Ignition timing when cruising

DEVONMAN · Post by **DEVONMAN** » Sat Sep 14, 2013 9:33 am

A question for any ignition timing experts.

My car is high geared and at 70mph in 6th is running at 1800 RPM.

I have a Megajolt system fitted and would like to set it to give good economy when cruising on the motorway.

The engine is a 5.0 RV8, CR 9.5 to 1 and the car weighs 1300kg.
The mixture ratio at cruise is 14.7 and the load varies 50-60 KPa.
The question is , what ignition advance can I dare to dial in for these conditions?

Thanks
Denis

r2d2hp · Post by **r2d2hp** » Sat Sep 14, 2013 10:14 am

Hi Dennis,

Think mine is set to around 33 degrees looking at a log file

regards
Reg

DaveEFI · Post by **DaveEFI** » Sat Sep 14, 2013 11:35 am

The factory mapping provides something like 4-6 degrees of extra advance at high vacuum. So that would be a good starting point. But if you've not got a cat, you could also lean off to about 15.5 or so at cruise.

SimpleSimon · Post by **SimpleSimon** » Sat Sep 14, 2013 6:51 pm

Currently running around 38/40 advance and AFR 15.1 on light cruise (50/70 ish)

hardly any throttle travel required but no forced induction or severe comp though

not suggesting you run the same but gives you some direction

DEVONMAN · Post by **DEVONMAN** » Sun Sep 15, 2013 9:58 am

Thanks for the input guys,

I will put in 33 degrees at 50 KPa and 30 degrees at 60 KPa and over the next week try uping those a few degrees.

The problem is that there is very little feel/feedback from the throttle due to the high gearing in 6th and trial and error is unlikely to find the best setting.

The Holley injection is controlled by a narrow band lambda and this allows the mixture to flicker each side of the 14.7 ratio on light throttle.

Regards Denis

stevieturbo · Post by **stevieturbo** » Sun Sep 15, 2013 8:09 pm

As Simon says, you could easily be in the high 30's or 40deg range, maybe even 50 in some cases

You wont do any harm though going too far, but Ive never seen an engine use more than 50

Some say you can tune these sites by increasing timing until maximum vacuum is achieved. That isnt always easy to do on the road though.

sidecar · Post by **sidecar** » Sun Sep 15, 2013 8:50 pm

I think 50 degrees would be OK for the figure at higher revs but we are only talking 1800 RPM here. The old vac advance on the Lucas dizzy would add about 12 degrees to whatever the mechanical advance had dialed in.

Having said that I think that Rover ran the system way too retarded most of the time anyway. I run 18 at idle on my 4.6, not the 6 that Rover set it to!

DaveEFI · Post by **DaveEFI** » Sun Sep 15, 2013 10:45 pm

sidecar wrote:I think 50 degrees would be OK for the figure at higher revs but we are only talking 1800 RPM here. The old vac advance on the Lucas dizzy would add about 12 degrees to whatever the mechanical advance had dialed in.

Having said that I think that Rover ran the system way too retarded most of the time anyway. I run 18 at idle on my 4.6, not the 6 that Rover set it to!

Think the problem with a mechanical advance system is you physically can't produce the ideal curve for starting under the worst conditions and advance at idle. With a mapped one, you can do near anything.

sidecar · Post by **sidecar** » Mon Sep 16, 2013 4:34 am

DaveEFI wrote:
sidecar wrote:I think 50 degrees would be OK for the figure at higher revs but we are only talking 1800 RPM here. The old vac advance on the Lucas dizzy would add about 12 degrees to whatever the mechanical advance had dialed in.

Having said that I think that Rover ran the system way too retarded most of the time anyway. I run 18 at idle on my 4.6, not the 6 that Rover set it to!
Think the problem with a mechanical advance system is you physically can't produce the ideal curve for starting under the worst conditions and advance at idle. With a mapped one, you can do near anything.

Yep, which is why I run a programmable MSD unit! I do run a dizzy but it's locked out. I was just quoting the figures from a mechanical system for ease.

DEVONMAN · Post by **DEVONMAN** » Mon Sep 16, 2013 7:56 am

Rover did have to be cautious to allow for crap fuel, driving at high and low altitudes and of course the odd driver who would floor it at 15MPH in top gear.

DaveEFI · Post by **DaveEFI** » Mon Sep 16, 2013 8:04 am

My SD1 is an auto with a near standard 3.5 Vitesse engine (only has stage1 heads different) with EDIS and Megasquirt. It can be made to pull very hard from low revs in second gear without changing down. And if anything, this is where it is light years better than the original Lucas EFI/dizzy.

SimpleSimon · Post by **SimpleSimon** » Mon Sep 16, 2013 10:53 am

sidecar wrote:I think 50 degrees would be OK for the figure at higher revs but we are only talking 1800 RPM here. The old vac advance on the Lucas dizzy would add about 12 degrees to whatever the mechanical advance had dialed in.

Having said that I think that Rover ran the system way too retarded most of the time anyway. I run 18 at idle on my 4.6, not the 6 that Rover set it to!

I also run 18 degrees at idle and it softens the cam lope too, I did try more (20+) but it made no real difference so stuck at 18 with my max total advance being 30 but sh*t load's at small throttle openings/cruise, that's the beauty of mapped ign knowing that soon as you jump on the gas it pulls all that advance away

can you imagine doing that with a bobweight/vac dizzy?

like DaveEFI says bonus

sidecar · Post by **sidecar** » Mon Sep 16, 2013 3:59 pm

SimpleSimon wrote: I also run 18 degrees at idle and it softens the cam lope too, I did try more (20+) but it made no real difference so stuck at 18 with my max total advance being 30 but sh*t load's at small throttle openings/cruise, that's the beauty of mapped ign knowing that soon as you jump on the gas it pulls all that advance away can you imagine doing that with a bobweight/vac dizzy? like DaveEFI says bonus

I'm glad that someone else runs as much timing as me at idle, I was half expecting people on the forum to 'kick back' with "that's way too much, Rover know best, bla, bla, bla" (No pun intended with 'kick back'!)

I run about 10 degrees from 0-600 RPM which helps with starting the engine, the MSD then ramps this up to 18 degrees and flat lines it to 1000 RPM. (My idle is at 850 RPM)

The problem with finding the cruise timing is that the engine will probably never detonate when under part load because the VE is so low, but that does not mean that you should just keep winding in more advance. The whole reason for advancing the timing under these conditions is that the peak cylinder pressure before the spark is also low because of the low VE, this means that the burn is slow which results in the peak cylinder pressure after the spark occurs is too late in the power stroke. My understanding is that is that any piston engine always wants the peak cylinder pressure on the power stroke to happen with a narrow crank angle margin, (15-20 degrees ATDC). The like of Ricardo came up with this figure, not me!

The problem is for the home enthusiast is that you really need a decent engine dyno and a load of diagnostic equipment in order to map all of this stuff out.

Really working out how to get the peak cylinder pressure to be 15-20 degree down the power stoke whilst on cruise (or under other conditions) without a dyno is always going to be a bit of a guess!