Advanced timing; more power but crap mileage. 22 mech adv too much?

[Hardwyre]

Car: '1976 280z

Ignition: MSD 6AL Ignition

Alternator: CS-144 GM

Dizzy: D6F4-02

 

I've been told these L engines love having their timing advanced, and will run more efficiently because of it. Here's what I was just running:

 

Initially I started with this:

Initial: 13° when warm, 7° when cold

Mech: +22° all in at 2600

Vacu: +20° all in at 4600

Total: 55° at 4600

Milage: ~17.3 mi/gal

 

Initial: 20° (No ping detectable)

Mech: +22° all in @ ~2500

Vacu: +20° all in @ ~4600

Milage: ~15.2 mi/gal

 

I've backed the timing down to 15° but I've only got about 90 miles on the trip and already I've used over a 1/3 of the tank.

 

I've got all the parts to put in a wideband O2 and find out just what kind of fuel:air she's running (smell rich), but I'd like to hear some advice on my distributor's settings (it's stock as far as I know). I'm wondering if the MSD ignition is changing the timing requirements.

 

Any advice?

[Hardwyre]

There is no one who has any information on this? Seriously?

[Maverick101]

What fuel delivery system are you using? I would imagine that most of the MPG lost might be nothing more then a case of lead foot due to the increase in power. I would leave the car to whatever setting makes the car run best by ear unless the car is a daily driver. Just my 2 pennies without knowing what you truly have under the hood.

[jhm]

Hardwyre-

As we all know, these old gauges can be highly inaccurate....but if we assume for the moment that yours is accurate, you've used approx 1/3 tank to go 90 miles. Doesn't that equate to roughly 19 mpg?

 

Regarding your original question, I think the answer's 'yes'. Most engines will run most efficiently with advanced timing, provided they're not advanced so far as to cause pre-detonation. Seems like there's a few threads on this subject in the 'Ignition and Electrical' sub-forum.

 

Good luck with it and happy motoring.

jhm

[Hardwyre]

What fuel delivery system are you using? I would imagine that most of the MPG lost might be nothing more then a case of lead foot due to the increase in power. I would leave the car to whatever setting makes the car run best by ear unless the car is a daily driver. Just my 2 pennies without knowing what you truly have under the hood.

Sorry, I didn't mean to sound bitchy. You're right, I did forget some information. It's the typical '76 280z set up as far as I understand; 2.8L with the EFI. N42 heads I believe. I do get into it some days, but only for brief periods that I wouldn't think would greatly impact the overall mileage (this is the first sports car I've had; before it was a camry and a suburban).

 

Moreover what I was looking for was if my 22º of mech. advance was unusual. I have heard these cars can get upwards of 28 miles a gallon and still be a lot of fun, that's kind of what I'm striving for.

 

Hardwyre-

As we all know, these old gauges can be highly inaccurate....but if we assume for the moment that yours is accurate, you've used approx 1/3 tank to go 90 miles. Doesn't that equate to roughly 19 mpg?

 

Regarding your original question, I think the answer's 'yes'. Most engines will run most efficiently with advanced timing, provided they're not advanced so far as to cause pre-detonation. Seems like there's a few threads on this subject in the 'Ignition and Electrical' sub-forum.

 

Good luck with it and happy motoring.

jhm

Thanks JHM, as I said to Maverick, I didn't mean to sound whiny. What's confusing me with this engine is I had it up to 20 degrees BTDC with as much as 50+ total advance and I still wasn't hearing detonation (I'm used to the *ping ping ping* of a chevy V8; I'm assuming it sounds similar on these motors); but when I ran that, my mileage tanked. I've since backed it town to 15 and seem to be doing better mileage, but I'm also running premium gas this time around.

 

I guess what I really need to do is find a guide for all the maintenance things to do before trying to adjust the car. Like checking resistances and such of all the various sensors and things that control the fuel delivery (smells really rich even after a TPS adjustment). I have a wideband A:F setup that I'm going to get into her soon.

[Hardwyre]

Interesting. By what this site says, my dizzy is for a '77 datsun truck:

http://www.newprotest.org/projects/510/dizzy_specs.htm

 

"'77 620 pickup w/ manual tranny; single breaker point non-EI dizzy w/Large cap; full mech 11.5 (x2 if measured by crankshaft rpm: thus the 22-23 mech adv I'm seeing.)

 

What's interesting is it is an electronic distributor. Wonder if it truly is a truck distributor.

[Lazeum]

Total advance of 55°????

 

That seems huge to me. I run around 30° on my high comp (bad) setup. I'm expecting 36° with my new correct build. That's 20° difference!!!

[sticky280zx]

Thats with vaccuum....you're suppose to unplug your advance and check it when its disconnected, and a GREAT engine builder on here says On 93 Premium the most you can hope for is 15-20at idle and 38-40 all in by 3k and just back it off if it pings. Remember this is with the vac adv DISconnected

[Lazeum]

My bad you're right

[Hardwyre]

Thats with vaccuum....you're suppose to unplug your advance and check it when its disconnected, and a GREAT engine builder on here says On 93 Premium the most you can hope for is 15-20at idle and 38-40 all in by 3k and just back it off if it pings. Remember this is with the vac adv DISconnected

 

My dizzy is putting out about 22 degrees of mechanical advance with another 20 or so in the vacuum. From the sound of it, it's a truck distributor (620 pickup?).

 

I'm running the traditional L28 engine with the dished (I assume) pistons and N47(?) heads which might explain the non-ping at ridiculous advanced. I need to check the actually cam timing to even make sure things are lined up properly.

 

I did get it to 110 mph today at 15 degrees initial without even thinking about it on level ground, so I should probably check all my other sensors to make sure it's not pouring out a bunch of excess fuel.

[rucus01]

Car: '1976 280z

Ignition: MSD 6AL Ignition

Alternator: CS-144 GM

Dizzy: D6F4-02

 

I've been told these L engines love having their timing advanced, and will run more efficiently because of it. Here's what I was just running:

 

Initially I started with this:

Initial: 13° when warm, 7° when cold

Mech: +22° all in at 2600

Vacu: +20° all in at 4600

Total: 55° at 4600

Milage: ~17.3 mi/gal

 

Initial: 20° (No ping detectable)

Mech: +22° all in @ ~2500

Vacu: +20° all in @ ~4600

Milage: ~15.2 mi/gal

 

I've backed the timing down to 15° but I've only got about 90 miles on the trip and already I've used over a 1/3 of the tank.

 

I've got all the parts to put in a wideband O2 and find out just what kind of fuel:air she's running (smell rich), but I'd like to hear some advice on my distributor's settings (it's stock as far as I know). I'm wondering if the MSD ignition is changing the timing requirements.

 

Any advice?

 

Engines typically run best at whats called minimum best timing. That is the least amount of timing that gets you the peak power. The things that determine where your minimum best timing are burn rate of the fuel, size and shape of the chamber, temperature of the heads, compression ratio, rod stroke ratio, and bore to stroke ratio. The better the combustion chamber is designed the less timing will be required. Most pump gas motors usually have a MBT in the 32-40 range. Since your motor probably is on the low compression side, and has a fairly open chamber it will probably increase in power slightly up to 38-40. If you run more than that like are likely well past your MBT and are likely causing your engine to lose power even if you arent detonating, since your peak pressure timing event is before optimal. Your MSD ignition is likely causing your MBT to be lower than it would be with stock ignition because it is giving you a hotter spark which will start the fire a bit quicker. The best way to determine what you should have your timing set at is with a chassis dyno. Set the timing to stock settings, then run it on the chassis and see what your WHP is. Bump it a degree or two in either direction and see if your WHP increases or decreases. Once you determine this you simply set it to the lowest advance that will give you peak HP.

[Hardwyre]

Total advance of 55°????

 

That seems huge to me. I run around 30° on my high comp (bad) setup. I'm expecting 36° with my new correct build. That's 20° difference!!!

 

I think I need a proper distributor for this engine. I was thinking of doing an 82 L28 motor with maybe an P90 head modified for high compression with Singh grooves in the quelch zones... but I haven't had a chance to look for an engine locally.

 

At the same time I want to do an LS1/T56 conversion on her so spending the money on the P90 headed engine doesn't make much sense unless I can find a decent engine/head for fairly cheap.

[Hardwyre]

Engines typically run best at whats called minimum best timing. That is the least amount of timing that gets you the peak power. The things that determine where your minimum best timing are burn rate of the fuel, size and shape of the chamber, temperature of the heads, compression ratio, rod stroke ratio, and bore to stroke ratio. The better the combustion chamber is designed the less timing will be required. Most pump gas motors usually have a MBT in the 32-40 range. Since your motor probably is on the low compression side, and has a fairly open chamber it will probably increase in power slightly up to 38-40. If you run more than that like are likely well past your MBT and are likely causing your engine to lose power even if you arent detonating, since your peak pressure timing event is before optimal. Your MSD ignition is likely causing your MBT to be lower than it would be with stock ignition because it is giving you a hotter spark which will start the fire a bit quicker. The best way to determine what you should have your timing set at is with a chassis dyno. Set the timing to stock settings, then run it on the chassis and see what your WHP is. Bump it a degree or two in either direction and see if your WHP increases or decreases. Once you determine this you simply set it to the lowest advance that will give you peak HP.

 

Absolutely awesome, thank you!

[Hardwyre]

Thats with vaccuum....you're suppose to unplug your advance and check it when its disconnected, and a GREAT engine builder on here says On 93 Premium the most you can hope for is 15-20at idle and 38-40 all in by 3k and just back it off if it pings. Remember this is with the vac adv DISconnected

 

 

That IS with the vacuum disconnected.

 

I was getting 22-23 degrees of mechanical advance and ANOTHER 20 with vacuum.

 

Here is what I get right now.

 

Initial: 15

Int+Mech: 37 @ 2600

Int+Mech+Vac: 57 @ 4600

 

I think I have a 610 truck distributor in mine (at least from what I'm finding on a couple sites)

 

Think I should try running without vacuum advance for a while?

[johnc]

Think I should try running without vacuum advance for a while?

 

Yes.

[neotech84]

 

I think I have a 610 truck distributor in mine (at least from what I'm finding on a couple sites)

 

 

 

I doubt you have the 620 dizzy. Unless the PO swapped just the housing...... but I HIGHLY doubt its a 4 cylinder dizzy.

[Xnke]

You can swap the points breaker and dizzy cap to make a four into a six, although you may have to swap out the whole shaft. I Have done this once to make a four cylinder dizzy for a buddy of mine out of a spare L6 distributor that I had; his had the casing broken, but the shaft intact, so we swapped everything over to make it an L4 distributor.

 

I'm not sure of any other reason to do that, other than maybe the one you have is rebuilt and they just grabbed the wrong casing?

[beermanpete]

I have a 1973 240Z. A used L24 engine I bought came with a distributor that turn out to have the incorrect points cam. Ran fine to about 5000 and then started missing and popping. I put the points cam from the old engine in and it runs great.

 

I know (assume) your car has electronic triggering. The point is that with cars of this age, unless you know the history it could have incorrect parts in it. Rebuilders don't necessarily respect the subtle differences between the hot-rod version, the regular version, and the truck version of part. A L28 is an L28 is an L28.

 

Detonation is not the only determining factor of timing limits. As always, a compromise needs to be found between two or more parameters. If you want good fuel mileage, set the time for that. If you want good power set the timing for that. You might not get both at the same time.

 

 

Peter