Rear end gearing tech question

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Zam1787

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I have a 1973 duster. I am installing a 360 that has a lunati voodoo cam, edebrock aluminum heads, Harland roller rockers, full electronic mad ignition system, hooker super comps and a procharger running 6-8lb boost. Have a new 904 transmission that has been built to
Hopefully withstand the hp I will be producing. This car will not see the drag strip, just a fun street car. I am concerned with the supercharger and motor work done, about what gear to run. I will be purchasing a locker kit, and was thinking 3.73 gear.. i want something that has good go but still keeps me modest enough to cruise highway.. if I'm off please tell me as any criticism is helpful. Like I said this is not a drag car, just a fun driver.. im also running fitech fuel injection.
 
Think about optimizing your setup based on torque curve. If you are going to dyno the motor, see where the torque really starts to climb and plug in your tire size and different gear ratios until you find the one that puts you at cruise(70mph) just into your particular torque curve. Example: If your combo don't start making power until 2500, don't be cruising at 1800. If you are not going to dyno, then at least you can use the cam manufactures rpm spread. Wallace racing .com has easy to use calculaters to help figure.
 
Dana said:
Think about optimizing your setup based on torque curve. If you are going to dyno the motor, see where the torque really starts to climb and plug in your tire size and different gear ratios until you find the one that puts you at cruise(70mph) just into your particular torque curve. Example: If your combo don't start making power until 2500, don't be cruising at 1800. If you are not going to dyno, then at least you can use the cam manufactures rpm spread. Wallace racing .com has easy to use calculaters to help figure.
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I've seen people recommend this before I don't get the reason why. Could you explain?

Especially since the dyno measure WOT power (torque) and on the highway you'd be part throttle.

My 380 hp create engine which probably peak torques around 4000 rpm. Will cruise fine at any speed as long I'm over 2000 rpm far from peak.

As for the OP you left out the main spec your cam size that will tell us where your powerband is.

My car unfortunately has 2.96 gears right now but are liveable and if in the right gear at the right speed can be fun but a set of 3.91 will make it fun in any gear or speed.

As for your car especially supercharged you pretty much could run any gear from 3.23-4.30. So 3.73 is a good choice most will probably say 3.55 for highway driving but it's up to you where to draw the line.
 
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Sorry I should have included cam info
Lunati voodoo cam
Advertised Duration (Int/Exh): 268/276
Duration @ .050 (Int/Exh): 226/234
Gross Valve Lift (Int/Exh): .494/.513
LSA/ICL: 110/106
Valve Lash (Int/Exh): Hyd/Hyd
RPM Range: 1800-6200
Running a 2400 stall converter

I appreciate you guys all contributing, this is my first time playing around with boost and didn't really think about how it may affect the gearing I may need (until I ripped it all apart of course haha)
 
All I'm saying is you want the BOTTOM of you power band to be right under your foot at cruising speed. A delay to wind up feels sluggish. And a 25" tire a 3.91 hear @ 70 mph sucks. I don't know a lot about superchargers, but I don't think you'll be able to pull that motor that low to see the bottom of the torque curve on the dyno.
 
Without an overdrive transmission you will grow weary of that 3.73. Quickly.
 
Obviously with boost I need a little rpm to keep the throttle response good.. I'll try to figure out the calculations for 3.55 and 3.73.. see which one will give me
A good 2k highway curve
 
Zam1787 said:
Sorry I should have included cam info
Lunati voodoo cam
Advertised Duration (Int/Exh): 268/276
Duration @ .050 (Int/Exh): 226/234
Gross Valve Lift (Int/Exh): .494/.513
LSA/ICL: 110/106
Valve Lash (Int/Exh): Hyd/Hyd
RPM Range: 1800-6200
Running a 2400 stall converter

I appreciate you guys all contributing, this is my first time playing around with boost and didn't really think about how it may affect the gearing I may need (until I ripped it all apart of course haha)
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With boost think of your engine more as a 500 cid motor than a 360 one, since your cramming that much air in there. Your peak hp and torque will be fairly low for the power you are making. I just find out the rpm of 3.73 at highway speed is if too high go 3.55 if your willing to turn more go 3.91 and if just right use them.
 
Dyno testing the engine does not just tell you what the maximum HP/Torque is at WOT. It is a graph of what those readings are at each rpm from starting to finishing RPM . What ever the max that the engine is capable of. That is very important in that you would generally want the cruising RPM's to be close to the best Torque/horsepower rpm to take advantage of that power. You can alter the rpms with trans gear ratios, manual trans gear ratios, tire diameter, Torque convertor stall speeds etc.. Or any combination of those variables.
 
Dana said:
All I'm saying is you want the BOTTOM of you power band to be right under your foot at cruising speed. A delay to wind up feels sluggish. And a 25" tire a 3.91 hear @ 70 mph sucks. I don't know a lot about superchargers, but I don't think you'll be able to pull that motor that low to see the bottom of the torque curve on the dyno.
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But peak torque isn't the bottom of the powerband, depending on the build torque is fairly flat. Theres a good 2500-3000 usable rpms under peak torque and 500 or so over peak hp making your engines powerband 4000-5000 usable rpms. Really the engine is at peak power at peak hp and the rpms above peak torque is stronger (more power) than at or below peak torque.
So as long your engine don't lug at speed its fine, which for me is anything over 2000 rpm and the OP probably lot lower even though with his gear choice will never know.
 
How tall are your tires and how fast do you want to run on the highway?

I run the Lunati cam one step up from that in my Duster with an NA 340. Ported 308 iron heads flowing 264cfm, 9.8:1 compression, dougs headers, eddy air gap intake, 750 double pumper and a 4 speed. Cam is
  • Advertised Duration (Int/Exh): 276/284
  • Duration @ .050 (Int/Exh): 234/242
  • Gross Valve Lift (Int/Exh): .513/.533
  • LSA/ICL: 110/106
  • Valve Lash (Int/Exh): Hyd/Hyd
  • RPM Range: 2200-6400
I run 3.55's and 26" tall tires. The cam wants more gear, Lunati recommends 3.73's for the cam I've got, but with 26" tall tires and the amount of time I spend on the freeway the 3.55's are about as low as I want to go. I can do ~75 mph at 3,500 rpm, so cruising on the freeway is usually between 3k and 3,500. That's about as wound up as I like to be for long periods of time.

Now, if you're going to run taller tires, or aren't worried about being able to do 70-75 mph for long periods of time, the 3.73's should be fine. Where I'm at in California if you're doing less than 65mph on the freeway you're a speed bump, and traffic usually moves closer to 70-75. With a daily driver like mine I'd rather have the cruising speed be where I need it and work out the first gear take off with the clutch. Either way, your engine should be just fine with the 3.73's unless you're rear tires are real tall.
 
Dana said:
OK. You're right I'm wrong.
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My bad, I reread your OP I thought you were saying peak torque which i've seen others recommend
which would be over 4000 rpm for me, which i've never understood, why I asked. But I now see we're actually said the same thing.
 
Gear it to run out of Rs at 65 in second gear; you will be fast like lightning. This will be 5.56 gears.

Or gear it to top out at 60 in first. This will be 3.23s. These cruise at about 2740

Or gear for about 32=4000. This will be 4.30s in first. These cruise at about 3650

Or gear for 2250@65 in hi, or 2.94s

As you can see, for a streeter and a 904 there is no perfect gear to hit all the bases.
Just slap in some 3.23s and get rid of the 2400. To get 4.30 performance, with 3.23s, all you gotta do is pump up the torque to get moving. How much? 4.30/3.23= plus 33% Ima guessing that's about 50ftlbs, so a 3200 to 3000 should do it.
But if you already have the 2400 installed, well I would never tell you to rip it out without at least trying it, especially with 3.73s or 3.55s, lol. And the reason is this; 3.73s are up 15.5% in TM, while 3.55s are up 9.9%, both over 3.23s So that makes the torque deficit a little more bearable.Plus that cam is pretty torquey as is. I ran one like that and loved it. It made huge bottom-end torque at 10.9 Scr.It was a 295-burner with just a 9.44 starter gear.(3.55 x 2.66)
You will need the biggest tires you can fit in the back.
Here's something like mine (which I rev to 7000 all the time); You'll have a wee bit more on the bottom,there; and a lot more on the top, and peaking a little earlier,probably. If you plan to regularly visit the 6400 plus region, you need to do some oiling mods.
This curve was with an HE3037AL 230/237/110cam, NA of course.Here's the link.

#11---360 Dyno Test (Feb 2000)
upload_2017-8-2_0-57-5.jpeg
 
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RustyRatRod said:
Without an overdrive transmission you will grow weary of that 3.73. Quickly.
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this; but it`s a good all around gear, I`m on my second hotrod w/ them. Yo "will" want tall tires on it for the hi-way. I run 29.5 tall drag radials on the currant one, had 30" tall tires on the previous hotrod. if u cant get that tall a tire in ur ride, I`d go w/ 3;55`s.
 
3.55 if you are running tires around 26". With bigger tires, adjust accordingly (3.73 would be O.K. with 27"-28" tire).
 
Depends on your tastes. I want the engine's cruise rpm at 55mph to be at or above the stall speed of the convertor if it's a street car. With OD and a 2400rpm stall you'll need more gear I think. If it were me I'd consider dropping the stall speed. IMO you don't need that much with a 360 in an often-driven street A body with a blower. A lot of guy with N/A might need it, but you won't. And crusing in OD below the stall speed will create a bit of heat in the trans.
 
moper said:
Depends on your tastes. I want the engine's cruise rpm at 55mph to be at or above the stall speed of the convertor if it's a street car. With OD and a 2400rpm stall you'll need more gear I think. If it were me I'd consider dropping the stall speed. IMO you don't need that much with a 360 in an often-driven street A body with a blower. A lot of guy with N/A might need it, but you won't. And crusing in OD below the stall speed will create a bit of heat in the trans.
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As you say "depends on your tastes". For some , anything looser than a stock converter feels mushy. Others don't mind cruising around with their 4800+ stall 8 inch. Personal experience has shown me that having a stall speed well above cruising rpm is not a problem. My 73 RR with 2.76 gears has a 175k MP converter that flashes to about 2800. My 68 Satellite with 3.23 gears has a 9.5 inch Dynamic converter that flashes to about 4500. Both cars are drivers (I don't own a modern car) and their transmissions haven't been out in over a dozen years. With a good cooler, heat is not a problem. Even some stock vehicles will have a converter with a stall speed a bit above what the cruise rpm at 55 mph is. My 318 duster had a 904 with a stock converter that flashed a bit over 2000. With the 2.45 rear gears , 55 mph was about 1700 rpm. I've known many people over the years that bought entry level 11 inch "street type" converters because the magazine articles of the eighties had them convinced that they would burn up their transmissions if their stall speed was over 2500. When they finally switch to a quality 9.5 or 10 converter they were amazed at the performance difference. And let's not forget all the guys that drive 1000 miles on Drag Week every year with those high stall converters coupled to Gear Venders overdrives. And I don't think the OP mentioned OD anyway. I'm not that knowledgeable about turbos, but I believe that more stall speed helps build boost. And I don't think they need much gear. There is a member on Moparts named Dizuster that has a badazz early B body with a turboed 360. I believe he runs 3.23 gears. For those that are interested, go to Moparts and search out his posts. they are very informative.
 
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