Engine/Transmission (1998.5 - 2002) Adding Timing

[NoSeeUm]

I got interested in timing today. Confusing ideas arise about the idea of advancing the timing by using performance boxes. This is old news for you diesel salts so just ignore me. But I learned quited a bit and might help some less schooled others along.



1) Increases HP, better mileage and decreases EGT's - seems good

2) Increasesd cylinder pressure dramatically (especially while towing) - seems bad



This TDR thread was the most sobering . I have decided that I do not want to walk this path. Read the thread, there is some banter included for free, but also some good lessons for us "future unplanned engine re-builders". Stay with the thread as Cliffman posts a picture of his engine damage around page 6-8 I believe (don't want to look for it again).



All in all, I think Hohn has it the most right. Thanks Justin!

Gary: The CR in an engine does not change as it is a mechanical feature built into the engine. At least when you are discussing static compression. So if you have a 17:1 engine, it will increase the pressure of the intake charge by at least 17 times. I say at least because the heating of the pistons causes an expansion of the air and actual pressure will be slightly higher. Notice the trend here-- there are MULTIPLE factors at work in any given situation.



Now DYNAMIC compression is a completely different animal. This CR accounts for the effect of intake intertia and delayed intake valve closure. For example, when you put in a big cam in a gasser, you need to raise static CR to maintain the same dynamic CR due to the later intake valve closure.



FYI-- ETh compression is 17:1 vs ETC compression of 16. 3:1 So 30psi of boost in an ETC is about the same as 28. 76psi of boost in an ETH. Thus, there is a difference, albeit slight.



To answer your question about doubling volume: since there is a linear relationship between pressure and volume (look at the units), when we double the pressure we half the volume (assuming the same mass). So if we DOUBLE the "volume" of air entering, we have necessarily doubled the pressure. It's worth noting that volume is a misleading term without pressure, since a gas will take the shape of its container. Mass flow is far more useful.



Let's revisit the issue of timing. Would NOW be a bad time for me to make my don't-stack-timing-and-fueling-boxes-with-injectors speech???



Seriously though, timing has a FAR greater impact than anything else on overall peak cylinder pressure. Just a few degrees too much advance can cause a doubling or even a tripling of peak pressures. After all, the peak pressure is what does the HG damage more than anything. The ETH is much more susceptible to really high peak cylinder pressures-- not because of its higher CR so much. Instead, it's the discharge curve of the pump-- a really short, spikey fuel injection event. This is the reason that ETHs respond so well to injectors. It's also the reason their HG are a little more fragile.



Ask yourself why the injectors on an ETh are SMALLER than on an ETC, even though the engine has had the power raised...



I do believe that timing boxes are generally safe on ETH engines even with injectors. But this is NOT the case if you tow very heavy. If the engine will see VERY heavy loads, what you want is LOWER compression and LESS timing while dialing in MORE boost. This gives you a high AVERAGE cylinder pressure to keep HP up, while also lowering the dangerous pressure spike at the moment of injection.



So does CR or boost play a more important role in determining cylinder pressure? Well, the answer is boost, because it varies from zero to whatever. The CR is fixed, so it ALWAYS multiplies cylinder pressure by that amount. The real variable is boost. But neither boost nor CR is as important as timing in determining peak cylinder pressure.



You have to be really careful when upping fueling with otherwise stock parts. You can either build a safe towing rig with less power, or build a hotrod. EGTs that are completely safe when running empty will soon melt something when towing.



Justin

Here Justin talks about setting up your truck for towing or for winning stoplight races... ...

Originally posted by CFAR

Increased fueling = Increased timing.

Yes and no. Increasing fueling alone has no effect on timing unless discussed along with HOW that fueling increase is achieved. Bigger injectors? Fuel box? Pulse width? Injection pressure?



Generally, a larger injector passes more fuel per unit time- duh. This tends to SHORTEN the time of the injection event, but it will not ADVANCE the timing, per se. The INITIATION of the injection event is still the same in both cases, thus the timing could be said to be the same. This shortening of the injection event tends to increase the peak cylinder pressure. Thus, less timing is NEEDED in the engine with larger injectors.



This is where we get to the different approach to building a tow truck vs. a race truck. Race truck= big injectors, high peak cylinder pressures.



Tow truck= smaller injectors, NO ADDITIONAL TIMING, and a longer pulse width.



Example:

1) sporty truck: Edge Comp, dd3s

2) TOW RIG: TST PMAX and dd1s or 275s



The tow rig needs to have less timing and smaller injectors to reduce peak cylinder pressures.



Justin

Justin again... .

Well, others will disagree with me on my turbo recommendations, because it varies so much with the other stuff done to the truck (and with transmission type):



Sporty truck:KSB-1B, PDR 35/12WG



Tow truck: PDR35/16, PDR40/18, B1, etc. .



It's hard to point to specfics because there are so many OTHER variables. All I can say is this: in a tow rig you MUST sacrifice some spool up to gain PEAK EGT control. The sporty truck can have the tighter housing because it can afford to momentarily flash to 1400-1500 degrees and sometimes higher.



The tow rig is going to be faced with the worst case scenario-- SUSTAINED high EGTs. That means at LEAST a larger housing than the sporty truck, but probably a larger turbo in general. The tow rig must operate with a greater cushion of air across the board. That means less fueling and/or more air.



The stock Dodge ECM has a great feature in that it will retard timing under peak loads. It also adances timing under light loads and steady druise, just like a vacuum advance works on a gasser to take the timing from 14-15 degrees to as much as 60!



When you tow with an EZ, you lose the ability to retard the timing at high load. The EZ will not allow the ECM to back it off. This gives you great torque, but also VERY high peak cylinder pressures. That's why the engine will give you that "thump thump thump" when you tow hard with the EZ.



Now with small injectors, the spike in cylinder pressure is still safe. There's just not enough fuel to hurt something.



But when you combine timing on an ETH with injectors that are 100hp or more, all bets are off. Towing heavy with this setup is torture on the bearings, crank, pistons, and rods. Lots of shock-loading.



If you think about it, there's almost never a towing situation where you NEED more power than you could get from a PMAX and Mach 1. 5s or so. Odds are you are ridiculously overloaded if you can't pull it with that.



I can't stress enough how each truck needs to be bult with the END USE in mind, and that there is a very complex interaction between components that should be considered when planning your build.



Justin

Included somewhere in the TDR link is a link to a Banks site that talks about the increase cylinder pressure and the staggering amount of force that is produced.



Edit: Cliffman added a pyrometer to the exhaust header on cylinder six.

Just got back from May Madness a week ago. Went easy on her going there but coming back I pushed her to 1200 degs while pulling my 5er. EGT's on #6 were 100-125 higher while pulling 15K 5er @1200 on the pyro. So, the average on 4, 5 and 6 was 1200 and #6 was running at 1325. I was surprised at the difference. I thought it would be higher but happy to see it only @ 1325.

Jim