Timing - round 2

[Dieselboy]

I have been waiting to time my engine again, and just had the opportunity last night.

I went from 1* BTDC @ 1400 RPM to 3* BTDC @ 1400 RPM. The engine clack is noticabely louder at idle to about 1500. Beyond that, the volume and quality of the clack sounds normal. My coworker's 96 Powersmoke is still ten time louder though.

As predicted, turbo lag increased slightly, and boost levels were down by about 1/2 psi. There was more smoke throughout the rpm range, but cruising EGTs were lower by 50-75 degrees. It also feels like I picked up just a little bit of power; nothing much though.

What puzzles me is the increase in smoke. I figured that with the fuel injected earlier, and a higher peak flame temp & cylinder pressure, there would be a more complete burn. I'm not flowing a lot of fuel, and have regular 1800 psi 6.2L injectors. No doubt much of this will clear up with a set of 6.5TD injectors, but for now...

I am going to drive it around today, and will get a feel for how it does during the day.

I am next going to try 5* BTDC @ 1400, and see where that gets me. I may play with the fuel rating later on as well.

I have a running record of my timing adjustments:
http://www.oliverdiesel.com/images/mt480/6.2_Engine_Timing_Figures.xls

BTW: I got the appropriate J series tools from Snap-On after waiting weeks for the special order. Some of you may have already noticed the holes in the IP on the passenger side. There is a tool that grabs those, and a 3/8" drive break-over attached to rotate the pump. I also got the pump wrench for the single t-stat 6.2L, and dual t-stat 6.5L. Due to the modifications on my truck's cooling system, both tools are required.

[Dieselboy]

I dug up this old post, so I may just go for a more conservative timing figure.


Originally posted by britannic:
Here are the results of experimenting with my timing:

Setting it to the milspec 4* BTDC @ 1300rpm, increased smoke, the shift points increased by about 500rpm and there was less power, boost and torque.

I found that 2.6* BTDC @ 1300rpm, gives hardly any smoke, plenty of torque and power with a slightly quieter engine.

YMMV, but in my case, because the forced aspiration increases combustion speed, it's necessary to slightly retard the timing as compared to a N/A engine.

Interestingly, Animal reported, that Cummins DI tuners retard the timing a little after increasing fuel and boost to prevent damage to the pistons and harvest extra power.

[gmctd]

I've found the GEARWRENCH 15mm flip-head wrench allows 15min roadside timing adj on the DS-4 with dual-stats housing - much more difficult task than the DB-2.

Some of that time is in resetting PCM parameters.

[Dieselboy]

I turned the timing down to 2* ATDC @ 1300 this afternoon. There is nearly zero black smoke unless you're really hammering down on it. Boost is very responsive throughout the rpm range, but power is down just slightly.

All this at the cost of EGTs. They are much higher than desired at around 750* cruising at 60 mph. They rise quickly on any hill too.

I would say that this kind of timing is the ticket for low speeds like a dedicated off-road rig.

Either tomorrow or the next day I'm going to experiment with advancing the engine to a few different points between 1 & 2.5 degrees BTDC @ 1300.

I am looking forward to the set of 6.5TD injectors that are on their way. :D I figure that the better atomization of fuel will really help out.

[Dieselboy]

It looks like we have a winner here.

I bumped the mark forward to 2* BTDC @ 1300 rpm. There's hardly any smoke, power is good, and turbo lag is almost non-exsistent. I think this is a good setup for an engine like mine - a conservative fuel rate and low pressure injectors like the 6.2 long style.

One thing that was of note:
I test three different positions today to find the right one, and though I moved the pump each time, the marks settled to the same place. There is no strong correlation between the position of the timing marks and the timing figure. Depending on how I jiggled the pump, I could get two timing results that were as mugh as 5* different, but at the same physical location. This just confirms that the factory-provided timing marks are a great ballpark figure for when one replaces their IP.

I'm going to get some highway miles on it, and see if the economy was affected at all.

Next up is getting some new 6.5TD injectors.

[Dieselboy]

Okay, so I went on a short 200 mile round trip yesterday, and here's what I found. Ambient temp was about 95 F, and I averaged 70 mph @ 2000 rpm. Cruising boost was 3.5 psi, and EGTs hovered at 600 F.

I found that with the newest timing of 2* BTDC @ 1300 rpm I got an average of 1 mpg better economy. One 100 mile leg has some decent hills, and economy was just under 17 mpg whereas it had previously been 16 even. On the second 100 leg, the road was nearly perfectly flat, and economy was 17.5 mpg.

[Stray Cat]

Where, approximately, is the mark on your pump in relation to the fixed mark with your final timing point? (2mm to passenger side, 2mm to drivers side, etc.). Also, I am curious about the 6.5TD injectors you mention. I have a 1983 K20 Suburban. Would the injectors work on mine? If so, what will they do for me? Do they flow more fuel, or simply atomize better? Thanks!

[Dieselboy]

Originally posted by Stray Cat:
Where, approximately, is the mark on your pump in relation to the fixed mark with your final timing point? (2mm to passenger side, 2mm to drivers side, etc.). Also, I am curious about the 6.5TD injectors you mention. I have a 1983 K20 Suburban. Would the injectors work on mine? If so, what will they do for me? Do they flow more fuel, or simply atomize better? Thanks! My marks show about 1 mm to the driver side.

The 6.5TD injectors have a higher pop pressure than the 6.2 long style. 2100 psi vs. 1800 psi. This results in better atomization, which gives a better and more complete burn, which allows one to crank up the fuel some more.

Brittanic's engine is an excellent example of how 6.5TD injectors & a 4911 can really wake up a 6.2L.

IIRC, 82 & 83 heads had a coarse thread injector. Later model years, and 6.5TD injectors are find thread, so they won't swap to your heads. Any set of 84+ 6.2/6.5 heads should work though.

[john8662]

Correction, only the 82 model 6.2's had the coarse thread injectors. So, an 83 model suburban (as long as its not one of the RED engines - 82 only) will have the fine thread injectors (so you can put 6.5L injectors in).

[Stray Cat]

Thanks for the reply! It looks like the injectors may be a worthwhile investment...any suggestions on a good source? Also, I have actually retarded my fuel timing a bit. It ran perfectly smooth and quiet, at the stock timing mark, before having the heads & pump/distributor changed. Since, it ran rough and loud, and started very hard (up to five minutes of cranking!) in mildly cool weather (45 degrees F over night). I retarded the timing to about 3mm to passeger side and it got smooth, quiet, and starts easily. I asked the shop if there was a chance that they could have been a tooth off when installing the distributor/pump, and they got angry and indignant. What are your thoughts? Am I really near the correct point because they were off; or, did something else change with the new pump/distributor and heads? By the way, the heads are fine thread, and I believe actually 6.5L heads as the fuel lines don't run exactly the same, they had to rig the fasteners that guide the lines. So, I think I can go with the 6.5L injectors.

[ropinfool]

I have been having an issue with egts for quite awhile, much like what Diesel Boy was discussing
here. I do not have access to a timing device and have paid big bucks ($200) to have it done only to have the same problem. I noticed after I got home the timing marks were aligned whereas it was slightly towards the drivers side when I took it in. I was also having problems with starting so I moved it back. Starts much better but still higher than norm egts. I believe they just aligned the marks and took my money. They wanted to turn my pump down since I was complaining of high egts, I wouldn't let them since I don't have enough power as it is. Is there anyone in the So.Ca. area that has a luminocity probe type timing meter I can borrow (I will pay shipping and insurance both ways)? I think that's what it's called anyway. Thanks. John

[Stray Cat]

I have a pulse-type timing adapter from Snap-On ($450! :mad: ). Instead of removing the glowplug and inserting the luminosity (light-sensing) probe, you clamp a probe around the metal fuel line to the #1 cylinder and it senses the fuel pulse. I live primarily in So. Cal.; but the probe is at our property in AZ. We are going there in October for a week. If you can wait until then, you are welcome to borrow it. Let me know! ;)

[ropinfool]

Thanks Stray Cat for the offer. I'll let you know before you head off to AZ. John