Rear end question

-
KJoeZ61

KJoeZ61

Well-Known Member
Joined
Apr 4, 2017
Messages
422
Reaction score
147
Location
Northampton Pennsylvania
Can anyone tell me what rear and gears should have come with on a 71 Scamp born with a 318 and 904 trans? The guy I bought it from a year ago said it was a 8-3/4 with 3:23 gears. The engine was changed and actually is a 360 which was supposedly "fresh", not true since I just had it rebuilt. Also I'm questioning the 3000 RPM stall converter, I probably should have changed the converter when the engine was pulled.

The bottom line is the trans is leaking pretty bad from several areas and has been to two shops Doesn't have the correct dip stick, so I'm not sure about the fluid Level. The trans is really the only real problem left short of whats going on with the rear. Stupid question but if I do change the trans going to a 727 probably wouldn't make much sense?

IMG_0108 (002).jpg


IMG_0943 (002).jpg


IMG_0051 (002).jpg


IMG_0934 (002).jpg
 
My slant six had 276 gears, so I imagine the 318 had something similar or lower. It probably had a 904 transmission too. I would Google ''1971 Plymouth Scamp 318 cu in, 904 trans, which rear end gears.'' or even ''1971 Scamp gears'' or something to that effect.
 
Last edited:
It depends on what was specified.

Do you have an original broadcast sheet for it?

If so, what are the D codes in line 2?
 
Picture of rear end from driveshaft side and back would help identify what housing. If there’s a cover on back held in place with bolts it is not 8 3/4. Depending on if sure grip etc there are ways to come close on ratio.
 
There’s a good chance it’s an 8 3/4, but for gears it could have almost anything. Nothing between 2.76 and 3.55 from the factory would be all that surprising.

But remember we’re talking about a 47 year old car. It could have anything in there at this point. Especially with an 8 3/4, it’s easy enough to just swap out the whole 3rd member.

86E84E13-B9FB-4216-8346-CF4D55F8DCEC.jpeg
 
Last edited:
Definitely an 8 3/4. Can’t make out the numbers in the second picture though for the ID on the 3rd member. Doesn’t look like it has a gear ratio tag either.
 
I'm going to do a spin test on it and see what that shows. For just regular driving what would be the best gear ratio? And I think by next year I'll change out the trans. I can get a rebuilt one from CRT shipped for around $1,100.00
 
8 3/4. There is a number on the drop out section. 741, 742 or 489 which basically defines the size of the pinion shaft. Listed Least to most desirable.
 
with both wheels off the ground, If when you rotate one wheel forward and the other rotates backward = Open diff. If they both rotate forward then Sure-grip or limited slip. If sure grip / limited slip - with both wheels off the ground and trans in neutral rotate one tire 360 deg and count the number of times the drive shaft rotates. If the shaft rotates ~2 3/4 times then 2.76, if ~ 3 then 2.93 if ~3 1/4 then 3.23 etc.

If open diff leave one wheel on the ground and do the same as above.
 
KJoeZ61 said:
I'ts an open diff. Is this even possible? One wheel turn to 1-1/4 shaft rotations.
Click to expand...

If it’s an open diff and both wheels are off the ground you get half as many rotations as the gear ratio. So, you’d double that to get ~2.5. That probably means what you have is a 2.76, which would be pretty standard for a 318/904 no frills car.
 
That sucks and now I see it says 741! So a 3:23 would be a big improvement? And it would be okay with the 360/904?
 
Last edited:
3.23 is a great driving gear. Peppy and freeway sane. Id say 3.23 is about the middle of the spectrum between 2.76 high way gears and 3.91 street gears not counting the drag 4.xx gears. Stick with the 9xx: light car, no engine mods and no slicks. Check dipstick O-ring, speedo gear O-ringS and the sector shaft seal.
 
The engine does have some modifications. It was rebuilt for a four barrel, has a quickfuel HR-650 and duel exhaust with 340 manifolds and bored 30 over.
 
The 904 will be plenty strong for your application, the 3k stall, if it’s even in there, will not. With no major mods done to the motor, a stock converter would not be out of the question, or something in the 1800-2200 range.
 
It sounds like the OP doesn't know for sure what is in the engine. If the cam is a little hotter than stock, then the 3000 stall converter might be a good match.
 
get the correct dipstick tube....search ebay......figure out where the leaks are coming from...pan...shifter linkage seal???
 
So going back to my original question. Would it be an improvement to go with a stock converter and maybe 3:55 gears? The engine is very healthy and just rebuilt, it’s just not delivering it to the ground. Look I’m not trying eat up tires, but I can barely spin them! Actually only one because it’s an open rear and I only have 14” tires on it.
 
Do you have a tach in the car? Go out and hammer it and see what the converter flashes to. Stock converters are for stock engines. Once an engine is modified to move the powerband into a higher rpm range, it needs a higher stall speed to get it there. And yes, more gear will help.
 
You must log in or register to reply here.
-

Back
Top