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700R-4 throttle valves- different for cars and trucks?

Blue85

Blue85

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I am having problems getting my 700 to shift right after converting to TPI.

I am using a normal TPI throttle body and linkage with my stock 1985 TH700R-4 TV cable. If I adjust it per procedure (so that WOT clicks the adjuster), it shifts super mushy and early. If I move the adjuster tighter until it shifts the way it did with the carb, the TV bottoms out and I can't open the throttle past 80%.

I have compared the geometries and both have a throttle radius of about 1.5" and a TV radius of about 1 1/8". In both cases, total stretch of the TV cable from idle to WOT is about 1.25". So my TPI must be making torque with smaller throttle movement than the Q-Jet. I don't know how else to explain this.

Here's a drawing of the Q-Jet linkage and the TPI linkage

PICT0093.jpg


So how do I fix it? I had two ideas:

1) Put a new attachment point on the throttle linkage with a smaller radius (closer to the throttle shaft) and adjust the cable tighter.
2) Put a stiff spring between the TV cable and the throttle, so that it opens normally until the TV is bottomed out. Then the spring stretches to let the throttle open all the way.

But is there some difference between the TV valve in a truck valve body and a Camaro valve body? Should I find one of these and swap it out?

I *should* check and see what rpm my WOT 1-2 shift occurs at so that I know the tranny is working the same as it did with the carb, but I have no reason to doubt that it does. Plus, it used to occur over 5000rpm (I have a Camaro governor in the tranny - I would always let up on the pedal to get a high load 1-2 shift) and this requires a high level of trust in my ECM tuning, which is still in progress.
 
Thinking more about this, it probably relates to the tiny primary throttles on a Q-Jet. This TPI setup does come off-idle a lot quicker. This is a stock 48mm TPI throttle body, but I have 52mm rebuild on order, which should make this problem worse.

Maybe I should change the whole geometry so that the TV moves quickly at first and slows down as the throttle opens further.

Somebody must have experience with this.
 
Have you had a search around on Thirdgen? If you're going to find your answers anywhere, it's going to be there. I've never seen anything about different TVs but then I've never really looked.
Interesting about the Camaro governer.....food for thought!
 
Even for the same year, different letter-code (YC, for example) 700R4s were intended for heavier, or more hi-po vehicles, and ran somewhat higher line pressures than some other 700R4s because of that. The important thing when changing your induction system is to have the correct TV cable brackets to keep the correct geometry and move the cable as it should be moved. They can probably give you more info here on the 700R4 and TPI:

http://www.chevythunder.com/

My hunch is that the TV cable hardware and brackets from a TPI/700R4 Corvette would work OK, but don't quote me on that (it's just a hunch). Talk to a real trans pro about the whole thing.
 
Well my TPI setup came from a Camaro that must have had a 700R-4 and I am using the bracket which came with that. The picture I posted above is from the B&M Super Transkit book and everything I have complies with the guidelines they have for setting up a TV on a custom install.

The thing is that the vacuum drops off really fast with throttle opening on the TPI. The Q-Jet is a different beast because the top flaps of the secondaries don't even open at low rpm. It has a flatter relationship between throttle position and load. With the TPI, 25% movement of the throttle must be 80% of the vacuum gone at low rpm.

I have searched all over thirdgen and didn't find anything. Their site really doesn't have as much real tech as our site does.

I fabbed up a bracket tonight that slips over the TV and the cruise control attachment points on the throttle linkage. It has a new attachment point for the TV cable with about 75% of the radius of the original attachment point. With this setup I am getting good shifting and I am able to reach about a 95% TPS reading (it is calibrated so that the mechanical limit of the throttle body is 100%). So something like this may work out.

I would like to find out if there really were any differences inside the trannies, but even if I find out what I need, I'm not looking forward to pulling the valve body out to swap some valves. Maybe the TV can be removed without pulling the valve body? I don't remember. Anyway, I have been trying to dream up an eccentric cam of somekind to give rapid movement where the throttle is nearly closed and slower movement approaching WOT. I don't have any great ideas yet.
 
Oh yeah, BTW, I was able to hit 5200rpm tonight testing this and everything looked good, so my tuning is progressing nicely.
 
Blue85 said:
Oh yeah, BTW, I was able to hit 5200rpm tonight testing this and everything looked good, so my tuning is progressing nicely.
Click to expand...
Was that the Camaro governer controlling that shift or did you shift manually?
My VE table only goes to 3200rpm,(7747) have you extended yours somehow and altered the BLMs all the way through?

Sorry for the hi-jack!
 
Oh yeah, BTW, I was able to hit 5200rpm tonight testing this and everything looked good, so my tuning is progressing nicely....

if that was the tranny controlling the shift you need to readjust the TV cable again.
if it was adj. properly i would say at WOT it should shift itself no later than 4500!

you can adjust the TV right by the bracket on your intake with the little button on the cable but if you do it wrong, GOOD BYE TRANNY CLUTCH BANDS!!!
LUKE
 
blackblazer717 said:
if that was the tranny controlling the shift you need to readjust the TV cable again.
if it was adj. properly i would say at WOT it should shift itself no later than 4500!

you can adjust the TV right by the bracket on your intake with the little button on the cable but if you do it wrong, GOOD BYE TRANNY CLUTCH BANDS!!!
LUKE
Click to expand...

Yes, this was the tranny controlling the shift. I run a Camaro governor for higher shift points. Compared to the (gas) truck governor, it has far less weight.

The point of this thread is that the "correct" procedure for adjusting the TV cable is not working for me.
 
Get a pressure guage and do it that way. I never have been fond of the ratchet method.

Just put the guage into one of the test ports. I cant remeber the procedure but you should adjust it so that as soon as you touch the throttle the pressure starts rising. A simple search would give you the correct instructions.
 
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