Here's the story. After a very long two year wait while piecing together my 89, I finally got to run it for the first time at a quarter mile strip today. I drove it the 120 miles up the Florida Turnpike to the Palm Beach Motor sports grounds, formerly known as Moroso. I guess you could say I was a bit pumped up since I was the first in line to get in, about an hour early. At home in Miami, there are only 1/8 mile strips which I run at frequently, so you can understand the intensity I was experiencing with the anticipation of finally getting my car on the 1/4 mile to see just what it would really do.

What a great track! Flat, fast, and sticky! On my first run the Nitto's hooked up beautifully and I got a good RT and strong run down the track with 3rd gear just topping out at about 5800 rpms I pushed it into Drive with about 60 feet to go, and my auto trans REFUSED to shift into 4th gear!! BUMMER!!! I ran it 3 times, and even tried to let off the gas at the top of third and then jump on it again to hopefully get it to go into 4th. Not a chance. What is the problem????

When driving it normally around town etc, the car shifts just fine and when I ran it on the 1/8 mile it never needed to shift to 4th gear, as I was still pulling strong in 3rd going through the 1/8 mile trap. You can imagine my disappointment when this little problem reared its ugly head! Any thoughts on how to fix this or what caused it??

Just a little background on my car and trans FYI.

The trans is a newly rebuilt and beefed up 700R-4 with a 2800 stall converter. It also has a BM shift kit. 3.73 posi, powered by a 383 stroker with a Super Ram, Comp's cam hydraulic roller rocker with1.6.1 rockers. and 2.10 -230 x .510- .510 @ 114 LSA, AFR 195 heads, Hedman long tube headers, no cat's dual exhaust into two Corsa mufflers, and a set of Nitto Drag Radials. The car ran 12.57 being stuck in 3rd for the last 50 feet or so, at 106 mph. My RT was .075 with a 1.81 60 ft. With the motor wound up near tight, it had to cost me some time without 4th gear in it's sweet spot for the finish.

Your thoughts please !!

Steve

 

Does your Corvette engage 4th gear OK driving around town & on the highway part throttle ?

My 87 Vert will not hit 4th gear- overdrive at WOT, foot on the floor till I hit 114 mph.
Then it holds in 4th till I hit top speed of 150.

Hitting overdrive in a C4 while racing down the 1/4 mile at the big end ( near end of 1/4 mile) is not productive to Low ET and MPH trap time & speeds in my opinion.

Brian R.

 

Its normal , my vette does it also. Fine driving around town but have to lift my foot of the accelerator slightly to get it to change on the track when around 200kph it is really annoying, i think they are trying to protect the overdrive gear during the 3-4 change
On the 1/4 mile i just keep it floored don't need to change to overdrive
but on a standard engine the torque band is best between around 1,500 to 4,800 rpm and needs the overdrive at around 4,800 rpm before the engine runs out of puff.

 

Sorry, I'm no expert but.........These transmissions won't shift into 4th at WOT. You can get a modified throttle valve sleeve for less that $50 that will allow full throttle 3-4 upshifts. I believe this was done because the 3-4 clutches didn't like a full power shift. It's very easy to install without removing the transmission. Hope this helps.

 

If you still have the original C4 trans in the car it should go into 4th. If the trans came from another body style, no. 1963SS is refering to the Vette valve that can go in another trans and allow the shift.

 

Thanks Jack. Oops, I must have missed that one. Sorry about that. If your transmission is a Vette transmission it should shift at WOT but since it doesn't, and you said you had a transmission rebuilt.....was it the same transmission or another core? Inquiring minds want to know

 

The trans is the original one that came with the car. It shifts just fine in normal driving. The problem is that I'm pretty much topped out in third gear at around 5800 rpm's, BEFORE I get to the end of the 1/4 mile run. I think 4th should kick in at about 5200 rpm's or so, instead of stressing the motor to it's max rpm's. stuck in 3rd. It probably wouldn't be a problem if I hadn't gone to a 3.73 rear end gear, as a lower ratio would not be maxing out in third, before the end of the run.

 

its not going to want to shift at WOT and the rpm drop could hurt your time. Try a taller tire instead

 

You're correct. I am sure it hurt my time, But why won't it shift into 4th? That's my question! I suppose I could go to a taller tire, or even drop back to a 3.54 rear gear, but I don't understand why the trans won't make the shift to 4th at WOT, since it does otherwise when just driving normally. And, is there a cure for the non shift to 4th at WOT?

Steve

 

Something doesn't add up. At 5800 rpm at the top of 3rd gear, you should be running a lot faster than 106mph with 3.73 gears. Your torque converter or tranny must be slipping big time or you are running go cart wheels. I ran for years with a 3.73 and ran 115+ at the top of 3rd and never needed 4th. Try locking up your converter in 3rd.

 

I'm running stock tire size= 275-40-17, with the Nitto drag radials on the rear end. How do you check to see if the converter is locking up, or how do you lock it up?

Steve

 

Since its a C4 trans, the other possibility is the governor needs to adjusted to make it shift at a lower RPM.

 

still looking for a definitive answer. I was told that 4 th is actually overdrive, and that as long as I am WOT, the trans won't shift to 4th. Is this true? Can it be by-passed to make it shift?

 

When they rebuilt the transmission did they rebuild yours or give you another core. If you bought one already built it may have not been from a corvette. In that case see post #4. Many vendors sell a TV sleeve that will fix this definitively. 4th is an overdrive but only real Corvette transmissions (not trucks, Firebirds or anything else) will shift to 4th at WOT. The reasons are explained above. Is this definitely definitively enough?

 

Ground the tcc lockup wire from the ecm to the tranny. I use a switch on the console and lock mine up after the 1-2 shift. It's good for a couple mph and .15 in the quarter.

 

It takes alot more specialized parts than that aftermarket TV valve sleeve to get the "C4 Corvette only WOT 3-4 upshift without lifting up the gas pedal.

Simply put,

C4's had a specially tuned valvebody unique from any other GM car or truck.

Bar None.

Brian

 

How does going to lock up in third produce a faster time? If you go to lock up, aren't you lcoking up converter and not getting any torque multiplication? I may be misunderstanding what is going on at lock up. Please explain so I can understand. Thanks

 

With the torque converter in lockup mode on a C4 700R4 or 4L60E auto trans,

There is no torque converter slippage present then.

It acts just like a 4, 5, or 6 - speed manual transmission,

True direct drive coupling between the engine & transmission.

 

They definitely rebuilt my core, as I stood there and watched them. I also remember before I had it rebuilt, that when I would drive the car on the turnpike and floor it while cruising in drive at about 70 mph, it would go into passing gear (3rd gear) and then when I hit about 85 mph, it would shift back into 4th gear and continue to accelerate (staying in 4th gear) until I let off the gas.

When the trans shop did the rebuild, they also beefed up the trans for the additional horsepower I had added with the engine mod's I had done, and I had them add a Hurst shift kit for a more positive gear change between 1st and 2nd. I'm thinking that there is something wrong or not adjusted correctly, that is keeping the trans from shifting properly.

Steve

 

Install a full manual valve body and you'll be in business.