bbraden

Wow... what I gained most out of that quote from Quad is that they just go the max on timing and they haven't put a whole lot of thought into how that works everyday driving/mileage, but only for max power. It's hard to say from the information there, but when he says "There is a too low and a too high number. If the IAT drops too low the ECU retards the timing just like too hot due to fears of detonation and stabilitiy of combustion in diesel fuels"... I would venture to say that "too low" is probably in the -degrees.

I was thinking about this again this morning... my DD ('93 Toyota pickup) is parked for the next couple mornings so I actually had to drive the Dodge again today. It was about 38 degrees here this morning and when I cranked the ol' gal up, she was just as clattery as usual. I was thinking about the attributes of advancing timing for a cold startup. 1) Cold air decreases flame speed which would require a little extra timing from the get-go. Hence "some" of the extra noise on a cold startup? 2) If an engineer were interested in getting an engine up to operating temperature more quickly, he would also advance the timing. Advancing timing increases the amount of time that the piston and head see flame frontage, driving more heat into both of them. This is common knowledge. If emissions required that our engine come to operating temperature sooner to minimize PM, this might be another driving force for running too much timing during cold starts. Advancing timing increases the amount of heat built in the cylinder, which forces more heat into the surrounding metal, which means that we see lower temperatures out the exhaust. Most people see this in lower EGT's when they run advanced, but they do not realize that much of that temperature is being ran through the radiator instead. Think about when you drive your pickup for a while (up to operating temp) and then you bring it back down to an idle. EGT sits around 350, right? Crank your cold engine up in the morning and see where the EGT sits. I'm not sure how conclusive that is... could be a matter of heat sink on the probe and exhaust stream and possibly not conclusive at all. This brings me back to the IAT fooler. We postulated that the foolers were advancing timing when set to higher temps, correct? If you need more advanced timing at colder temperatures, would it not be working the opposite way, by advancing during cold temps and retarding at hot temps? I think another experiment I would try if I had an IAT fooler would be to get the motor up to operating temp and let it idle for a while. Check your EGT at different intake temperatures (using the fooler) and see which ones increase EGT and decrease EGT. Might be worth a try?

I'm not familiar with Singh grooves... But yeah, I'm talking similar things to the super squish pistons/heads.

CSM, I agree 100% with your post. I didn't know that diesel burned that much slower than gasoline, even when artificially pressurized (turbo or super boost). Many VW gas engines run 30-32 degrees total advance when cruising under part throttle conditions. If we can increase the burn speed - whether by compression, smaller droplets, or more swirl - we can pull out more of the timing and create a more efficient engine. I was thinking about the difference in piston/head design between the 12v and 24v. I know that both engines have a more or less flat top piston with a combustion bowl built in, but would the injector direction cause a big difference? I know that the 12v injector sits at an angle... much like the plugs do on a fast burn head. I know with gasoline engines, head designs like the "fast burn" and Vortec heads lend themselves to better fuel economy. The smaller the intial combustion area, the higher pressure this little area sees. In the fast burn heads, the spark plug is directed in such a fashion as to be right in the middle of the smaller initial combustion area, and the valves are situated in such a way as to enhance swirl. Ideally, the combustion chamber is a wedge shape with the exhaust valve sitting at the smallest part of the wedge, so that exhaust is funneled out of the chamber more efficiently during the exhaust stroke. All of these things make for a more efficient combustion design that allows you to pull more timing out, but doesn't create knock. - - - Updated - - - Or maybe... it's as simple as combustion pressure. A 12v has 17.5:1 CR as far as I know. At 5 psi boost > 19.7 PSIA * 17.5 = 344.5 PSIA 24v non-HO has 16.3:1 CR. At 5 psi boost > 19.7 PSIA *16.3 = 321.11 PSIA. I'm sure this would have an effect on flame speed, and thus efficiency. If the 12v has higher compression, it should theoretically require less total advance - but that may be made up by the smaller droplet size on a 24v.

As most of ya'll know, Crazy Carl "Turbolver" put a supercharger on his P-pumped 24v. I've followed it very closely with great curiousity but haven't seen any talk about the timing. I read where he bumped timing from 15 degrees to 24 degrees, but he never reported back with results. How would a supercharger affect timing, and how could you maximize efficiency using one?My thoughts are: since extra boost (especially down low) increases flame speed of diesel, you might actually be able to retard timing and maintain similar EGT's and increase torque. Am I way off?

How interesting... I wonder where the extra injection events fall on those maps?

Absolutely. Peak pressure BTDC is not good at all. The CR injection system should theoretically be able to inject just about anywhere to get peak pressure wherever needed. Low boost levels + high RPM (where you would normally be during a cruising situation) should have the highest amount of advance, which is probably the majority of injection BTDC.

Have you optimized for fuel mileage yet? What mpg are you seeing?

Well, I haven't revisited this in a while, but it's way past time to stop back by. I've been reading a lot about one of my other hobbies - VW aircooled engines. I built my own dune buggy a few years back and have helped work on numerous others. About a year ago, I converted mine to run on propane like many rock crawlers do, and ever since then I have been on a quest to make it run on the least amount of fuel possible. In doing so, I've read and read about how to make a VW engine more efficient and have stumbled on lots of good information about timing. Granted, this is on a gasoline engine, but the theories are similar. Truthfully, it seems that as a whole, the gasoline tuner crowd has a much better understanding of timing than we do. As it turns out, timing BTDC is BAD. Ya'll may all understand this already, but it's really sunk into my head now. I've said this before and stood by it, but now I have a little better comprehension. In theory, the perfect engine would never have any combustion happening BTDC. Any pressure built BTDC is wasted as negative work against the piston. However, with our physically limited fuels, we have to start injecting BTDC in order to have peak pressures ATDC when the piston can actually make use of the fuel. On the same note, when we want to start increasing fuel input, it must be injected somewhere, so we advance timing and lengthen the event so that we can squeeze more fuel into the combustion chamber during the same event. Given too much timing, we are once again performing negative work on the piston. The only way to make a diesel more efficient is to perform more work ATDC without carrying over any of the event into the exhaust stroke. Decrease the amount of time it takes to make a complete burn in the power stroke, and you begin to see better efficiency. We do so by increasing cylinder pressure (boost). The higher the pressure, the faster the flame front. We can atomize the diesel more, making smaller droplets and a faster flame front. We can introduce swirl into the chamber (whether by valve structure, or piston/head design) which brings the fire around more quickly. We can decrease the viscosity of diesel to make it separate and atomize quicker and into smaller droplets. Particulate matter slows the combustion process, so we can evacuate the cylinder more quickly during the exhaust stroke to ensure all PM is out of the cylinder (difficult for the those EGR guys). NOS increases burn speed by increasing oxygen content, creating faster flame front. All of these things have a beneficial effect on timing, by making the fuel burn more quickly. Quicker burn = less timing required = more POSITIVE work on the piston = better fuel economy. Of course, as RPM increases, more timing will be required because the flame speed won't change, but the piston speed increases. This will require extra timing BTDC to continue building highest pressure ATDC. As the Cummins injection systems have continued improving and seeing higher and higher injection pressures, it becomes obvious that the newer (and higher the pressure) systems will NOT require as much timing advance because the flame speed is fast enough that the event can be over with before the end of the power stroke. BUT... we worked backwards by lengthening the single event out into three actual events for "noise control" or "pollution control" as is the case in the CR. In-cylinder EGR camshafts also work backwards to our goal by not evacuating all PM during the exhaust stroke. My thoughts are, if you were to get rid of all the extra events, in-cylinder EGR, and run the highest pressure possible for fastest flame speed and retard timing as close as you could to exert the most work on the piston ATDC, you would have an extremely efficient CR engine. Maybe everyone here knew all this, but it seems as if there is a LOT of hocus pocus about timing and nobody really understands the negative effect it can actually have.

Funny how these old threads get dug up! I'm curious as well. I'm getting ready to adjust my valves (never been adjusted and I'm at 230k miles) Yikes! My two cents: First - when you were asking about the necessity for valve lash - On Volkswagen motors, the rockers are designed such that everytime the lifter comes down to contact the valve, it actually causes the valve to rotate ever so slightly so that the valve is contantly rotating during operation. This keeps it from wearing abnormally. Cummins valves may have a similar design. Second - when talking about intake valve vs boost, don't forget that all things equal, if you open the intake valve sooner in the intake stroke, your turbo boost (measured at intake) should be lower. Many people don't understand that boost pressure is truly a measure of restriction. If your turbo were allowed to flow from compressor directly to atmosphere, it would make 0 psi boost, regardless of how fast it spun or how big the compressor is. Confine that airflow and that's how boost pressure is made. If you were to set your intake where it opened sooner, it's going to allow more air into the cylinder, which means some of that air volume creating backpressure outside the cylinder is going to get consumed inside the cylinder. Cylinder pressure goes up, but boost pressure goes down.

Then I guess the next logical question is - how does the VP, or any of the pumps for that matter, vary injection pressure if pump displacement is fixed?

Well... I think that the VP44 can do something very similarly to the CR by varying max injection pressure... but I don't have any evidence to back that up.

Is the pressure actually always the same in the rail of a CR? I thought that it fluctuated quite a bit with fueling requirements? Where it should be low at idle and higher at RPM's.

Taken off the Cummins Forum (NOT MY WORK!) This is actually a tuning "how-to" for the Quadzilla Adrenaline: _________________________________________________________________________________________________________ Pump Stretch. This will make the biggest difference in overall power and smoke. This is the amount of time the injector is actually opened and spraying fuel into the cylinder. I have this set pretty low on the 1000 tune that I sent you. The COMP file is set to 1800. More than 1800 creates more torque and a lot more smoke but, actually makes less power on the upper end based on all the trucks I have had on the dyno. This is where the COMP file is set to. Feel free to run it to 2400 anytime you want lots of low end power and smoke but, realize you are giving power up on the top. This does not mean you will get this amount of fuel all the time. There is a map, in percentages based off of this number as the maximum. Beyond that the map is also scaled by the TPS. So even if the map commands 100% but, you are at 50% throttle you will be getting 50% of the max stretch. If the map calls for 25% and you are at 50% TPS you will be getting 12.5% of the max stretch. ________________________________________________________________________________________________________ ..... This makes me question just exactly what capabilities the VP has. If it can vary how "long" the injector is open... could it make it such that the injection event is lower pressure (and lower volume) across the span of the event, but make the event so long that you get the same amount of volume in the end? This could possibly create an ignition event that is started way too early (causing timing knock), yet continue long enough into the power stroke that it's wasting fuel. From what I understand, one hot and fast event also creates NOx. The cooler you can keep the event, the more fuel you can give it, creating the least amount of NOx. If the event was lengthened out so that it never got extremely hot at one particular point, you could possibly make the same amount of power with more fuel, but with less NOx. Couple that with timing that's too advanced, and you have less fuel efficient combustion (with less NOx) at lower RPM and load levels. I guess it works like this in my head - 12v - pop set at 260 bar, pressure in injector lines exceed 260 bar and opens the pintle at say... 14 degrees BTDC. Injector remains open and injection pressure continues upward until a max of 1000 bar at say... 6 deg BTDC. Pressure spike begins decreasing at 6 deg BTDC, decreasing below 260 bar by TDC, closing the pintle. 24v - pop set at 310 bar, pressure in injector lines exceed 310 bar and opens the pintle at say... 24 degrees BTDC (timing knock). Injector remains open and injection pressure continues much more slowly upward until max of 500 bar at say... TDC. Pressure spike begins decreasing at TDC, decreasing below 310 bar by say... 24 deg ATDC, closing the pintle and ending the event (but burning much more inefficiently). If the VP44 can control pressure at any given time in the injection event, it should be able to accomplish such a thing. Just because the VP is capable of more MAX injection pressure than the P7100, doesn't mean that it necessarily uses that pressure all the time.

Boost, more often than not, also increases as RPM increases, which would additionally advance effective timing. So to get back to our VP-44 conundrum... what is it that makes the VP burn more fuel when it's cold. Obviously it's something computer controlled, so what all parameters can the VP-44 control/manipulate to make power? 1 - Timing Advance/Retard 2 - Event duration 3 - Injection Pressure? How does the VP vary the amount of fuel injected if not by duration? Pressure?