Tuning Ignition for Economy

[XPC66]

You got me wrong, most of the wild cams I put in can't get down to 1000 rpm.

Never punctured a piston.

To get best performance max cyilnder pressure should occur ~13°. Lean mixtures burn slower, there is no way around that.

What I think you are referring to insofar as advancing with lean mixtures, is actually the gross dilution of A/F at idle with big cams... the mixture going in is say 11.5:1, but the effective fill mixture is lean?

The fact that a dizzy loses vacuum advance under load (as VE rises) is a clear indication of the requirement to retard as A/F increases.

Just programming an ECU readily shows the relationship between lean burn and ignition advance.

 

[jamyers]

OK, so electronic controls are the way to go...

On the ignition side, is there anything that will provide active feedback, like an oxygen sensor does for the fuel system?

Seems like without either a dyno or a ton of trial-error, it'd still be hard to get that "perfect" advance programmed.

 

[XPC66]

Yeah a knock sensor

 

[jamyers]

But you could still have "too much" advance without knocking, particularly at low loads - igniting the mix too early and losing power / economy.

 

[XPC66]

In the real world that tends not to happen. If the mixture is really lean, you are usually running part throttle and power is not the primary objective.

The O2 sensor is really only good for measuring at stoich and above, but if your program has a target of 15.5 for cruise/part throttle it will reduce fueling and therefore allow more spark advance. That is of course if your setup is for closed loop off idle.

Below 14.7 a CO sensor is the go for accuracy. For instance if you are in the power band the Lambda is about 0.85 = 12.5 AFR = 34.588 - (2.3529 x 12.5) = 5.18% CO

When you plant the foot, the TPS sends a signal for fuel enrichment, the spark retards and the O2 is cut out of the equation.

EGT as I mentioned previously is way of measuring efficiency so long as you account for the bathtub effect.

Hard limits for normally aspirated engines are:

knock
max cylinder pressure
ignition limit
egt

 

[wsa111]

Tony, i hope you got some good info about your post.
What i listed was some of the main concerns on parts available to help you.
Scott at reincarnation can custom curve your distributor if you feel its beyond your expertise.
I see you have cruise control, that alone will help your over the road fuel miliage by at least 10%.
You need to find someone local to you to do an A/F ratio check on your HW. That will help you to get the miliage & power you are looking for. Bill

 

[XPC66]

....

Seems like without either a dyno or a ton of trial-error, ......

Well you can't just buy a resprung unit offshelf and hope it will give you the best results. As wsa111 pointed out the curve will be peculiar to your setup, so the dyno is the best way to establish the curve and test it, otherwise you'll be going up hills for the best part of a weekend trying to derive a map.

However there is a method whereby you can make a rough appoximation using a timing light, that us older guys employed back when dynos weren't readily accessible and money was expensive:

find the best advance at idle, by adjusting for maximum smooth rpm e.g. 10°. Your vacuum advance shouldn't be a factor here, because the vacuum port shouldn't be active with the throttle plate shut;

with constant revs up a hill, find the the maximum advance at peak VE (which is around 400 rpm shy of the peak torque rpm) without inducing det. e.g. 41°. Your weights would then be notched to (41° -10°)/2 = 15.5°. To be on the safe side, the old rule of thumb was to knock 4° off the topend so for a conservative governor limit it would be (37°-10°)/2 = 13.5°. (Advance weights reference cam degrees).


Springs in parallel are like capacitors so to get a smooth curve that doesn't lead or lag you need to find the opitimal rate of advance. One spring should be lighter duty than the other to get the ranging right, while the overall stiffness of both will establish the setpoint. Once again you trial and error starting with weak springs, until accelerating up the hill doesn't result in pinking/det/bogging.

 

[dieselmaster]

sounds more like you want to look at transmission and rearend gearing for bet mpg. Create just enough power that allows you to pull the hill without downshifting, staying in your torque curve while at an acceptable road speed.

 

[BIGREDRASA]

I guess I should have posted this in the small six forum. This got way beyond what I was seeking. We've had a number of posts on resetting the curves on the DS II, with most aiming at improved power, and RPMs beyond highway cruising (70-80 MPH).

Specifically, I have a Stock engine and C4 (+/-), 2.83 axle, and 215/60-15 tires. Due to current financial/marital situations (nearly moribund mortgage business), I'm holding off on buying more toys for a while. In short, I was looking at setting max advance to correspond to 80 MPH on level ground, and let the vacuum take care of retarding advance on uphill runs.

My next toy will lilkely be a basic AFR meter. I've seen some advertised to work with the "Stock" narrow band O2 sensors. Will that be totally worthless?

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