high rpm
Thread Starter
Advanced
Joined: May 2007
Posts: 62
Likes: 0
From: scarborough maine
high rpm
I have a '78 with a 454 conversion. The automatic transmission shifts fine and works great up to the 3rd gear at 40 or 50 mph, then the rpm's just keep getting higher and it sounds like i am in a drag race going 55. It is extremley annoying and i avoid going on the highway with it or cruising at high speeds. I think it may have the tranmission from when there was a 350 in it, but i can't tell. This thing has about 600 hp in it im sure it doesn't have to work this hard. it seems like i need about 3 more gears........
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
What transmission do you have installed behind the motor? I'm guessing it's a TH350 or TH400. In that case, if it's really in 3rd gear, and all you hear after that is engine RPM, then you just need different rear gears or an overdrive transmission.
What stock rear gears do you have now?
What stock rear gears do you have now?
Thread Starter
Advanced
Joined: May 2007
Posts: 62
Likes: 0
From: scarborough maine
first let me start by saying i dont have a whole lot of mechanical knowledge. I know the tranny is probably from a 350 but i dont know exactly what kind it is. i dont know anything about going about the rear gears checking and or replacing them.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Most good local transmission shops will do a free test drive to help you diagnose your car's problems. They will be able to tell you what your transmission is and they will even be able to tell you what your rear gears are.
If they feel there is a problem they will let you know. If what you are experiencing is just normal for a loud engine and a three speed automatic transmission then they will suggest either an over drive transmission or different rear gears.
CF JASOC Member
Joined: Feb 2006
Posts: 8,040
Likes: 34
From: Downtown Annapolis, MD. The Future is where we all have to live. Let's not screw it up.
St. Jude Donor '08-'09-'10-'11
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
I don't think he's going to know that. Not too hard to walk him through it so long as he doesn't mind jacking up the rear end and getting his back dirty on the garage floor.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Burning Brakes
Joined: Dec 2002
Posts: 1,069
Likes: 2
From: Sherwood Park AB
My 78 4 speed runs 3200 rpm with 3.70 gears at 60 mph. Maybe you have 4.11's. Is the tranny going through all gears?
Lift the rear end, make a mark on the drive shaft and spin the rear tires once. Count the rotations of the driveshaft... If the driveshaft turns 4.11 times, you have 4.11 gears... 3.7 times, 3.7 gears... or spin the wheels 10 times and divide the driveshaft rotations by 10.
Lift the rear end, make a mark on the drive shaft and spin the rear tires once. Count the rotations of the driveshaft... If the driveshaft turns 4.11 times, you have 4.11 gears... 3.7 times, 3.7 gears... or spin the wheels 10 times and divide the driveshaft rotations by 10.
Thread Starter
Advanced
Joined: May 2007
Posts: 62
Likes: 0
From: scarborough maine
I checked it out and they are 3.7 gears. I wouldn't think having the tranny from the 350 with the 454 would have this effect until high speeds. Was this ratio used for racing or something? It is shifting through all three gears fine tho, it just seems i need more. Where is the torque converter and what exactly does it do?
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
I checked it out and they are 3.7 gears. I wouldn't think having the tranny from the 350 with the 454 would have this effect until high speeds. Was this ratio used for racing or something? It is shifting through all three gears fine tho, it just seems i need more. Where is the torque converter and what exactly does it do?
The torque converter is a unit that connects the engine crank and transmission input shaft. It is filled with a viscous fluid. The engine and trans are not mated with a hard connection. Just the two spinning sides in the torque converter and the fluid driven by the crank turns the input shaft in the transmission.
I think you either need an over drive transmission or different gears. Maybe both.
