What rear end gears for 904?

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1ToughDuster

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I'm at odds with what rear end gear to run in my Duster. The engine is a 360 with a 268XE cam with aluminum eddy RPM heads, aluminum RPM intake, 750 Holley Double Pumper, and produces about 400hp on the motor. Rear end is a built 8 1/4. A 100 shot of NOS is also in the plan.

The trans is a 904 with a 2400 stall and a shift kit. Tire size is 28" tall (275/60/15).

The car is primarily designed to be a street brawler with occasional interstate use and an occasionally a trip to the strip.

I'm curious how a 3.55 gear would perform (i.e launch, cruise, and mid range pickup) versus a 3.73. Which gear will perform better with the NOS? I don't care about gas mileage I'm all about the fun factor and butt kicking factor on the street.
 
4:11 or 4:56 would make that combination a lot of fun. 3 series gears give the car a lazy feel.
 
3.73 if you've got much interstate driving planned, but 3.92 or 4.10 would be much better for the street brawler feel. I myself would go for 3.73 and 2.74 1st and 1.55 second gears, especially with the 268 XE grind.
 
Have either of you ran nitrous on 3.73, 3.91,or 4.11 geared cars? From what I've researched, higher gears (in this case 3.55) do better with nitrous, than really low gears.
 
Have either of you ran nitrous on 3.73, 3.91,or 4.11 geared cars? From what I've researched, higher gears (in this case 3.55) do better with nitrous, than really low gears.



See post 4. That's number 1 to get correct.

Second, you don't make a bunch of power and you ain't hitting it with a big shot of nitrous. So gear and converter should be chosen accordingly.

Also, you didn't chose a cam that is ground for nitrous. You'll make more power on the nitrous (and less power N/A) and have a much wider tune up window if you buy a cam for nitrous.
 
YR beat me to it, but post number four indeed. To me, really low gears start at 4.56...But the low gear set (and I highly recommend a clutch pack upgrade) in the transmission is definitely a plus for the shot of nitrous. I would be looking at a999 upgrades and a converter (preferably custom) with anti-ballooning plates before worrying about gears. Everyone I know that starts running nitrous gets the boost bug and starts adding more heat to the chili eventually.
 
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2400 is a stock 340 converter stall speed. Go 3600 and 4.30 gears and have fun.
 
The engine and trans are already built and it's unlikely that I'll be going back inside of either one of them unless something breaks.

As far as the engine, the cam manufacturers recommend specific gear ratios from 3.55 up through 3.91 if I remember correctly.

The nitrous is being added for fun factor. So far I'm hearing 3.91 and 4.10, but I'm not sure that I'll be happy buzzing down the interstate at 70mph with 3.91s or 4.10s.
 
So far I'm hearing 3.91 and 4.10, but I'm not sure that I'll be happy buzzing down the interstate at 70mph with 3.91s or 4.10s.
You kindof gave cruising up, when the 400hp cam went in. I bought an overdrive to solve that. You may get away with a trailer; mine is/was a DD
 
You kind of gave cruising up, when the 400hp cam went in. I bought an overdrive to solve that. You may get away with a trailer; mine is/was a DD

BOOM! Bang on! Mike drop!
AJ with the big mallet hitting the nail on the head.
Gotta love this.

Everyone is suggesting a higher than normal gear based on two factors. Your statement of you don’t care about mileage, just the fun factor & you too low of a stall converter rating. The extra gear will help a lot with a stall too low & add a good amount of zip on take off.

AJ’s OD suggestion may be a bit pricey and such since you all ready have your trans done but the advice makes sense. Do the basic math.

OD Ratio multipled by rear end ratio =‘s final drive ratio.
4.30 X .69 = 2.96
4.56 X .69 = 3.14
4.88 X .69 = 3.36
See where there going....
It’s pricey but the best of both worlds is now coming to you really well. Re sell the current 904 & converter when your done for a recoups of funds.
 
Also u should have a converter that’s built for nos. It has anti ballooning plates. When the converter balloons it will take out the crank thrust bearing in the motor. Kim
 
3.55 is good all around gear for highway cruising and "decent" street pick-up.
My DD has a 3.21 with an A999 transmission, most likely with different 1st, 2nd gear ratios than yours. (2.74, 1.54 vs. 2.45, 1.45).
My rear tires are only 26 inches (245/60R15) diameter. I can squeak the tires off the line and she does have decent off the line acceleration, but she is not a 1/4 mile blaster.
I can do 70-72 mph and be right at or just below 3000 rpm. The A999 has been modified with non-lockup mod & TA Cheetah Valve body.
I do a few trips from UT to CO and back each year (1,000 mile round trip) with no issues. 15.5-17+ mpg (depending on how bad the WY cross winds are).
That said...
I'm currently rebuilding an A500 out of a 92 2wd Dakota, which is a project because it involves some fabrication mod of the transmission crossmember and tunnel area.
Once done the rear gears are going to change. likely to either 3.90 or 4.11.
My 360 is no slouch either but it was a budget build with Ebay specials and hand me down used parts. (
1990 Roller cam with low miles.
Lunati Voodoo: 270/279 dur. gross valve lift .515/.530 (Going 1 step bigger soon)
Speed-Pro flat topped pistons .020 below deck.
Weiand action+ intake (Going Eddy Air-gap soon)
Ported 340 2.02 valve J-heads for now, (not the best but hey they were cheap and already ported).
Street Demon 750.
MSD 6A CDI system.
Hooker Comp Headers (free, was in the trunk when I bought her)
 
Okay, I see why everyone is recommending the lower gears. Thanks for everyone's input.
I like my 2.94's on the highway... Gear it as much as you can live with. If your car see's a lot of 70 mph road trips for lengths at time, I doubt you'll like ANY gear in the 4's without O.D.
  • but, the OP hasn't been back to FABO since Dec 26, 2019
 
Gotta make up your mind what you want to do. In post #1 you said you were after "fun factor" and in post #10 you said you may not be happy with 3.91-4.10 down the interstate at 70. I can guarantee you won't be happy with low gears down the interstate but with that low stall converter you have you probably not going to be happy with 3.55's for butt kicking fun factor either. Changing the converter to 3500 stall may make as much difference as the 100 HP shot of nitrous and as Kim pointed out, unless the converter was made specifically to take NOS it'll probably balloon and take out the mains. I had a 360 built just about like yours and can tell you from experience that you need at least a 3:73 gears or it won't be that quick out of the hole. If you swap to a 4000 stall you could probably get away with 3:55 gears and it won't be a monster at 70 mph
 
I can't comment on the ballooning converter but with nitrous you have torque in a bottle and do not see a lot of benefits to running a high gear ratio unless the car is being built specifically to race. I think at 3:55 or 3:54 and with your camshaft specs would be a sweet setup
 
Well, I'm going with 3.92 gears. I pulled a 3.92 sure grip rear end out of an '01 Durango 5.9 R/T from Pull a Part for $128. It's a 29 spline 8.25 that's not leaking and looks pretty clean. Sure, I'll have to have it narrowed, but after an LPW cover and extra strength alloy axles, it should serve my needs just fine.
 
With 3.92s ; 70=~3300 rpm@ zero-slip with 28s,
which are about the tallest tires you can put in the factory tubs. But you won't be running zero-slip at 70. You can expect 165 to 330 additional rpm. All going into the trans oil, so you probably should get yourself a trans oil cooler.

with the more typical 25.5 tires the rpm could be as high as 4000.

I ran 3.91s for a while,a long time ago, and was actually happy when they blew up.
 
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With 3.92s and 27" tires;
and with an A904;
3.92x 2.45x 1.1 in the TC = a starter gear of 10.56; and
70=3550@4% slip

But for a streeter, a starter gear of 1056 with an automatic is overkill, because at zero mph, the TC ratio can easily climb to 1.8, so at zero mph, the instantaneous starter ratio is actually;
3.92 x2.45x1.8= 17.29, totally overkill. At this point, you are just giving away cruise rpm for tire smoke.
So you will never be able to use Nos in first gear.
With 3.92s and that 268* cam, top of first is ~48 mph. On the shift, the Rs fall to 3400, which is already nearing max torque, say 380 ftlbs x1.45x3.92x1.2 in the TC =2590 ftlbs, by itself still enough to keep the tires spinning, so kindof a waste to hit the Nos just yet; and the torque is still climbing. By 60 mph, the Rs are back up to 4100 just about peak torque, say 410 ftlbs, so; 410x1.45x3.92x1.2=2563; the tires may have stopped spinning, can't say for sure.
In any case, now you are getting close to hitting the Nos button........ at 60 mph. And when you do, you can expect about 100ftlbs to kick in so;
505x1.45x3.92x1.1=3160 ftlbs out the back, and the tires are probably gonna light up again...... and top of second is coming at 78 mph.
So what's my point?
Well, to my way of thinking, what's the point of Nos on the street, if the motor by itself, can spin the tires to the speed limit? You got too much motor, or too much gear,or I estimated not sticky enough tires; to use Nos below ~60 mph. Gears are easier and cheaper to change.
Or
Just save the Nos for the track.
I guess time will tell.......
 
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I'll see how the car runs with the 3.92s before I go for nitrous. If it doesn't run right for me I'll go with a 3.55. Pull a Part here is full of 3.55 rear ends under Dakotas and Jeep Libertys.

I may not need nitrous after all.
 
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With 3.92s ; 70=~3300 rpm@ zero-slip with 28s,
which are about the tallest tires you can put in the factory tubs. But you won't be running zero-slip at 70. You can expect 165 to 330 additional rpm. All going into the trans oil, so you probably should get yourself a trans oil cooler.

with the more typical 25.5 tires the rpm could be as high as 4000.

I ran 3.91s for a while,a long time ago, and was actually happy when they blew up.

A trans cooler was put on the car years ago when I built it. The car is built to run.
 
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