Advanced timing-lower boost

[rdodd]

After advancing timing 4* over what i think was stock my wot boost went down about 3 or 4 lbs. Has anyone else noticed that boost went down after advancing timing? It seems to me that boost and hp should go up after timing advance to a certain degree unless one goes to far.

 

[Harlan]

Did you advance the timing 4 degees at the crank or 4. 0 mm at the pump.

Or was it 4 degrees at the pump.

Harlan

 

[rdodd]

All i know is the shop said i was at 10* and they advanced it to 14* with a timing light.

 

[Harlan]

From 10 to 14 should be a good advance. Have you run in your power screw. If not try runing it in one turn and see what that will do. You may not be giving it the fuil it needs for the tining it

is set for now .

Just thinking.

Harlan

 

[rdodd]

My power screw was set up some time ago up against the stop collar. I,ll have to remove collar to go more. Is it usually required to give it more fuel after advancing timing to get the boost back to wear it was before advancing it?Does running less boost now mean that my power is down lower than before advancing timing?

 

[Power Wagon]

This is a common misunderstanding... Boost does NOT equal horsepower.



Boost is a function of hot gases expanding against the turbo wheel and spinning it fast, so as to make the other end compress air into the intake.



Now, imagine this: If your timing were excessively late the fuel burn occurs very close to the opening moment of the exhaust valve. The gasses in the combustion chamber/cylinder would be hotter than if injection occurred earlier, and the heat and pressure were dissipated more by driving the piston down, and would have more time to cool, both from the decompression of the piston falling, and by there being more time between injection and the exhaust valve opening.



This explains the phonomenon so many of us are aware of... advancing the timing often lowers the EGT's.



Now after advancing the timing, since there is less heat being expelled as burning or just burned fuel into the exhaust manifold, there is less force to spin the turbo, meaning the turbo will spin just a little slower, make a little less boost. This is NOT a problem, since it's generally observed that smoke and exhaust temperature are STILL reduced, even though we often see the boost #'s fall slightly.



So, it is not a problem at all. In fact, it generally means things are in better shape. Hopefully, this makes sense...

 

[BushWakr - RIP]

Ok, PW,



Will increasing the pop-off pressures help with idle/low speed smoke? Does the internal pump pressure actually increase markedly as RPM/demand goes up?



Any insight would be great in the search for solutions here.





Bushy

 

[rdodd]

Thanks PW for the information about lower boost after advancing timing. Maybe now i will even get better fuel miliage with timing change, too soon to tell at this time as iv,e been into the pedal a lot lately testing things after the change.

 

[Bob Beauchaine]

After having my new pump installed by Wentland Diesel I too saw a drop in boost - however I do have more - no - MUCH MORE power than before. I felt that my truck just did not have to work as hard to produce the same power as before. I have not really tested it with the trailer on a serious mountain pass as yet.



After meeting and talking with Powerwagon I feel as if I am right in my thinking. My '93 has one H#!! of a lot more power now than before. BTW my mileage is now running about 1 mpg better than before - it's all in how and when you use that extra power. Just my 2 cents worth.



Happy trails

Bob

 

[Power Wagon]

Originally posted by BushWakr

Ok, PW,



Will increasing the pop-off pressures help with idle/low speed smoke? Does the internal pump pressure actually increase markedly as RPM/demand goes up?



Any insight would be great in the search for solutions here.





Bushy

I'm not sure which pump pressures you mean, so I'll address both.



The pump contains an internal vane-type supply pump makes 1-10 psi at cranking, and then starts a relatively flat curve upward as the rpm's climb. This pressure encloses the pumping plunger and helps ensure it fills completely each injection stroke. It also pushes agains the advance piston, which is loaded with operating torque and spring loading. When the fuel pressure exceeds the load of those two, the timing begins to advance.



The "other" pressure in the pump is injection pressure. This is generated by the hydraulic head, routed out to the outlets by the rotation of the pumping plunger, and through the lines to the injectors. This pressure is only enough to get the fuel through the nozzle tip. (There's resistance to flow in the lines and other things, but for our purposes, just ignore it for now) At cranking, for instance, the pump goes into "crank" mode, where the governor sleeve drops way back and the plunger pumps for a very long stroke. It helps overcome high pressure injectors, it puts in a lot of fuel to help start (notice that big black puff when you start?), and the moment the engine rpms get above roughly double cranking speed, it's cut off and back to normal. As soon as you get faster than the beginning of cranking, a second factor comes into play which determines the ultimate pressure generated for injection, and that's the resistance of the nozzle. The nozzle, for our purposes, is just an orofice with multiple holes. The more fuel you try to cram through it over a given amount of time, the more pressure is needed. At cranking and idle, the amount of fuel is low and the speed at which the pump attempts to shove it through the injector is low enough that the pressures do not spike sharply. They overcome opening pressure of the nozzle and some (I don't know how much) more. The larger your nozzles are, the less is needed.



As you go up in rpm, the plunger attempts to inject fuel at higher and higher rates because the plunger itself is moving faster and faster. It pumps for less and less time as your speed goes up. At a certain point (and memory fails at the precise number), of around 18,000 psi, the pump just won't pump any more pressure. Leakage in the head and other factors cause the pressure rise associated with either the amount of fuel or the speed (rate) of injection to begin to level off.



Apparently, this happens with our stock injectors on the 91--up engines quite soon. We turn up the fuel screw, but the pump can't make much more rate of injection, so we are making it pump for longer periods of time at a maximum load (pressure). This long period of injection is relatively easy on the engine (it IS hard on the pump), but it results in injecting fuel beyond the efficient timing point and after the oxygen is depleted... We get smoke. By putting in bigger injectors (pw, pod), we allow more fuel to get through with a lower pressure spike.



Will increasing pop-off improve low speed and low load efficency, reducing smoke? Hard to say. If it smokes because the nozzles are not atomizing fuel well, it might. I have not tried it. raising that pop-off pressure means the fuel charge released will be a higher pressure, and therefor at least some greater velocity... both of which tend to improve atomization.



But, if you're using a hole-sac type nozzle, or one where the needle seat and needle leave a lot of fuel trapped underneath them at closure, you will never clean up completely. On the other hand, if not one of those two types, raising opening might help after all. Raising it too far will have the effect of making the pump take a long time to spike the pressure that far, delaying injection point a bit and perhaps again, causing the very thing you seek to avoid. Only one way to find out. Experiment.



You CANNOT do this in your back yard. The injectors need a clamping jig to dissassemble and reassemble, the shims inside come in a large number of sizes, and they are expensive - some approaching a dollar a shim, and you have to have the whole array of shims, plus several of each. And, you need a true lapping block, solvent tank for cleaning, torque wrench and adapter, and a pop-off tester. Lack any of these and you could be asking for broken injectors or other serious things to happen, up to and including destroying your injection pump.



I've never had the chance to experiment. I've never seen a set of POD injectors, much less had a rig sitting around with time and money to experiment with... so...



Long answer to not say much, it seems, but I hope it makes sense, again

 

[BushWakr - RIP]

PW,





:--) :--) :--) Ok, what is the velocity of a coconut laden swallow



Now that is so gonna take some thinking and pondering, at least for me anyway.



PW, thanks a million. Those are the type of answers that make me look beneath the surface, to the principles of how a system works in detail.

I need to go away and think about this some more...



Thank U. Bob.

 

[Lil' Dog]

Just for the record....

My truck lost about 2 lbs boost accross the board with the timing set up to the factory 10° from where it was at 1°.



Power is a little more subdued, but it smokes way less. .



Good lesson on injection PW... :--) :--)





J-eh

 

[Power Wagon]

Originally posted by BushWakr

PW,





:--) :--) :--) Ok, what is the velocity of a coconut laden swallow



Now that is so gonna take some thinking and pondering, at least for me anyway.



PW, thanks a million. Those are the type of answers that make me look beneath the surface, to the principles of how a system works in detail.

I need to go away and think about this some more...



Thank U. Bob.

If I could secure the cooperation of my former employer, and someone with a truck with POD injectors who has time on their hands, I might be able to try tweaking the set a bit to see if we can produce better idle and startup smoke characteristics.



I know how to do the job, and am familiar with all the parts and procedures - after all, I did injectors for more than a dozen years

 

[hdm48]

Hey PW, I've got an idea. If you can get your old boss to let you play with injectors, how about doing that little project you and I were talking about with your truck. I just happen to have POD's in my truck. hint-hint-nudge-nudge-wink-wink

maybe we could do all this in his shop?? On a weekend??



--dave--

 

[Power Wagon]

There might be a possibility something vaguely similar.



I'll email you after Boise, and let's see if we can work something out.



I KNOW his shop (neitehr new or old one) is NOT available on Saturday. In all likelyhood, we'd have to do this during the week, but I can ask him about using his old shop.

 

[Mel Fuelling]

I adjusted the rpm from 2,500 to3,100 on my truck, but it didn't

really welcome the added rpm until I advanced the pump spill port

timing from 1. 25 mm to 1. 5mm. Since then it comes on much better above 2,500. If my unscientific impressions are correct,

perhaps my Getrag will live longer with less torque below and

more torque above 2,500.



I believe you need to adjust your max rpm to see the complete

change and improvement that advanced pump timing offers.



I was disappointed that I haven't gotten better mpg with

advanced pump timing, but then the motor rpm's so much

more freely now, maybe my right foot is the culprit.



I didn't notice a drop in boost, but I did see a drop in egt, so

I turned the fuel delivery rate up with the aneroid diaphram

which feels like it gives me better acceleration with a medium

load(8,870#) on a work truck than it had before when it was

empty.

Mel

 

[bflood]

I had the same experience a couple of weeks ago. I think the previous owner tried to advance the pump but turned it the wrong way. I was getting 25-27# of boost and several members found it hard to believe but . . . I was having trouble with excessive EGT's too.



I advanced the timing (by ear sorta) to 1/16" advanced from the factory timing mark on the pump. It was 1/8" retarded from the factory mark.



Here are my results:

Boost dropped to 19-20# from 25-27#

EGT's dropped "big time" (roughly 200 degrees pre-turbo).

Fuel mileage, around town - no load, went from 17mpg to 18. 5mpg.

Pulling power exploded. Oo.



I thought my rig was good before but getting the timing closer made a tremendous improvement, in spite of the lower boost.



I'm not through learning and tweeking but I'm going to stay with what I have for a bit.



Power Wagon, thanks for your explanation! It confirmed what I suspected and you made it so clear. BZ!



Happy hauling, Bruce