changing timing

[norfolk boy]

Hi
Hope someone out there can help me, I have just converted my Elan 130s big valve head to unleaded fuel. Do I have to advance or retard timing? I can vagely remember that unleaded fuel burns slower than leaded? Any advice greatly appreciated.

[bill308]

As I understand it, fuel burn rate is a function of octane rating. A higher octane rating means aslower burn rate and all other things assumed equal, leaded versus unleaded, with the same octane rating, will burn at the same rate. If a sample of unleaded has a higher octane rating than a sample of leaded gas, it will burn slower in the combustion chamber.

So, if the octane rating of you gas is lower than sthat pecified by Lotus, you may have to back off (retard) your ignition timing a little. If octane rating is the same, no change in ignition timing is required. If octane rating higher, presumabley you may gain some benefits by advancing your ignition timing a little.

I hope this helps.

Bill

[tower of strength]

unleaded burns faster and a lot hotter than true leaded fuel, I believe this is what causes a lot of problems with older engines(its the temperature issue) Having an Alloy head helps with the heat dispersal, and is why all modern engines have alloy heads at least.
Normally you'd knock the timing back a couple of degrees (retard it) but if you are using some of the premium Super Unleadeds all of the time, you may not have to. My old 130s was very sensitive to fuels, it would pink on some super unleadeds but not on others, Shell Optimax was its favourite tipple. I decided to run it on std unleaded with an octane booster and slightly retarded ignition as it seemed happier like this. Also trying to use a specific fuel at the time was hit and miss as not all filling stations had super unleaded at the time. I used the castrol lead additive with octane booster.

hope this is of interest/help

regards

Mark

[bill308]

Mark,

I'm a degreed mechanical engineer, specializing in heat transfer, though not in automotive combustion thermodynamics. Granted, heat conduction through aluminum is much faster than through cast iron. What's important though is the state of the fuel/air mixture.

Octane rating can be viewed as resitance to knock. Knock is the unconrolled burning of the air/fuel mixture end gas (that left after a controlled burn), usually at the end of the combustion cycle. That is, flame propogation begins at the spark and expands shpherically toward the edges of the combustion chamber and piston top boundaries. Knock occurs when the remaining or residual air/fuel in a cylinder explodes, because it is too hot. This results are a shock wave that propagates throughout the combustion chamber, piston, head, and block.

I believe octane rating is determined in an instrumented single cylinder test engine. The engine doesn't know if the fuel/air mixture has lead in it or not. It only knows if it feels the explosive shock wave or not. Octane is the deciding factor. Given enough octane rating, explosion of the end gas will be eliminated or minimized. It matters not whether octrane rating is derived from tetra ethel lead or alcohol or some other additive. What matters is whether the end gas explodes or not.

Any chemical or combustion engineers care to chime in?

Bill

[twincamman]

IM just a an owner builder but I set my super 7 T C timing at 5 degrees advance static set with a bulb----runs 85 octain unleaded fine at 190 degrees f or so on a hot day with no problem--same with 26R s2 33 - -ed

[bill308]

Thanks for your input twincamman.

So, your static is 5 BTDC. What's your full advance and at what rpm?

There are aparently 2-ways of measuring octane rating for pump gas, motor (M) and research (R). In the US, pump octane is measured as the average of R and M, or (R+M)/2 ,at the pump. Locally, (in CT, USA), typically one has the choice of 87. 89, 91, or 93 octane choices, based upon the (R+M)/2 measure. My experience says that for a given compression ratio, hemi head engines, like my elan, like the 93 octane gas. My Ferrai 308, also with a hemi head, will get by with the 91 octane gas due to it's lower compression ratio, about 8.8:1 verses the Elan's 9+:1 compression ratio. In the Elan, I'm running about 36 degrees BTDC at full advance, while the F-car is running about 38 degrees BTDC at full advance. The F-car engine is a bit tired at this point in time, so seems to run better with a little more total advance. Pent roof 4-valve heads seem to like a little less total advance. This may be because the flame front moves a little slower in this design. I think the hemi heads are classified as a fast burn combustion chamber. The flame front is quite large relative to the pent roof designs.

Bill

[tower of strength]

UK unleaded is 95 ron octane, super unleaded either 97 or 98 ron, I'm going to try to run my twink (standard plus 2 or s/e webber spec) on the 95 ron stuff with an octane booster additive. I hope with the lower compression ratio compared to my old S130 it should be ok.As for the timing I'll run as much advance as I can without detonation, I should be able to tweek the dizzy over a couple of days use to achieve this, I'll let you know what setting I end up with, 5 degrees sounds about right , as If I recall correctly, the standard setting is 7 degrees for our old 4 star leaded. Another thing that may help, I know the NGK rep for the UK, he recomended using thier new Iridium spark plugs, he rekons they would help with reducing pinking as well, allowing slightly more advance over standard plugs, they are also less prone to fouling, another problem I had with my old S130 (it wasn't in the best of health I admit!).
For the sake of ?20 quid its got to be worth a go, they should last a lot longer than std plugs.

regards

Mark

[twincamman]

24 degrees at 6500 comes to mind --ed

[norfolk boy]

Hi all,
Thanks very much for your input, from what has been posted I shouldnt need to alter the timing very much from my workshop manual settings.
Will let you know how I get on. I have in the last couple of years done several mods to the car such as a more efficent fan to assist cooling and electrically driven head lights rather than vaccum which was always a bug bear with seals breaking down and lights slowly creeping up. I know it looses originality but at least they stay put!
Regards
N.B

[Jason1]

Hi N.B.

This is a good site have a read it will answer all of you question a possible add a few more; http://www.roverp4.com/unleaded.htm

Any chemical or combustion engineers care to chime in?

I have a B.Eng Automotive System any good???? I also know a Chemical Engineer if it helps I worked as an Emissions Technician in a Emissions Lab a while ago but only for a week (long story).

There are lots of good sites on the internet and lots of people on here that have a lot of knowledge. Don't be afraid to ask, there is always someone happy to help.

Jason

[Jason1]

Hi

I just read my response back

Sorry Bill I was not having a go at you, just saying that I am very well qualified but feel like an amateur compared with some of the guys posting on here. Many of the people here have stripped their Lotus down to every nut and bolt many times and know the car inside and out.

I promise to read back my postings before posting next time

Jason

[john.p.clegg]

Sorry to hijack this thread (should be in the things in common?) but looking at the Miles book,advance curves vary from 7 to 10 degrees static to 18 -34 degrees fully advanced.....and knowing that things have changed quite a bit since the sixties....was wondering if any of you experts out there....without baffling me with science....could tell me a generic timing curve for todays twinc on todays low octane unleaded..
I do realise there are plenty of variables and am possibly asking the impossible as no two twincs are the same,but thought I'd give it a go...

John

P.S.
reason for asking as am going to EFI the twinc and would like a base map for the ignition..
probably the best enswer would be to set the curve on a rolling road?

[rgh0]

The starting point I use for road twin cams running premium unleaded and around 10:1 compression ratio is as follows. Where you end up after a few dyno seasons varies for each specific engine as they are all different and the specific fuel you are using

10 to 12 engine crank degrees static advance
15 to 18 engine crank degrees centrifugal advance coming in a straight line from 1000 to 3500 engine rpm.

Maximum total advance 25 to 28 engine crank degrees above 3500 rpm

cheers
Rohan

[denicholls2]

Tower of Strength wrote:

I'm going to try to run my twink (standard plus 2 or s/e webber spec) on the 95 ron stuff with an octane booster additive.

I'll assume the three grades RON correspond to the U.S. R+M/2 grades 87-93 except for our silly practice of formulating special mixes to meet state-imposed gasoline requirements.

If your additives contain lead, I'll opine that your engine will be much happier and longer-lived (and you will spend less) if you simply run the grade of unleaded without them that your engine is happy with (probably 97 or 98.) Here in the states, the delta between low-grade and top has narrowed as price overall has increased. YMMV.

I know transition produces anxiety, but lead shortens engine life. Valve recession has been discussed to death and is basically a non-issue.

You can find a detailed technical discussion of the technical reasons in the Europa site archives. I run my Renault Europa on the best stuff I can find and it's happy, but could run more advance with higher octane for more power. The Renault is 10.5:1, which is typically high for a Twink.

[tower of strength]

Interesting, that the spectre of valve seat recession hasn't reared its head over the water, clearly you guys have several years more experience than us. I think, and i stand to be corrected here, that the lead valve seat issue only applies to low grade inserts in alloy heads in cheaper cars and low grade cast iron heads. As the TC was destined for a high performance application, it would make sense that the valve seats were of a higher quality and therefore would be tollerant to unleaded running, I'm sure that this observation will get me a right thonging though!!LOL

Mark