RPM's too high at 70 mph for 2.75 gears

[jbojo]

I am thinking it's a 3.25 rear too. At 60mph the RPM's would be around 2500 and 2900 at 70mph with 26.3" dia tires.

 

[luxstang]

Did you check you tire diameter? If the tires are too small the wheels will spin faster thus causing higher revs. Shouldn't make that much of a difference though but maybe it's a few things adding up.

It's a long shot but worth eliminating.

 

[toastintx]

I am thinking it's a 3.25 rear too. At 60mph the RPM's would be around 2500 and 2900 at 70mph with 26.3" dia tires.

well i did the test on the tires. 2 complete rotations on the tire = 2 3/4 turns on the yoke!

 

[toastintx]

the tires measure 26.6" according to the tire calculator. they are 245/60/15. marty report shows that it came with the E70 X 14 which is 25.8" so according to the tire calculator original tire at 70 mph = new tire at 73.6 mph. that shouldnt be enough to increase that much RPM's

 

[72HCODE]

when my trans started to go i noticed the rpms were higher then usual, but the car still had 3 gears at that point. i thought i was imagining it, but then about a week later i lost 1st gear completely and the trans wouldn't go into 3rd. it was running in 2nd my direct drum fused my clutch plates to the steels and that completely locked 1st gear, the forward drum was slipping and basically the trans was actually running on the 2nd gear band clutch and it was slipping on that as well.

i believe what was happening was all 3 gears were working before the direct drum fused but the direct drum was slipping an enormous amount of power(reason why it fused from heat), and 3rd was slipping power so my trans was kind of in between running in 2nd gear on the clutch band and slipping in 3rd which explains the higher then normal RPMS on my tach.

before the failure from a stop sign i counted what felt like 2 gear changes which is correct. you would feel the shift from 1st to 2nd and 2nd to 3rd, but it was slipping at that point so just feeling the engagement of 3 gears didn't really tell me much.

what can happen is you have VERY old seals that have hardened inside the transmission, the fluid pressure is leaking causing the drum pistons to not fully engage, thus under low load you feel 1, 2 ,3 but on the highway it is slipping power and the rpms are a sign it is going.

 

[toastintx]

good info 72hcode i guess i need to have tranny serviced or rebuilt before its too late. i hope the doesnt cost an arm and leg and i hope i can find a place to do it and do it correctly

 

[barnett468]

if you have the trans done, i would make sure they use good plates even if you dont beat on it . borg warner and raybestos both have a high quality one available, but they have crummy budget ones also so make sure to get their high end ones. if you beat on it, i would use red alto plates and kolene steel plates.

it costs around $1800.00 to drive the car in with the worn trans and out with a rebuilt trans and new torque converter where i am and that isn't a deal.

 

[will e]

Some timing lights also show RPM. Many autoparts stores will loan these to you. I would verify everything before spending money on a rebuild.

I swapped a 3.25 into my car and later a 3.5. I also had a tach that read wrong after I broke the tip off the needle. With 40 plus years on your car goodness knows what has happened to it.

Finally, if you determine the transmission is going, you need to decide if you want to spend some extra coin on an upgrade. This will depend on your long term plans and use of the car.

www.grimmjeeper.com << one of the best calculators I know. It is for 4wd but you don't have to choose a transfer case.

 

[toastintx]

thanks again guys for the advice. just took her out and used the phone gps speedometer dead on, and i noticed that it didnt seem to be shifting into 3rd, i think i found the prob. Good advise 72HCODE. now the decision begins rebuild or upgrade and if upgrade to what??????

 

[73pony]

My FMX was going and leaking like it was made of swiss cheese last year. I took the opportunity to swap out for a T-5.

 

[TommyK]

Have you checked the level and condition of the fluid? The FMX is very sensitive to fluid level. It's a long shot but easy to do and doesn't cost anything.

 

[Qcode351mach]

now the decision begins rebuild or upgrade and if upgrade to what??????

No brainer if your staying auto 4r70w (4spd with overdrive) (you can source from a 1999-2001 Mustang 6 cyl or 8 cyl almost a direct bolt in and not much more $$$. You'd be crazy to spend money on the fmx UNLESS your doing a factory correct restoration.

 

[72HCODE]

i did my own trans last year 1800$ is a great price. i spent more in tools and parts but i wanted to do it myself.

my car originally had a FMX (not the one that blew) somebody swapped it for a junkyard 1974 C6 from a station wagon.

i had that junk yard 74 c6 rebuilt in 2008 by the worst trans shop in NY, in 2015 it grenaded itself.

I got a 1973 mustang C6 from a fellow forum member and rebuilt it myself.

the issue with the FMX is no real aftermarket, basically you rebuilt it as it was and getting the parts is more difficult. so there really isn't any performance clutches and bands and steels designed for it.

for the C6 you have many options including modifications using late model parts.

or you can go the route of a modern trans.

nothing wrong with a FMX though, rebuilt they will take like 400 HP, plus if the trans is original to the car and you want to keep some value for resale then it will be the cheapest route to have it serviced.

you can also increase performance just installing a higher stall convertor during the trans rebuild.

for me i never had my cars original FMX it was long gone by the time i bought the car.

the 74 c6 it had was in Terrible internal shape and so it was junked and a correct floor shifter 73 mustang C6 was used as my core.

i got a 72 Q code C6 transmission from another member as well, and originally i was going to use it until i found the entire case was cracked from a previous car accident,, so i used the Lever and Servo from that transmission as it was much better.

I had some complaints about the Clutch and steel kits offered by raybestos and Alto which to a Novice made things much harder in the end i used raybestos and raybestos steels, RED Alto and kolene steel plates caused me clearance issues that required additional machining, i did use a wider RED Alto 2nd gear clutch band in my transmission....

you can go crazy on a transmission rebuild and spend like 10K in upgrades.

if the trans is internally in good shape very little mechanical damage then a rebuild will get you on the road again quickly.

now a trans shop can also diagnose if there really is a problem before they tear into it. they can install pressure gauges on the transmission using the diagnostic ports and they can see if the line pressure is low, they can also inspect the pan and fluid to see how much clutch material is there. and pull the valve body for testing and inspection all before the decision is made to tear the trans out completely.

for me once the trans failed it was obvious as if i put the car in 1st gear manually the rear end locked up and the engine would almost stall with the car going nowhere. and there was no more shifting or 3rd gear.

here you can see what failed in my trans



some of my saga

http://www.7173mustangs.com/thread-the-curse-of-the-blown-transmissions-strikes-again

 

[toastintx]

A BIG THANKS to all the guys who gave there input on my little problem. Problem solved! bought a cheap Tach at auto store, red necked hooked it up with wires running from under hood into through passenger window lol, anyway old tach reading at 60mph 2600 at 70mph 3000. new tach at 60 mph 2200 and at 70 mph 2650 rpms. if you do the calculation for rpm and my size tires (new tires) 60 mph 2189 and 70 mph 2554. Thats more like it! i guess now just need to get Tach calibrated.

Thanks again everyone

Johnny

 

[73pony]

Good news.