spark 1,5 ms
Total time from timer is conducting to peak pressure 3,1 ms
Crank angle = 0,0031 x 4800 = 14,8 degrees
Peak pressure at 15 degrees after TDC, so timer must be set at TDC

Second calculation at 1600 rpm which is 1600 x 360 / 60 = 9600 crankshaft degrees/sec

Standard head at 1600 rpm
Burning time 4,25/800 = 5,3 ms
Time to spark 1,5 ms
Total time from timer is conducting to peak pressure 6,8 ms
Crank angle = 0,0068 x 9600 = 66 degrees
Peak pressure at 15 degrees after TDC, so timer must be set at 51 degrees before TDC

Prus head at 1600 rpm
Burning time 2/1200 = 1,6 ms
Time to spark 1,5 ms
Total time from timer is conducting to peak pressure 3,1 ms
Crank angle = 0,0031 x 9600 = 29,7 degrees
Peak pressure at 15 degrees after TDC, so timer must be set at 14,7 degrees before TDC

The Z-head has a somewhat different shape and the distance from spark plug to the outer edge is a bit larger. This head needs a bit more advance than the Prus head, but still a lot less than a standard head.

These calculations shows the advance of the ignition with a HC head is much less than with the standard head.

Because the time to burn all of the mixture is much shorter with a turbulent head, the burning can start later and the pressure build up before TDC is much less. That means you lose less power during compression and that leaves more horses for the propulsion.
Because the compression ratio is higher too, you lose a bit more power on compression, but the expansion is also bigger and you extract more power from the burnt mixture. The net result however is quite positive. A CR of 5 versus 4 gives roughly 15% higher thermal efficiency, 6 versus 4 gives 25%.

[TXGOAT2]

I used to run a lot of "drip" gasoline. It varied in octane, but it was low at best. Correct timing was what ever gave the best performance with minimal "knocking, and near-spec initial timing gave the best overall result. Limiting spark retard was best, since very late timing reduced power and caused higher combustion chamber temperatures, which promoted knocking. As for compression, I found that a high compression ratio with a squish type head worked best. All throttle governed engines have variable compression when running, and using the throttle to control detonation gave the best power and mpg with the spark timing set near spec. If dieseling was a problem on shut-down, I'd put the transmission in gear and let the clutch out as I switched the ignition off. With an automatic transmission, turning on the air conditioner or stopping the engine in drive would usually kill it. I don't have drip anymore, but if I had some clean, low sulfur drip, I would definitely run it in the T.

[Jerry VanOoteghem]

I used to run a lot of "drip" gasoline.

Pat,

Maybe explain what "drip" gasoline is...

[TXGOAT2]

"Drip" or casinghead gasoline is gasoline that naturally condenses in the oil field gathering lines from "wet" natural gas, usually in cool weather. Quality varies widely. Around here, for many decades, the natural condensate gasoline approximated pump gasoline as far as vapor pressure and general characteristics, except that it lacked the octane equivalency of pump gasoline. By paying attention, you could drive thousands of miles on it without doing any harm to the vehicle. Again, not all natural gasoline is suited for use in gasoline engines. Some of it is loaded with sulfur, and some is closer to kerosene than gasoline. Ideally, you'd mix some pump gas with it, but as a kid with little cash, I usually ran it straight. At one time, you could obtain a "tax sticker" to burn it legally, but I never heard of anyone getting in legal trouble for using it. Around here, the cops called it "white lightning".

[Allan]

Lex, if you have specifications/part numbers of the needle and spring provided, I would appreciate details. My son has an early SU which he wishes to use on a speedster build some time.

Allan from down under.

[Chris Barker]

Studying Lex's excellent essay above, it's interesting that the timing he calculates for the stock head (18BTDC at 800, and 51BTDC at 1600) make it clear that Henry's magneto timings of 18 or 41 are, by luck or test, about right for normal T driving.

[Lexveen]

Lex, if you have specifications/part numbers of the needle and spring provided, I would appreciate details. My son has an early SU which he wishes to use on a speedster build some time.

Allan from down under.

I don’t have them, but meanwhile asked the guy who rebuilt the SU. But…. No garantee it works the same as in my case. It strongly depends on the engine. What head is on it? What manifolds? What bore size? What cam? What valves? I guess I had some luck it worked out fine. However, if I get the productnumbers, I’ll post it on this forum.

[Allan]

Thank you Lex. Any thing is a better place to start from than absolute scratch. With the right jet tube, needle, spring and oil in the dashpot, those SU carbs can be used on almost anything. My son had a 1.5" one on a 6 cyl 250ci Ford Falcon ute, and it ran and performed very well.

Allan from down under.

[Lexveen]

Allan, I got the reply. It’s a ‘M’ code needle with an 8oz Yellow code spring. The carb is a 1 1 1/8” HV2 type with the vertical throttle spindle, originally fitted to the Morris cowley (bullnose & flatnose). Hope it works out well for you.

[Allan]

Lex, that's great news. Duly noted for future refernece. Many thanks.

Allan from down under.