I have heard but not verified that the stock 5.9 cam was 250/264/110+19/37* overlap( 12* effective)/182/194@.050/.411-.430 lift.
But take note of the +19 advance. this makes the compression to be 116* with ab Ica of 64* and the power extraction to be 127*. Read it again; 64* . the intake valve is closing ridiculously late, killing cylinder pressure, and bottom end torque with it. Remember, I have not verified this. I got it off some other Mopar forums.
I can't say why the factory would do this but I can guess.
Firstly the 5.9 is advertised at 9.4 Scr, and perhaps they needed to give up some pressure.
Secondly the extraordinarily long extraction cycle may be their way of increasing the fuel mileage to some target they may have had in mind.
Thirdly they killed the overlap from a maximum possible of 37* to to just 12* Effective.
Why is this important?
because in the stock engine this marriage of 64*Ica and 9.4Scr predicts a cylinder pressure of just ;
Static compression ratio of 9.4:1.
Ica of 64* @800ft elevation
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 7.55:1 .
Your dynamic cranking pressure is 147.37 PSI. ............................. 147
V/P (Volume to Pressure Index) is 124 ....................................... 124VP
and a mediocre VP of just 124
now lets advance that cam to split overlap; and I get;
Static compression ratio of 9.4:1.
Ica of 51*,still 800ft
Effective stroke is 3.08 inches.
Your dynamic compression ratio is 8.23:1 .
Your dynamic cranking pressure is 165.42 PSI. ............................. 165
V/P (Volume to Pressure Index) is 154 ..................................... 154VP
Check it out! To read about VP click on the blue V/P in my sig.
This is an increase in cylinder pressure of 12.2% and more importantly, an increase in VP of 24.2%.
The compression duration is now 129* and extraction is more than adequate at 114*, and the overlap has grown to an Effective of 36*......... which means headers are now optional, and will make a power difference.
The downside to this is that the stock engine ran on 87 gas, but the new cylinder pressure may require 91 at WOT.
But make particular note of the over 24% increase in VP. This is like increasing your engine torque by a like amount, or could also be likened to adding a similar amount of rear gear. And..1.24x2.76=3.43s
You don't have to retard the cam timing to split overlap. Say you want to top out at 160psi pressure to run 89 gas ( I can't imagine why, but lets just say)
Here it is at
Static compression ratio of 9.4:1.
Ica of 55*,still 800ft
Effective stroke is 2.99 inches.
Your dynamic compression ratio is 8.02:1 .
Your dynamic cranking pressure is 159.81 PSI. ........................ 160
V/P (Volume to Pressure Index) is 144 .................................... 144
Notice the VP fell from 154 to 144. which is still waaaay better than 124. I like to say that 140 is where the fun starts.
This will liven up your bottom end like 3.17 rear gears would. So still winning!
Ok now, I hate to try and make you be the GuineaPig, but no one has yet spoken against this overly advanced factory install, soooooooo you might want to check it out. You know you need a chain-kit anyway right........ If this info I'm presenting is correct, and you end up correcting it, it will certainly plump up your torque throughout the rpm range, and especially at zero mph to ~3500rpm.......... which with 2.76s is about 35mph;BadaBoom: just try not to spin the tires ALL the time (154VP is incredibly potent off the line). It will not, however, make much significant power over the top. I mean it's still just a250* cam /182*@.050. But retiming the cam usually costs no more than a gasket and a couple of hours......... if you have or can borrow the tools.But in your case,an install change from 119* to 106* is 13* and you won't find an offset cam key for that...... So you are gonna need the cam/crank gears with pre-machined timing grooves, in addition to the offset keys... And not sure if that combo will even go 13*.
And most importantly; I don't know what this will do to your piston to valve clearance. I can say this; retarding a cam usually increases the valve to piston clearance, but you need to verify this; I will not buy you a new set of valves if they crash.
Is it worth it? On a budget,IMHO, absolutely yes. No contest, and I would go all the way to split overlap with the cam centerline in at 106*.
But if you can afford a new cam, then no; forget it. Just buy a cam with the same Ica and more duration in both intake and exhaust and then you get more power over the top, in addition to the stronger bottom end. This is kindof a unique situation. In the lightweight A, with a 360, you don't ,for street, actually need VP into the 150s ; on account of it will just end up in tiresmoke. 140VP would be lots. And that gets you an Ica of about 56*. So Any cam you might chose that can be installed at that 56* Ica will already have a strong bottom-end. So then you can concentrate on putting the power at whatever rpm that works out to. Just remember, with 2.76s It's kindof dumb to have a power peak at 75 mph in first gear, if the speed limit is 65.
Also with that overdrive trans, it's also kindof dumb to marry it to 2.76s with a hi-rpm cam. You are in a unique position with that .69 overdrive and loc-up TC, to do some pretty unusual things.
I would not bother with increasing the rocker-arm ratios.
Ok you said not the usual advice wanted so lets work this backwards From a few posts down, we discovered that the factory cam is in straight up, and I'll go out on a limb and say the cylinder pressure has come in at 155psi, Just a guess, based on this
Static compression ratio of 9.4:1.
Ica of 55*, still 800ft
Effective stroke is 2.99 inches.
Your dynamic compression ratio is 8.02:1 .
Your dynamic cranking pressure is 159.81 PSI.............. 160
V/P (Volume to Pressure Index) is 144
And I will arbitrarily deduct 5psi for worn rings; so that makes 155psi.
This is just a baseline.It doesn't need to be accurate at this point.
So two things we know; the engine should be at 160psi, and the LSA is 110*. We also know ;
250/264/110+0, so that spits out the Ica of 55, and yur not happy with the launch from zero mph,right? So we need to either raise the compression ratio,to get more pressure; or increase the stall to where the engine has more power; or increase the cylinder pressure with an earlier closing intake valve. Earlier closing intake usually means smaller cam so less power over the top.
Of these three the compression ratio is the most effective.... but I theorized that you already had 155 psi. A re-ring alone might get you 160 psi...... But I arbitrarily chose 800 ft as your elevation....so if you are not at 800ft, then the pressure could be a wee bit higher or a lot lower. But since this is just an exercise, lets say I guessed right on the money. Ok so I see you have already freshened the engine, so now you are up to 160 and are using 89 gas, and lets say you want to stay there.
So now we have a written-in-stone Ica of 55*, see how easy that was?
Next, since I'm on the unusual road, lets look at fuel economy. To get good fuel economy we need to stretch out the power extraction cycle, as far as we can. Numbers less than 100 are pigs on gas. Numbers over 130 are silly targets, they never get the mileage you think they will. So half way in-between is a good target, and I get 115*
See; we are building a cam and now we have the compression duration set at 180less 55=125*, and the extraction is 115* .
Next, lets talk about overlap, cuz this cycle can make lots of power or none at all, in addition to what the intake duration is gonna do. Ok, so lets say you are running log manifolds, and not looking to set the world on fire, so how about we just say; don't care about overlap. Great that opens up a whole big world to intake and exhaust, the two remaining durations to hammer out.
This is getting easier all the time. Ok, so to complete all 4 cycles we have exactly two crank revolutions, plus whatever overlap we are gonna run. Since the Ica of 55* is written in stone, we are gonna have to install this theoretical cam at split overlap. Fine. So we start with Two revolutions which is 720degrees and lets throw in lots of overlap cuz the logs are gonna kill it anyway. I chose ummmmmmmmmmmm 50*, that totals 770 available degrees, from which we subtract the already knowns of 125and 115 so that leaves 530.
Next, because you have logs we're gonna give the exhaust 10 extra to get outta there, leaving 520 to be divided equally between intake and exhaust, so 260 each. But we gotta add back the 10 to the exhaust, so it will be 270. Lets write it out;
260/270/***/50 overlap( /***/125 compression/115 power, Ica of ***, hyup that looks pretty good. Lets fill in the blanks.
First we'll find the intake opening point which is 360 less the compression and intake degrees, so;
360-(125+260) = minus 25 so that is exactly half the 50 I allocated to it, so that is 25* to each ; intake and exhaust so the cam is in straight up with an effective overlap equal to its maximum potential , so still 50*, Ok pencil that in, and I get
260/270/***/50 overlap (50effective)/125/115
ok that just leaves the LSA, which is the average of intake plus exhaust duration less Twice the LSA= overlap so I get
{(260+270)/2 less 50 }2= LSA; or 107.5, pencil that in
260/270/107.5/50(50*Effective)/125/115
and finally the required install to make it all happen,which is half the intake duration(260) less the number of overlap degrees (25) previously allocated to intake; =105; which is 2.5* from 107.5 so the cam goes in 2.5 degrees advanced, so pencil that in and I get;
260/270/107.5+2.5/50 overlap(50Effective)/125 compression/115 exhaust/ Ica of 55* badaboom we're done
Now let's analyze this; first the factory cam, then the theoretical cam
250/264/110.0+0.0/37 overlap(30effective)/125 comp/118 exhaust, Ica of 55*
260/270/107.5+2.5/50 overlap(50 effective)/125 comp/115exhaust, Ica of 55*
So first things first, we have to assume both cams are being measured at the same tappet lift spec, whatever it is.
Then since the new cam is 10* larger on the intake duration we can assume this is 10/7=1.4 cam sizes bigger, and so the power has moved up about 1.4x200=286 rpm higher.Maybe. Lets suppose. And that might be, if the rest of the combo supports it, plus 15hp.
But from idle to 3000/3500 there may be no power difference at all,.... because the ICa, and therefore the cylinder pressure, remain exactly the same. Ohhhh that's gotta hurt,lol.
That's why this is just an exercise.
See,at your compression which I assumed to be 160psi,you are about maxed out on cylinder pressure for 89 gas and iron heads, so there is no place to go. And that is why the theoretical cam had to have the same Ica as the current cam.. the exercise is not valid if your real world pressure is not the same 160 psi, or if your real-world Ica is not 55*.
But now you know why things are the way they are.
But be of good cheer this 260 cam, will make more absolute power for starters. And there are two more things to consider. The 107.5 LSA and the 50* overlap.
These two together with long-tube headers are gonna make a nice lil power bulge over the top , in addition to the possible 15hp already mentioned, and the tighter LSA will also increase the midrange from around peak torque and beyond. How much? IDK, cuz it's gonna be lost in whatever the headers bring to the table. But I could guess; New cam plus headers, if the rest of the combo supports it, say maybe 30hp total, at around 4700 rpm. Perhaps a tad more.
The headers, now, should also fatten up the bottom end just a lil bit.
So what's wrong with this picture?
Two things;
First is the factory stall TC, which forces the engine to work at a very low, terrible for performance, rpm.
And second is the 2.76s which keep the engine at a very low power level forever and a day. I mean 39 to 42 mph is a long wait. To be fair the torque is already working in the mid to hi 3000s so make it 32 and 35 mph when the power begins to be felt; still a forever wait.
With the current power peak of perhaps 4000, and with 26" tires, means 4000~39mph.... so that is a long wait to find the power. And the theoretical replacement cam is nearly 300 higher so now the peak is hitting at 4300 or 42mph, 3 more mph to wait! sux to be the guy with his foot in the throttle-body,lol.
So what's it all mean?
My answer is;
The first go-to with the 2.76s is about a 3500 stall TC, minimum.
OOPs I missed that this is not an overdrive combo, so ignore the following, perhaps just put it on the back-burner.
But why in the world are you keeping the 2.76s with that overdrive?
Lets say you retimed that factory cam to split overlap. At 36*Effective overlap, it will make a few more horsepower.... and a bit of torque, with headers and dual exhaust. Leaving that cam in, just retimed means about 115* of extraction, and so a target cruise rpm of 1800 to 2000. With a .69 overdrive, this means 3.55s and 26s will get you a cruise rpm of 2035 in loc-up @65mph. or 2350@75mph
Now 3.55s are still not great but they are 28.6% better than the 2.76s, allowing your engine to spool up much faster, cuz it thinks it is now 28.6% more powerful. And that means you no longer need the 3500 TC. I would try a 2800, but if the one you already have is 2500 or more then just start with that. It might satisfy you.
end of od discussion.
So what you see here is juggling the starter gear with the Stall-speed. Usually you would juggle this with cylinder pressure as well, but in your case, you may be maxed out, or close to it, on pressure, as is.
Logically then, it follows that if the combo needs a TC and gears already, before any engine changes, then you might as well get that over with and out of the way.................... First.
As always
Happy HotRodding.
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