longtimedead

Hi ,I have a 79 Corvette with a L48 engine and th350 auto box and have just changed the engine to a new 383,i kept the same Edelbrock carb and inlet manifold and hedman headers from the old engine and they are now set up on the new engine which has now run in and running well ,my concern is that the wot shift used to be 5000 to 5500 ( can't remember for sure) but is now 7k.
I'm not sure if this is simply due to more power of the 383 or if the 7k is ok for the engine so would appreciate some advice on this ,it has flat top pistons and roller rockers so i presume it will rev higher without a problem ,the pipe from the manifold to the modulator was replaced with a new one and the kickdown cable reset but other than that nothing else was done .
I have the 4.11 axle which is great fun and makes the 7000k come up fast but i intend to change the ring and pinion or get a 700r4 at some stage in the near future to keep the highway revs lower,I have also read that you cant simply change the 4.11 ring and pinion on its own as the carrier for the 3's is different and was wondering if that is true as finding any parts over here in the UK is not easy
Many Thanks in advance

JimLentz

Check with the builder of the engine, but I doubt you should be running that engine up to 7000 RPM. How does it shift if you aren't at full throttle?

bashcraft

You'll need to ask the engine builder if it's safe to run it up to 7000 rpm. It depends on many things that you haven't shared with us.

3 and 4 series carriers are different, but you may be able to trade yours for a 3 series.

longtimedead

Hi Jim, it changes exactly as it should at all speeds ,very smooth and no odd noises ,the only other thing it does do occasionally when it changes at wot at 7k is sometimes gives a "floaty" feel when it changed up as though its slipping when it goes into the next gear ,if i back of the throttle slightly it settles down and pulls hard and smooth again ,it only does this on wot and not when driven normal or even hard ,i did wonder if this down to the fact the gearbox is not able to cope with the 7k change as i have never changed above 5500 on the old engine .
The story of the engine is a long one but the long story short is that i had an opportunity to buy this engine at a very good price ,it had never been run and was built in the usa before it came over and was checked before fitting but no one knows who the original builder was and therefore we dont know the spec

Ibanez540r

"having roller rockers and flat top pistons" DOES NOT mean the motor is capable of 7000 rpms. To keep it short and sweet, it takes a lot of plans and quality parts to turn a 383 to 7000 rpm. Until you know exactly what you have I would not spin it much over 6-6500.

cagotzmann

I have a GM ZZ383 which is only rated for a 6000 RPM Redline. Look at its specs and you will see for what you have shown us I wouldn't run to 7000 RPM. I also have beehive springs in the heads to improve high RPM issues, but 7000 RPM ?

Now when I had a TH350 in my 78 it would shift at 4500-5000 WOT. With the Stock L48 it shifted 4000-4500. What kind of vacuum at idle with this motor.

Soon after the install the 2nd-3rd shift starting getting soft.(slipping) pull the TH350 and replaced with a T56.

7T1vette

Chances are that the tach filter (little can mounted near the distributor on the intake housing) is no good and your tachometer is reading high all the time. Find someone with a Dwell Meter/Tachometer, hook it up to your engine, and compare RPM readings between that meter and your car's tachometer. If yours checks higher, you need to replace the tach filter.

bluedawg

I also think that the tach filter isn't making a good conection which I did after doing the top end on the l48, but if it is connected making good connection then we'd need to know a few things to know that it is good to 7k. If you had the engine built by someone, they should know. If not the biggest question would be what type of lifters are you running? What are the cam specs? What type of crank, rods and pistons are you running? Forged or cast? Weather you built the engine or you had some one else do it maybe you can share the build details for the engine? Just building the engine won't change the shift points at WOT. More than likely the tach filter is loose or there is something wrong with the govenor.

longtimedead

Hi again and thanks for the reply's ,Just to answer a few of the questions you have asked .
I do not know the spec of the engine build ,all i know is it was built in the states and then purchased by a small reputable hot rod builder who had shipped it over from the states in a crate with a load of other stuf and just took a chance on it ,they checked it over in there workshop and were prepared to stand by it if i had any problems as it was an "unknown engine ", i paid just one third of the normal price of an engine over here and thought it was a risk worth taking ,i agree 7000 is high and would like some help and ideas on why its gone from a shift at 5k to 7k after i changed the engine and how to get it back down to around 6k .
I have checked the tach filter and refitted making sure its a good connection to the engine but it has made no difference ,I have not seen any for sale in the uk so would have to order from the states to get a replacement ,my vacuum is around 16/17 my timing light is a accuspark sp8000 it has a rpm dial but i do not think they work on V8's so cant check if its reading correct but it definitely reads different to my rev counter with it being either higher or lower ,depending on the setting its on

bluedawg

When it shifts does it rev super fast when it goes from 5k to 7k rpm and not seem to accelerate during the last increase in rpm? Basically does it sound like it hits neutral? I believe Wilcox has a paper on how to check the filter.

427390

You probably need to change the governor weights in the transmission - Google "TH 350 Governor weights" and you'll see there is a B&M kit for about $60 that allows you to move the shift points up and down. I recently did this on a Hughes TH400 that was shifting at too low an RPM and it's a relatively easy fix

bluedawg

I'm less likely to think the govenor all though it could be, I'll bet the tach filter is starting to go out.

diehrd

At wot the governor controls shift points . Nothing else . Get a spring kit read how it affects shifting and swap out springs as needed it may take a few tries but is easy to do.

bluedawg

The govenor worked be for which leads me to think that it should be working now unless it's got trash or debris holding it in the wot position which would make all shifts happen later in the rpm and not just wot, i had that happen once, after a piece of s41t tranny company rebuilt the 700r4. I find it hard to believe that more horsepower would cause the govenor to shift latter in the rpms let alone 1500 rpm latter when nothing had been done to the transmission except disconnecting and connecting the vaccum for the module and the kick down cable. So if it's truly shifting at 7k, i would investigate those two areas before adjusting the govenor weights and springs, i could be wrong. I really don't think it's shifting that high and he's getting false readings on his tach.

longtimedead

Thanks for the ideas I think i will try the filter first as its the easiest option and then the governor ,to be honest it never really feels/sounds like its at 7k and pulls hard all the way to 7k without feeling its running out of breath or sounding strained .

BKbroiler

You guys who are saying its easy to change governor weights and springs on a TH350 trans - are you talking about with the trans installed, in a C3?
Without cutting a hole in the floor, it looks nearly impossible.

diehrd

No cutting required

bluedawg

I'm not saying it's easy and know that it's a pita, i don't figure that's what's wrong with it.

BKbroiler

Thanks. I know where it is but the trans tunnel is very close to the governor. I don't think its doable on my car. My shift points are good as is, but if I was able to get at the governor I might play with the adjustment a little.

7T1vette

You don't really "play" with the "adjustment". There is no adjustment, other than to remove the governor and change out springs and weights...and that's not just a simple task, as you need to cut off the ends of the pins holding the weights in place and replace them with something when you rebuild the governor (each time you do it....complete trial-and-error, unless you have pro equipment).

If your problem has anything to do with the governor, it is that the valve in the governor shaft is stuck, or the little filter screen in the governor pressure line is plugged/blocked. If either of those things happen, the governor may not shift at all--which is NOT what you are describing.

I still have trouble believing that your engine is actually turning 7000 rpm.