Ignition timing on XK120 FHC 669085

(Johnball) #1

I am interested to know what numbers people use for their ignition timing and any thoughts or experiences.

My engine is 3400cc, B type head, Hayward and Scott 6 branch SS exhaust, 9/1 compression, cold cylinder pressures170lbs, plugs BP6ES, carbs 2x 2”HD8’s, UO needles, Rob Beere Standard Road camshafts designed for SU carbs 0.41lift, ignition system CSI from Denis Welch ( a sort of copy of the 123 system but with their own curves ). I have it set at 10BTDC static and at 700rpm without vacuum connected, to set up.
But with vacuum connected, the curve gives 22 degrees at 2,000 rpm,
26 degrees at 3,000 rpm, 30 degrees at 4,000 rpm, Maximum of 34 degrees from 4,500 upwards.

Not suggesting mine is right or wrong, just want to know what you all think?
The car is for fast road use to annoy neighbours, but not track uses.

(Johnball) #2

No comments, advice, thoughts, arguments, criticism- a bit odd !? Or are you all sitting by the Christmas tree drinking hot mulled wine !?

(Gary Grant) #3

Hi John,
OK I’ll bite.

For what its worth, your advance curve seems ok to me. However, the info you gave is slightly unclear.
Are you saying that you get a different timing advance number when the vacuum is disconnected, versus vacuum disconnected ?
You should not be able to observe any difference in timing (at idle speed) when you disconnect(and plug the source of vacuum) and when you connect the vacuum, when running the standard Lucas distributor.

A little more info would help understand what is going on…what distributor are you using, and exactly where is the location of where the vacuum comes from? We need to know if the vacuum source is ported or not ported.
Can you hear any pinging or knock when driving the vehicle?

(Johnball) #4

Hello Gary,

Thank you for your reply,

To answer your questions.

The vacuum I had disconnected and blocked off when running the engine to obtain 10 BTDC. Then reconnected for the various advance numbers.
The ignition system I have is a copy of the 123 Dutch setup. It is made by a rival new company also in Holland, who I believe were a break up of the original people ? It is sold in U.K. by Denis Welch. It is adjustable from under the cap with a cross head screwdriver, with several curves depending whether road or race. DW put their own curves in the system. They are well known for racing Jaguars and Healey’s. I also have one fitted to my Healey 100 which has made a remarkable difference to the torque.

On the Jaguar the vacuum comes from the port behind and on top of the manifold behind the front HD8. It is the standard little plate with the brass outlet and two hold down screws.

Not sure what you mean by ported or not ported.

I am fairly satisfied, by the performance of my Jaguar, but looking for any advice to improve. It is quite hard to tell on pulling away from standstill as I have a fairly high first gear and slightly lightened flywheel, so it needs a bit of a rev to pulled away cleanly without stalling.

(Johnball) #5

Forgot to say the make of Distributor is CSI, it has a base and cap that is the same as the Lucas original, unlike the 123 which appears to have a Bosch cap.

(Lee140FHC) #6

Sounds like you have manifold (UNported) vacuum…ported vacuum would have the vacuum source blocked by the carb butterfly at idle. HOWEVER, since I have never had my hands on an HD8, it could be that the hole under the plate/vacuum tubing you describe leads down to the butterfly/carb body interface so I would use a length of small wire to poke through down into that hole(port) to see how deep it goes…this will tell you for sure. Then again, the advance figures you quoted are rather small IF you have any vacuum at all…I would expect to see figures approaching 50 degrees at high, no-load RPM.
Why not retry the test with vacuum blocked?

(Gary Grant) #7

John, just to clarify, the source of vacuum can be either directly from the intake manifold (unported) or from a port inside the carburetor which becomes blocked at closed or small throttle openings (ported).
If you use a ported vacuum source, the ignition timing shouldn’t change at idle rpm whether the vacuum source is connected to the distributor or not. This is because no vacuum is available from the ported source under any conditions at idle rpm.

The opposite happens when using an unported vacuum source. Since the vacuum is available under all conditions (throttle open or closed), the timing will be more advanced when the vacuum source is connected, than when the vacuum source is disconnected.

im unfamiliar with HD8s and the manifold you are using. Another way to determine if your vacuum source is ported or unported, is just by idling the engine then disconnecting the vacuum pipe from the advance capsule on the distributor and seeing or listening to any air rushing into the open end of the pipe.
If there is suction at the end of the pipe you have unported vacuum.

The ported systems have the advantage of not having to disconnect the vacuum every time you wish to measure the ignition advance.

Just for kicks, one day I drove around in my 140 with a vacuum gauge hooked up to the (ported) vacuum source. I found it impossible to measure any vacuum to the gauge (at any throttle opening or rpm), when the car was stationary.
In order to see any vacuum at the gauge, the engine had to be under the load of moving the car over maybe at least 25 mph.

(Johnball) #8
![image|669x499](upload://leMIoqfdHA8am096iBpQ3o6RXG0.jpeg) ![image|669x499](upload://gMx2YdKQUkcafmMVsWoQHTE1UOx.jpeg) 

Hello Gary and Lee,

The photo shows the copper tube going from the manifold side of the carb to the distributor. I believe that the plate for the tube is standard on the
E Type and other HD8 equipped cars. Under the plate is a very small hole about the size of locking wire, that goes right through to the manifold tube on the engine side of the carb.

(Johnball) #9

Not sure how to download pictures ?

(Mike Balch) #10

John, with so many engine mods, I doubt you will ever optimize the package unless you tune it on a dyno. The dyno will give you HP, torque, and air fuel ratio across the entire loaded rpm range. The dyno will allow you to select the optimum advance curve for your distributor and will tell you if the HD8 carbs are adjusted correctly and have the optimum needles.

Last summer I dyno-tuned my XK120, which is far closer to stock than your car. It made a huge difference. The engine now idles happily at 600 rpm once warmed up, and pulls strongly from 900 rpm in fourth gear. I paid $120 for a one hour session on the dyno. Best money I ever spent on the car.

(Johnball) #11

Yes, I think you are right. I am going to book the car in for a rolling road Dyno run in the new year. I am going to use CCKhistoric in Uckfield, Sussex, England as they do all the runs for CKL Jaguar who look after many historic C’s and D’s and also the famous Croswaite and Gardiner engine builders. So, it will be interesting to see how my engine performs and what adjustments can be made to improve the the torque and power.
It maybe that the car is at its optimum, but it would be good to check for the cost of a couple of hours with the experts in a controlled environment ?

(Johnball) #12

Hello all,

This afternoon I decided to take off the little square block in photo 2 where the copper vacuum tube comes from. In fact the hole is not under the copper tube where it connects, but across to the middle of the plate. Which seems to be directly above the butterfly. The hole is very small and a piece of locking wire did fit through without obstruction. The HD8 carb opens the opposite way to the usual H6 carb as standard fit on 120/140/150. You can see that in the photo the throttle is closed. I decided to also recheck all the vacuum tube connections and also redid the timing with the strobe light, to check 10 BTDC at about 700rpm. Then went for a fast drive around the neighbourhood, no pinging, pulls hard and lots of noise to annoy everyone.

(Johnball) #13

So, today took the car down CCK Historics in Uckfield. They have a rolling road and the expertise on Jaguars. In fact they tune many of the very well known cars and the big well known names that race at Goodwood, including D types, C types, E types and Mk2’s. The car was checked over and the ignition timing checked, then the first slow speed run showed too rich at tick over, but far too lean at 2,000 rpm. The needles were taken out and adjusted three times and then run again to provide optimum torque and power for a road car.

As they explained it is a myth about the power that Jaguar boasted.
They were very pleased with the torque of my engine, which was a maximum of 205ft/lbs at 3,535rpm and was almost completely flat from 1,900rpm to 4,800rpm. I was a bit disappointed by the maximum power of 180 BHP at 4,730rpm, ( maximum rpm attained 5,005 but power was dropping off ) but they said that was very good and showed me several other previously done road cars graphs that for were all less in the 155 to 172 range. So, mine is good for a 3.4 engine. The previous graphs for 3.8’s mostly only showed another 15 to 20 BHP. I was quite surprised how low some of the 4.2 E Type powers were. Not anywhere near the mythical 265BHP.

The Rob Beere Cam’s I have are designed for good torque and a bit more than standard power.

Pretty happy with the results and the car felt different driving home, certainly in the mid range pulling power.

(Roger Payne) #14

Johnball - there is no myth regarding Jaguars claimed power ratings.
The quoted 1950s/60s Jaguar Power ratings were all accurate and honest, but you need to appreciate they were tested in strict accordance with the then industry standard - which was an SAE GROSS Power rating, and if you want the details of what that means, Google it, but it is an accurate and precise Standard. Essentially it is the power achieved by just the bare engine, in a environmentally controlled laboratory, and a purpose designed and controlled test bed, but it is a GROSS figure, with no losses due to the engine driving any ancillaries, such as generators, and water pumps, let alone clutches, gearboxes and rear axles.
Your CCK Historics will not be doing a similar test, so there is no comparison possible. Once you do a power test on a ‘rolling road’ - it is far removed from an SAE Gross power test as you get a huge amount of losses by the engine having to drive through a gearbox, rear axle, and tyres rolling on a road, so quoted figures will be NET power (of variable accuracy depending on whether done properly or not).

The question that really matters does CCK actually conduct their tests in a fully controlled environment (this can have a significant impact) to a recognised Test Standard? Otherwise the figures they quote are even less meaningful than Jaguars 1950s/60s SAE Gross figures, which can be compared accurately to other SAE Gross tests quoted by other manufacturers and other test facilities.

But the relativity of figures is significant and real. So on its introduction, an XK120 engine achieved 160bhp (SAE Gross rating). The upgraded XK120 SE engine produced 180bhp (SAE Gross), so a real 20 bhp (SAE Gross) improvement. The standard XK140 engine produced 190bhp (SAE Gross), so again another real 10 bhp (SAE Gross) improvement, and the XK140MC (with C-type head) produced 210 bhp (SAE Gross) and so on, up to the 265 bhp (SAE Gross) quoted for a 3.8 litre/straight-port-head/triple HD8 carburetter E-type engine. So the 160 to 265 bhp (SAE Gross) represents a pretty amazing development of an engine that remained road-tractable, acceptable fuel economy, and in a vehicle with satisfactory cooling system to cope.

CCK Historics horsepower numbers cannot be compared with Jaguars SAE Gross figures, but again their relativity to other figures they quote can most definitely be relevant comparisons - subject to one huge qualification. How accurate is their equipment, is their test environment controlled, operator skills, and is test done in strict accordance with a recognised Test Standard. Most private workshops simply do a reasonable test, where they can assess their own relativities, and thus do what you asked them to do - optimise your own particular engine, but I wouldn’t get to hung up quoting numbers in absolute terms to compare against other uncontrolled test numbers; of little value apart from good debate and bragging rights at the local pub.

But still money well spent, if CCK Historics now has your engine running to its optimum (in accordance with your priorities)

Also keep in mind, that when you modify your engine, to produce more power than the original Jaguar solution, its not free. There is a down side that may or may not matter depending on other aspects. XK120s were and are known to have marginal cooling system, and increasing the power of an XK120 engine installation, will add additional load to the cooling system, which may or may not matter depending on how vehicle is used, and indeed where re prevailing ambient conditions.

(CP120) #15


Thank you.


(Johnball) #16

Roger, thank you for very thorough explanation. All I can say further is that having discovered that CCK do virtually all the tuning for the top well known racers in UK, they must know what they are doing, as their results speak for themselves in the race wins at Goodwood Revival and Silverstone Classic.
They do not advertise and don’t need to ! They appear to have all the right equipment, but more importantly, they can analyse the result and know how to correct it. Which is a must.

I find the problem with Jaguar specialists over here is that there are so many, with different opinions, different abilities, different knowledge.
I have owned my Healey 100 for 25 years and think I know most of the best specialists. The XK I have owned 5 years and this is a much bigger ‘Mine field’ I have had some real bodgers try to tell me what they can do ! Slowly I am unearthing the specialists, not all of whom are the most obvious who shout the loudest. The XK certainly is a quantum leap in complication over the Healey 100. As a side issue I also have the CSI ignition ( copy of 123 ) on my Healey and it really has transformed the car.

Lastly, yes you are absolutely right about not getting power increases for free. I am looking at cooling issues next, as the car does run hot in the summer and needs the electric fan switched on in traffic. I wish I had a lovered bonnet (hood), but don’t want to risk changing and paint miss matches.