transmission (mostly gear ratio) issue...

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razorback

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ok so im working on a project, i will be using a mopar 360 engine, not sure of year yet but not a magnum. anyway, i was going to use the 904, but after putting in my rear gear ration at 4.1 and tire size 25" i realized that my gas mileage will be horrible and i will have high rpms at cruising. what are my options automatic wise if i decide to go with auto? i would like it to hold at least some power, so i dont want something that is anemic and that will break easily. any way i can even change gears in the 904 to make it more..cruise speed friendly?

also, how hard would it be to use a viper 6spd on a 360? any other manual trans options?

i will post photos of my project when i am all done.
 
First off you will need to run at least 28" tires to begin with.A 727 tanny runs a 2.45 first gear,1.45 second gear.and a 1.0 third gear.What you need to do is find a 518 tranny and take advantage of the overdrive of .69.This will mean you can bring your 4.10 gears you are running now,to a 2.9 highway gear.So you have the best of both worlds.mrmopartech
 
as mentioned above is either taller tires or an automatic transmissions. there are aftermarket tailshaft additions that include a overdrive gear. made by gear vendors. pricey, but a good unit. even at 28.5" tall rear tires and 3.91s, highway driving sucks with no o/d
 
i entered info on this website, my rear will be 4.10 (for a while until i swap in solid rear), 24.5 tires, and the 4 gear ratios of the 518. that looks more like what i am looking for. now how would i mate the 518 to the 360 la? just like a 904 trans or do i need a special convertor?


RPM 1st Gear 2nd Gear 3rd Gear 4th Gear 5th Gear 6th Gear
500 4 6 9 13
600 4 7 11 15
700 5 9 12 18
800 6 10 14 21
900 7 11 16 23
1000 7 12 18 26
1100 8 13 20 28
1200 9 15 21 31
1300 9 16 23 33
1400 10 17 25 36
1500 11 18 27 39
1600 12 20 28 41
1700 12 21 30 44
1800 13 22 32 46
1900 14 23 34 49
2000 15 25 36 52
2100 15 26 37 54
2200 16 27 39 57
2300 17 28 41 59
2400 17 29 43 62
2500 18 31 44 64
2600 19 32 46 67
2700 20 33 48 70
2800 20 34 50 72
2900 21 36 52 75
3000 22 37 53 77
3100 22 38 55 80
3200 23 39 57 82
3300 24 40 59 85
3400 25 42 60 88
3500 25 43 62 90
3600 26 44 64 93
3700 27 45 66 95
3800 28 47 68 98
3900 28 48 69 100
4000 29 49 71 103
4100 30 50 73 106
4200 30 51 75 108
4300 31 53 76 111
4400 32 54 78 113
4500 33 55 80 116
4600 33 56 82 119
4700 34 58 84 121
4800 35 59 85 124
4900 36 60 87 126
5000 36 61 89 129
5100 37 63 91 131
5200 38 64 92 134
5300 38 65 94 137
5400 39 66 96 139
5500 40 67 98 142
5600 41 69 100 144
5700 41 70 101 147
5800 42 71 103 149
5900 43 72 105 152
 
You just need a 518 converter balanced for an old style 360 or use a B&M flexplate and neutral balanced converter. You'll also need to do some electrical wiring and install a switch to operate the overdrive and have a custom driveshaft made since the 518 is longer than either a 727 or 904. Make sure and get a 93-95 518 as the later ones (96 and up) were electronically controlled by the computer and the 91 and 92's had some issues. Any way you go about it the 518 isn't a drop in and go transmission.

BTW: What car is this going into? I ask because a 518 is a tight fit in a 67 and up A-body and won't fit at all in a 64-66 A-body unless you modify the trans. tunnel. You have to do some crossmember surgery in any car to make it work because the tailshaft of the 518 is quite porky compared to a 727.

The 904 you mentioned is a strong transmission when set up right. A properly rebuilt 904 will handle 400 hp no problem. It also has a lower first and second gear ratio that a 727 so you can run taller gears such as 3.55's or 3:23"s and highway cruising is decent since it still has a 1:1 3rd gear and you get take-off like lower gears.

This is just my .02 but the easy way to go is keep the 904 and have it rebuilt with Red Eagle clutch plates and Kolene steels and install a Trans-Go shift kit and install some 3:23 or 3:55 gears in the rear. The 518 weighs about 80 lbs. more than a 904 so you'd have a lighter car with the 904 also. It also requires less hp just to spin it. Not a huge diff there but that added to 80 lbs. lighter and allot less work and the fact you can't hardly touch a used 518 for less than $400 equals allot. You'll easily save enough money to buy a gear set by keeping the 904 since it will only cost a rebuild and shift kit.

BTW: in your last post you mentioned changing the rear to a "Solid" type. If you mean a spool I hope you don't intend on driving it on the street. That's dangerous since there is no slippage. I tried that when I was a dump 19 year old kid and ended up in the ditch soon after leaving my house and I want even getting on it hard.
 
i would like to keep the 904. it is compact enough and i will be looking to save weight, i dont like the extra 80lbs with the 518. i guess that will be a good reason to invest in some taller gears and put the solid rear in. and when i said put in a solid rear i have an IRS in there now, only 2 gear options, 3.9 and 4.10 it is going in an rx7.
 
razorback said:
i would like to keep the 904. it is compact enough and i will be looking to save weight, i dont like the extra 80lbs with the 518. i guess that will be a good reason to invest in some taller gears and put the solid rear in. and when i said put in a solid rear i have an IRS in there now, only 2 gear options, 3.9 and 4.10 it is going in an rx7.
Click to expand...

OK gotcha on the rear suspension. If this is going into an RX7 you can forget about getting a 518 in there unless you get happy with the torch.
 
yeah the 904 is kind of a tight fit now. its the linkage that is is sticking out the most. ive also thought of a manual trans, that would probably give me the most room/desireable gear ratios (most rx7 guys use the ls1 with t56)
 
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