Technical Ford 8"

Considering replacing my 8" center set with a new Posi 3.80 gearing for my now single leg 300 ratio, my engine is a SBC 330 hp. I was looking for a little better launch as well as other things. The car is a town driver but what are your thoughts on the highway with speeds at 65-70mph? Would this be taxing on the engine for a long drive (500miles) with 3.80's?

 

Overdrive?

 

I'm running 3.55 Trac-Loc, 60 mph around 3100 rpm w/ 205/60-15. That's @ the low end of my power band & at maximum spark advance. Purrs like a lion really smooth. Plenty of power to break the tires on the 1>2 shift @ 4K rpm on the low end. Power around 400 gross hp. Curb weight 3000 lbs. w/ 4-speed stick.

Joe

 

I guess that depends on what you consider a "long drive". A few thousand miles...the engine won't really care. If you're talking about 50,000 miles, yeah, it'll wear things out sooner.

 

no

 

@Roger Loupias , tire size?

If it is only the rear end ratio you will be changing, and all other things stay the same, there is an easy way to know what your rpm will be for any given speed. Just divide the rpm by your original ratio (3.00) and then multiply by the new ratio (3.80)

Example.
65 mph, 28" tire, 3.00 ratio would give 2403 rpm.
2403 rpm divided by 3.00 = 801.
801 times 3.80 = 3043.8
Rpm with new ratio is 3044. (But only if nothing else is changed)

 

Years ago a mechanic friend of mine called desperately needing an 8" pumpkin, apparently he changed the clutch out in an otherwise stock Mustang with a factory 3-speed manual and for some reason simply had to test his work by taking the car out on the main drag in the busiest part of the day for a little "tire frying", unfortunately when he revved it up and dumped the clutch the rear-end spit the pinion out the front of the differential case.

If you are interested in a better "launch", my advice would be to dump the 8 and get a 9.

 

The 8" ford differential in my ,T has just a tad over 120K miles on it , it also has a lokrite ratcheting type traction device , I run a four speed Muncie behind a reasonably strong 388" bored & stroked ,SBC ( see avatar) rear tires 275 60r15 , car weighs a bit over 2000 pounds depending on passenger . I've "launched" the car many times over the last 22 years & 50K miles.
I think the " tire frying" mechanic blamed the differential for his shoddy work &lack of operator skill, then again , some over zealous folks can break an anvil
To the OP , 3000 highway cruising speed will not adversely affect your engine , in fact , it was the preferred cruising speed for many , many cars of the muscle car era. The difference between a 3.80 & a 3.55 is about 5 mph @ 3000 rpm ,( 65-70)

 

There is a big difference between preloading the drivetrain a little before launch, and just dumping the clutch. One makes things go, and the other makes things go bang...........

 

Strange offers a 3.43 for the 8" which IMO would be a perfect street ratio behind a 350/350 combo in a lighter car like your roadster. That will give a marked improvement in zip off the line without being as 'touchy' about how much throttle is applied compared to the 3.80 or even the 3.55; both are a pretty steep ratio for an automatic car w/o OD. That would give you about 1/2 the RPM increase at freeway speeds, and won't have you wishing for an overdrive as much. My distant second choice would be 3.55.

 

I second the 3.43.

 

I was sitting here with morning coffee wondering why you were thinking such a big ratio jump. Was thinking you would be happier with 3.50 plus or minus. I was to slow to respond, but Crazy Steve gave you a great suggestion and explanation.

 

I'll ask again. What is your tire size? It can make quite a difference to the final rpm.

 

Roger Loupias,
You will definitely get a better launch with that ratio. It will bring a smile to your face.
65 - 75 will be the comfortable area on the highway. (You will get passed by everything except RV campers)

My answer is based on an off topic daily driven S-code Mercury that I swapped an open 3.00 gear to a 3.70 track loc. That one change woke that car up! I do suspect the new RPM range taxed my engine on the highway some but that was limited to an eight mile stretch. My driving mostly consisted of 4 lane and back roads. Sure was fun.

 

Are you running an auto transmission? If so ( consider costs) I’ve seen references to switch pitch converters that change stall speed. Unsure if it’s relevant for you or not or even available.

 

{ 235-65-17" }

 

I put a 4.something in an 8" for a guy with one those cars named after a horse. The motor was give or take 375hp, he had a 4 spd. Tried to talk him out of it, "You'll be back for a 9", sticky tires and 8" rears are an oxymoron." He didn't know what it meant, thought I called him a moron til I explained. After a short day at the track that Sat he was back. Spit damned near the whole center section out on his 2nd pass.
RPM, we're all so conditioned and accustomed to sub 2500 highway revs any more seems like screaming. I think 3.80 is a big jump. I think YOU will need to be "tuned" to the new revs at 70MPH, and I suspect theres likely some deal to be had on this to consider it. 31" tall tire, 1:1 3rd gear, 3.78 rear equates to 2650 give or take a few at 65MPH.

 

^^^^^^^^ Exactly.

 

I have never spent money on an 8" rearend, if I'm gonna upgrade one, I start with a 9". An 8 can work very well within it's limitations, light loads, auto trans, easy on the clutch, it'll run forever- but why spend a lot of bux on it when spending the money on a 9" gets you the real deal? I've seen folks spend a lot on 10-bolt GM units too- start with a better basic unit if you're gonna lean on it

 

Here's a helpful calculator, so you can plug in some "what if" numbers to help with your consideration and gear ratio selection.

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

Lynn

 

The Roadster came with the Mustang 8" and I have a chance for a new center set with these gears at a screaming deal. Don't care to gut the back end to go to the nine install. This car wont go to the drags so I would think nothing should grenade.

 

Take the "screaming deal"! Put'm in and drive it. If your happy your done, if not then put different gears in the original center. Changing the gear section isn't a big deal.

 

think of it as having an overdrive for road trips, but it takes you an hour to engage or disengage the overdrive

not that I would ever do anything that stupid....I had my son along, he helped me change the 3.00 gears to 4.11 and back again on Drag Week 2015. We had the highway gears in for the 1500 mile drive to the event, the racing gears in for the 1200 miles of the event, and put the highway gears back in for the drive home.

 

Agreed given those parameters. Here's another thought to share. Suppose at your tire hgt it's a bit much at freeway speeds. Get a taller tire. That calculator above is pretty dadgum accurate. Maybe a screamin deal (no pun intended) can be had a on taller tires and you end up with a nice compromise, still zippy in 1st and 2nd gear. Assuming you have the room and adjustability for em. I've actually been on this thought off and on for a bit since my 39 has 3.78 gears and 31" tall tires, but I'm using a flatty, not a SBC.

 

The "taller tires" thing really doesn't do much, compared to changing gears....and you still want a tire that looks right on the car....

 

I have a friend who runs a 8" Ford in the rear of his Nova gasser with a 350 Chevy and 6-71 supercharger that's probably around 600 hp. Even with slicks at the drags his 8" has held up fine. Most people poo-poo the 8", but they're not as weak as most would think they are.

I run 3.73 gears with a 29" tire on the rear of my car and I personally don't like running hundreds of miles at 70 mph at 3100 rpm's. The engine doesn't seem to care, but I just don't care for it myself.

 

Perfect car for an 8". It is a very good unit, just not a HD deal

 

I had my 3:25 single legger 8" switched to a 3:80 trac lok 235 75 15 tires, with a crate 350/turbo 350 the RPM at 65-70 was at 3200
I have a 200-4R that I am almost done installing and figure in 4th it will be around 2200 a lot less strain on the engine lock up converter with a override switch to dis-engage 4th at lower speeds I had the 8" set up by "8 & 9" ford rears in Pennsville New Jersey (609) 420-7113 and the owner (Allen) claims the 8" is good for up to 500HP Mu crate engine is only 290 HP so I have no worries Very good service, he is on the internet.

 

That's conditioning 3grand @ 70 sounds perfect to me, but I started driving in 1962 !

 

There are gear-ratio calculators online. Enter transmission & axle ratios, tire diameter, and rpm, and it will calculate speed. I can tell you with those gears, you will be looking at over 3000 rpm @ 65.