Help me pick a gear for my combo

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Dusterdude72

Dusterdude72

IN MOPAR MUSCLE MAGAZINE
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Hey guys, I got a pumped up small block in my 72 duster, 904 auto trans, tci 2500 stall converter,b&m pro ratchet shifter,trans go tf2 shift kit, 28" tall rear tires, 8 3/4 rear with 742 center section and a clutch sure grip.4.56 gears

Here is the thing, Ive been restoring the car over the last 4 years almost and have only got to drive it a hand full of times. But im close to having it ready for the road so I would like to figure this out soon and get a clear idea of what I should be doing.

The Duster is a BEAST of the line...super fun. only problem is.....from what I can tell, its going to be clinging pretty high up in the rpm's at back road cruising speed of 45mph-55mph (not to mention what kind of rpm's it would be running on the highway if I were to ever decide to take it on the highway that is.).

so I am looking for some input from people who have basically the same driving needs as I do and what setup they have similar to mine and how it performs?.

Im mostly going to be doing around town cruising , and lots of back road crusing (45-55 mph). Id like to be able to occasionally get on the highway if need be and without having to listen to my engine screaming towards red line lol. I also would like to be able to take it to the 1/8th mile or 1/4 mile strip at some point. not to mention with the 4.56's running high r's so often at higher speeds I am sure it will ruin what gas mileage I might be able to pull out of it lol.

so what would be a good all around gear for me for my setup...keeping in mind it will mostly be a around town cruiser that wont see alot of highway driving and that may see an occasional trip to the track. and I still want that seat of the pants performance feel.

3.55?, 3.73?, 3.91?

all info is appreciated ,thanks in advance guys.
 
i guess 3.55s .but i would leave the 456s just put those gears in mine and it's a rocket!
 
I agree, from what little I have got to drive it.....the thing takes off like a rocket and pulls really hard. but it sounds like at higher speeds I am going to be running really high in the rpm's and I dont want to put my engine through constant high rpm's or for it to eat through gas like a pig. but at the same time I dont want to lose that seat of the pants performance. so I am hoping maybe someone can recommend something that they think would be good with my combo and that would be a happy medium.
 
a lot of it depends on your engine combo. will a 3.55 or 3.91 kill your combo?

some kind of OD would be the ideal thing.
 
I'd go with a 3.91, maybe even a 4.10. I agree with Joe for once thsat an overdrive would be ideal. Plus, your 2,500 rpm converter with that engine is going to be mismatch with lower numerical gears. Small blocks need gears to get going and that converter aint gonna help. Its probably covered up right now since your rpms climb pretty quick with the 4.56 but if you change to a lower numerical set, you will probably notice a difference.

I currently have a 4.30 gear in my car, 28" tire (275/60/15). RPM @ 55 mph in high gear is 3,000. Previous owner built it for the 1/8 mile. Switching to 4.10 gear sometime this week. I drove it on the highway once just to see what it was like, it sucked. It was lugging when I was on and off the gas and it was hotter than usual. Its not set up for cruising so I didn't care but it was slow lane all the way.

Use a gear ratio calculator to see what different ratios will do.

http://www.wallaceracing.com/calcrgr.php
 
likewise the previous owner of my duster had it set up for the 1/8th mile aswell.

it had 275/60/15 bfg drag radials on it when I got it and I liked the stance of it and how it filled out the wheel well so well I actually just got all new bfg radial T/a's put on it....so new tires is out of the question at the moment lol.

And I agree, if I could have 1 more gear it would be a perfect set up. But a 4 speed swap or an automatic with over drive swap is no where in the budget.

As for my engine....its pretty spunky and climbs through the RPM's pretty quickly on its own...im sure the gears help it scoot as quick as it does from a stop but I dont think down sizing a gear will hurt it TOO bad when it comes to it ability to get up an go.

its a 67 318 thats been punched out,stroked,dished pistons,blocks been decked,heads have been milled, the good breathing factory heads with a slight bit of work done to them,purple shaft,edelbrock performer intake,750 carb,long tube headers,true dual 2.5" exhaust all the way back to welded mufflers. accel ignition coil,electronic ignition conversion.....along with the other drivetrain goodies I previously mentioned. Its a quick reving small block and makes good power. I havent had it dynoed yet but my guess would be right around the 350hp mark....nothing to radical but spunky.

so far its looking like 3.91 is in the lead, which is kind of what I was leaning toward already....just looking for some help from the experts here that know more about this area than myself lol.

Thanks guys for the input so far!
 
Oh and here is a picture of the duster to give you guys an idea of my wheel/tire combo and stance
 
Overdrive of some sorts.........gear choice without one is a compromise......
 
i think 3.91 would be perfect for your combo. a little more stall would compliment it but you will still be happy with the combo as is.
 
Why not go with an overdrive transmission? Then you could keep your gear. The 200R4 would be a fantastic choice.
 
If you reversed your transmission and rear combo to something more prevalent of today's standards you would have exactly what you are looking for in both respects by using an overdrive transmission.

Today you are running a 3 speed with a 2:45 1st gear and a 4:56 rear

An example of what you are looking for is a 3:07 1st gear and a 3:23 to 3:55 rear which would launch like you do now and land into the 4th gear overdrive which would put little to no strain on your setup. Being able to cruise at 75 in 4th @ around 2000rpm

From where you are today, you'll be fumbling around with rear gear and tire changes for the rest of your days.

The 200 4R 1st gear is a 2:66 and the 700R4 is a 3:07 1st. gear. Both built correctly will last a very long time.
 
BTW, one of my vehicles runs a T56 with a 2:66 first gear a 3:73 rear and launches like a rocket. Its' 5th gear is .74 overdrive and 6th is .50 overdrive. It cruises at 2000 rpm in 6th. at around 75 mph and it MPG has dramatically increased..
 
3.91's with that tire for the street. My tires are 28" tall with 3.91 gears. I stroll down the freeway about 65MPH at 3300RPM. This is a 340 that has a 484 Purple cam and A904 Trans.
 
Problem is, I've already dunked enough cash in the car over the years and there just isn't that kind of cash laying around for trams swaps. Not to mention the horror stories I've seen of installing newer over drive transmission, adapter plates,trams tunnel modifications,custom drive shafts...etc...etc.... If anything I would probably convert to a 4 speed stick before dealing with those headaches.

Maybe one day I will look into the "ideal" set up, but for now I'd just like a financially reasonable happy medium solution in picking a gear that won't give it a doggy bottom end but will also not be pushing the redline at 55 mph or forcing me to wear ear plugs haha
 
rmchrgr said:
Small blocks need gears to get going and that converter aint gonna help. Its probably covered up right now since your rpms climb pretty quick with the 4.56 but if you change to a lower numerical set, you will probably notice a difference.
Click to expand...


So small blocks need more gear huh......

help me understand this. so if I have 340 putting out 450hp/400tq, it will need more gear than a small big block putting out the same numbers?

In this case, Size(cid) doesn't matter. It's the power curve and amount of power an engine has to offer. I see built small blocks with small gears pull as hard off the line as near stock big blocks running fairly tall gears.

It all has to do with your combo. Everything needs to work in harmony to give you what you want. Engine combo, cam size, auto or stick, converter stall speed, trans gear ratio's, and final drive ratio all need to be considered. remember with the limitation of only having 4 gears it will be hard to get good highway gas mileage/ low rpm's. This is why i'm adapting a 5 speed trans in my 71 Scamp. It will drop my overall RPM's down by 800 at 70 MPH to around 2200rpm's. Plus the first gear is taller than the factory 4 speed making it possible to run a smaller final drive while giving the same effect of having a tall final drive gear. Correct me if wrong, but this would also strengthen the rear gear because taller gears have smaller teeth that are easier to sheer off.
 
DusterDude, I been through that with a 9 second street car and vowed never to let it happen again. After that I had a 4 speed in my 340 65 Barracuda and went with a Doug Nash 5 speed ( No Overdrive ) yet a 3:27 first gear and ran a 2:74 sure grip and it was awesome to say the least.
 
Money aside, the tunnel mod I did for the 42RH a-500 wasn't that bad, it's a weekend worth of work. Don't let that scare you away. It also opens up the possibility for a wide range of transmissions to fit as well.

Back to the question 3.55, should serve you well for street/strip duty.

Here is a handy dandy calculator to help you decide, it even has a tic-toc-tac:

http://vexer.com/automotive-tools/speed-rpm-calculator

Happy trails, :burnout:
 
71Scamper said:
So small blocks need more gear huh......

help me understand this. so if I have 340 putting out 450hp/400tq, it will need more gear than a small big block putting out the same numbers?
Click to expand...

OK, I'll bite.

Depending on your performance goal, yes.

To use your example, a 450 hp 340 (3.31 stroke) generally won't start making big power (overcoming inertia) until the engine reaches a higher rpm. 450 hp from a 340 is pretty stout. That's going to take a pretty sizable cam that probably has an operating range above 3,000 rpm. What happens below that? You need torque multiplication, i.e. gears.

The amount of torque big blocks can produce at a lower rpm is generally going to be greater than what the small block can produce at the same rpm. That's engine architecture and essentially volume, nothing else. If you stroke a 360 to 400+ inches, then you start reaping the benefits of more torque at a lower rpm. That's not to say that big blocks won't rpm or small blocks have no torque but that's the general idea.

The torque multiplication that the steeper gears create helps to overcome inertia more efficiently, there's less parasitic driveline loss trying to get the car moving. The O.P. has a street machine 318 based engine which still has a small bore. Needs gears.

So yeah, I guess all the NHRA 340 stockers in the 10s that use 5.38 gears are doing it wrong. Those cars are not streetable because of the combo. High stall converters, low gears in the trans and super steep gears to take advantage of whatever torque multiplication they can harness. Those combos are proven to work.
 
Dude, my first thought was the 3.91's. I know you don't want to do the trans swap thing at this point and i admit i just skimmed thru the previous posts, but you said it was stroked.......meaning 4"? If that's the case, I think you can back it down a hair more. 3.73's if your concerned about performance, and even 3.55's if you really want to drive it a lot. That 4" arm will really cover any low end softness.
 
[I know a couple of Nhra 340 stockers..with 4.86 gears in the rear....but he uses a 2.89 first gear 904...and other uses a 2.66 first gear 904.
 
Hey sorry for not chiming in sooner but Fabo didnt notify me of there being any recent posts to the thread for some reason.

Like I said before....I am aware that there is a far more ideal set up....but I cant swing the expense of something as steep as a trans swap/conversion (atleast not for awhile) and the duster is not going to be the daily driver...just a weekend cruiser.

mostly looking for a good gear that I wont have to sacrifice alot of bottom end power and the "pull" through the rpm's but also without my engine running high R's at higher speeds.

so far from what I have been reading it sounds like maybe the 3.91 would be a good "happy medium" gear to use.

still going to wait to see what everyone has to say though before I go making any final decisions.

And thanks for the input so far guys, alot of knowledgeable answers in here so far by the sounds of it!
 
check out wallace racing calculators. Have you punched in your info to see what kind of RPM drop you will get with each ratio? That would be what I would do first chances are the 3.91 wont give you as much of a drop as you are after. EXP: plugging your info in assuming you are around 3500 RPM at 55mph 3.91 will only drop that maybe 500rpm.
 
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