Cheapest way to change ratios on an 8-3/4, and what ratio to choose?

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MOPARoldtimer

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Here's a dumb question fer ya. Please indulge me.

When I built my Valiant, I opted for a 3.73 Dana Power-Lok posi diff. But now I'm getting a little tired of my engine having to rev so high on the freeway. So I'd like to replace the ring and pinion with a higher overall (numerically lower) ratio, but after spending so damned much money on the Power-Lok, I want to go cheap!

The car is a '62 Valiant with a 408 having 10.5:1 compression and what we used to call a "292 degree" cam (before the days of measuring duration at .050"). ThermoQuad carb.

How can I get into some new ratio cheaply, and what ratio would you recommend for lower RPMs and better mileage? It can be an open diff. It doesn't have to be a posi.
 
get another open chunk with gears you need (SWAP PIGS AS NEEDED)
i have FOR SALE
741 case 3.23 gear chunk
489 case 3.23 gear chunk
742 case 2.76 gear chunk

i'm a good trip away though, but you may be able to find fabo members closer to ya that can sell ya one.
 
With that cam, there is no mileage.
Is it a pure cam?

The easy way is to order one from a place like Randy's ring and pinion a s drop it in.

The cheapest way is to our use the parts and do it yourself.
 
"292" could mean anything from 230 @ .050 to 250 @ .050. That's the bad part. How radical an idle does it have? I think rumblefish meant purple cam. If you have the 292 purple cam, he's right. No amount of gear change is gonna help a lot. If you can give us maybe duration @ .050 numbers........

The bad thing is if the cam is stout @ .050, regearing to even say a 3.55 might make it a slug on the bottom end. It all depends.
 
It has a Comp Cams no. 20-243-4, which has 244 degrees of duration and .334" of lift at .050". The total lift at .006" is .501", with 292 degrees of duration. I'm running "J" heads that have been ported just about as much as possible, and the 2.02" intake valves. I guess there's no way I'm going to get any economy with an engine like that, is there?

And yet, Cadillac improved its gas mileage when they went to higher-compression engines in the late '50s. I suppose it all depends on the cam profile.
 
I think you've answered your own question. If the purpose of wanting to regear is economy, pretty much, it ain't gonna happen. Now an OD transmission on the other hand would help, but IMO it's not gonna help enough to justify the cost.

As far as the higher compression comment, efficiency goes up with compression. Always has. Of course so does the cost of fuel.
 
How tall are your tires? That plays a big part in the "final drive ratio". That cam may idle kinda rough but it's not that big at all in a 408. Just 4 degrees bigger than what is in my 408 and mine is a stump puller. With decent 275-60/15 drag radials mine will break em loose in 2nd gear at a 20-25 mph punch and leave 30-40 foot black marks on the road. And that's with 3.55 gears. I'd be willing to be that unless you have some really tall tires you could put some 3.23's in that'll drop the cruise rpm a whole bunch. As mentioned fuel mileage will never be great but for example mine with just a slightly smaller cam and slightly less compression (9.8 to 1) averages 13.5 mpg. When I keep my foot out of it it'll pull down 15 mpg as long as I don't go over 60. 3.23 geared chunks are plentiful and usually fairly cheap even if they have a sure grip. I'd bet 3.23's will help some on fuel mileage, maybe 2 mpg. Not a huge increase but every little bit helps
 
The "final drive ratio" thing means a taller tire reduces the gear ratio? If I am running a 235/75/15 that makes my 2.94 an even more of a hwy gear? These are questions....Help me fishy68.
 
kahuna1949 said:
The "final drive ratio" thing means a taller tire reduces the gear ratio? If I am running a 235/75/15 that makes my 2.94 an even more of a hwy gear? These are questions....Help me fishy68.
Click to expand...

It doesn't actually "reduce the gear ratio" because that is a set figure due to the gears. But it basically does the same thing so yes your tall 235/75/15 tires make a 2.94 gear seem really tall. For instance a car with 2.76 gears and 225/60/15 tires will seem a lot lower geared than yours even though it isn't.
 
Thanks much, so maybe where I thought a 3.73 might be marginal it might be what I was looking for. I know a lot of factors play, but for now.
 
oldtimer, i'd go 3.23 like "fishy" spoke of. You should still have plenty of torque with the 4" arm and the combo you have. More comfortable, less wear & tear, and better mileage at speed.
 
What folks are getting to is camshaft, torque converter and final drive train gearing have to be matched for best compromise between performance, and fuel economy. In other words if your cam positions power and torque curve high up the rpm band as most lumpy overlapping cams do, than a rear gear ratio and tire diameter has to chosen to keep the engine in the torque and power band under most driving conditions for optimal performance and fuel economy. Anything less will result in a dog, pig, or both.

If one of the four components is changed, most likely other adjustments will be needed to the other three to keep a good balance. You need to know what your engine’s torque curve looks like, that is, what its operational rpm band, what general speed car is to be driven, and work from there using an online calculator, or call a torque converter supplier and get their take on your combination.
 
i just did the 8.8 explorer swap n was running 3.73 s the car was twistn up pretty high crusing 70 on interstate so i went to 3.55s it helped some but with a 26 inch tire it was still a lil hi i have a abundance of 8.8 stuff so i again swapped the gears this time to 3,30s i am very content with em car feels good still but can cruise 70 75 mph n not feel like your killing it twisting it so hi for long periods a time. i work 20 miles from home n its all interstate speed limit is 70 n doing 70 people still buzz right by lol i felt like i was impeding traffic at 55 n 60
 
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