Yeah I learned soon after I bought the car that the British use a different rating system than we do. So technically speaking I agree with you that without extra advanced timing using high octane fuel isn’t strictly necessary. But I continue to for a couple reasons. For one, about the third time I ever filled up the car when it was quite new to me, it knocked a little bit on 87. It’s never knocked on 93. And beyond that, I drive only a few thousand miles a year since I don’t need the car for my daily commute so the cost difference just doesn’t amount to much of anything.
Another question I’ve wondered about though is ethanol. What do you think of it? Would our V12s prefer E0? There is nothing but E10 (regardless of the octane rating) around me as NoVa/DC have heaps of environmental regulations but there are a handful of E0 87 stations maybe 30 to 40 minutes away in more rural Virginia–too far to use regularly but maybe I will try their fuel sometime if I happen to be out that way.
Yeah, any knocking is definitely worth going up on octane. Perhaps with carbon deposits, etc, the compression is a wee bit higher? Going too high with octane will only hurt your wallet. Better safe than sorry. I’ve always ran super (92AKI) in mine, because I misunderstood the 91 RON to be 91 AKI. When I advanced timing, I stuck with super.
I’ve heard some say the V12 would prefer non-ethanol, but they still seem to run fine with it. Once the fuel system is renewed and cleaned up, I can’t see any issues with running ethanol based fuel. I could see switching from non-ethanol to ethanol on an old engine could cause dirt to get dislodged and into fuel injectors. At least ethanol is better for keeping water out of the system. But perhaps it doesn’t burn as well, so maybe that’s why a higher octane was needed?
Right the fact that it doesn’t burn as well is kind of what I was thinking of. But basically I don’t give it much thought. The decision is made easier by the lack of nearby E0 when it comes time to fill up anyhow.
And since I have gone carefully through the whole fuel system on my car I know the rubber lines are all in good shape (and are something I keep an eye on). Besides, I think one of the biggest ethanol grievances I’ve heard is it’s bad for cars that sit a lot. My car doesn’t get a ton of annual miles but it certainly runs and drives more days than not so again not a big concern. My Mark X sits a lot more (though I’ve gone through the whole fuel system on it too at this point) so I just add some fuel stabilizer to a full tank when it gets one.
Other key is to avoid going below a 1/4 tank–something I try to do with all my cars. Whatever is at the bottom of the tanks (though hopefully there is nothing there) is just that much less likely to get sucked up and cause trouble that way.
So, after my car was definitely idling a wee rough in P but not D yesterday and I noticed an intermittent random misfire on bank A in Park only, this morning I went ahead and disconnected the whole vacuum advance system (brown thing with the two vacuum inputs and dump valve) and hooked vacuum advance hose simply to top of throttle body, as Kirby’s book points out as an alternative. So there is now 0 inHg going to vacuum advance at idle (before there was 10-12 inHG). So now I’m effectively retarding the timing a bit at idle (But since I’ve advanced my overall timing from 18 to 22 @3000rpm, my timing at idle is probably very close to others who are at factory settings)
I had to turn the AAV idle screw one full rotation open to get the idle back up to 800-850rpm in Park.
Well, guess what? Still idles nicely in D, but now it also idles smooth in P! No misfire at all. And maybe I’m imagining things, but the exhaust does not smell as bad as it did before.
I have read that a retarded timing gives more time to burn the fuel at lower rpms, maybe you should check out your vacuum advance system. Perhaps yours is giving the car too much advance at idle, causing a rough idle in P. Going into D (closed loop) lets the ECU change the mixture enough to get rid of roughness.
I also notice a slightly quicker throttle response - before I would occasionally get a wee hesitation when giving it a bit of throttle from idle. Maybe due to the complicated system of 1. advancing the timing a bit at idle then 2. dumping the vacuum on throttle and retarding the timing to 3) slowly advancing the timing again as vacuum goes up. This way it simply goes from retarded timing to advanced timing. Much simpler. There’s gotta be some delay in that little brown valve doing it’s job.
BTW, I had tried this setup last year with no help in idle, but that was before I later found my main issue of a flakey injector harness/possible flakey injector.
That’s really interesting. I will take another look through my vacuum advance system and might try that same modification to see what results.
It is a complex system so I agree it could easily delay throttle response slightly although that is not something I have specifically noticed.
Do you intend to leave the system modified in your car or put it back to stock and allow for a slightly rough idle sometimes/all the time in park? Any reason the modification would make other systems unhappy in the long run? Nothing comes to my mind offhand.
I will leave it as is for a while and test drive over the next few days/weeks. Eliminating another vacuum device from the car would be a win for me!
I don’t see how it could make any other problems while driving. When having the throttles open, the vacuum from the throttle tap would advance the vacuum advance timing a bit. About the same if it were still hooked up stock? And the centrifugal vacuum advance does a lot of the work when in high RPM.
This setup should only change idle conditions. And running a tiny bit more retarded will just make it run a tiny bit warmer at idle, but it should be negligible. Not sure how much the vacuum advance is advancing the timing by with 10inHg applied to it at idle. And I’ve already introduced 4 degrees advance at distributor, so I can’t be that far from a stock car setup now. Doing this on a car that has not had it’s timing advanced at the distributer like mine, may push it too retarded of a timing at idle. Not sure.
2nd day test drive, so far so good. The idle in D is slightly improved too, I think.
Wanted to add to this, I have decided to stop messing around with non-stock settings, and bring the timing back to 18 BTDC @3000RPM. I will then try hooking back up vacuum regulator. I have to assume the engineers (Roger Bywater included) set up these regulators around precise settings. Also, these high compression engines are not to be messed with, regarding too advanced timing.
I am running my V12 on E10 for more than ten years now, no problem at all with the engine.
Two caveats though:
Fuel hoses: must be Ethanol compatible. Regular hoses will swell and become very soft.
Fuel Pump: If it ever stays dry for more some it will leak right away next time you use it. Ethanol expands the o-rings, but when they get dry they contract much more than their initial size.
The new Bosch pumps are Ethanol compatible though.
So I did some experimenting today with vacuum advance at idle and timing. I hope this is good info.
I decided to put my V12 back to 18 BTDC @3000RPM so I’m not messing around anymore. I also put back vacuum regulator system, which of course shot up the advancement at idle. I adjusted the RPM back to about 825RPM. Here are my timing readings. I also applied my own vacuum to vacuum advance unit with a pump/gauge.
All these readings are at idle in Park::
Stock regulator hookup vacuum advance (10 inHG): 16 BTDC - random misfire on A bank
Vacuum pump (0 inHg): 2 BTDC - no misfire, but choppy idle
Vacuum pump (6 inHg): 8 BTDC - no misfire, but slightly choppy idle
Vacuum pump (7 inHg): 10 BTDC - no misfire, smooth idle
Vacuum pump (8 inHG): 12 BTDC - hints of misfire on A bank, decent idle
Vacuum pump (9 inHG): 14 BTDC - random misfire on A bank
Vacuum pump (10inHG): 16 BTDC - random misfire on A bank
My random A bank misfire at idle in Park returned as soon as the idle hit 14 BTDC or more advanced. It didn’t misfire, but idled choppy at 8 BTDC and lower. The best idle was at 10 BTDC. Not choppy, and no misfire.
I’m wondering why my engine idles smoother with a slightly retarded timing than stock. Is there a way to lower my regulator down to 7 inHg? Will the XJ6 regulators work (they have a lower regulation, right?) I see the AC Delco 214-1674/GM 14006719 looks like the exact same part as the XJ6 (off white). There are a few NOS available on Ebay. I’ll buy one and report back.
Ehm. Apologies for not reading full story above, not tha I ain’t gotta time… But just willing to ask simple question. What’s the mileage on the clock of your state-of-art British beauty?
Well, everyone seems to be focused on th engine. Maybe it’s worth to investigate tranny… Highway ride heats it up a bit, thst includes torque converter and pump. Just saying.
Transmission is perfect as it was rebuilt last fall less than 1000 miles ago! Got it done by an expert near me who did a wonderful job! Issue only seems to be occurring in particularly cold weather. I need to follow some of the step Greg suggested above to test it all out but I’m quite convinced it’s engine related!
Mine has about 54,000 miles, similar to yours. Transmission is original, but shifts perfect. Anyways, I would think any transmission issues would show up in Drive when the torque converter is engaged?
It will be a week until I get the XJ6 vacuum regulator which has a lower inHg at idle. It’s also used on old Chevy V6 engines of the 70s/80s, so is easier to obtaint than the V12 one…I ended up buying an actual New Old Stock GM part, which is identical to Jag XJ6 one. I’ll post back with my results then.
I wonder if they increased advance at idle for the V12’s simply to get better gas mileage and keep engine cooler at idle? I know if you advance too much at idle, you can get what they call a ‘nervous idle’. Our V12s also idle at a very low RPM (750-850), where most fuel injected cars I own idle at 900-1000rpm. I don’t understand the science, but too much advance at a low rpm can cause misfire.
You should definitely check what your timing is at idle in Park with vacuum advance regulator engaged. Maybe centrifugal advance in distributor, being engaged much on your highway drive, got a bit sticky when you got home and advanced too much at idle? And then unstuck itself later and all was fine?
Did you rebuild the distributor/centrifugal advance as outlined in Kirby’s book?
Yeah I would agree with that. My transmission had been abused/poorly maintained by some PO. I suspect the fluid was never changed ever in 31 years. It was black and burnt. Actually the car shifted ok even so but eventually 3rd gear was slipping and, eventually, not engaging. I wanted a full rebuild for piece of mind and had someone do it since I don’t have any transmission experience, a lift (would be hard to drop the trans on jack stands), or the time!
Nervous idle–hadn’t heard of that before but I get the idea. Interesting thought.
I will check timing at idle hopefully this weekend! I did address the distributor right when I got the car after I saw the warning in Kirby’s book! It was fine then (signs that a PO had worked on it at the same time he redid the injector hoses). But I haven’t gone digging under the cap in about a year–worth pulling it to make sure it hasn’t since become stuck!
Or just remove the cap and add a bit of synthetic oil (you DID use synthetic as Kirby recommended?) to your cotton ball. I’ve done that four times since rebuilding the distributor two years ago. A bit too much oil will simply fall into the engine?
I’m sure someone here can explain too much advance and a low rpm? Something about the fuel mixture not burning as well?
Yes synthetic!! I continue to run conventional in the engine though. I might switch at some point–I know you’ve had good success with that!
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
39
Apparently not. IIRC, we had a guy on here some years back who had been chasing overheating issues with his V12 for quite a while with no luck. Then one day his tranny started acting up, so he took it to a shop and they rebuilt the transmission. Voila! No more engine overheating!
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
40
A bit too much oil will spin out within the distributor base and eventually be ignited by a spark, blowing the distributor cap off.