Gear Ratio and RPM deciding on gears

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Not talkin about the OP here, BUT.........

I know I've preached it till I am blue in the face. People come on here wantin to know "what gear" or "what converter" is best. They have (for example) a 340, 240-260 @ .050 cam, all the goodies and then curl up in the fetal position like a little sissie when I say "3600-3800 stall and 4.30 gears".

If you're gonna build one with "all that" up front than "what's out back" needs to match.

if your willing to gear it build it or if not go bigger.
 
if your willing to gear it build it or if not go bigger.

Yeah people talk a good game but then get all skeered to do what something "really needs".
 
Yeah people talk a good game but then get all skeered to do what something "really needs".

True, unlike a race car where you can gear and stall it to your needs, Most street cars car have to do the opposite and build it to the gear and stall your willing to run.
 
Yeah people talk a good game but then get all skeered to do what something "really needs".
Kinda surprised and not sure how to take some of the comments here. I have a set of 3.55's ready to go but am leaning to buy a set of 4.11's because I feel the cam and intake is set up for a higher RPM peak and with the 2.71 its rather flat. Was thinking that the 4.11's would help with that. I posted the curves to try and figure out if 4.11's might be too much in the only way I knew how and that was cruise RPM. I plan to take some data on RPM VS speed and do an AF mix per RPM/speed later. Then plan to do a leak down and compression test. I really dont want to yank the cam just to see what I have.......
 
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Op should pour a bottle of slick50 in the ole chrome valve cover and go n do burnouts into a brick wall somewhere over the rainbow.
Sciencing out a cruise rpm is mental masturbation
How should I take this post MOPAROFFICIAL?
 
Kinda surprised and not sure how to take some of the comments here. I have a set of 3.55's ready to go but am leaning to buy a set of 4.11's because I feel the cam and intake is set up for a higher RPM peak and with the 2.71 its rather flat. Was thinking that the 4.11's would help with that. I posted the curves to try and figure out if 4.11's might be too much in the only way I knew how and that was cruise RPM. I plan to take some data on RPM VS speed and do an AF mix per RPM/speed later. Then plan to do a leak down and compression test. I really dont want to yank the cam just to see what I have.......

Did you not see where I said I was not talking about you?
 
Don't run a 4:10 (or higher) and expect to drive 4 hours to a car show at interstate speeds without a overdrive. You have to choose your battles and then decide.
 
I have 3:23's in both my cars but with tall tires. I don't need to abuse my equipment and I don't race. They work good for my use.
 
I want a hot street car that I can take on the track a few times a year at most since I don't have a lot of money to replace parts etc. But I would like to be able to ride a decent torque curve around town and the torque of the 340 starts above 3000 probably more like what I show in the graph at 3800. Wouldn't 4.11's have more torque around town than a 3.55? I don't plan on leaving the Phoenix metro area and tour with. I DO want performance and the 2.71 I have is a dog with the torque converter as is. I plan on working with Lenny AFTER I get the rear ratio in. There is always a split on what rear gear ration is best. All I know is I have a 904A 3 speed trans that idles at 35MPH lol which is not good.


What do you not have any power under 3000? Seriously, that makes no sense.

You may need to work on your tune up or something.
 
Yeah I was gonna say either this ^^^^^ or 3.91s.

HOWEVER, you STILL have not answered the question asked. WHAT'S YOUR ENGINE COMBO? Are we just "supposed" to know somehow? The cats knocked off my crystal ball and broke it a long time ago. Are we supposed to remember YOUR combo from another thread? Fat chance for my "old butt". PLEASE enlighten us if you want an answer remotely "accurate".


Dang kitties!!! They've been known to knock stuff over!!
 
What do you want and be happy? I ran 4:56 for a long, long time. I rebuilt and thought what I wanted was the same Car that jumped on a right foot press and a great 1/8 mile . Always made me smile! Older I am and 3:55 is just fine for good a cruise car and a romp when I feel so.
 
What do you want and be happy? I ran 4:56 for a long, long time. I rebuilt and thought what I wanted was the same Car that jumped on a right foot press and a great 1/8 mile . Always made me smile! Older I am and 3:55 is just fine for good a cruise car and a romp when I feel so.

Yup. I ran 4.30s in my 65 Valiant for almost four years. 12 miles one way to work. On the interstate. There and back. 28/11/50/15 rear tires. It would spin like 2700 at 55 MPH. Not bad at all. I drove it several times to Indian Springs about 55 miles away. With a tall tire, like what is "correct" to run with those type gears it is really not bad at all. Of course, everybody is in a hurry and nobody wants to go "just 55". I am probably going with a "little less" in the Ford 9" I am putting in my 64 Valiant I have now. Most likely going with a 3.89 gear.
 
If you want some original Chrysler 4.56 ge
Yup. I ran 4.30s in my 65 Valiant for almost four years. 12 miles one way to work. On the interstate. There and back. 28/11/50/15 rear tires. It would spin like 2700 at 55 MPH. Not bad at all. I drove it several times to Indian Springs about 55 miles away. With a tall tire, like what is "correct" to run with those type gears it is really not bad at all. Of course, everybody is in a hurry and nobody wants to go "just 55". I am probably going with a "little less" in the Ford 9" I am putting in my 64 Valiant I have now. Most likely going with a 3.89 gear.

Never been happier in my older age than the 3:55 set. Great compromise.
 
problem is it not what you are happy with but the OP

Do not disagree! Just an opinion and should have posted as such! My sorrow and condolences!
 
Do not disagree! Just an opinion and should have posted as such! My sorrow and condolences!


Sorry I took it out on your post, it’s just most seem to want peeps to do as they would instead of what’s probably best for the OP, or even what’s not best but what the OP, wants.
 
Sorry I took it out on your post, it’s just most seem to want peeps to do as they would instead of what’s probably best for the OP, or even what’s not best but what the OP, wants.

No problem for me, I was giving an honest opinion to the OP. 28” tires and too much testosterone in my younger days. 3.55’s are a great compromise!
 
OP keeps writing 4.11 gear as if it was ever a ratio available. It was not. It still isn't. 4.10, sure....
These cars never had POSI either.
Back on point:
If this isn't a daily driver, gear it for the type of driving that you intend to do. Road race? tall gears like 2.94 to 3.23. Drag race? 4.10, 4.30. If you just want a reasonably snappy cruiser, aim for the middle like 3.55 to 3.91.
I had 4.10s in my Charger with a 28" tire. It was snappy at all speeds but freeway speeds of 60-65 felt slow! I swapped in some 2.76s and I was surprised at how it still felt peppy at freeway speeds. Torque is a great equalizer. If you build a small cubic inch engine, you need rpms to make power and the ratios under 3.55 may leave you wanting more gear.
 
Kinda surprised and not sure how to take some of the comments here. I have a set of 3.55's ready to go but am leaning to buy a set of 4.11's because I feel the cam and intake is set up for a higher RPM peak and with the 2.71 its rather flat. Was thinking that the 4.11's would help with that. I posted the curves to try and figure out if 4.11's might be too much in the only way I knew how and that was cruise RPM. I plan to take some data on RPM VS speed and do an AF mix per RPM/speed later. Then plan to do a leak down and compression test. I really dont want to yank the cam just to see what I have.......

if you have a 3.55 ready to go in then throw it in and see how you like it. can do all the computer crap in the world and its still not gonna tell what it feels like..
 
Not a huge difference between the 3.55 and 4.11 at cruise 70 MPH

What is a reasonable RPM at cruise 65ish for a 340?

View attachment 1715454038
Who determines what is reasonable? Opinions are all over the map, just like asking if a cam is too big or too small. You mentioned it’s not a huge difference between a 3.55 and a 4.10, so I say go with the 4.10. Why do I say that? Because I run a 4.10 w/27” tires and run down the interstate every now and then on 30+ mile jaunts to work during rush hour, above the speed limit to keep up with traffic or in most cases just blowing by everyone for extended periods. These posts saying it gets old running low gears at higher speeds for any length of time just sound like old softies to me. Is it a hot rod or what?! Some of these guys should just get a loaded Chrysler Newport or similar if low rpm sailing is what one wants! LOL:p
 
Guys looking to decide over a 3.55 or 4.11 still. Have been reviewing my old threads as well as others. Ran some numbers and wanted bounce off conclusions from this and if I am missing anything. I used the Spicer calculator. Red dotted lines are approximated power band.

1. My 2.71 never makes it into the power band!
2. The 4.56 could hit my rev limiter just over 100MPH say if I was doing the 1/4 mile
3. Not a huge difference between the 3.55 and 4.11 at cruise 70 MPH

What is a reasonable RPM at cruise 65ish for a 340?
Am I missing anything? Does load or type of transmission alter the plots?

Thanks

View attachment 1715454038
I don't understand this pictorial, as to why the rpm changes at the top; what is your torque convertor doing?
The speed to rpm ratio should be a linear relationship, and can be written as xxxmph per 1000rpm. So then if your gears bring 2000rpm @50 mph, then 100mph should be 4000 rpm. It's a straight line.

A) I don't care about your combo. You say your 340 is lazy below 3000 and I believe it. There are a couple of cures for that; #1) is more cylinder pressure and #2) is a higher stall TC so your engine never has to pull in that soft zone.
As to #1) there are a couple of ways to achieve that. Obviously the first is a higher compression ratio, and the second is an earlier closing Intake valve to trap more pressure.
As to Ica, there are several ways to get an earlier closing Ica; one is to retime your current cam and 2) is a smaller cam, and 3) is a faster cam, and 4) is a different type of cam
Hold that thought.
B) The engine is already built, and it already has a detonation issue.
From this one can surmise that 1) the cylinder pressure may already be excessive, or 2) the timing curve may be too aggressive, or 3) you need to get a higher octane gas, or 4) the engine is too heavily loaded. Hold that thought
C) powerband; for a performance streeter, you are only concerned with first gear, the bottom of second gear and the 1-2 powerloss at the shift. The speed limit is ~65mph so if second gear takes you to 107 mph @5500 with 2.71s that is, as you already know, a crummy gear.
Here's what's happening to make your bottom end soft;
The low stall forces your engine to work from a too low point on the power curve, and the 2.71s keep it there for what seems forever. If the tires don't spin, it's a friggen long wait to where the power starts. I get it. I've been there. Hang on to that.
D) where do you want the power? You gotta make a choice, cuz without an overdrive, you just can't have it all. Furthermore, with an automatic you already have a gear handicap. So is it ; 1) off the line, or 2) at ~32mph, or 3) for passing at say 55mph? The set-ups are each different
E) cruise rpm. This can make or break your combo. And cruise rpm is a highly personal decision.
For you;
with a 340 that has a performance cam; forget about selecting a cruise rpm for economy, especially if the cylinder pressure is down.
the 340, in street trim, is torque-handicapped compared to a longer stroke engine. So it's gonna need more stall, and more starter gear, and this is a problem with the A904 because the low gear is just 2.45. aaaaand this drives the cruise rpm ever higher. So, you gotta make a decision; do you want a city bomber . Or do you need that combo to cruise long distance in comfort. If the latter then only you can define comfort. Some guys are OK at 3500. Some guys get cranky by 2500. I like 2200. This is where the highly personal comes in.
I have full length duals the really sing at 2800. That sounds great around town with a 4-speed. But I can't cruise at 2800. I cruised 4 years at 56=2600 and it drove me crazy. I finally sprung for an overdrive and now 65=2240, and I am a happy guy. But here's the thing, I live 20 miles from the nearest urban center, and I used to drive this car 35 minutes one way, every day to work and back. So that again points to the highly personal cruise rpm. If you only plan one roadtrip per summer, and it's just an hour away, then that is a different situation.
Also, if you don't have dual 3inchers, your exhaust may be quieter. NVH is the deal here; Noise,Vibration, and Harshness. It's not just rpm. Moving stuff vibrates. and shakes and oscillates. My Barracuda has a very annoying natural body oscillation at 55 mph. It doesn't matter what rpm or what gear I install, or how many times I balance the driveshaft or adjust the driveline angles... it just has a natural hiway rhythm at 55 mph. Just 3 mph either way and it sorta disappears.
F) ok lets pull it together.
#1) you gotta fix your detonation issue. Rear gear and stall may help you, but you gotta eliminate it.
#2) you gotta rate your cruise rpm as to importance. Every next decision depends on this
#3) you select the rear gear to get the cruise rpm
#4) this establishes your starter gear
#5) if the take-off is boring, you increase the stall-rpm,until it's not.
#6), if kick-down into low at 32mph is boring, you increase the stall, until it's not
#7) if KD at 55mph gets you a screaming 340 but no rapid transit, your rear gears are wrong;start over.
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on another note;
since you have 2.71 gears, you have a rolling dyno. A dyno measure torque over time and pukes out a convenient picture of horsepower. With 2.71 gears, your mph at 6000 will be close to 70mph in first gear. The only other piece of information you need is your on-the-start-line weight. Then you just measure the time required to accelerate between any two mph points, and you can assign a horsepower to that, which was required to do that. Ten you repeat as often as you want. Then plot the points on a graph and voila, there is your dyno-chart. It doesn't have to be dead-nuts accurate. What you want is the shape of the graph.and a rough idea of the power at various mphs, which you are gonna convert to rpm. Now when you go to select a stall-speed, you are gonna look on your chart to be able to compare the power at the current stall, to help select that higher stall TC, to achieve the take-off power with the new stall.

As to powerband; your transmission, in conjunction with the engine-combo, determines the powerband. The 904 ratios are 2.45-1.45-1.00
So if your engine combo peaks at 5500rpm, and the power plateau is 400rpm wide, and you then arbitrarily choose a 6000 rpm shift, take a breath, then at the 1-2 shift, your Rs will fall to 1.45/2.45x6000=3550. Ergo; your powerband has to be from 3550 to 6000rpm. That is a flippin' huge 2450rpm powerband.
In comparison, The regular 4-speed has ratios of 2.66-1.92-1.40-1.00. At the same 6000 shift,the rpm-drop is 1.92/2.66x6000=4330, which is just 1670rpm.
If you look on a typical dyno curve, you are gonna see that at 4330rpm the power could be easily 50Plus horsepower higher than at 3550.
So for a guy with a 904 and a 6000 shift,at the track; he's not concerned with any power numbers below 3550rpm.
But you, as a streeter, I'm gonna bet, that 95% of your 340's life is gonna be spent below 3550, so for you this rpm band is of prime importance. If you get this wrong you will not be a happy camper.

My opinion;
My hi-compression 367 like a minimum starter gear of 10/1 with a 4-speed. It likes 11/1 better. I have a clutch so I can rev the engine up to whatever I want , to find whatever power I need, to accelerate from zero mpg, at whatever mood I'm in. I like to dump the clutch and go. So I blip the throttle, and let the flywheel do the work, then accelerate with whatever throttle setting I need. So, I could be "stalling" at 1600 or 3000
But you, with a 904, don't have that luxury. However you do have something, as good or better. Your TC is a Torque Multiplier, as well as a fluid coupling. The internal TM in that TC varies with input and load and speed, so it's kindof an automatic trans all by itself. At zero mph, the torque differential thru it is the greatest, and may be as high as 1.8.. That is to say, for every 1.0 ftlb that goes into the TC 1.8 may come out. But this ratio rapidly diminishes as the car starts moving, and still more in each subsequent gear, and eventually might stabilize at somewhere between 1.08 and 1.05.
Ok so how do you make that work for you?
Well lets say my engine makes 140 ftlbs at 1800rpm. and it like the 11/1 starter gear, so at zero mph, I slip the clutch out. the rear axkes are gonna be seeing;
140x11=1540 ftlbs and away I go. 1800 rpm with this combo is 13mph, so by 13 mph my engine is putting down 1540ftlbs to the rear tires. There is no TC to multiply so that is all I got.
Lets say your engine also puts out 140@1800rpm. So at zero mph, with the same 11/1 starter gear,and an 1800TC, you would expect the same 1540ftlbs. But with the internal TC ratio of 1.8, this is increased to 2770.... for just an instant. Then as your car begins to move, the TC ratio begins ramp downwards, but by 13mph the ratio might still be 1.4 (I'm guessing), and your rpm may have increased to 2400 so at this time your engine may be putting out 180 ftlbs so now;
180x1.4x11=2770 ftlbs. From this point on, the TM in the TC is gonna continue to drop, and the engine torque is gonna continue to increase... upto maybe the high 3000s RPM, depending on your combo, where the torque will plateau for a bit, then begin to decrease.
What's the point? Well 2770 ftlbs is more than enough to break traction of any sized street tire you can fit in your stock tubs. Lets say your 340 peaks at 340 ftlbs and the TC gets down to 1.1TM at that rpm and speed, still in 11/1 starter gear, this maths out to
340x1.1x11=4110 ftlbs. This might be 28 mph, and obviously, at WOT, the tires are still spinning.......... ergo it follows that the 11/1 starter gear, for your combo, is overkill.
So what is 11/1 for you? ------ 11/2.45=4.49..

Ok so if 4.49 is overkill, then what is a good gear?
Well, the target has to include the internal TM of the TC. So if 1540 is enough for me to power away briskly then we can do the math backwards and get a real-world number for you. Suppose you currently have a 2400stall TC and your engine puts out 180ftlbs there; just suppose. Then 1540/(180x1.8x2.45)=1.94 rear gear...... to power away briskly.
But 1540 is not enough for me to break traction. I need about 2200. So
2200/(180x1.8x2.45)=2.77 now. But your TC is gonna lose TM as soon as the tires spin, because the torque differential is lost. So we have to swap out the 1.8 for something closser to1.4 say, and
2200/(180x1.4x2.45)=3.56s. Hmmmmmmm were have we seen this number before? lol. Sure everybody loves the 3.55s.... because with them, your 340 can bust the tires loose with the 1.8TM ratio, and as the rpm rises the TM in the TC is falling, but the torque output of the engine is rising, so the spin can be maintained, and your engine does not need to be at WOT to make it happen.
Bottom-line;
So while I need 4.10s to make this happen with a regular 4-speed, your 904 equipped engine might only need 3.55s. I say might, because IDK if your engine can make 180ftlbs at 2400rpm. I pulled those numbers out my arse.
But here's the trick; If your engine pulls 10% less, then you'll need either 10% more rear gear, or 'SOME" more stall, or some combination.
Opinion coming
I've had hi-compression 340s for most of my driving life since 1969. I know they are soft below 3000. This is particularly noticeable with a 4-speed. And that is why they usually got 3.55s or better. Whereas the auto cars usually got 3.23s and 2400 to 2600 TCs. When you stick a performance cam in 'em, the autos feel it right away. I don't have a clue how you ended up with 2.71s lol, but 3.55s will make a 3.55/2.71=31% improvement. But a hi-stall can easily surpass that. Consider the numbers I used 180/140=plus 28.6% more available engine torque. Even if the numbers are not dead-nuts accurate to your combo; the percentage should be close. If you add the two, which you can, then by swapping to both 3.55s AND a hi-stall, you could see a 60% improvement in footpounds to the tires. That will really wake up your otherwise lazy bottom end. Don't forget, the TC will make little to no difference once it has reached "coupling". After that, the TM is strictly in the gears. And finally with 3.55s and 27" tires 65=2870, which only you can decide if it is acceptable. My cure was to drive slower. And eventually, I went to an overdrive.

Oh, I gotta mention this;(more opinion)
My 367 combo is "fierce". First gear at WOT is just a tireburner. So I like to nail it at 30 to 35 mph, in second gear, to see what happens. my rpm here is just 2700. But it still lights up the BFG 295/50-15s
For you with 3.55s; 32 mph in second will be ~2150, so you'll want the KD to work, and jump it up to 3650 and POW! yur gone. 5500 will get you 48 with the 3.55s. Maybe your cam will get you 60@6800,lol, but I think it's better to shift earlier. Then 60mph in second will get you about 4000.
As you can see; this is no where near your power peak. Remember in the very beginning of this post, I said that the autos were a performance limitation for a streeter? This is why I said that. Your auto only goes thru the power peak once, on the way to 60mph, and on the street, the tires will be spinning most,if not all, of the way thru first gear, and you end up near peak torque, instead of peak power. This will kill your average power from zero to 60mph. And the ET will suffer, because of it. About the best cure for this is to bring the suspension up to speed to try and reduce your tire slip; spinning ain't winning..... Sometimes a softstart ain't a bad thing.........
Maybe you can imagine, that with 3.55s, if you could build a combo to make peak power at 4000, then you could go thru the power-peak twice on the way to 60mph. And if the average power was the same between this low-rpm combo, and your 340 combo....... you both might hit 60 at the same time. That would probably require more cubes tho.

Conversly; it might be better to gear for peak power at 60mph in first gear, and jump up the stall to get off the line better.
Suppose your engine falls off the cam at 6200, then you would need 3.23s to top out at 60=6200, and you could cruise at 65=2610 @zero-slip.
Off the line it would be a lil lazy..... unless you jumped up the stall. I like a 3500 for this. But remember this is a pretty big cam to peak at around 5900. That is a solid two sizes, maybe three, over the factory cam, say a 292*. Hyup, I had one of those in my 367 at 11.3Scr and with 3.55s it was ferocious at the top. But I got rid of it PDQ, on account of it was unacceptable on the bottom,for me, with a regular 4 speed.
I hope you don't have a 292 in a 10/1 iron headed 340,lol. 3.55s will never cut it.
How do you know? Well firstly tune the heck out of it with a properly synced up transfer slot. then crank the timing back to get about 750 rpm in neutral. Then measure the manifold vacuum. If it's down at 8 inches or less, and out the tailpipes it is singing a beautiful rumpiddy-rumpiddy rump, that's the one.
But I doubt you have this on account of your idle timing was 19* right? And you said the power-timing was around 31 IIRC, and delayed to prevent detonation; so the 292 I don't think would behave that way, even with 2.71 gears.
That timing curve is also killing your low-rpm power
 
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