David Roe

[David Roe]

Can a 1956 mk 7m run on unleaded petrol?

[rayUK]

Can a 1956 mk 7 run on unleaded petrol?

The theory behind lead substitute is that when the engine in your classic car was designed and built, gasoline had lead in it—more specifically, tetraethyl lead, or TEL. That lead served several functions. It boosted the octane rating, allowing for higher compression ratios; helped reduce knocking; and reduced wear on the valve seats. (It did so by helping to prevent "microwelds" from forming between the hot valve surfaces and the seats in the cylinder head as the valve closed.) The process of constant welding and subsequent tearing free when the valve opened again could wear the valve seats over time, requiring expensive repairs.

There are two important points to consider: Has the car been running for many years on unleaded petrol, if yes then it is highly likely that the valve seats are of a hardened quality sufficient enough. Secondly does the engine ping (some say pink) or knock under acceleration? If it does it is because the octane rating is not high enough and you can try to use a premium higher octane rated fuel. DO NOT USE ETHANOL MIXED PETROL! You can also retard the ignition timing slightly to stop any pinging or knocking if it still continues after upping the octane.
You can of course use an additive. Lead substitutes use a variety of proprietary formulas, often based on manganese, sodium, phosphate, or iron, rather than lead, to fulfill the function of lead without the toxic side effects. It is available in garages and auto part dealers and if the car hasn't run for many years it may be better to be safe than sorry and use one. You can always check with Jaguar first.
Wish you all the best with that lovely MK7. It was a fine car indeed.

[Ian D]

Welcome to the forum!

Thread moved to MkV to MkX section.

[SV8Predator]

Has the car been running for many years on unleaded petrol, if yes then it is highly likely that the valve seats are of a hardened quality sufficient enough.

Great post Ray, but of course you forgot to mention that the Jaguar XK engine's cylinder head is made from aluminium. Therefore it has hardened steel valve seats fitted and thus is not the slightest bit concerned about lead-fee fuel!

[rayUK]

Great post Ray, but of course you forgot to mention that the Jaguar XK engine's cylinder head is made from aluminium. Therefore it has hardened steel valve seats fitted and thus is not the slightest bit concerned about lead-fee fuel!

Thank you SV, I didn't realize the MK7 had the XK engine, but yes, 1956 puts it in that era. Oh well, something I learned today. Hope you told David that, it will save him a lot of agro. However he still must stay away from the Ethanol mix which is a carb killer.

[SV8Predator]

However he still must stay away from the Ethanol mix which is a carb killer.

Why?

In the UK, from 1953 with the ending of pool petrol, ethanol was blended into a petrol that was widely available at the rate of 20% - 25%. Cleveland Discol was an unleaded ethanol blend available nationwide and advertised as the cleaner petrol with more power. For once this was true, ethanol blended petrol had a higher octane rating and it was unleaded so no lead pollution. How come we didn't have disaster from all the Jaguars of the 1950s (and earlier) that obviously will have used this brand of petrol?

[rayUK]

Why?

In the UK, from 1953 with the ending of pool petrol, ethanol was blended into a petrol that was widely available at the rate of 20% - 25%. Cleveland Discol was an unleaded ethanol blend available nationwide and advertised as the cleaner petrol with more power. For once this was true, ethanol blended petrol had a higher octane rating and it was unleaded so no lead pollution. How come we didn't have disaster from all the Jaguars of the 1950s (and earlier) that obviously will have used this brand of petrol?

Sorry SV but have to disagree with you on that one. I well remember Cleveland Discol and because of the higher octane rating had used it. I had both carburetor and rubber fuel line problems after using it for some time but didn't know why back then, and I would suspect many others had the same problems without associating it to the fuel itself. The following article is one of many that can be found on the internet on this subject.

The problem for owners of older cars is that ethanol is hygroscopic, which means it absorbs water. Ethanol-blended fuel will naturally hold 0.5 percent water in suspension, but once the water content exceeds this percentage, the water/ethanol mix becomes heavier than the gasoline portion of the fuel. This leads to what experts call “phase separation,” which is the point at which the water/ethanol mix drops out of suspension and sinks to the bottom of the fuel tank.

Because the fuel pickup is located on the bottom of the tank, the layer of water/ethanol mix is also often the first thing to get sucked up when the engine is cranked over. That means the corrosive mixture is circulated through your fuel system and into the engine, leading to possible corrosion of your fuel pump and other components. Although ethanol has been known to gum up and cause clogging in fuel injectors and fuel filters, the corrosion problem is typically limited to carbureted engines, because carburetors are typically made from alloys that are more susceptible to corrosion–zinc, aluminum, and brass.”according to Scott Diehl, National Sales Manager at Driven Racing Oil.
According to Ed Callis, V.P., Technology at Red Line Synthetic Oil, other components are at risk, too.

“Fuel hoses, O-rings, seals commonly installed in older engines and fuel systems are negatively affected by ethanol,” Callis said. “Leakage can result, so these components should be changed out with ethanol-resistant components.”
Callis says the makeup of ethanol-blended fuels can lead to more problems and potential engine damage down the road as well.
“Other than the possible elastomer compatibility problems, which I feel should be the most serious concern of older car owners using E10, an additional issue is the extra oxygen present in E10,” he said. “Since older engines lack the computer control and oxygen sensors of newer cars, adjustments are not made to fuel flow rates based on exhaust emissions. Knock sensors were also not generally installed in older engines, and the higher air/fuel ratio caused by oxygen present in E10 can cause a leaner-running engine. That can produce more heat as well as poorer fuel economy.”

Hence my resistance to using ethanol in older cars.

[ponsaloti]

Just run it on unleaded.
You may have to adjust the timing.