xk engine

[piquet]

guys

I have a 3.8 e type and the engine was modified by the previous owner, the company that did it have subsequently gone out of business, but before they did I phoned them and they remembered in dyno'd at 290bhp (I can only assume this was a gross figure to compare to jaguars gross). To achieve this as well as all the balancing, I know they gas flowed the head and also increased the compression ratio. When I got it was fitted with triple webber 48s which made the car awesome from 3000 upward, but drank fuel and made the car very flat low down, this has subsequently been replaced with efi running an emerald unit and triple twin jenvey throttle bodies. This has improved the low end enormously and also doubled the average fuel economy.

At some point in the near future I’m going to be fitting a 5 speed box and since the engine will be coming out to do so, I was going to get the engine refreshed, I was just wondering what your opinion was about certain things, since you're not trying to sell me anything.

1) The car presently runs on classic 20/50w oil, when rebuilt is there any reason it couldn’t run on something thinner and more modern?

2) As the bore is 87mm and the stroke 96mm what kind or revs would be safe to cruise at on the motorway without breaking down the lubrication, would this be higher with a modern synthetic oil?

3) Because the spark plug is offset from the centre of the hemi chamber some of the rebuilders use asymmetric pistons to concentrate the fuel air mix to the side with the spark plug, as in effect the flame front is hitting the piston at an angle would this result in more and faster bore wear?

4) It has been suggested to remove the offset plugs and put a small surface discharge plug between the valves in the centre of the chamber, how would this compare to the asymmetric pistons

5) The car tends to pink and is very sensitive to increase in air temperature (like in the cold air now it doesn't, but in the summer it does big time) when the engine is rebuilt, am I better of getting the compression ratio dropped or getting the ignition retarded more

pk

[Marquis_Rex]

piquet said:

I have a 3.8 e type and the engine was modified by the previous owner, the company that did it have subsequently gone out of business, but before they did I phoned them and they remembered in dyno'd at 290bhp (I can only assume this was a gross figure to compare to jaguars gross). To achieve this as well as all the balancing, I know they gas flowed the head and also increased the compression ratio. When I got it was fitted with triple webber 48s which made the car awesome from 3000 upward, but drank fuel and made the car very flat low down, this has subsequently been replaced with efi running an emerald unit and triple twin jenvey throttle bodies. This has improved the low end enormously and also doubled the average fuel economy.

First off, I tend to ignore quoted dyno figures, unless it’s someone I know first hand or it’s a dyno I have faith in. You only have to look at the stupid claims on forums like these. The chassis dyno figures on forums are banded about like they’re gospel, totally ignoring the massive differences you’ll get one uncalibrated dyno to another back street dyno.
The better way to get meaningful figures is to calculate it from measured data, perhaps from a data logger or to V Max a car and calculate the figures from frontal area, gear ratios and drag etc etc.

Tripple Webbers in themselves wouldn’t have made the fuel economy worse, however poor set up, getting rid of part load ignition advance because of a lack of vacuum port (very common) will hit fuel economy a huge amount. In any case the triple webber set up couldn’t hope to match the fuel economy of a fully mapped system with mapped ignition.

piquet said:

At some point in the near future I’m going to be fitting a 5 speed box and since the engine will be coming out to do so, I was going to get the engine refreshed, I was just wondering what your opinion was about certain things, since you're not trying to sell me anything.

1) The car presently runs on classic 20/50w oil, when rebuilt is there any reason it couldn’t run on something thinner and more modern?

Better to perhaps ask Opeil oil man- the resident oil expert, but in my opinion when an engine is totally rebuilt with tight clearances, I see no harm in using an oil with a low “W” value- which denotes a figure that is relevant to the cold cranking viscosity.
The only problem I see is if you have leaky gaskets and seals, which is likely on an engine design of this vintage- in which case perhaps go for a 10-60 synthetic

piquet said:

2) As the bore is 87mm and the stroke 96mm what kind or revs would be safe to cruise at on the motorway without breaking down the lubrication, would this be higher with a modern synthetic oil?

What block are you using? The old XK block used to crack and the later blocks were slotted between the bores to try to stop crack propagation
Modern synthetic oils do help at high oil temperatures but those temperatures are not usually exceeded at normal motorway speeds unless the ambient temperature is also very high. It is usually in crank bearings where the oil reaches the highest temperatures, being about 20°C higher than the bulk oil temperature. Good synthetic oils such as Mobil 1 can work up to about 180°C in the bearings compared to about 160°C for older oils.
The XK engine was not as durable as the engines that followed it (AJ6 etc). I wouldn't recommend running it continuously above 4500 rpm for long periods of time on the motorway. I would probably keep to 3500 to 4000 rpm if it was my car and just use higher speeds briefly during accelerations. I assume with that relatively long stroke the peak power is at about 5000 rpm unless the engine is modified?

piquet said:

3) Because the spark plug is offset from the centre of the hemi chamber some of the rebuilders use asymmetric pistons to concentrate the fuel air mix to the side with the spark plug, as in effect the flame front is hitting the piston at an angle would this result in more and faster bore wear?

This idea actually makes sense. BMW have done it on , their admittedly shallow angle, hemi on the E30 M20 325i. Because the spark plug is offset, the piston has an offset dish which concentrates the charge around the plug. It will NOT lead to faster bore wear, but more on that later.

piquet said:

4) It has been suggested to remove the offset plugs and put a small surface discharge plug between the valves in the centre of the chamber, how would this compare to the asymmetric pistons

This is my summary on some spark plug test work I once looked into

www.eng-tips.com/viewthread.cfm?qid=158647

Bottom line was that I found that conventional “J” plugs were the best for combustion stability and lean mixture/EGR tolerance if perhaps not longevity. This test evaluated surface discharge plugs also. I wouldn’t mess around with spark plug bosses between the valves, as the current position was optimized with heat dissipation rates in mind.

piquet said:

5) The car tends to pink and is very sensitive to increase in air temperature (like in the cold air now it doesn't, but in the summer it does big time) when the engine is rebuilt, am I better of getting the compression ratio dropped or getting the ignition retarded more

What compression ratio are you running? Depending on what your answer to this is, I would be very hesitant in lowering the CR as you’ll be dropping your engine efficiency at part load (CRs were often dropped by the factory in low octane parts of the world by using two head gaskets). Again depending on what your CR currently is, Jag XKs usually like around 35 to 38 degrees total advance at full load. If you’re above that- I’d start to worry. The biggest issue on the Jag XK chamber is that the valve angle is very deep and the piston is usually domed to get the CR up- which isn’t a nice shape from a thermodynamic efficiency point of view- but it gives GOOD valve area- which is very important on an long stroke engine such as yours.
The XK would originally have been designed to run on 97 RON leaded fuel. Later Jaguar engines have been tested on leaded versus unleaded fuels and it was found that a higher octane rating was required for unleaded fuel to avoid knock. Obviously the AFR has to be correct (richer at high loads) to give good knock resistance. How was your engine mapped? This is why I don’t have any faith in the after market. Most of them don’t know the first thing about engine mapping. Ideally it would be worth checking the calibration on a rolling road. It's not good for durability if the engine runs excessively rich (which could also happen with worn jets). Air leaks in the intake manifold area could cause it to run lean and make knock worse. If you hear knock clearly during tip-in then retard the spark timing very slightly. Don't run with any knock continuously above 2500 rpm. You should run on the highest octane fuel available .Be careful if running at high loads for long periods of time with hot ambient temperatures (above about 30°C).

It is usually pre-ignition that causes engine failures rather than knock. Changing to a cooler spark plug in the summer when running at high speeds can be an advantage. A cooler spark plug may cause plugs to foul in the winter.

[GavinPearson]

You really ought to read this article, it gives a pretty good guide to the XK engine and the designed in 'fixes' to the big ends which might make running thin oil undesirable.
www.jagweb.com/jagworld/xk-engine/index.html

I think you also ought to think about how to trigger the ignition from the crank, this will be more accurate than using the distributor so you can use more advance without worrying about a detonation problem.

I would also think about entrusting the work to a specialist such as AJ6 Engineering or Chesman Engineering who can give you something they know will work, rather than guessing and having something you are less than happy with.

[piquet]

thanks for the answers

it's a standard 3.8 iron block, unfortunatelty i'm not sure what the compression is, i just know it's raised, i think it's around 10.5:1

the car is running wih around 33/34 degrees of advance at full load, it was mapped on a rolling road, but i think it was mapped to a constant afr rather then making it richer under load, i did kind of feel when they did map it they were kind of in a hurry, does anyone know anywhere that's reasonable and really good to map it fully? althoug i may hang on until after i've had the engine redone and it's getting a new exhuast system

i run it on the tesco 99ron, like i say in the cold weather it doesn't pink, but under load in the heat it does, there is an option to retard the ignition with the air temprature, come the spring i may try ramping that up to see if it can fix it

forgive me for asking, what the difference between the a normal and cooler spark plug?

so i have nothing to fear from the assymetic pistons, excellent, what would happen if you put a second spark plug in? interestingly whenver you pull the pistons out of an xk engine you always have carbon build up on the non spark plug side

oh the igintion is triggered off the crank, although it does use the distributer rather then coil packs

pk

[jagsport]

No problems with cracking with the 3.8 block it was the 4.2 and not the early or late ones that did this. The off set piston domes are usually supplied to the 4.2 engine as this has offset and siamesed bores to get the larger pistons in, this means that piston 2 and 5 are in the middle of the dome of the hemi head and the other 4 are offset. Not applicable to the 3.4 0r 3.8 as these pistons (being of the original design) are centered. As we used to run Castrol R40 in race XKs I can see no reason why you cant run a stronger synthetic 40 grade like a 5w-40 which would be better for resisting shear. Jaguar made some twin plug heads for the 'd' type running Lucas injection in the late 50's they where of little benifit as they ran then with points and they would get out of faze with each other very quickly. However..... I made a twin plug head by machining a seconf hole and putting in an adapter. This has been done using one large and one small plug but it . You can do it with 2 x normal plugs but the tolerences are tight. Ignition needs either 2 coils (or I gues 1 wasted spark double ender, feeding somthing like a magnetti marelli V12 twin king lead type of dissy. You could adapt one which would be considerably easier than making one from scratch as I did before they made this distributor. An alternative would be loads of coil packs. If you get this far then sorting a pick up from the front pulley or flywheel will present little problem.

Edited by jagsport on Saturday 2nd December 00:44

Edited by jagsport on Saturday 2nd December 00:45

[smhmotorsport]

agree with the 4.2 blocks cracking, just had one re-sleeved with top-hat liners. Oil wise I use castrol rs, and have put it in several other xk motors while at Lynx. If your looking at 'boxes, Derek Watson is one to speak to, used to supply us with the 5 speed borg warner with the speedo driving the right way.. Check to see what your final drive is if you want a motorway cruiser.

[Pigeon]

jagsport said:

Jaguar made some twin plug heads for the 'd' type running Lucas injection in the late 50's they where of little benifit as they ran then with points and they would get out of faze with each other very quickly. However..... I made a twin plug head by machining a seconf hole and putting in an adapter. This has been done using one large and one small plug but it . You can do it with 2 x normal plugs but the tolerences are tight. Ignition needs either 2 coils (or I gues 1 wasted spark double ender, feeding somthing like a magnetti marelli V12 twin king lead type of dissy. You could adapt one which would be considerably easier than making one from scratch as I did before they made this distributor. An alternative would be loads of coil packs.

Factory racing MZs had twin plug heads, not for ignition improvements but for redundancy - two completely separate ignition systems, aircraft style. Twin plug heads occasionally turn up on ebay for some silly price. I did what you did, and drilled a second plug hole in an ordinary side-plug head. The two plugs are fed from a wasted-spark coil (half a Mondeo coil pack) driven by a 6V CDI unit which gives a humungous spark (160mJ - 80mJ per plug - the original points/coil system gave about 40mJ per plug - in all cases less coil losses).

The improvement is all at the low end - low speeds, small throttle openings, the region where scavenging is poor and the mixture difficult to ignite. The bike is much easier to trickle along in town in a high gear at minimal revs. At full whack it makes no discernible difference. The racing heads would indeed have been purely for reliability.

As an experiment I tried shorting one plug lead to the head so I just had one huge spark. This gave some improvement over the original condition but not as much as the full twin spark setup.

I found it was possible to increase the compression ratio slightly, by reducing the head shim from 1.4mm to 1.0mm. Further reduction to 0.8mm made the engine too "tight", so 1.0mm it was. The ignition needed to be retarded about 1mm from the standard 2.5mmBTDC - with increasing advance, output started to drop off long before the knock limit. Combustion temperature seems to have fallen; on standard MZs most people use grade 8 (NGK) with a few using 7; mine is happy on 6.

For an XK I'd be much happier using multiple coil packs than a great complex dizzy and one big highly-stressed coil... you could use three Mondeo coil packs and achieve a pleasing visual correspondence with the triple carbs

[piquet]

so the important question, in a choice between offset dished pistons ( rob beere fir them to all the xk engines due to the offset spark plugs, not just the 4.2) or a twin spark set up with coil packs?

i don't know which 5 spped gearbox derek watson used to supply, i wonder if it was the jt5 which is the only on i've heard of with the tach drive off at the top, i know they stopped supplying thier uk distributor because thier designs were being copied, but jt5 is the main one with a choice of ratios, especially in 5th

i'm pretty sure my engine was mapped to a constant EFR how much should richer should it go, who do people trust to do it and do it properly?

pk

[smhmotorsport]

piquet said:

i don't know which 5 spped gearbox derek watson used to supply, i wonder if it was the jt5 which is the only on i've heard of with the tach drive off at the top, i know they stopped supplying thier uk distributor because thier designs were being copied, but jt5 is the main one with a choice of ratios, especially in 5th
pk

Its the Borg warner T5, needs a little fettling if its a diy job, fittted one to an E type and one went into the last xkss from Lynx. Derek is a really nice bloke, give him a call if you have any questions.
www.derek-watson.co.uk/5speedboxes.htm