Basic Magnum question: Safe maximum RPM on stock internals?

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MRGTX

MRGTX

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Well, I have been driving my Dart all over the place and the 5.9L Magnum 300 crate motor is running like dream.

How high can you safely rev the stock rotating assembly? Where do the stock motors tend to reach their hp/torque peak?

I have a 5700 RPM governor in the rebuilt 904 but I haven't come anywhere near that RPM yet.
 
MRGTX said:
Well, I have been driving my Dart all over the place and the 5.9L Magnum 300 crate motor is running like dream.

How high can you safely rev the stock rotating assembly? Where do the stock motors tend to reach their hp/torque peak?

I have a 5700 RPM governor in the rebuilt 904 but I haven't come anywhere near that RPM yet.
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I was told the factory ECM rev limit is 6500 rpm. Without some significant headwork and larger cam, it will run out of breath well before that.
 
My stock 300hp crate ran out of breath at 5K.
I ported the heads, swapped the M1 dual plane for a Pro Comp airgap, stuck a [email protected] roller cam in and the motor pulls to around 6500.
I run a 6400 chip in an old MSD 6AL box and hit the limit quite a lot on the 2-3 shift. 150+ runs and no issues.
 
5500 is generally the limit of a stock hydraulic lifter.
 
Rev it until the oil pressure drops or you get valve float. Back it off 500rpm to be safe.

LMAO!!!!
 
What cam do the 300HPs have? Power backs off after the peak, on stock motors it starts dropping at the latest around 4500. There's a lot of minor factors like having more air intake or better exhaust that will extend it a little or limit it more if there's a restriction but the stock Mags lay over before 5K. How high it can turn- I really can't help but think so much of that has to do with maintenance. A good motor that's always been well-kept, seeing up to 6500 doesn't hurt many cast crank short blocks but that doesn't describe every motor you see and sometimes the better kept can be significantly higher miles and still be less likely to fail.
 
Thanks for the input, guys!
The stock cam is pretty mild. I don't know the specs off hand but I gather that it's the same as whatever Chrysler used in the Ram/Dak/Grand Cherokee versions. The idle is very mild. The power dropping off after 4,500 sounds likely.

I have a M2 dual plane intake, 650 cfm carb, 2600 RPM TCI stall, Doug's headers...but the motor feels pretty lazy everywhere.

I have no idea what kind of care the motor had before it fell into my hands. The seller claimed that only had a "couple thousand" miles on it but it sat a lot...and there is a slight/occasional lifter tick that I will have to tend to.
 
MRGTX said:
Thanks for the input, guys!
The stock cam is pretty mild. I don't know the specs off hand but I gather that it's the same as whatever Chrysler used in the Ram/Dak/Grand Cherokee versions. The idle is very mild. The power dropping off after 4,500 sounds likely.

I have a M2 dual plane intake, 650 cfm carb, 2600 RPM TCI stall, Doug's headers...but the motor feels pretty lazy everywhere.

I have no idea what kind of care the motor had before it fell into my hands. The seller claimed that only had a "couple thousand" miles on it but it sat a lot...and there is a slight/occasional lifter tick that I will have to tend to.
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My old 360 magnum circle track engine had pushrod issues at anything over 6000 rpm. I built it much like the first batch of crate engines, basically a stock engine with a bigger cam and MP valvesprings - 2.2L retainers etc. I used a hughes cam similar to the MP one. I over revved it a few times and the weak link seemed to be pushrod flex which would result in a bent pushrod and a gouged rocker arm. So to keep it safe, in my opinion keep 'er under 6!
 
If it's lazy everywhere, it's probably rich everywhere, and of course the timing may be way off. A little tuning here and there, should wake her up;(baring chassis issues like 2.45 gears or a 1600TC, or an overweight situation)
300hp is plenty to have fun with, when all 300 are ready to play
 
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that cam is pretty tame. I believe with those valve springs you can go closer to .500 lift which would be a big improvement . google your engine and find specs on valve springs and total lift heads can handle. Stock magnum limit is about .470, yours should be higher
 
AJ/FormS said:
If it's lazy everywhere, it's probably rich everywhere, and of course the timing may be way off. A little tuning here and there, should wake her up;baring chassis issues like 2.45 gears and a 1600TC, or an overweight situation.
300hp is plenty to have fun with, when all 300 are ready to play
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I'm thinking that you're right on all counts here...
The car isn't overweight...I mean, it's an Abody! Even a V8 big bumper car came in under 3,200, IIRC...that's probably 1000lbs lighter than a 5.9L Grand Cherokee or 1500lbs lighter than a 5.9L Ram. I do have a higher RPM stall so that's probably not it either but there's no doubt that the 2.73 ratio is a big source of the sluggishness. Still, the motor feels like it just doesn't want to rev. Holding it in lower gears helps some but not enough... So yeah timing and carb tuning are certainly contributing.
 
Yeah I shoulda bracketed that part about chassis issues, and written ors instead of ands.I'll go fix it. The only info missing at that post was the gear size.
The 2.73s are really only an issue A) outta the gate, and B) over the long haul.
As to A) The lack of torque multiplication at the start line makes it lazy until the motor comes into it's powerband, say about 35/40mph.
As to B) The long gears force the engine to spend a lotta time in each gear, at rpms outside of it's powerband. That makes for a poor E.T.

Then there's the timing thing.Insufficient power-timing will not help performance; too much will damage the engine.
And fueling? Fat is safe, but lazy. If you had 420 hp you can afford to give some up. But at 300 it's nice to have them all snorting.Lean is mean, but can be dangerous.
The 2.76 gear can be a tremendous tuning aid, exactly cuz the engine stays in one gear for such a long period of time. This gives the plugs time to build a picture you can read. Or gives the O2 sensor a nice stream to read.
First gear should be good to over 60mph, second to 110.
Just 2 gears in the Qtr will seem lazy alright.
 
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MRGTX said:
I'm thinking that you're right on all counts here...
The car isn't overweight...I mean, it's an Abody! Even a V8 big bumper car came in under 3,200, IIRC...that's probably 1000lbs lighter than a 5.9L Grand Cherokee or 1500lbs lighter than a 5.9L Ram. I do have a higher RPM stall so that's probably not it either but there's no doubt that the 2.73 ratio is a big source of the sluggishness. Still, the motor feels like it just doesn't want to rev. Holding it in lower gears helps some but not enough... So yeah timing and carb tuning are certainly contributing.
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Niners were like 3,900ish. The heavy late cars with all the options would hit almost 3,600.
 
Give it some gear and a cam to match the converter. Then tune that carb and dial in the timing right. Then tune the carb again.
 
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