2:91 or 3:55?

-
I daily drive with 3.55 gears and an A518 (OD).
If your engine makes nice torque down low a 3.23 would be good enough.
 
Here’s a little something I came across recently while doing some 1/8 mile gear ratio calculations for a friend. It has a wide range of transmissions as well as selection for custom ratios. It’s the most satisfying gear calculator I have come across yet... hit auto shift and it runs the rpm and shift points for you.
Gear Ratios, RPM and Tire Diameter to MPH with Shift Points Calculator
 
I think this is a second gear question. With an A500:
In the city;
Almost anytime you want to accelerate, the KD will be into First gear. So if this is currently working for you, then leave it alone.
One of the nicest second gears I have used is a 6.82 in a manual trans. The TC would make this with a 6.50 ratio, and with a 1.54 second, this would take a 4.22 rear gear. The closer you get to that, the more fun you are gonna have.
Your 3.55s and 1.54second make 5.50, which is a lil shy, but your torquey cam will likely get it done.
But I gotta say; KD into first at 30mph with 3.55s will get you very close to peak torque (I get about 3800) and that is gonna light up the tires at WOT.
And the final drive is still 3.55x.69=2.45, a great cruiser gear.
So that leaves on the hiway, and specifically, passing.
At 60 dropping into second with 3.55s will get you 4300, and you can hold that to about peak power with a 218@.050 cam;at 5000=70, not the best but it works. You should be able to hold that to 5500@77mph
The alternative 2.91s makes KD into second at 60 to be 3500, at or near peak torque with a 218@.050 cam; giving you plenty of headroom. Peak power comes around 5000=85 mph. Heckuva passing gear.
 
AJ/FormS, I just want to jump in here a second, cause I have a very similar setup to TrailBeast. Or at least I will soon. My '68 Dart has a carburated 5.9mag, stock bottom end, right now stock cam, edelbrock air gap and a 1406 carb. Dougs headers.

This weekend I'll be pulling my engine and tranny to do both an A500 swap, and a cam swap to a Lunati voodoo 213@.050
My car currently has 3:23's but I have a brand new set of 3:55's I was going to throw in soon as well. Just haven't done the gears yet cause I'm saving up for suregrip to throw in there too.

I think I might be running a smaller rear tire, most guys here seem to be running a 27-28 inch tire. My 225/60-15's are 25.5"

In my case, would you recommend just leaving the 3:23's in there till I get the cam and tranny swap done and see how it feels? Or go ahead and throw in the 3:55's at the same time?

Note: I do a lot more highway driving than in town.
 
I would
Leave the cam in there. and install the 3.55s to hit 65=2100, 75=2400
The A500 has ratios of 2.74-1.54-1.00-.69.. So
the starter gear is 3.55x2.74=9.73 and the TC will pump that up by at least 5 to 10 % at WOT, after the initial hit; so say 10.50.
Compare this to your 68TF and 3.23s which make 2.45x3.23x1.08=8.55
That is a huge improvement of 22.8%. This is the same improvement as if you had installed 3.23x1.228=3.97s in the current combo. Whatever torque your engine currently puts out the crank at stall, this 22.8% is directly comparable to increasing it by that same 22.8%. So if 250 currently, then it will feel like 307 after the swap.
But it doesn't stop there, in second gear you are going from 3.23x1.45x1.05=4.92 ratio to; 1.54x3.55x1.05=5.74... which is a 16.78% increase.
And direct goes from 3.23x1.00x1.05=3.39 to 3.55x1.00x1.05=3.73= 10% increase.

Ok now lets look at it a bit differently; If your 5.9 Magnum puts out say 280hp right now at say 4600rpm (just guessing) , then with the new gears, second will feel 16.78% bigger ALL the WAY THRU that gear and peaks like it had 280x1.16788=327hp. OK hang on to your hat; that is the equivalent of 47hp; and to get that with a cam will take at least 3, count'em 3 cam sizes, if the heads will support that cam. I think the stock 5.9 cam is 250/264/110, so the gears will make it feel like it has a 271/285/110... WITHOUT any power loss at lower rpm, but rather a continuous boost of 16.78%, from stall to shift.

And one more way;
that 250/110 cam is gonna power-peak at near 4600. But the power peak is so flat up there that she will keep right on pulling to an easy 5000, as long as the lifters don't pump up. Have you tried it? Am I right? Well let's say you sprung the valves and are good to go. Let's make a run at WOT with the new combo of A500/3.55s;
Now with a 9.73starter gear, you run it up to 5000 and with those 25.5s you hit 36mph. On the shift the Rs drop to 2800, and your stock cam is gonna torque-peak at about 3100, so it would seem like yur sunk; but that's not true! the torque peak of that small 250 cam can easily pull you up to 3100@41mph, it's only 5 mph and your car is already accelerating. Ok so second will take you to 60@4550, or 66@5000 perfect.

Ok now whats gonna happen with that Lunati?
Well,um; Since you didn't say, I'm gonna guess yur talking about this one;
Voodoo Hydraulic Roller Cam - Chrysler 273-360 (W/ Long Snout) 264/270 SKU 20200715
Hydraulic Roller Cam. Great performance cam in 340+ c.i. applications. Works well with computer modifications or increased induction systems. Likes headers and 3.42+ gearing.; Advertised Duration (Int/Exh): 264/270; Duration @ .050 (Int/Exh): 213/219; Gross Valve Lift (Int/Exh): .485/.485; LSA/ICL: 112/106; Valve Lash (Int/Exh): Hyd/Hyd; RPM Range: 1200-5500; Includes: Cam Only

OK then this cam is almost two sizes bigger, but the LSA is down 2 degrees so looks like 20hp (I'm guessing again) ...... but it's gonna be 300 to 400 rpm higher so say 4800. So with the new gears this will be 63 mph, and taking that out to 5500 will get you a shift speed of 72 mph.
Now think about that; The stock cam peaked at ~61 mph and coasted to 65/66.
And this Lunati peaks at 63 and coasts to 72, making about 20hp more in that zone somewhere, and the speed-limit is 65mph... is that 7 mph gonna be worth it to you? EG how often are you gonna be charging thru that zone? I'm not preaching to keep your speed below 65, I don't care; I'm asking is that 20hp and 7 mph worth the cost of the cam-kit
What about first gear you might ask? Fair question. With the new gears you should be spinning the tires straight thru first with those small street tires, so 20hp at 36mph is gonna make only a tiny difference in ET if any. If you can't make first gear stick, it's a throw-away gear. It's a lotta fun, but it only gets you into second, where the action is gonna be.
Now the bad news; This bigger cam,going from 250* intake to 264*, with no other changes is guaranteed to lose cylinder pressure , and that means a loss of power in the lower rpms somewhere below 3000 in your case. So if you have a stock 2000/2200 stall TC, then whenever you are operating in that zone, it's gonna feel soft; not quite sucked out with only a 2 cam-size change on the advertised, but had you not swapped the gears, you would have easily noticed it. You can get past this with a higher stall TC. But now you have to add the cost of that TC to the cost of the CamKit, and again I ask; Is it worth it to you? I'll guess yur up to about $1200 on the cam kit and TC.. and it will not reduce your zero to 60 time even 1/2 second.
It would not be worth it to me in this situation. There are cheaper ways to get a half-second.
There is one nice thing with this combo; 30mph is about 4100rpm at the top of first,so when you are cruising around at 30mph,not in first, and you suddenly stomp it; the KD should get you instant tirespin,and the shift into second doesn't come until ~40/41 mph. So that's gonna be FUN
Parting shot;
Since the 3.23s are already in there, I would try the combo with them; maybe the 2.74 low gear will be enough for you. But if not; then I would , if you already have them, install the 3.55s. BUT; if you don't already own the 3.55s........
Well then;
3.73s would get you 60=4800 in second with the stock cam, probably quicker than that Lunati and 3.55s. And the 3.73s will cruise 65=2200, 75=2550

IDK why my board went to double spaced, and IDK how to put it back,sorry. It's not double-spaced in the draft.But it is in the preview
 
Last edited:
Thanks very much AJ, you are always a wealth of information..

You are correct on the Lunati, it is the 20200715
Already have it, got a smoking deal at $150, and have already bought Hughes 1110 valve springs, retainers, locks. But I suppose that stuff could be resold,..

I also already have the brand new 3:55 gear set. Just haven't installed them yet, cause I want to add a suregrip at the same time which I don't have yet, but soon.

If I go ahead with the cam swap, I was also intending to change the head gaskets from the .050 thick felpro thats on it, to the thin .028 gaskets to help with any cylinder pressure drop, Or stay with the same gaskets and have the heads shaved. 020 or so, but thats more money.

Anyway, in my first post I had a brain fart, or a type. Right now it's a stock 5.2 cam not the 5.9 cam. It's 184/193 at .050 and its a 112 lsa.
Don't know that that really changes the scenario much, just wanted to throw it out there.

I"ll probably take your advice and just do the A500 swap first and see how she feels, then the 3:55's. May not even need a cam at that point.
 
Oh yeah that makes a mondo huge difference. Kidding,lol.I have the cam listed as the same for both Magnums; IDK if that is true.
Is that 5.2 still at 9.4 Scr?
At the top right of every page you open you will see your name. If you care to, click on it and find "signature" click on that and there you can enter all the pertinent info of your combo, about 5 lines worth. After you close it, that sig will appear on every post, just like mine does down there on the bottom..Now whenever you have a question about your combo, everything we need is right there. You can edit it in the future, and it will change every post you ever wrote to reflect the change. I used to do that often.

Ok so for me to try and show you what is going on, I need the exact cam spec the exact known, not estimated,nor believed to be; compression ratio, or all the specs that are used to calculate the Scr. If you don't have these, I'm gonna guess, and then you know how that goes garbage-in/garbage-out.

I have the cam listed as the same for both Magnums;
250/264 advertised; 182/194@.050, I see it with an LSA sometimes at 112, sometimes at 110, and now you bring it as;
184/193 at .050 and its a 112 LSA. Lets go with your numbers and with 9.4 Scr, an I'll use the 250/264 advertised numbers.
OK; I have on file that this cam is factory installed at 118 Ica. Which IMHO, is ridiculous. here's what that installs like
250/264/112-7/29 overlap/116 compression/119 extraction/64 Ica
Now here's why I think this is ridiculous.
A) it puts the Ica way past BDC and the piston is ~1/3 of the way up before the Advertised says it's closed. But that cam has such slow ramps, that the valve is not closed on-the-seat-and-not-leaking until PERHAPS 12 degrees later or say 76* ABDC That's ridiculous. I'm sure they are using that to create EGR and is a Pollution thing, but it's nuts.
B) the overlap is spec'd as 29 but with the 119*installed centerline, the actual, EFFECTIVE overlap is just 12* which is fine for log-manifolds, but gives headers nothing to work with.
C) add up the compression and extraction degrees and you get 235*. This is a good number, but check out that the extraction is hoggin more than half of it!, more ridiculousness.

OK ,I tell you what lets put this cam in as it should be. All I'm gonna do is retime it to an install of 111*, and here's how that looks;
250/264/112+1 /29 overlap(28 Effective)/124 comp/111 extr/ 56 Ica
Compare that to the stock install of
250/264/112-7/29 overlap (12 Effective)/116 comp./119 extr/64Ica

So what happened?
We traded away 8* of extraction to get the same 8 more degrees of compression, which also gave us 16 more degrees of effective overlap, which only helps if you have headers. Recall that all I did was retime the cam.
So what's it matter?
Glad you asked.
Lets install them into an 9.4Scr 5.9 Magnum and see what happens; and no I haven't lost sight of the fact that 182*@.050 is extremely small; we'll get to that inaminit.
First the factory install
Static compression ratio of 9.4:1.
Ica of 64, 500ft elevation
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 7.55:1 .
Your dynamic cranking pressure is 148.95 PSI.................. 149
V/P (Volume to Pressure Index) is 125 ............................ 125

and next, the retime
Static compression ratio of 9.4:1.
Ica of 56*, still 500ft
Effective stroke is 2.97 inches.
Your dynamic compression ratio is 7.97:1 .
Your dynamic cranking pressure is 160.08 PSI..................... 160
V/P (Volume to Pressure Index) is 143 ............................... 143

What's mean?
Notice the pressure increased from 149 to 160. That's huge and makes your throttle response come alive and punches up the midrange power.
Notice the VP rose from a dismal 125 to a more than acceptable 143, which is where the fun starts.
Notice the Dcr rose from 7.55 to 7.97. this is only 5.56% but you are really gonna feel it. It also means that, at WOT, you might require best gas; it depends on a lotta factors, specifically to load at low-rpm.
And finally, lets look at the overlap, which rose from an effective12*, worthless with manifolds; to 28* with headers which ain't much. but you're gonna get a slight kick in the midrange, to after the power beak, all the same.
All that from just a cam retime?
You better believe it.

Ok so now we know what coulddabin, lets see what the new cam will do;
264/270/112+4/43 overlap(34* Effective)/122 comp/107 extr/Ica 58
compared to the retimed stocker
250/264/112+1 /29 overlap(28* Effective)/124 comp/111 extr/Ica 56

First notice that the retimed stocker is only 6* short on effective overlap
Second; compression plus extraction is 229 compared to the stocker's 235, by itself not bad, but notice that the new cam gave up 2* of compression.
Third is the advertised intake duration of 264 versus 250. This is fully two cam sizes bigger, but there's a hitch interpreting that, which is that I don't know the measuring point of either cam. No matter, we have the .050s which is all that matters.... they say.
The new cam is
213/219, and lift of .485/.485.. versus
184/193, and lifts of .411/.430
which tells us that the new cam is 29 degrees bigger on the intake, which is 4 sizes bigger, which usually equates to the power peak being about 800 rpm higher..... cuz the only way to make more power with all other things remaining, is with more rpm. That's the way it is.
On the exhaust side, the factory cam was made artificially large on the exhaust side so they could run the dang thing so retarded so in this case the difference is relatively meaningless.
So what's it all mean?
Static compression ratio of 9.4:1.
Ica of 58*, still 500ft
Effective stroke is 2.93 inches.
Your dynamic compression ratio is 7.87:1 .
Your dynamic cranking pressure is 157.42 PSI........................ 157
V/P (Volume to Pressure Index) is 139 .................................. 139
WEll wouldja lookit that!
Because the retimed factory cam, had an Ica close to what the new cam has, the pressure only fell 3psi, which caused the VP to fall just 4 points.
With closed chamber heads, and tight-Q this should run on midgrade at WOT, maybe even 87, if you get the timing just right.

But here's the deal; below 3000 the retimed factory cam has a slight edge on power. But it's only slight. But the new cam has Huge power advantage over the stock, retarded cam, below that same 3000, now increased to perhaps 3500.
So since the cam is already in there, it's gonna be a HUGE improvement to the stock one at low rpm, AND since it's 4 sizes bigger at .050, it's also gonna be a huge improvement every where in the Rpm band. There is no down-side to this combo.
Except one.
While I'm sure this combo will run well with a carefully thought out and installed timing curve, you better run best gas until you get it. And you cannot arbitrarily increase the compression past the the stock 9.4, for the same reason. Yet, this combo will do best with the Quench set into the normal target of .040 +/- .005.. And if it's not there, then you are gonna need to be even more careful to avoid detonation.
Once you get all tied together, it will run gangbusters; a VP of 139 is right on the lower edge of big fun..

And that takes us right into Torque Multiplication. Since this combo, in fact increased the VP from the retarded factory number of 125 to 139, Yur gonna have an 11% stronger bottom end with the new cam .............. so if you were happy with the old set-up, yur gonna be even happier with the new combo. Even with the stock-geared 904 and 3.23s This is kindof an oddity,and has only happened because the stock cam was so retarded, as I showed you right in the beginning.
It will however be slightly harder on gas because of the reduced extraction duration; 107* is not too bad tho.

Parting shot
This cam has the unusual ability to run at a much slower than usual cruising speed... because of the meager 34* of overlap, the early closing intake, and the reasonably late exhaust opening. This also does not often happen. So if the mood strikes you to install that A500, your cruising speed with the 3.23s is gonna come in at around 65=1880 in loc-up. And if you're a sharp tuner, I see up to mid 20s mpg, in point to point cruising. The stock timing systems won't get you there; cuz at 65=1880 she's gonna want in the neighborhood of, I'm guessing, close to or more than 50 degrees of advance............... And it has to drop out the instant you floor it, or you could be finding piston parts in the oilpan.
Is it worth the chase? I thought so, and after 125,000 miles, I have saved a lotta money.

Ok hope that's been helpful, and
HappyHotRodding

Click on refresh , but I'm done for tonight, let the questions begin.OOPs I see a math error I gotta fix before you notice it, Got it!
 
Last edited:
Probably a bad time to tell you that I"m at 5000' elevation.
That's why I thought I could get away with trying to bump the compression a bit with the thinner head gaskets.
And thanks again for all the calculations and input. Not sure I can say I understand all of it, but it helps for sure.

Obviously cause the engine is together right now I can't measure piston dish volume, or combustion chamber volume right now to figure static compression, But I did run a compression test on it this morning just to see where i'm at. Engine is only a couple years old, maybe 6000mi on it since build.
Numbers are all between 139 and 144, most were 140
 
Last edited:
MileHighDart said:
Probably a bad time to tell you that I"m at 5000' elevation.
That's why I thought I could get away with trying to bump the compression a bit with the thinner head gaskets.
And thanks again for all the calculations and input. Not sure I can say I understand all of it, but it helps for sure.

Obviously cause the engine is together right now I can't measure piston dish volume, or combustion chamber volume right now to figure static compression, But I did run a compression test on it this morning just to see where i'm at. Engine is only a couple years old, maybe 6000mi on it since build.
Numbers are all between 139 and 144, most were 140
Click to expand...
5000 ft eh?
140psi, eh?
ok then lets start over lol.
 
Last edited:
MileHighDart said:
My '68 Dart has a carburated 5.9mag, stock bottom end, right now stock cam, edelbrock air gap and a 1406 carb. Dougs headers.
the stock install of
250/264/112-7/29 overlap (12 Effective)/116 comp./119 extr/64Ica
>
>A500 swap, and a cam swap to a Lunati voodoo 213@.050
264/270/112+4/43 overlap(34* Effective)/122 comp/107 extr/Ica 58
>car currently has 3:23's but I have a brand new set of 3:55's I was going to throw in soon as well.
> My 225/60-15's are 25.5" (80" roll-out)
>In my case, would you recommend just leaving the 3:23's in there till I get the cam and tranny swap done and see how it feels? Or go ahead and throw in the 3:55's at the same time?
>Note: I do a lot more highway driving than in town.
>you are correct on the Lunati, it is the 20200715
>a stock 5.2 cam not the 5.9 cam. It's 184/193 at .050 and its a 112 lsa.
>I"m at 5000' elevation
>Numbers are all between 139 and 144, most were 140
Click to expand...

Ok then
with the factory cam in at Ica of 64* as per the 119* installed centerline, and the stock Scr of 9.4; your pressure is predicted to be;
Static compression ratio of 9.4:1.
Ica of 64*, 5000 ft elevation.
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 7.55:1 .
Your dynamic cranking pressure is 125.52 PSI................ 125
V/P (Volume to Pressure Index) is 106........................... 106
So your reading of 140ish is telling a different story. I'm just gonna advance the cam until it lines up with your numbers

Static compression ratio of 9.4:1.
Ica of 53*. 5000 ft, factory cam
Effective stroke is 3.04 inches.
Your dynamic compression ratio is 8.13:1 ........................ 8.13
Your dynamic cranking pressure is 140.55 PSI.................. 140
V/P (Volume to Pressure Index) is 129 ............................. 129
So it looks like somebody has been inside there all ready.
A 53* Ica calls for
250/264/112+4/33overlap (32Effective)/127comp/112 extraction/53* Ica so that looks like someone already gave her the perfect cam-timing.
So now lets see the Lunati, per their install,at 4* advanced.

Static compression ratio of 9.4:1.
Ica of 58*, 5000 ft
Effective stroke is 2.93 inches.
Your dynamic compression ratio is 7.87:1 .
Your dynamic cranking pressure is 133.78 PSI. ..................... 134
V/P (Volume to Pressure Index) is 118 ................................. 118
So looks like a bad deal below 3500rpm or so, she be needing more compression. How much?
here yago
Static compression ratio of 10.4:1.
Ica of 58*, 5000ft
Effective stroke is 2.93 inches.
Your dynamic compression ratio is 8.69:1 .
Your dynamic cranking pressure is 155.29 PSI. ..................... 155
V/P (Volume to Pressure Index) is 137 ................................. 137
now you got something!

Ok so now if the current engine is in fact a 9.4 Scr, then the total chamber volume will be 87.76cc. With your gasket swap of -4.87cc, the Scr becomes 9.89, lets check it out;

Static compression ratio of 9.89:1.
Ica of 58*, 5000 ft, Lunati cam
Effective stroke is 2.93 inches.
Your dynamic compression ratio is 8.25:1 ............................... 8.25
Your dynamic cranking pressure is 143.69 PSI........................ 144
V/P (Volume to Pressure Index) is 127................................... 127
So that compares favorably to the factory numbers of 140/129, as seen above. So now at least the bottom is as strong after the cam swap as before, Good deal. and the 213 @.050 compared to the 184@.050 is no comparison at all; the Lunati will rock with 29* more intake duration, which is 4 sizes, and I'll guess an easy 40 horsepower stronger,possibly more, at 800 rpm higher. The new power peak will be at about 4900, so yur gonna have to rev there to get the power. The peak will stretch out fairly flat with a 112 LSA cam so, with an automatic, your 1-2 shift rpm might be over 5500; you'll have to run some shift loops to see.
I see the Dcr has risen to 8.25, which might present a timing problem; But at most you might have to delay the All-in power-timing.
All in all, an excellent upgrade.
I say do-it
So again, the bottom end is pretty much the same so I would run the A999/3.23 combo, but I would first leave whatever is in there, in there. Cuz you said mostly hiway. Keep in mind that the power is not gonna be noticeably different until after about 3500, so don't get cranky!,lol. with A904/2.94s this will be 35mph in first. With the A999/3.23s it will come down to 28.5 mph. This is the beginning of the power increase. With the A904/3.23s it might be 32 mph.
You're in charge of maintaining the Piston to head and to valve clearances to prevent your engine from self-destructing.I think you'll be fine, but saying it doesn't make it so.

Ok so that's corrected to 5000 ft now.
 
Last edited:
Thanks so much.
If my brain had been functioning better I might have saved you some calculations on the stock 5.2 cam. When you were talking about it being installed at 118 I think you were using a term that didn't click in my mind or something. I have a long post in the Magnum swap section of the site from when I was putting this engine together. So I went back and checked it out, and indeed I did install that cam at 110 Installed center line. Actually 109.5 was as close as I could get it.
Are you doing all the calculations by hand or do you have a program or something to help out?
 
Mostly napkin scribbles, basic calculator key-strokes,lots of formulas in my head, and a few notes, and this on-line
calculator below,for the icing on the cake.
Wallace Racing: Dynamic Compression Ratio Calculator
Amazing that the math found your install by the calculators predicted pressure. Simply amazing. Many thanks to the Wallace crew. I love this stuff!
 
You must log in or register to reply here.
-

Back
Top