Spark Timing

[bobc455]

Has anyone experimented with engine timing?

My car is a 1969 Buick that has been converted to EFI so I have full control over all parameters via a laptop. I know a lot of OBDII cars have similar software available.

My timing for highway cruising was around 51-53 degrees, but I noticed a misfire. My brother has a '98 Supercharged Riviera so I asked him where his timing was for highway cruising, and his timing map tops out at about 40 degrees.

In the "old days" where you had initial+centrifugal+vacuum advance, I thought it was fairly common to have well over 50 degrees of timing on the highway (10 degrees initial + 22 degrees centrifugal + 20 degrees vac advance), so I was surprised that Buick only programmed 40 degrees.

I have backed off my timing to about 40 degrees for now, but I want to achieve maximum FE, and I've read somewhere else on this site that every degree of timing can cost 1/2 MPG (although that seems quite high)

Anyone have some insight that could help?

Thanks,

-Bob Cunningham

[retrorocket]

My buddy back in the early 80's had a 69 Buick that got some pretty good mileage.

It had a 350 With a Qjet and a 2.73 gear.

He had a hot box on the factory distributor and a 2 1/2 in single exhaust with a

Turbo muffler. It would get 24 MPG very often.

It impressed me at the time because I could not get over 21 with my Olds.

I had a Holley carb and that was a good part of the problem.


The timing that works best depends on several factors for you car.

One of the absolute most important is how powerful the ignition system is.

You need more ignition lead with a weak spark and less with a strong one.

That is one of the reasons the timing map on a newer car does not have as high of part throttle cruise timing as we ran in the old days.
The other is oxides of nitrogen.

It is better for mileage to not have too much timing because you just waste power compressing a mixture that is already expanding.

But it is very important that you have enough timing or you do not reach your max combustion temp at the best time in the power stroke.

This is very much a test and test and test procedure.

The load is very important to timing requirements as well.

Is your Buick a 350 or a 430?

[bobc455]

Actually it has a 455, and I have a 2.56 rearend. Your friend's 24 MPG is impressive with the 350!

The other question, then, is how do I tell when the cylinder reaches maximum pressure? I understand that it should be several degrees after TDC, but I'm not sure how to measure this (without some sort of fancy in-cylinder pressure transducer).

I guess I could watch my injector pulse width to see if tweaking the timing affects the gas mileage, however the changes might be so slight that I can't read the difference... (with a 455, I am SO light on the throttle to maintain 75 MPH that the slightest muscle twitch to my foot will affect injector PW).

I have a relatively strong ignition.

Unless someone has some other insight, I will try adding some timing one degree at a time until it starts to misfire.

-Bob C.

[omgwtfbyobbq]

Whoa, 455? Forget ignition timing, you need that puppy idling down the freeway at whatever your highway cruise is to see significantly better mpg.

[retrorocket]

Get a vacuum gauge and see if you can monitor the pulse width of your injectors. Someone on this site should be able to help you there.

If you had an msd ignition box then you could buy an adjustable timing control and adjust it from inside the car and monitor what is happening to the injector pulse width while cruising down the road.
other wise just keep adjusting the distributor, vacuum canister, weight travel, and springs

Get a dial back timing light and plot your timing curve.

Keep playing with it till your mileage seems its best.

I assume (but do not know) that you have a throttle body injection on a dual plane manifold. If that is true and your two two plenum's are completely isolated (or can be) then you could kill the injector signal to one side of the manifold. That will take out every other cylinder in the firing order and still run smooth but will give you a a tiny 3.75 liter engine.

Listen for detonation and if you run a hot ignition system and your engine does not have any oil control problems you could probably run one or two heat ranges cooler on your plugs to reduce pinging is you have that problem.

hope to hear more

[omgwtfbyobbq]

It depends... If you do it w/ a small displacement auto, and the darn thing downshifts the majority of the time then there probably won't be much of a gain, as we've seen. Otoh, given that the OP is at barely any throttle at 70mph, I think running half the injectors could help as long as the trans doesn't kick down, since all the cylinders, specifically the four firing, will be pulling in twice as much air and pumping losses will drop by a bit.

[omgwtfbyobbq]

Iirc, everyone who has tried has had a small engine with an auto transmission, so, given that the transmission is likely to kick down way more often, of course there wouldn't be an improvement. Unless someone has consistently tried it with a manual trans, or a large disp auto that won't kick down w/ it in place, I'm stickin' to mah gunz. You and whoever had the auto Acura tried it, and I know the Acura spent most of the time a gear down, w/o the tcc locked, as I'm guessing you did. The OP has your engine, times three, so it shouldn't kick down, and he should see a noticable increase as long as the trans does kick down and *tcc stays locked imo, especially if he can see/log the duty cycle and do a few alternating runs to look for a difference.

*if it has one

[Gary Palmer]

I have to agree with Clencher. Unless you find some way to shut off air flow to the cylinders which are not receiving injection, it is not going to be more fuel efficient, it's just going to run lousy.

[omgwtfbyobbq]

No way! How/when did you kill the injectors? Didn't you say yours wouldn't even idle and you had to stomp the gas to keep it going? That may play a big part.

[omgwtfbyobbq]

It can't be pumping losses from the other cylinders. I already asked about that and there's little to no difference between an engine that's off, or cylinder that's off in this case, pumping or not pumping air.

Also, would having the TB wide open with no fuel being injected (assuming one could "trick" the ECU into thinking it's closed) during a coast reduce the losses compared to TB closed coasting?

So it can't be pumping losses from those cylinders that are just moving air. You know what I bet it is? A rich mixture. I saw a significant mpg increase from leaning out my car during closed loop because the afr was around 12:1, and according to this graph from an article SVOboy linked for me, efficiency drops by nearly 10% for each unit decrease in afr.

So if you needed WOT/open loop for even a third of the time, and your open loop afr was like mine was, your mileage would drop way more than the increase of ~6-12% in cruise FE, and you'll see worse mileage. Iono, I still wanna see a relatively large displacement vehicle that can cut half the injectors w/o going into open loop or downshifting/etc try this.