Dumb question about advance

[Ratman]

Ok, here's a dumb question that's been in the back of my head now for quite some time. Why is it that on our IP's (particularly on the DB2), when we increase fuel rate, -we retard timing?

I understand that mechanically pulling in a bunch of advance if you were lugging the engine along in high gear and suddenly "matted" the go pedal, -I don't imagine the engine would appreciate that.

But, -it sure seems to me that some extra power and drivability could be had if there wasn't so much retard built into the cam.

I have read some info on the 7.3 IDI site about people tweaking the ramp angle and tuning the retard cam for a little more "go".

Anyone care to shed some light?

BTW, my rig is still chuggin away running great with almost 40k on the rebuild.

Rich

 

[Ratman]

Just to add a little more, -I was in a hurry and running out the door when I was making the original post. The way I understand it that advance is desired as engine speed increases, because you obviously want to give the flame front more of a head-start (so to speak) to make the most practical use of combustion-pressure.

If I'm not mistaken, that is what the internal mechanical advance is supposed to do in our DB-2's (and what the stepper controlled advance is supposed to do on our DS-4's).

Is the mechanical-retard on the fuel control shaft of the DB-2 simply there to protect the engine from a situation like if you were rolling along at 30 mph in 4th gear with your NV4500 and you simply rolled on the fuel and lugged it?

As we all know, -that would be one of the toughest things to make your drivetrain and engine endure, (not to mention the smoke and high EGT's) -sloppy and foolish and lazy at best.

None of us would ever consider doing that, -if you need a gear, you grab it, plain and simple.

I guess what I'm wondering, -could the retard cam be defeated without serious consequences if the person driving it used a little common sense? Or, is the mechanical retard there for a completely different reason that I am totally missing?

Comments, -suggestions, -and even insults, -welcome. :agreed:

 

[Burning oil]

Not a dumb question but a great one. Tuning in.

 

[3500GMC]

I think you've got the advance/retard thing backwards.

The 'ramp' on a DB2 is for advance. It actuates a valve that rotates the cam ring internally with housing pressure. It also works in conjunction with the governor. It's hydraulic tug-of-war. A similar system is used in an automatic trans that uses a throttle valve and governor, ala TH700R4.

You want advance 'lead' for higher engine speeds simply because the fuel (gassers too) needs to be lit sooner to reach optimum cylinder pressure with faster moving air and pistons.

I think..:hihi: Hell maybe I'm bass ackwards.

 

[buddy]

I think he meant some physically retard the base timing by turning the pump. I'm not sure you need to do that.

 

[Ratman]

I think he meant some physically retard the base timing by turning the pump. I'm not sure you need to do that.

Nah, -what I meant was that I am the retard!!:hihi:

 

[Ratman]

I think you've got the advance/retard thing backwards.

The 'ramp' on a DB2 is for advance. It actuates a valve that rotates the cam ring internally with housing pressure. It also works in conjunction with the governor. It's hydraulic tug-of-war. A similar system is used in an automatic trans that uses a throttle valve and governor, ala TH700R4.

You want advance 'lead' for higher engine speeds simply because the fuel (gassers too) needs to be lit sooner to reach optimum cylinder pressure with faster moving air and pistons.

I think..:hihi: Hell maybe I'm bass ackwards.

View attachment 26096View attachment 26095View attachment 26097

Hey, those pictures sort of clear things up a little, -good stuff, thank you. It looks to me like the advance piston advances the cam ring by using transfer pump pressure. That makes sense, because the cold-advance solenoid opens and closes a valve that supplies the advance piston the fuel pressure it needs to advance the cam ring when the engine is cold.

Now, that being said, -I'm looking at the arrow that shows advance direction, and it would appear that the mechanical plunger (that is hooked to the retard-cam that I am talking about) is actually mechanically linked to the cam ring, not hydraulically. It pushes against the pressure created by the hydraulic advance piston on the opposite side, -but does so through a small coil spring.

The coil spring rests in what is referred to as a piston, -but really, it looks like it's just a spring-cup.

If I'm looking at it correctly, the cam on the outside of the IP would still retard timing.

That is the part that confuses me. It would be interesting to defeat that and see what happens.

Still have cold advance, -and still have mechanical advance, -but lose the mechanical retard thing.

This may sound way off base, but I worked on a 366 BBC in a GMC flatbed that had the vacuum advance can on the HEI distributor hooked to manifold vacuum (from the factory). I asked a tech about why they did it that way, -and he said it was to prevent spark-knock under heavy load. I know that doesn't pertain to our diesels really, -or does it?

Maybe there is some merit to backing timing off under heavy-fuel/loaded conditions?

Could someone please tell me I'm not insane?

 

[ak diesel driver]

gassers tend to pre ignite so they retard the timing in a low vacuum situation (wot). this wouldn't hold true for diesels it would just bog alot which isn't necessarily good either

 

[NVW]

Also, the gas has a mechanical advance which is affected by rpm.

 

[buddy]

Factory programming for L65 engines is similar where GM retarded the timing at higher fuel rates. However, that doesnt mean it need to be in a diesel. The only reason I retard timing in my programming is for towing, where sustained fuel rates will heat up the engine and cylinder walls, heavy boost will heat up the air charge, and the ignition delay is reduced for that reason. Also even if it was making more power by advancing, more heat would be transferred to the heads if ignition starts too much before TDC, just the amount of time combustion and high pressure is being applied to the heads is more. If ignition is at or slightly after TDC then the heads would be exposed to lesser combustion pressures and less time at combustion temps.

But in my normal programming I dont retard timing, I steadily advance it. This is the way many of the stock GM L56 engines were programmed as well. I suppose GM didnt think that the 1/2 tons would be towing as much weight or as often. However, the L65 programming does advance timing again near max fueling. In fact right about 3300rpm is where it really jumps up in timing advance at around 60-65mm3, and that is where the stock engines make the most horsepower, 3400rpm at max fueling.

 

[Ratman]

Factory programming for L65 engines is similar where GM retarded the timing at higher fuel rates. However, that doesnt mean it need to be in a diesel. The only reason I retard timing in my programming is for towing, where sustained fuel rates will heat up the engine and cylinder walls, heavy boost will heat up the air charge, and the ignition delay is reduced for that reason. Also even if it was making more power by advancing, more heat would be transferred to the heads if ignition starts too much before TDC, just the amount of time combustion and high pressure is being applied to the heads is more. If ignition is at or slightly after TDC then the heads would be exposed to lesser combustion pressures and less time at combustion temps.

But in my normal programming I dont retard timing, I steadily advance it. This is the way many of the stock GM L56 engines were programmed as well. I suppose GM didnt think that the 1/2 tons would be towing as much weight or as often. However, the L65 programming does advance timing again near max fueling. In fact right about 3300rpm is where it really jumps up in timing advance at around 60-65mm3, and that is where the stock engines make the most horsepower, 3400rpm at max fueling.

Everything you said makes sense, -and I would understand about heat being a potential problem under sustained high-fuel operating conditions, -absolutely. But, it would be cool to have a means of controlling that mechanical timing retard with a dash-mounted cable or something. Then, if you wanted to hot-foot around town empty bangin' gears in the 4500, -you could lock it out and take advantage of the timing advance and spunky response. If you were going to pull a grade, or pull a heavy load, -you could manually back the timing off via the cable.

I know that us DB2 guys are in the stone age, -but does what I'm talking about doing seem like a reasonable thing to do to pick up some responsiveness and gain a little more power?

I appreciate your programming experience, because whether it be electronic or mechanical, -the same theories still apply.

Thoughts, opinions, and criticism please........

 

[WarWagon]

Take a bar and push the advance piston in when the engine is at idle.
The chugging and near stall will both confirm and illustrate the need for advance and that the IP advance system is working.

Temperature of the fuel is compensated for in the IP to keep idle RPM steady. However temperature for sustained high load conditions Buddy describes is not compensated for as of 2008. Your engine can advance too much when things get hot enough from extended high load and air temperatures. If the load was quick and gone this heat build up would not happen and self advance the timing by less ignition delay. So the designers put in a safe amount of timing having no way to back it off under sustained high load. (My 2008 would start to knock under a steady trailer hill climb as the ECT and inter cooler heated up.)

In any case the DB2 advances as required. You can mess with timing by turning up the housing pressure screw on the inlet of the IP.