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Old 07-15-2010, 10:42 PM   #1 (permalink)
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RPM limits ?

During some of my reading, I've noticed that tacking 3000+ rpm's seems fairly common.
Mine redlines at 3000 according to the tack. I'm considering a 3k spring kit but don't really have any intentions of running it that high. Just want to enhance the driveability potential in the low to middle 2k's where it initially starts defueling.

I'm curious though, with the 4k and even 5k governor kits available, how many RPM's can these 12v's handle ??
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Old 07-15-2010, 11:07 PM   #2 (permalink)
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Generally, you want to keep it below 3800rpm unless you have heavier valve springs. Spinning much higher than 4k you start to get resonance through the crank (aftermarket dampner will help this) and you typically see a rotating assembly fail past 5.5k
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Old 07-15-2010, 11:09 PM   #3 (permalink)
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Long story short, a 3k kit is a good way to go if you dont want to open up a big can of worms.

The 4k does fuel harder, but it also makes the throttle a bit touchy, and of course requires the use of HD valve springs. even if you dont plan on spinning the engine that high, its good insurance incase you accidentally spin it up that high!

A 5k kit is not something you would want to be using on the street, not to mention they can be wickedly hard to set up and most of the time have to be set on the bench
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Old 07-16-2010, 01:35 AM   #4 (permalink)
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Thanks ykdave.

Informative reply !
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Old 07-16-2010, 02:18 AM   #5 (permalink)
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It's fine to take your engine up to 3200RPMs (w/ a 4k GSK) and not have HD valve springs.
So the answer to your question is:

1. if you don't have HD valve springs, 3200 is the maximum "safe" RPM for running your engine.

2. if you DO have HD valve springs, 3700 is about the maximum "safe" RPM for preventing bent pushrods etc...

People who do truck pulling like to run 4-5k RPM's so they do things like Fluidamper's or HD pushrods and block girdles, head girdles, HD Connecting Rods etc...

And the answer to why you need those upper RPM's with such a torquey, low-revving engine, is that there is a LOT of good power to be had at those upper revs, and you can shift later which amounts to much quicker 0-60 and 1/4 mile times... especially when you advance the timing (which shifts the powerband ^UP^ in RPMs significantly ~ lowering power at low revs) and if you have a 4k GSK (I hate 3k GSK's!) you will really want to make it scream through 3200RPM before shifting. It's so fun to drive after timing, injectors and a 4k GSK.
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Old 07-16-2010, 11:35 AM   #6 (permalink)
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Again, good info !

I had thought about advancing the timing along with the gov kit but if it is going to hurt my low-end power/torque, I'm going to reconsider that option.
That is helpful info because if I hadn't read that and had went ahead and advanced the timing, expecting it to help throughtout the entire power band only to find out that it hurt the lower p/band, I would have been disappointed.
I'm not really looking to turn it into a fire breathing, red light to red light race truck.
What I do want to do is open up some of the lower powerband capabilities(up to 3k) that have been muted with factory detuning.
Strangely, I usually want the fastest boat, motorcycle and UTV but after having gone thru the phase with earlier mod'd 4x4 trucks and jeeps, big lift kits, big tires, etc, tearing them up on the weekends and having to wrench on them during the week, only to go bk out and tear something up the following weekend, I really just want to keep this truck driveable and reliable but still have access to some of the safe power available in the lower rpm range.
Thanks again for the info.
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Old 07-16-2010, 12:32 PM   #7 (permalink)
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Problem is you're looking at this wrong... you can't compare these trucks being power-modded to a 4x4 jeep (or another truck) getting torn up on the trails.

These engines are meant for huge power and from the factory, they are SO severely detuned that it's just silly. Don't be so quick to sacrifice power for reliability when you can have both.

And for the lower-revs vs higher revs thing.. it's actually WORSE for your drivetrain to push the engine hard in the low RPM's. The immense torque of the Cummins loves to destroy things when you push it below 1500-1700RPM.

Also, a bigger turbo (even as small as a 62mm) won't spool-up until above 1500RPM unless you do all kinds of other work, like intercooler, cam, P&P, exh manifold, heat-wrap etc... all the quickspool tricks. Even then, the difference isn't gonna be huge.

The stock turbo is pretty much spooled by 1300-1400RPM so that also adds to the feeling of low-end loss when you not only upgrade to a bigger turbo, but also advance the timing. Most people don't consider this before upgrading to a bigger turbo/injectors... then you feel the loss cruising on the freeway at low revs, locked-up in OD. You will smoke people out behind you if you don't downshift to 3rd below 65MPH. That's in my truck w/ 3.5 gears and 35" tires. It's only turning about 1500RPM at 65MPH.

Honestly, the stock turbo (or compounds) is the best daily-driver. I miss my truck when it was stock-ish. But a 4k GSK, no plate, built-trans... all of that is great.

A 350HP truck is nothing to laugh at. Especially when everything works fine without babysitting.
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Old 07-16-2010, 01:13 PM   #8 (permalink)
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totally true about low end torque eating things. Spread the same amount of work over a few more Revs will add life to the rest of the drivetrain.
I've seen big truck axles that look like candycanes before they snapped, due to improper driving and near unlimited low end torque.
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Old 07-16-2010, 01:47 PM   #9 (permalink)
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Yeah, I should've added...

On stock timing, you have a lot of power from spool-up until 2500RPM and above that, it tends to taper off quite a bit. However on these engines, the best power zones are between like 2200 and 3500RPM.

When you advance timing, (of course, it depends on how much, but let's use 17 as a happy medium) it will greatly improve the revs from 2000-3200+ (depends on if you have HD exh valve springs). It will drop power a little bit below 1800 and it's around the same between 1800-2000.
Above 2000 though, advanced timing really helps.

And honestly, those high revs are really where it's at... especially after you get bigger injectors.

The headgasket and high EGT's are the only real concerns unless you're really turning high revs, above like 3700RPMs. And the headgasket is a touchy subject. Some people swear that either highly advanced timing (18 or over) or high drive pressures (differential between exhaust "drive" pressure and intake "boost" pressure) hurts the HG, but it's really a matter boost pressures and also letting the engine (head & block both) warm-up to at least 165-170 before exceeding 20PSI boost.

Nitrous is also up for debate as to whether IT hurts a stock HG. I know 12-valve mechanics who have run 25 timing, nitrous and a 66mm turbo (pushing 60# boost) on a stock HG & headbolts. They are crazy, but they prove it's a soft-science.

Also, a couple of those guys don't believe in re-torquing the stock headbolts. They just leave it how it is because they believe that can actually increase the risk of HG failure (stretching old screws and disturbing the gasket as well).

I don't know... my point is just that you can do alot more to these things than you think.
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Old 07-16-2010, 02:43 PM   #10 (permalink)
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You bring up a good question right there.
With a mild build and towing in mind, would 15.5 be a good timing?
I realize it is a lot of opinions but it makes sense that since we are stuck with fixed timing there is a proper place for a certain job.
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Old 07-16-2010, 03:07 PM   #11 (permalink)
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Absolutely. On stock injectors & turbo... (or on a 160 pump'd engine w/ marine injectors ~ for example), 15-15.5 is about the sweet spot (or max) from what I've heard.

It all depends on what you want to do. I think 15.5 should be a nice middle ground between stock and 17 (where low-end power noticeably starts dropping).

If you're running bigger injectors, the ideal timing starts going up... like with 5x12's, it might be 17-18, where with 5x14's you may need 20, or w/ 5x16's you may be needing 22 and more (it all depends even more, the bigger you go) depending on the use (abuse).

If you're trying to turn 4-5k RPM and comp sledpull, you're going to want to squeeze the horsepower out of those upper RPM's by going 25-30. So it's kind of a non-linear sliding scale. More exponential maybe.
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Old 07-16-2010, 03:15 PM   #12 (permalink)
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Cool! I love this place for the calm discussions. I have heard similar things, and also 16 was kicked about, but my trans is good, my TC good so I can stand to keep the low end. I don't have the tools so when I get access to them I will try out 15.5
I have 3.55's and slightly larger 265 tires, but I also have a two speed underdrive in the works that will give me 4.40s as well. That will positively kick arse on the nasty twisty mountain passes out west, where I will be around 35~45 MPH a lot of the time with the pacbrake on
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