Rebuild or leave trans?

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DartVadar

DartVadar

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I am adding some power to my motor and adding some 3.91 gears to the rear of my 73 dart, and just started thinking about the transmission. The trans is a 999 with a low first and second gears. My old 360 had mid to high 300s for power, I really am just putting in a bigger cam (hydo flat tappet to solid roller) so It will probably have a tad over 400HP. Last time when I put it in the only thing that I changed with the trans was add a transgo tf2 shift kit, otherwise the trans was stock as far as I know.

The transmission shifted really nice and firm, and it never slipped or anything, seemed to run just like it should and easily handled the power (no racing but some spirited street driving). Because it ran good before and I am not adding a ton more power should I just leave it as is and run it again? Or should I rebuild it first? from what I've been reading they are pretty easy transmission to rebuild, but I've never done it before.
 
My buddy ran the same set up for years and put about 40,000 miles on his with some track time and I think he is still using it in a different car.
 
These cars are mighty light. Some people don't like the low first gear set with short gears. It's almost too much on the bottom and a big jump from one to two. IF you have it freshened up, you might consider the stock first gear set. The A999 with the low first gear was made for the pig 5th Avenue that had like 2.45 rear gears. Your car is really light in comparison, plus you're going to a 3.91. Just food for thought.
 
RustyRatRod said:
These cars are mighty light. Some people don't like the low first gear set with short gears. It's almost too much on the bottom and a big jump from one to two. IF you have it freshened up, you might consider the stock first gear set. The A999 with the low first gear was made for the pig 5th Avenue that had like 2.45 rear gears. Your car is really light in comparison, plus you're going to a 3.91. Just food for thought.
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That's true I didn't think of that, that is something to consider. The lower trans gears didn't really do too much before as I had 2.76 rear gears, so it was pretty slow off the line. Wonder if the change will be too substantial like you mentioned.
 
Have you used the gear ratio's and your new cam's power band top end, and computed where your RPM's will be when you shift from 1st to 2nd? If the RPM's are well into the cam & engine's torque band when you go into 2nd with present gears, then the jump should be OK; I imagine you'll just need to be a lot quicker on the shifter. Then check the same thing for the 2-3 shift.

Then recheck the road speeds for the light throttle, low RPM shifts for everyday driving and see if the shift speeds for 1-2 and 2-3 seem too low.

I'd try it and see what you think; you can always change it later and you will learn the feel of that particular setup, which will add to your overall knowledge for the future.
 
nm9stheham said:
Have you used the gear ratio's and your new cam's power band top end, and computed where your RPM's will be when you shift from 1st to 2nd? If the RPM's are well into the cam & engine's torque band when you go into 2nd with present gears, then the jump should be OK; I imagine you'll just need to be a lot quicker on the shifter. Then check the same thing for the 2-3 shift.

Then recheck the road speeds for the light throttle, low RPM shifts for everyday driving and see if the shift speeds for 1-2 and 2-3 seem too low.

I'd try it and see what you think; you can always change it later and you will learn the feel of that particular setup, which will add to your overall knowledge for the future.
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I've calculated a few things, but mainly the change in RPMs when I'm driving. But changing the gears wouldn't change the shift points though? Or would it? Currently the trans shifts at around 5600 and it could be a bit higher, but matches decently with the new cam. I'll have to calculate a few things still, but I assume I check a lot of that stuff by actually driving the car?
 
Now going back to the strength of the trans. I could take the time to rebuild it, I'm sure it's not too hard just time consuming. In completely stock form with the addition of a shift kit would it live happily behind 400 ish hp? Majority street driven. Does the shift kit add strength?

Now I assume if I rebuild it with some better parts it would handle that power with ease, but I am not reaaly wanting to tear into if I don't have to. From what I understand shift kits aren't terribly expensive anyways.
 
The shift kit doesn't add any strength but since it reduces clutch and band slippage it will add life. I say if it's working good now why mess with rebuilding it? It's not like you have to take it out to do the other mods your doing. If it croaks later take it out and rebuild it. Even in stock form a 904 with a good shift kit like you have will handle 400 hp if it's in healthy condition. If it were an original trans from 73 that has 42 yr old seals in it and lord knows how many miles on it that'd a different story but I know it's not original cause in 73 they didn't have the low gear set yet. It's either a newer 999 or someone has done major mods to the original 904 to install the low gear set
 
fishy68 said:
The shift kit doesn't add any strength but since it reduces clutch and band slippage it will add life. I say if it's working good now why mess with rebuilding it? It's not like you have to take it out to do the other mods your doing. If it croaks later take it out and rebuild it. Even in stock form a 904 with a good shift kit like you have will handle 400 hp if it's in healthy condition. If it were an original trans from 73 that has 42 yr old seals in it and lord knows how many miles on it that'd a different story but I know it's not original cause in 73 they didn't have the low gear set yet. It's either a newer 999 or someone has done major mods to the original 904 to install the low gear set
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That makes sense, and the motor and trans are out of the car, (some other issues needed to be fixed so it was easiest) but it wouldn't be hard to take it out later if it does fry. It hasn't missed a beat yet, so I may just leave it as is and continue running it.

The trans is actually from the 80s, it came with a roller 318 already swapped in the car when I got it, when I pulled the motor out and the trans, the motor was original, but seemed to have very low miles, the trans I assume was low miles as well, it ran perfectly before as well, and once I added the shift kit it made it that much better.

Now what would happen if the trans does start to go? Would it just start slipping? Or would there be other issues, I'm installing a MSD so I'll have a rev limiter, so that should be good in the event something major lets loose.
 
Fishy68 described the kit benefits well.

Auto trans failure symptoms can be all over the map; it all dedends on which part actually fails/starts to fail. More often than not, you will feel something odd in a shift, or the unit wil slip in an odd way. It can go on and on for a while, or do it a few times and then bam-o. I had a GM350 that slipped when cold for several months; just had a worn clutch pack. But 40k miles after a simple rebuild, it shifted once oddly and then 2 mintues later, it locked in 1st and 2nd gear simultaneously and tried to tear itself apart; that was just a simple seal that went the 2nd time and with very little warning.
 
I think I'll just leave it then. And if it craps out ill rebuild it, I dont really want to tear open a perfectly working transmission. Now if It did fail is there any chance that it could take out the motor? What casues a converter to baloon? or would that only occur from overrevving as the trans fails? Will install an MSD ignition so ill have a rev limiter anyways.

But yea it needs a few new seals, and the pan leaks! I'll need to get a different pan, but thats no big deal.
 
It would have to be an awfully odd failure where something jammed in the TC area and suddenly stopped the motor dead..... and it would like just bust the AL case anyway. Rare..... don't even worry about it.
 
Converter ballooning only happens with pretty high horsepower if I understand it right. I'm no torque converter guru so the best I can explain it is the torque converter swells up and pushes forward hard enough to take out the thrust bearing in the engine.

One question, when you pulled the pan to install the shift kit how much "mud" and metal flakes did it have in the bottom of the pan? If it wasn't a lot chances are it'll last a long time. If it had mud 1/8" or more deep and metallic flakes things are worn bad.
 
fishy68 said:
Converter ballooning only happens with pretty high horsepower if I understand it right. I'm no torque converter guru so the best I can explain it is the torque converter swells up and pushes forward hard enough to take out the thrust bearing in the engine.

One question, when you pulled the pan to install the shift kit how much "mud" and metal flakes did it have in the bottom of the pan? If it wasn't a lot chances are it'll last a long time. If it had mud 1/8" or more deep and metallic flakes things are worn bad.
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That's good to know that's it's not really much of a concern for the motor, so I won't worry about any type of failure that will damage the engine.

And when I took the pan off there was only a little bit of "mud" in the pan, really just a film, that wiped off with my finger. Not even close to 1/8" too thin to even measure, and I didn't notice any metal flakes either. When I took the pan off after running it for a while to fix a leak, it was still pretty much clean. So that's a good sign.
 
So I took the pan off the transmission just to take a look at everything, and this is what I found. Little bit of "mud" and some small little shavings, the fluid didnt smell burnt either.

This trans has had the pan off before, but I've never flushed out all the fluid so it could have just been crap lying around the first time. But are these pieces of metal cause for concern? And that little green thing in the picture isnt metal, its from a gasket or something, and some of the little black pieces is silicone from the gasket as well.
 

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DartVadar said:
I think I'll just leave it then. And if it craps out ill rebuild it, I dont really want to tear open a perfectly working transmission. Now if It did fail is there any chance that it could take out the motor? What casues a converter to baloon? or would that only occur from overrevving as the trans fails? Will install an MSD ignition so ill have a rev limiter anyways.

But yea it needs a few new seals, and the pan leaks! I'll need to get a different pan, but thats no big deal.
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Balooning converters can be caused by line pressure too high, lots of HP/TORQUE without the converter having anti balooning plates on the front and rear, internal clearances being wrong, poor manufacteuring procedures(cheaply done), converter pump metal fatiqued (old age).
 
Thats good that it doesn't seem abnormal, I will just clean everything up and keep running it then,

That makes sense, I never fully understood what caused it or really what it was. Is it ever an issue on street cars?
 
I am just wondering now about the torque converter. I remembered a while ago I made a thread that was talking about the torque converter and about getting an aftermarket one.

You emailed me a sheet and some phone numbers Transman, and I was planning on changing out the converter over the winter as I have the stock one still in there. But just like everything there is always a budget, and now that my engine had to be taken out and fixed my budget is a bit smaller. I know its not ideal for performance, but would it be ok to continue running my stock converter in the trans? It was a new converter when I got the trans, but I dont know the specs on it, but it just looks new. I want to get a new one soon, but want to save up for a good one. When I was driving the car last summer I didnt have an issue with the converter, it performed quite well, the car was just held back by the gears. Thats why I was wondering about balooning, I dont want to have to pull the motor again because the thrust bearing is gone.

Currently the main priority is the engine, but I have all the parts and it is just a matter of putting it together. And the rear end as I am still running the stock 7 1/4 rear end with 2.76 gears, I feel that is where I am losing most of my performance. An 8 3/4 with 3.91 gears is going in soon.
 
DartVadar said:
Thats good that it doesn't seem abnormal, I will just clean everything up and keep running it then,

That makes sense, I never fully understood what caused it or really what it was. Is it ever an issue on street cars?
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There was only one instance were a replacement converter (stock rebuilt) supposedly caused a problem that I was aware of. My customer insisted that our converter caused the engine (a small block 400 Chev with a TH400 transmission) to take out the thrust bearings. I took it back to the shop, and asked the shop foreman to check it out without cutting it open. All dimensons were fine and in his opinion, the converter had not ballooned. My customer of course called me and the company every name under the sun, and insisted we were lying to him. At the time, there was another converter rebuilder in our trading area, so we sent it to him and asked him to inspect it and give us a written report. His findings were the same as ours, the converter was fine. Again the customer wasn't a happy camper. I later found out that the engine was in a 1 ton GM van and the owner of the van thought he had a 5 ton truck. Everything on the truck was overworked big time. The engine was on it's way out before the transmission was rebuilt and the converter replaced.

The other thing that came up about converters ballooning was they wouldn't just balloon in one drirection(towards the engine), but would also expand back into the transmission and would take the transmision pump out at the same time. Hence the reason on HP high stalls, have anti-ballooning plates on the front side and back side of any quality high stall.
 
That seems like it was a very extreme case and caused by using something to extremes that it was not meant for. So from that the event of a failure would be very unlikely for a light car.

My old cam said it could be used with a stock converter, but a 2000 would be best. I think my converter stalled at a bit over 2k, maybe 2200 or so by holding my foot on the brake, maybe more, I dont really remember(I didnt think factory ones were that high). My new cam recommends about 2700 stall or so. It would probably hinder performance more with the stock converter, but I can always change it out in the summer.
 
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