Less is more? No...

[Dieselboy]

...More is more!

I just ordered a 4911 and injectors from Tim at Accurate Diesel. :D 76 ccs & 2150 psi injectors. This will be a nice improvement from the 59 ccs and 1800 psi injectors I have now.

I asked Tim if he had any references for timing. He found a spec that called out 4* ATDC @ 1200. I'll give it a try in addition to a few other figures that have been discussed recently.

[doncannon]

Hi dieselboy,
I have just emailed Tim about injectors and a pump. what were they used on? if you dont mind me asking? why are they better?
thanks for the help,
Don

[britannic]

Is that supposed to be 4* BTDC @ 1200 - otherwise it seems very retarded.


Originally posted by Dieselboy:
...More is more!

I just ordered a 4911 and injectors from Tim at Accurate Diesel. :D 76 ccs & 2150 psi injectors. This will be a nice improvement from the 59 ccs and 1800 psi injectors I have now.

I asked Tim if he had any references for timing. He found a spec that called out 4* ATDC @ 1200. I'll give it a try in addition to a few other figures that have been discussed recently.

[Dieselboy]

Casey pioneered the 4* ATDC for his hot-rod engine.

Once the engine is running, the first timing mark will duplicate the -1.94 offset that 6.5 guys use. That translates into about 8.5* BTDC at idle.

[arveetek]

Originally posted by britannic:
Is that supposed to be 4* BTDC @ 1200 - otherwise it seems very retarded.
We must not forget, when considering timing, as to what way we will be measuring the timing: either by a clamp-on injection pulse meter, or a screw-in combustion probe. When measuring timing as I did, by measuring when the actual combustion takes place, then the readings MUST be ATDC, or else the engine would be running backwards! Combustion happens AFTER the fuel has already been injected. However, if measuring timing with a clamp-on injection pulse meter, you'll be measuring the injection taking place BEFORE combustion, so the timing measurements would end up being BTDC.

So, when giving timing recommendations, the type of meter being used MUST also be stated, otherwise the numbers are useless. Setting the timing at 4* ATDC when using a clamp-on meter would end up a with a really sluggish, smoking engine. Setting the timing to 4* BTDC with a screw-in combustion probe meter would result in the engine trying to run backwards, or serious engine damage. It's physically impossible for the engine to be running when trying to set the combustion to take place BTDC.

Casey

[john8662]

Originally posted by Dieselboy:
Once the engine is running, the first timing mark will duplicate the -1.94 offset that 6.5 guys use. That translates into about 8.5* BTDC at idle. With a clamp-on type as Casey states (great write-up by the way) you could see 8.5* but I don't think you'll be happy.

I had my timing set at 6.5* BTDC on my '86 non-turbo rig and it was too much advance in my opinion. The problem I got was that it started to "rattle" at full throttle right before the trans would shift at red-line. This to me means I had too much advance, although I was supposed to be right on the money, 4.5* plus 2* for high altitude = 6.5*.

What I was experiencing (the high end rattle) was confirmed by Kent on the 6.5 forum, because he said he experienced the same thing when running the -1.94 TDC offset on his 6.5. His experience with other Diesels has led him to the impression that when you have the rattle at high end like that, that you're taking a chance with the engine each time (Hopefully he'll elaborate here).

[Dieselboy]

The pump and injectors are in, and I fired it off today. There's a slight miss, but that's normal for fresh injectors. I usually have to go around bleeding every single one to purge that last bit of air. It definitely needs timing as it sounds retarded.

At the same time, I installed a 96+ serpentine setup with a HO water pump and dual alternators. That's working very well.

[britannic]

I set my timing at 2.6* BTDC using a combustion probe style meter. Remember, it takes a few milliseconds for combustion to reach full pressure and non-turbo engines will have a slower burn, so they usually require around 4* BTDC at 1400rpm. Those few milliseconds will see the piston pass through TDC, so the engine won't be running backwards.

The injection event likely started in my engine slightly before the probe saw it light off, then continued through several degrees of engine rotation.

Gasoline engines use BTDC spark timing to combat combustion latency as well, which is why the spark timing advance can range from 3-14* at idle to 60-70* (or even higher) at high engine speeds.


Originally posted by arveetek:
</font><blockquote>quote:</font><hr />Originally posted by britannic:
Is that supposed to be 4* BTDC @ 1200 - otherwise it seems very retarded.
We must not forget, when considering timing, as to what way we will be measuring the timing: either by a clamp-on injection pulse meter, or a screw-in combustion probe. When measuring timing as I did, by measuring when the actual combustion takes place, then the readings MUST be ATDC, or else the engine would be running backwards! Combustion happens AFTER the fuel has already been injected. However, if measuring timing with a clamp-on injection pulse meter, you'll be measuring the injection taking place BEFORE combustion, so the timing measurements would end up being BTDC.

So, when giving timing recommendations, the type of meter being used MUST also be stated, otherwise the numbers are useless. Setting the timing at 4* ATDC when using a clamp-on meter would end up a with a really sluggish, smoking engine. Setting the timing to 4* BTDC with a screw-in combustion probe meter would result in the engine trying to run backwards, or serious engine damage. It's physically impossible for the engine to be running when trying to set the combustion to take place BTDC.

Casey </font>[/QUOTE]

[arveetek]

Originally posted by britannic:
I set my timing at 2.6* BTDC using a combustion probe style meter. Remember, it takes a few milliseconds for combustion to reach full pressure and non-turbo engines will have a slower burn, so they usually require around 4* BTDC at 1400rpm. Those few milliseconds will see the piston pass through TDC, so the engine won't be running backwards.

The injection event likely started in my engine slightly before the probe saw it light off, then continued through several degrees of engine rotation.

Gasoline engines use BTDC spark timing to combat combustion latency as well, which is why the spark timing advance can range from 3-14* at idle to 60-70* (or even higher) at high engine speeds.

If I had tried to set my engine to anything BTDC at 1400 RPM, I am convinced I would have done serious engine damage. At 2.5* ATDC, my engine was rattling terribly, and it made me cringe. I can't even begin to imagine trying to set it any more advanced than that. I still believe that serious engine damage would have occured.

Yes, gasoline engines are timed measuring spark, not ignition/combustion, hence the BTDC settings. Yes, some time does pass between the beginning and ending of combustion, but some time also passes between spark/injection and combustion. Therefore, it's impossible to use the same settings when measuring injection or combustion. Measuring injection pulse on a diesel would be the same as measuring spark in a gasser.

Is it possible that you may have misread your meter? It took me a while to figure out exactly what -4.0 degrees meant on the digital MT 1480A. I finally discovered that a (-) sign in front of the number meant ATDC. IIRC, you were using the older analog meter?

One thing's for sure...timing these engines is really more of an art than an exact science. Both Tim Wagner and John Kennedy have stated that timing these DB2 equipped trucks is done more by ear and readings on the boost/egt/coolant guages than by timing equipment. But it's good to have a refrence point.

When reading through the various TDP posts on timing, recommendations vary widely, from 4* BTDC to 6* ATDC. That's quite a range. I soon realized that some readings were taken using the clamp-on injection pulse meters, and others were using the lumy probe. I was simply able to find out that my particular 6.2L turbo likes 4* ATDC when measuring combustion with a lumy probe. Any more retarded than that, boost falls and EGT's climb. Any more advanced than that, and it rattles terribly, and approaches piston damage from too early of combustion.

Casey

[arveetek]

One other thing I just thought of: the timing mark tabs on our 6.2L engines are held on with a couple of bolts, and the timing tabs have slots in them, allowing adjustment. If the tabs are not installed properly, then when the harmonic balancer shows TDC on the timing mark tab, it may be BTDC or ATDC in reality. That could also be a cause for the wide range of readings. I will be honest and say that I don't for sure that my timing tab is located correctly. If mine's installed a little ATDC and yours is a little BTDC, then that would cause a mess as well.

In fact, now that I think about it, I installed a different timing cover when I rebuilt my engine. I had another cover already cleaned and painted, so the timing marks may not be correct. Hmmmmm....

Casey

[britannic]

I have timed 4 different engines using 4* BTDC for NA and 2.6* BTDC for Turbo using both an MT480 and a Rotunda timing meter, both combustion probe/mag pick up style. There wasn't a mistake about BTDC/ATDC in my case and in fact as the rpms were increased above 2000rpm, the meters actually show the combustion point approaching 0* or TDC (no load).

Your observation about differences in timing cover timing tabs is right on the money - to ensure accuracy, it should be set correctly.

With the HPCA engaged, the meters both showed 7-11* BTDC for the 4 engines I worked on.

My military manual stipulates 4* BTDC @ 1300rpm for the Hummer 6.2L NA - excerpt follows:

[b]"14. Position sensor switches (1) to clamp-on and magnetic pickup positions.

For vehicles with 1990 engines and above, timing must be 5

[JeepSJ]

I'm suffering from the timing pointer quandry myself. My engine didn't have one when I bought it. I figured that the timing pointer would have holes, not slots, just like the pointer on my 350. Had I known about the slots, I would have found true TDC while I had the engine apart and set the pointer.

We were discussing this on the 6.5 board and Kennedy's response was to just run it. Set the timing by sound and feel and when it sounds and feels right just make note of the indicated timing as a reference for future tuning.

Question - when using a clamp-on pickup, does it go on the pump end or the injector end of the injection line? The manual for my timing meter says to refer to the vehicle manufacturer to determine which end of the tube for the measurement.

[john8662]

JeepSJ,

The clamp on probe goes onto the line close to the injector itself.