The V8 Owners Forum -

Trying to wring the maximum MPG out of my SD1 fitted with a MegaSquirt, and wondered what would be the best target AFRs at various temperatures during warmup?
It runs just fine - but I suspect is rather richer than it need be.

Trouble is you don't get long to try different settings before it has warmed up.

DaveEFI wrote:Trying to wring the maximum MPG out of my SD1 fitted with a MegaSquirt, and wondered what would be the best target AFRs at various temperatures during warmup?
It runs just fine - but I suspect is rather richer than it need be.

Trouble is you don't get long to try different settings before it has warmed up.

When I set mine up it took about a week of cold starts. I backed the settings off towwards lean until it ran ok on fast idle until warm.
My 5.0 does not like anthing leaner than 13.5 at hot tick over, and at the moment the cold start is 12.5 for the current ambient temperature.
In mid winter it's set at about 12.

Cheers Denis

Right. Mine idles pretty well at the factory setting for a Vitesse - 14.1:1. It will idle well quite a bit richer - but as I said I'm trying to get the best mpg, as it does get used for quite short journeys in town.

I'll try your settings tomorrow, thanks.

DaveEFI wrote:Trying to wring the maximum MPG out of my SD1 fitted with a MegaSquirt, and wondered what would be the best target AFRs at various temperatures during warmup?
It runs just fine - but I suspect is rather richer than it need be.

Trouble is you don't get long to try different settings before it has warmed up.

Simple, none.

Use whatever is necessary to make it run to a satisfactory level for you. Every engine will be different.

Some might need down to 11's, others might be happy at stioch

Mine seems to be happy at stoich more or less from the coolant getting to 40 degrees C. In fact closed loop lambda is active from 45 C.

It takes more fuel to get to a stoich. mixture when cold, slightly more range than the lambda correction is allowed to have, but that's why there's a temperature compensation table... It's generally not, in my limited understanding, to run a lot richer when cold unless the engine has a particularly unusual level of tune.

So in other words, the AFR shown shouldn't vary with temperature even although the amount of fuel injected does? So once the engine is running at idle, I should ideally see the same figure regardless of temp?

There is no right AFR.

If you're aiming to save fuel, tune it to run the leanest usable AFR that will allow you to drive it in an acceptable manner.

stevieturbo wrote:There is no right AFR.

If you're aiming to save fuel, tune it to run the leanest usable AFR that will allow you to drive it in an acceptable manner.

I understand that. However, I'm asking about some guidelines for warm-up.
It's difficult to experiment since it doesn't last long.

I know, but that's my point.

My LS is pretty happy in the 14's from cold

I do a lot of work at Subarus, and most of them wont be happy anything leaner than 12's when cold.

Pretty sure my RV8's also liked to be fairly rich

Easiest thing to do is if the engine starts and runs well when hot. Remove all cold start enrichments and start from zero.

If it doesnt start, slowly add fuel.

MegaSquirt allows totally different cranking fuelling.

Cranking fuelling and cold running enrichments after startup are two separate areas though.

Again if saving fuel is paramount, zero everything and add fuel as required to achieve acceptable start and run.

stevieturbo wrote:Cranking fuelling and cold running enrichments after startup are two separate areas though.

Again if saving fuel is paramount, zero everything and add fuel as required to achieve acceptable start and run.

As stated earlier that's what I did, but, I worked towards lean so that the thing would actually start. Once I got that sorted I then kept reducing the temp at which the Oxygen Sensor came in but that would be pointless if you have a tuned motor which will not idle as lean as 14.7.

Regards Denis