V12 Ignition Timing

gsleyland · Post by **gsleyland** » Thu Mar 20, 2008 5:15 pm

Have a 73 V12 imported from the US last year. I've owned it since 94. Have installed the later ignition from Barratts. The car was set in the US at 6 btdc with the vacuum off, but the original handbook says 10 btdc with vacuum off and 4 atdc at idle. Also have a shop manual which reccommends 12 btdc. Given that I'm now running it on 97 unleaded in the UK what is the best setting for starting/running.

MarkE · Post by **MarkE** » Fri Mar 28, 2008 7:16 pm

What else has been done to the engine, and are you sure that you?re looking up the spec for the ?73 North American version? These are low compression engines compared to the UK spec, and have different carburettors and manifolds / cross pipes to accommodate the emission control systems.

Unless you change the pistons, carbs, manifolds and junk all the emission stuff, you should probably set it up as per book for the North American spec, which from memory is 10 degrees static. Using a higher octane fuel won?t make any difference, and may lead to a lot of wasted fuel due to low burn. You don?t need that on a V12!
Mark

Phil · Post by **Phil** » Fri Mar 28, 2008 7:37 pm

Mark

regarding your comment on octane level :

I have a 1972 S3 (from Canada) , all emission stuff has been removed.
The rest is original.

What would you recommend? 98 octane or 95 octane ? and why?
I read previously (can't remember where /when/why, most probably on Jag Lovers) that 98 would be better.

So far I use 98 oct. but going on 95 oct. if trouble free, would
save some cents/km

MarkE · Post by **MarkE** » Fri Mar 28, 2008 8:51 pm

I don't know what spec the Canadian market had...it could well have been the UK / European 9:1 compression ratio rather than the 7.5: 1 or 8:1 (can't remember which) of the North American cars.

9:1 is about the change point to higher octane.

The compression ratio is calculated as the total volume of the cylinder and combustion chamber above the piston at the bottom of its travel (BDC), divided by the combustion chamber volume above the piston at top dead center (TDC). The higher the ratio, the more compressed is the fuel / air mix....the starting volume is being compressed into a smaller space. The more compressed the mix is, the higher is the heat generated, the earlier the detonation occours...hence the pre-ignition or pinking if you're using too low an octane rating. High octane just slows down the combustion point, which used to be achieved by adding lead.

On the V12, the early pre-HE engines have a flat head. The compression ratio is therefor governed by the depth of the bowl in the piston. when I stripped my ex-Californian engine down, I was very suprised how dished the pistons are...the UK spec pistons look very different.

With most engines, you can hear the difference on cranking speed, but with the V12, it sounds like there's no compression at all...with either ratio!

For your car, I would just try the two different octanes and see if you can feel or hear any difference. I can't with my 9:1 engine.

Mark

Phil · Post by **Phil** » Fri Mar 28, 2008 10:40 pm

Mark,

thank you for the clarification.
I had found previously the following info on compression ratios :

you can find the info from the engine N? :
for example mine is 7S5671SB

the last two letters are giving the info :

S is 9.0:1 which was fitted to all e-types for 1971 and 1972, non US
e-types throughout. Factory rated at 272 BHP SAE (later 285 HP with fuel
injection).

L is 7.8:1, fitted to US (maybe Canada) market e-types from 1973 model
year. Approximately 240 BHP SAE

and Two type of pistions are listed, A or B

so my engine should be 9,0:1 with pistons type B

I remember trying the 95/98 oct. when I had the car 6 years ago (and still anti-emission fittings)and the car did some pinking on 95 oct. so I went for 98 intuitively.

gsleyland · Post by **gsleyland** » Sat Mar 29, 2008 5:20 am

Mark/Phil

Appreciate hearing from you guys. Car is low compression US model and other than the ignition upgrade is running as it came from the factory. The original owners manual that came with the car talks about 10btdc and 4atdc at idle with vac on. Am going to try this set up with 95 octane per your comment Mark. After all, prior to the ignition upgrade the car ran fine on 91 Octane in the US with no 'pinging'. I am not sure why the guys who installed the ignition upgrade in the US set it at 6btdc, but then they did a lot of other things wrong that I've now corrected.

MarkE · Post by **MarkE** » Sun Mar 30, 2008 9:38 am

I really shouldn't post on a Friday evening after a nice bottle of vino collapso...sorry for the rambling response! But we all got there in the end.

I've just had a thought that in upgrading the Cal engine to UK spec I may have missed a trick...I didn't touch the distributor. Anybody know if the dynamic advance characteristics are any different between the two engines?

Mark

Phil · Post by **Phil** » Sun Mar 30, 2008 9:51 am

Mark

no rambling at all, just interesting sharing of info for all of us.

No idea/knowledge on my side regarding the dynamic advance characteristics, sorry.

redranch · Post by **redranch** » Tue Jul 22, 2008 9:07 pm

Interesting thread (now that I have my very own 1971 S3)

ENGINE NO: 7S 2902 SA

I guess mine the factory with the type of pistons that are listed as A

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