OKay, so I have a 406CID SBC with a 750CFM holley carb vacuum secondaries hooked up to a Th400 in a C4. Problem is that I cant stop the tranny from leakin, sometimes it doesnt, sometimes it does. :crazy: Also when I have it in 2nd gear or even 1st, their is really no difference between 1/2 throttle and full throttle. This is really ticking me off b/c I worked 10 weeks on this car and it doesnt even pull for s h i t. I mean sure it pulls in 1st gear when I get on it, but after a while from driving it, say 10 mins it doesnt react the same way.

Also the tranny doesnt upshift ever into 3rd gear unless I stomp on it hard and the revs finally get to 6000rpm. Another thing is it feels like the transmission is slipping, being that its out of a UPS style van, I hope I can solve this problem soon.

Plus for anyone out there that has bought a rebuild or brand new high HP capacity TH400, does it suck up a ton of power or does it not when you swap it out for a rebuilt one? :confused:

Comeone guys, help me out here, after installing a 406, it has nothing to show for itself b/c it doesnt even pull hard. Im starting to think that a 406 is possible of only making 200+HP with my setup even though I have an airgap intake and block hugger headers. Maybe I need some new heads? :confused:

 

I would check a few things in this order:

1: Check fluid level (hot)
2: Check to make sure vacuum modulator has a manifold vacuum source and it is hooked up.
3: Check adjustment of vacuum modulator (assuming its adjustable).
4: Drop pan and look for metal

I had my vacuum line come unhooked on me while on a short trip and it felt exactly like what your describing. Crawled under the car and saw the vacuum line hanging down, plugged it in and I was off and running.

 

Wheres the best place to T up the vacuum line? Also how much hP can my stock UPS style Th400 eat up? COUld I gain more power from getting a brand new High HP/ Rebuilt Th400? :confused: I mean it feels pretty slugish. :nonod: :boxing

 

I'm not sure where you would hook this up on a C4, you just need manifold vacuum so it gets a signal related to engine load.

Information sources I can find argue that basing power loss on a % basis isn't accurate. I cannot argue for this position as I don't have any experiance with testing these. It is said it consumes roughly 40-60hp.
A converter that slips a lot will obviously increase this number.
Also, if your TH400 came out of an industrial vehicle it may have straight cut planetary gears and should be very stout if/when you decide you want more power.

If your 400 is slipping badly (could be), yes. If its in working order there are some things that can be done to reduce loses, but it gets spendy.


If you don't have the vacuum modulator hooked up it is going to run like *(#($. Is it hooked up?


Good Luck!



[Modified by VETDRMS, 9:45 PM 10/29/2003]

 

I hooked it to a T fitting from a vacuum line that came out of the canister, but it did not help at all. SO being desperate, I drilled a small hole in the base of the air cleaner and plugged the hose into the hole. :crazy: Yeah, even after that it did not help, the tranny would not shift, could it be that my Vacuum shifter is all screwed up?

Also I bought a downshift kit, but I read the instructions and I dont get it. IT says that it must pull the switch out, but the rod in the picture and the way thet describe it would just push it and rotate it? :confused: Is there anyway to get a C3 Style Downshifte button? INstead of one that requires an odd looking deal that goes on the carb and the linkage of it? :confused:

 

OH yeah, I watched your videos, man thats so baddazz, I mean how hard does that thing accelerate? :confused: The vid when you where rolling by and hammered it and almost lifted the front tires up off the ground, what gear and how hard did you open the throttle on that run? :confused:

 

Light, you still haven't tapped into a valid vacumn source, that is your (first) problem. If there are no threaded holes into the intake manifold plenum/runners, then you will need to drill one. You need to have a vacumn source that is BELOW the carburator. The air cleaner is not a source of vacumn.

The TH400 is extremely tunable on shifting, it uses both the vacumn modulator and an internal shift governer. Go to http://www.chevelles.com and in the forums area you will find both a performance section and a transmission section. Do a search and you will find a TON of TH400 information.

Thomas

 

Absolutely! You haven't hit vacuum, yet. The modulator signel "tells" the trans how far open the throttle is. At full throttle it gives you later (it winds out) and harder shifts, like you want at WOT. Manifold vacuum drops as the throttle is opened. Without a proper vacuum signel to the modulater, the trans thinks the gas is floored, all the time. That doesn't mean it doesn't have other problems, but there is no point in trying to assess them until you get the modulstor working.

Do you have any history on this trans? You mention it being a "UPS special". Are you sure it isn't a TH475? The TH475 was used in heavy duty applications, like trucks (other than pick ups). It will hold up as well or better than a stock TH400. It's a heavy duty mother. One thing to realize, however, is that the TH475 has an extra low ratio gear set. The TH400 uses a 2.48:1 first gear. I think the TH475 has a 2.75:1 first. This, plus the inoperable modulator, may give you unexpected driving impressions. You have some adjustments, to do. Good luck, and...

RACE ON!!!

 

The transmission is heavy, but how can I really tell if its a Th400 and not a Th475? Do the pans change between the two? IF they donot, then mine is a Th400 b/c I did change the pan and it fit perfectly. :yesnod:

Also, I'll just buy a new modulator and try to find a better vacuum source and let you guys know how it goes. Thanks for all the help. :yesnod:

 

The two transmissions are identical. The differences are strickly internal. Just like... don't want to go too deep, here. They are identical, externally.

Why would you buy a new modulator, before you even hook up the old one?

I saw a post of yours on the Tech/Performance section, where you asked jburnett about the play in your "C" beam/rear trans mount. Didn't we address that a month or so ago? Is that thing still flopping around in there?

Good luck, and...

RACE ON!!!

 

Its constantly on my mind that It might break somehow, I actually rechecked it today and its only 1/4inch play, about 1/8 down and 1/8 up. I dunno if this might affect the stress on the drivetrain so this is why I bring it up many times to hear others opinions on what they think about it.

Also I hooked up my vacuum line with a T fitting and drove it, its pretty good seeing how it upshifts smoothly and now goes into 3rd gear and I can keep it in 3rd gear at low speeds so I dont waste too much gas cruising around local. :yesnod:


Other then that heres my problem, I dont feel any kind of difference between part throttle acceleration and full throttle acceleration. I tried this in 2nd and 3rd gear and 25% throttle has no difference compared to say a 75% or even full throttle pull. I seriously think the tranny is slipping but my dad insists that it may need a tune and buying a new tranny wont help. What do you think? :confused:

 

Well, from here, I can clearly tell that... You're kidding! Right???

Good luck. You're gonna need it, and...

RACE ON!!!

 

Why not put in a full manual valve body and then you don't need a vacume hooked up to it. Of coarse you will need to put in a ratcheting shifter like the B&M Megashifter. I have a manual valve body in my th400 and enjoy ratcheting up and down the gears under full throttle. If you do decide to go manual valve body make sure you get one that is a forward pattern because those are the only kind that provide engine braking. The popular reverse pattern ones are only suitable for drag racing because they don't give you any engine braking. Also, I hear people saying that the th400 robs you of too much horsepower compared to the 700r4. I don't buy it. I have had both in my C4 and I don't think you can tell the difference in parasitic loss. My th400 even has a Gear Vendors overdrive bolted to it and it pulls just as strong if not stronger than my 700r4's ever did (notice the 700r4 is plural, because I demolished a couple of them). Good luck getting you car set up.

 

Thanks Bob, but the thing is their is really no difference between 1/4 throttle and full throttle acceleration in either gear at any speed. So I'm not going to take my chances and I'll just buy a rebuilt Th400 and install that one and see if it still doesnt pull which I Hope that wont happen. :crazy:

 

Light, let me gently suggest that you enlist help on a local level. The TH400 is a farm simple transmission, there is virtually no town in America without a professional trans tech that couldn't diagnose and repair that transmission practically while you were reading the newspaper in the customer lobby. Wouldn't matter if the tech was into performance or not, there are still so many TH400's in operation that all trans techs have worked on more than a few. Your issues have nothing to do with your application being a hot rod, just basic stuff.

I say this only because I worry that someone who thought vacumn could drawn off the air cleaner might also be missing other fundementals to good TH400 operation.

Vacumn modulator and governer - that's it, somewhere between one, the other, or both lay all of your probable issues.

Trans slippage if not manifested in burnt particle laden fluid, can easily be computed mathmatically - the tricky part about trans slippage calculations is the source of the slippage: converter or trans internals. For first gear slippage calculations you will need to know for sure if yours is a TH400 or TH475 because of ratio differences. Third gear is one to one in either version.

Formula one:
Theoretical no-slip high gear (1:1) MPH = (rpm X tire diameter X 2.96) all divided by (differential ratio X 1,000)

Formula two:
Theoretical no-slip high gear (1:1) RPM's = (MPH X gear ratio X 336) all divided by tire diameter

To adjust either formula for first gear, simply multiply differential gear ratio by first gear ratio.

Now just compare observed mph or rpm against theoretical - the difference is your slippage.

Slippage that does not occur under steady cruise, but does under acceleration, is simple enough: rpm's rise without corrosponding increase in speed, perhaps with a noticeable lag in speed buildup as rpm's hold *steady* after pressing down on the throttle.

Good luck, Thomas

 

Thanks for the help, I guess my transmission is not slipping but my engine must not be as powerful as I once thought. Thats what you get when you buy a rebuilt 406, oh well. :rolleyes:

Also, I tried wiring my downshift module, but I dont know if it is supposed to take positive current or negative? :confused: