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what gear ratio is standard in a 7.25" a-904 slant-six?
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Author:  73ScampSSS [ Fri Sep 02, 2005 12:24 pm ]
Post subject:  what gear ratio is standard in a 7.25" a-904 slant-six?
Hi I was wondering what was the standard gear ratio in my 1973 Plymouth Scamp with /6 a-904 and the pinner 7.25" axle. I am having the engine rebuilt to accomplish 250 horse (hopefully more) and am looking into getting some posi for the rear. I cannot find posi for these 7.25" rears so I am considering the Dana 60 (which is 50lbs heavier :( ). I am a bay-area driver and like to do around 80 mph on the freeway and would like to accomplish that without over revving the engine. I have seen a transmission kit with modified planetarys that gives a numerically higher ratio in 1st and 2nd while maintaining the lower ratio in the rear for freeway speeds. I have also seen the infamous Kiesler trans which would be nice but cannot afford 3k for the trans and 3k for the axle. Also wouldnt want to cut up my floor for the Kiesler shifter. Can anyone recommend a good gear ratio or some mod that would give me the high freeway speeds but not bog out at the stoplight. ALot of questions, Thanks
Author:  mcnoople [ Fri Sep 02, 2005 1:39 pm ]
Post subject: 
The dana is way overkill for slant. Find a 8.75 or 8.25 from a A-body either can handle you power levels and suregrips (posi is chebby cretin speak) are readily available for both. Do you have large bolt pattern or small bolt pattern wheels now. If front is disc you prolly have lbp. The 8.25 is lbp the 8.75 was only used with sbp. I would expect you current ratio to be 2.94 or 3.23, but the only way to know is a tag on the axle or pull the cover and read it off the ring gear.
Author:  SlantSixDan [ Fri Sep 02, 2005 5:02 pm ]
Post subject:  Re: what gear ratio is standard in a 7.25" a-904 slant-
Quote:
Hi I was wondering what was the standard gear ratio in my 1973 Plymouth Scamp with /6 a-904 and the pinner 7.25" axle.
2.76.
Quote:
I am having the engine rebuilt to accomplish 250 horse (hopefully more) and am looking into getting some posi for the rear. I cannot find posi for these 7.25" rears
They're scarce, and not worth looking for. 250hp will chew up a 7-1/4" in a moderate hurry.
Quote:
I am considering the Dana 60
Gross, massive overkill in weight, size and dollars.
Quote:
I have seen a transmission kit with modified planetarys that gives a numerically higher ratio in 1st and 2nd
'80-up A904s all have the lower (numerically higher) 1st and 2nd.
Quote:
Can anyone recommend a good gear ratio or some mod that would give me the high freeway speeds but not bog out at the stoplight.
8-1/4" 2.94 ratio. Sure Grip is not necessary.
Author:  73ScampSSS [ Sun Sep 04, 2005 8:19 pm ]
Post subject: 
Ive been inside those 904s and 727s and know they are basically the same from the 1950s to the 1990s. If I was to pull a gearset out of a 1980s a-904 do you think all the parts would go back into my 73 a-904 with no problems, to accomplish that lower gear ratio. Also you said the suregrip would not be needed, but I want to be able to do a burn out and leave two lines. This is going to be like a autocross/street car made for hitting hard corners. I already installed the boxed lcas, flaming river manual steer box, big bolt 11.75" discs on the front, .99 torsion bars and 1" sway, koni shocks, aluminum strut rods & tierod sleeves, tubular adj ucas, and all poly bushings except the lcas I went with the stock rubber. I almost ordered a Dana 60 with wilwood discs from Gearsandrears.com that would have cost $2800 but I cancelled at the last minute, after I found a 8.75 in San Mateo for $100. 2800 or 100 was not a hard decision, but the 8.75 is from a b-body, so I am still on the lookout. The guy with the 8.75 said to look for slant-sixs with the 8.75 rear but Ive never heard of that. I was at pick-n-pull this weekend and didn't find squat but a 4-door 72 dart, but no 8.75s on any a-body.
Author:  SlantSixDan [ Mon Sep 05, 2005 8:18 am ]
Post subject: 
Quote:
Ive been inside those 904s and 727s and know they are basically the same from the 1950s to the 1990s. If I was to pull a gearset out of a 1980s a-904 do you think all the parts would go back into my 73 a-904 with no problems
If you're trying to put together a wide-ratio 904, you want to install the gearset and the outer and inner governor weights, which are different. If you don't change to the wide-ratio governor, you won't get 2-3 upshift under full throttle—that can be annoying.
Quote:
going to be like a autocross/street car
Oh, I was thinking "street". Yeah, get Sure Grip.
Quote:
The guy with the 8.75 said to look for slant-sixs with the 8.75 rear but Ive never heard of that
Can't figure out why he'd say such a thing. The rear axle does not know what engine is installed. You could order an 8-3/4" in your A-body with any engine, and more of them got put behind V8s than behind \6s.
Quote:
I was at pick-n-pull this weekend and didn't find squat but a 4-door 72 dart, but no 8.75s on any a-body.
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