Folks Of Interest Rear gears for flathead and t5

Being that this is the only flathead I've ever seen up close or worked on, I don't know much about them. Since that's the case let me ask this…what's the best cruising RPM for a Flathead? As far as I know this motor hasn't been rebuilt but it runs super strong and when I drained the oil it was black but nice and clean. No sludge whatsoever came out of the drain plug and I got my pinky up in there as far as I could and didn't feel any. All 8 plugs are a dark even tan. Let's just ***ume it's a strong motor... what RPM should I be shooting for

 

Mine cruises at 2000 but the power band is up around 3500 when it is boosting. I have a 5500 rev limiter so it doesn't all fall through the oil pan. Around town I hang on to my gears a little longer so it has more throttle response and I like the noise it makes up around 3000 rpm. I put a stereo in it but between the blower and the quick change I cant hear it with the top down. I prefer this noise better anyway.

 

I would keep it at, or just over 2000rpm at the speed limit, in top gear, on flat ground.

It sounds like you have a strong runner. Toss all of your variables into one of the calculators online, and see what your intended combo will produce.

 

I feel like those calculators give you a certain number which sounds good on paper but once you get out there in the real world it's always different. I did it and it said 3.73 was my best option which would put me at about 2200 and I think 70 miles an hour or something like that. Unfortunately the calculators don't differentiate between a 1500 pound car or a 4000 pound car which of course makes a huge difference

 

Here's what I just came up with in the gear calculator. I also typed in 4th gear and it had me at about 50 at 2300ish. It looks like running 4.10 would put me about 5 miles an hour slower. The speed limit on some of the roads around here is 70 and of course traffic goes 75 to 80. I know cruising RPM should be right around 2000. To get to 70 with 3.73 I'd need to turn 2400 and with 4.10 I'd need to turn 2600. Are those rpms safe to do for extended periods of time? One of the shows I go to every year is about 2 hours away, all highway.

 

Yeah, those calculators only do RPM. The heavier your car is the lower the gear would need to be. You don't have an open-wheel roadster. You have a full-bodied car. Granted it is not over-large, and it does have nice rounded surfaces, but it has a bunch of weight.

As for 2600rpm, on the freeway with a flathead in good health, no problem. It will hum along nicely all day long., so 2400 would be fine, too. Of course, this is RPM alone.

3.73 an 4.10 are not all that far apart in ratio. On some axles there may not even be an available ration in-between.

Try this calculator. You can go gear by gear: https://www.tremec.com/calculadora.php

 

I'd say that normally going to an overdrive will help most cars. The thing is, your stock flathead is going to make significantly less HP than the Chevy you had..........and you already felt like it was underpowered sometimes. The T5 may help you, but with minimal HP/torque available, you just about have to give up something even with the new transmission. I'd say plan on running 2500 rpms at 70 mph instead of 2000.
While Bandit Billy has a flathead, his car is significantly lighter than yours and his flathead is making way more power than yours. Not sure what your goal is, but my guess is you just want an enjoyable driver that doesn't feel underpowered in most instances. The equation for that means you either have to find more HP to work with, or gear it to work better and at a higher rpm. Thats basically why so many of these cars got converted to later engines.

 

If gimpys is right, and I trust his judgment/advice 100%, and the car can run at 24 or 2600 with no problems I'll do 4.10 for sure. I'm just trying to get it equivalent to the Chevy as that thing that thing was pretty much perfect for daily driving except up hills in overdrive. I can pick up axles for about $200 so worst case scenario I do 4.10 and if I don't like it I'll switch it out

 

What is your stock rear ratio?

 

There's your answer right there.

 

If I could buy axles out here for $200, I would buy the yards out.

 

Lol what do you guys pay out there?

 

I haven't checked it yet but as far as my research goes it's 3.7X

 

A lot of desirable rear axles have crept up into the $450-500 range at a yard.

 

Which rear axle are you looking for?

I think the stock axle width was 57" WMS-to-WMS.

That leaves you with just a few choices. Early Ford 9", Maverick 5-lug 8-inch, which are 57" or 1990-1992 7.5 or 8.8. which are both 56-1/2".

If your wheels are not excessively wide, you might be able to use a later Ranger, 8.8, which is 58-1/2"

The 7.5 and 8.8 in that era had 3.55, 3.73, 4.10, and 4.56 for ratios, and many had a clutch style limited slip differential. If there is an L on the ratio tag, within the ratio stamping, like 3L73, and not 3.73, it has a limited-slip.

These had two brake sizes, 9" and 10". See if you can get the bigger ones.

It does not matter if it has ABS. That just means that there is a tone ring on the differential carrier, and a sensor in the housing. It can simply be ignored. I use mine to drive my speedometer.

 

I'm actually going with a 2 wheel drive S10 axle. I'm doing that for a few (bunch of) reasons.
1) It's an inch narrower on each side than the factory axle (I'll be buying 2 new wheels from summit with 3" backspacing and a chevy bolt pattern) so it will make getting the wheels on and off easier with it being lowered.
2) They're super easy to find anywhere in the country I might go.
3) I can run standard S10 u joints which are also easy to find anywhere in the country. On the Chevy I had an S10 rear and T5 with a '57 Chevy drive shaft and had to have conversion u joints that were about $40 a piece and had to be ordered
4) They're dirt cheap at $200 a pop
5) They're fairly new as far as axle swap axles go so they should be in better overall condition without needing a rebuild
6) I have a better shot of finding brake parts over the counter as opposed to needing them to be ordered in

 

If memory serves, that axle is 54" wide, or 3" under stock. You will want to make sure that the backing plates will clear the springs, frame rails, and body work, etc. I believe that the stack of the drum and backing plate is about 3" on these, so the gap between the backing plates would be about 48".

You'll probably be fine, but I'd throw a tape across it and double-check.

 

The wheels I'm going with are 3" backspacing so it'll make the back of the wheel a quarter inch to a half inch closer to the inner wheel well. I measured it all before I decided on that axle and I think the factory axle drum to the leaf Springs is something like 4 or 5" or something like that so it should be good. I measured all this stuff like a month ago so I can't really remember all the numbers

 

No worries. Just making sure you did.

Do-overs are not fun.

 

True, especially since after the axle, new brakes, new wheels and mounting and balancing I'll be at like 600 bucks

 

One thing that I always do in building cars is that I get the wheels and tires that I want first, and then I put them in the ch***is where I want them. Then I pull a tape across and figure out how wide the axle needs to be. That's the final check as the weather or not what you want to do will work. Like I said you're probably okay. But that's one thing that you can check before committing to a rear axle.

Incidentally the Ford 8.8 and 7.5 are both incredibly common, and have very cheap easy to get parts. One thing that they also have is axle flanges that have no holes in them except for the five that have the original lug studs in them. What that means is they can be redrilled to any other pattern by a competent machine shop, or or by you, with a drill jig.

As for the u-joint situation, both of those Ford axles do use a companion flange in a yoke on the pinion, but that yoke takes a 1310 u-joint. Transmission yokes for 1310 u-joints are easy to get. A 1310 u-joint is one of the most common ones on the market. With that joint that both ends, you can buy a drive shaft from a circle track vendor, already made and balanced. They are sold in half inch increments from stubby to stupid long.

So just in case the GM axle doesn't work out, there are other options there.

 

These are the wheels I plan on using in the rear. They'll be behind a set of Foxcraft skirts or full caps so the extra holes won't look like s#!t. I'm going to talk to the yard I'm buying the axle from and explain to them what I'm doing. I've gotten a few axles from them for various Jeep projects over the years so hopefully if it won't work they'll let me return it.
Speaking of Ford axles, do the Ford and Chevy output yokes interchange on the t5 or are the shafts different splines/diameters? Whichever rear I use I'd want the front and rear u-joints to be the same.

 

I'm going to ***ume this is a stock or near stock flathead. The stock 3 speed has 2.82/1.60/1.00 gears. The axle has 3.73 or possibly 4.10 gears. The stock setup is good for driving on all roads except maybe the highway. Rev's will be up pretty good there. Can do it, but why?

If you run a T5 with 2.95/1.99/1.34/1.00/0.63 you will have a little better than stock like first gear acceleration coupled with better 2nd and 3rd gear acceleration, the same in high gear and much lower revs in 5th. In fact at 65mph you would be running pretty close to the rpm at which peak torque happens (~2000 rpm). A little less at 60. A little more at 70.

If you run a stock Ford 87-93 T5 with 3.35/1.99/1.34/1.00/.68 it'll jump off the line run strong through 4th and still turn a relatively low rpm at speed.

 

I'm running an s10 t5 with a .72 od so not far off the Ford ratio.

 

It's a Ford and Chevy yokes do not interchange on the T5. The splines and the seal diameter are different.

There are, however, yokes for 27 spline GM transmissions, like the T5, in just about every u-joint yoke you can possibly think of.

 

What's factory, 1310?

 

Factory GM u-joints in this application are 3R joints.

 

I haven't ever heard of that until now. So the rear s10 yoke is the same then? Anyone have any spare trans yokes without a worn in groove they want to part with?

 

I'm not sure that the front and rear you joints are technically identical on these. They both had the same cross with and cap diameter, but I think one has an inner snap ring and the other has an outer snap ring. My memories a little fuzzy here so I might be wrong.

More often than not I standardize on the 1310 u-joint, because I can buy drive shafts in half inch increments pre-made, and balanced.

I'm not sure but I might be single-handedly responsible for the shortage of usable T5s in Northern California.

I bought them all, and put them into hot rods. There's always something T5 going on in my shop.

Just today:

 

I just checked Rockauto and it looks like the joints are the same. I found a conversion joint from 3r to 1310 I could use for the axle but that could get me into trouble out on the road if I need a replacement. I guess I'll stick with 3r ujoints and a 3r slip yoke to keep it consistent, after all that's one of the reasons I want all s10 stuff.