How much ignition advance?

[Tim Falce]

I was looking at a new trick distributor at the weekend and the question of top end advance came up.
My engine is a 4.2 XK Jaguar lump with standard 9:1 pistons, mild duration but very high lift cams with triple Weber carbs.
I run about 36-39deg of advance at around 3-3.5K revs with about 12 at tick over. The top end advance is pretty much what is recommended for a road engine.
The instructions on the dizzy I looked at said to run 10deg bottom to 27deg top end for a full race setup. I would have thought that this is no where near enough advance at both ends, especially the top.
A couple of fellow racers who were looking at the distributor as well said that 27deg top end is enough, have I got it wrong and why?

[phoenix]

Originally Posted by falcemob

I was looking at a new trick distributor at the weekend and the question of top end advance came up.
My engine is a 4.2 XK Jaguar lump with standard 9:1 pistons, mild duration but very high lift cams with triple Weber carbs.
I run about 36-39deg of advance at around 3-3.5K revs with about 12 at tick over. The top end advance is pretty much what is recommended for a road engine.
The instructions on the dizzy I looked at said to run 10deg bottom to 27deg top end for a full race setup. I would have thought that this is no where near enough advance at both ends, especially the top.
A couple of fellow racers who were looking at the distributor as well said that 27deg top end is enough, have I got it wrong and why?

I am looking into the same question for a different engine. I don't have an answer for you, but this is a summary of what I have read an understand so maybe it will help!

1) The shape of the combustion chamber and the position of the plug are major influences on the amount of advance required, so if they remain unmodified the total advance required will stay the same unless:

a) the CR is raised
b) cylinder filling is improved
d) higher octane fuel is used

All three of the above will require less advance than standard.

From what I have read, if your cylinder heads have a hemi combustion chamber and an offset spark plug the 36-38 degrees total advance would seem to be in the right ball park.

2) Modified cams may (often do) result in poorer cylinder filling at lower RPM. This will require more advance at lower RPM than standard. This usually means having full advance at a lower engine speed than a standard engine, but usually in the range 3200 - 3800 RPM.

3) If there is a combination of 1) and 2) above you may need less total advance but with the maximum advance reached at a lower RPM than standard.

As the objective is to have the minimum advance for the maximum torque, the only real way of finding the optimum advance is on a dyno. But if you don't have any pinking under heavy load at say 3000 rpm, you have probably got the timing about right as it is. Retarding the spark, by the 9 to 12 degrees you might be thinking of, would probably lose power IMHO without a higher CR and much improved cylinder filling.

I am keen to hear what other people with experience have to say on this matter. The engine I am looking at is an 8 valve Fiat Twincam which has a hemi head and offset spark plug. The standard timing curve is given as 10 degrees at 600 rpm, 20 degrees at 1600 rpm with 38 degrees maximum at 5400 rpm. I am thinking that with higher CR, higer octane fuel, headwork and cams, the maximum advance needs to be in by 3800 rpm, but reduced a little due to improved cylinder filling, the higher CR and the better fuel.

I also have an old Porsche 912 which has hemi heads and offset spark plugs. The standard max timing is 36 degrees with 5 degrees static, but all in by 3200 rpm. With raised CR and a rally cam, ported heads and better carbs and 95 octane fuel we found on the dyno that MBT (minimum best timing?) was achieved with only 29 degrees maximum advance, again at 3200 rpm.

Now over to the experts!

Thanks

Martin

[Tim Falce]

Originally Posted by phoenix

d) higher octane fuel is used

All three of the above will require less advance than standard.

I always thought the higher the octane rating the more advance you need, hence us having to retard our road cars when 95 ron unleaded became the norm in the UK.

[phoenix]

Originally Posted by falcemob

I always thought the higher the octane rating the more advance you need, hence us having to retard our road cars when 95 ron unleaded became the norm in the UK.

Sorry! BIG mistake by me! I ought to wake up before I start typing!

Raised CR and/or increased cylinder filling = less advance
Higher octane = more advance

To late to amend my original post of course!

Martin

p.s. hope the rest of my post was useful/interesting?

p.p.s. - at least you didn't ask what happened to c)

[Al Weyman]

Well a lot of informed sources tell me 30 degree advance all in on my V8 chevys and thats what I have always stuck to and they sound nice and cisp at that. A word of warning though, I had an engine on EDA's dyno once and it was making good power at 30 degrees advance and against the advise of the owner but taking the advise of an engineer he had there we 'swung' the dizzy to advance it a little to try to make more power, the outcome was we detonated the pistons in about 10 seconds flat and had only advanced it a further few degrees max!

[zefarelly]

I've been running 14 static 32 max @ 11.5:1 chamber in head, flat top pistons

I'm thinking 12-30 would be better with the new cam and 12:1

minimum 100 octane motion lotion

most twinks run 28-30 max advance in full race trim . . . but Gordon will be on in a minute as his engine ( flat head ) sparks before BDC @ 20:1 running on home made cider thats 36 degrees & 13:1 in old money

[phoenix]

A couple of sources for further reading:

http://www.veloce.co.uk/shop/graphics/pdf/V291.pdf

http://www.autospeed.com/cms/A_1609/article.html

http://members.aol.com/DVAndrews/timing.htm

Martin

[phoenix]

Originally Posted by zefarelly

most twinks run 28-30 max advance in full race trim . . .

Lotus twinks I take it?

The Fiat is 1995 cc with a 90 mm stroke and 145 mm rod length whereas the (standard) Lotus has a 72.746 mm stroke and 121.922 mm rod.

As I understand it, this results in a piston speed about 25% higher in the Fiat than the Lotus for a given engine speed and also the piston spends less time around TDC. I am told this requires that the spark be ignited a bit earlier with the Fiat engine compared to the Lotus.

Could all be bull of course!

That being said, a full race 2 litre Lancia engine I know of also ran at 30 degrees max.

In my search for answers I came across the fact that Cosworth quote 34 degrees for the FVA which seems a lot to me.

[GORDON STREETER]

You woke me up Zef ! Yes I do run 36 deg at the top, and I have tried fixed timing as its never below 4000, but its difficult to start without a gizmo to knock it back initially. Its true that the better the combustion the less advance you need. I would like to think that my engine is as powerful as most of a similar ilk, but everything can be improved on as it has a big hole in the power band below 6000 due to poor cylinder filling.
That's running on LARKINS not cider

[zefarelly]

mine drives through but really pulls from 5-7k It might be that a little more advance would make it less drivable, with more power at a point, but as its got freeform jazz piston crowns now it'll all go in the bin as soon as I've bolted the new one together so I'll never find out, and I'm going from shopping to supermarket sweep in the cam stakes so the same disyy will do for starters.

[dtype38]

My Jag engine guru advised me to go for 12-13 deg static advance and 35-36 max for all states of tune on the XK lump. The question then is how quick to ramp it up. On the mechanical dizzy I used to run the ramp up started at about 1500rpm and went in pretty much a straight line to around 5000rpm (my peak power was at 6400rpm, max revs 6600rpm). Since then I've gone to managed spark and although I'm starting and finishing at the same advance, I'm ramping it up earlier and getting to max way down at around 3000rpm. I've had the car on the rolling road and increasing or decreasing advance both lose power, so that's pretty much optimum I'd say. :-)

[Al Weyman]

All in asap (around 3000rpm) just retarded to start the thing as Gordon says.

[GORDON STREETER]

On my engine I have found I can almost put all the advance in once the engine has started. Because of the silly cam duration I use it is useless down low anyway regardless of what I do with it. One thing I will say about timing is that once you have found the optimum max on the dyno or rolling road, find out what it is on your own timing gun, because I have seen a few deg variation between them and that could spell disaster if you re timed it at the track for any reason.

[zefarelly]

I tend to make marks on the pulley/front cover when I build, before I take the gauges etc off so I always have a static reference point.

of all the stupid things this week . . .new engine, fired and started and ran like a sack of crap . . . . turned out to be the fuel hose from bulkhead to carbs was twisted inside the braid and starved the engine of proper fuel flow !!!

I've been wracking my brains about ignition timing advance and carb set up for two days trying to work it out

[GORDON STREETER]

Do your brain's take a lot of wracking

[zefarelly]

turned out to be a twisted, and subsequently blocked fuel pipe, runs well now, at least it did on the rollers earlier until a core plug blew . . .they're held in with bolts and 1/4" bar now . .back to the rollers in the morning. . . .