3:55 gears
Thread Starter
8th Gear
Joined: Aug 2017
Posts: 8
Likes: 0
3:55 gears
I have a 74 corvette with a zz4 crate engine (about 365HP) I can idle it in the heat and it stays below 200 degrees. However, on the highway even going 65 MPH has the RPMS over 3,000 - it's screaming and going above 200 degrees. I know I would lose some power but would I be better off with a 3:36 or so? It's running a 3:55 now
Last edited by SKROCK51; Aug 8, 2017 at 08:43 PM.
Melting Slicks
Joined: Jul 2006
Posts: 2,285
Likes: 487
From: Salmon Arm, BC
It will be better but not by much. When I had 3:36 in my car it was 2750 @ 60 MPH.
The best thing is to move up to a 5 speed trans, much more expensive than a gear change, but hands down the best mod I've made to my car.
Oh....welcome to the forum
The best thing is to move up to a 5 speed trans, much more expensive than a gear change, but hands down the best mod I've made to my car.
Oh....welcome to the forum
Last edited by OMF; Aug 8, 2017 at 11:45 PM.
Le Mans Master
Joined: Jun 2013
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From: Colorado
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If it's the heat you're concerned about, consider a Dewitt's dual core radiator. I have one and never go above 200 degrees, even in parades. 
Melting Slicks
Joined: Jun 2006
Posts: 2,071
Likes: 147
From: S Nevada
I have a 3:55 ratio in my diff. With a 27" tall tire, it runs about 3200 at 70. The engine temp would be what it should be, 190 or so. It was a 383. New engine going in soon, much more performance. I too run a Dewitts. Ive been using the stock mechanical clutch fan. That will soon change to dual 11" electric fans. How accurate is your temp indicator? The factory sender and indicator can have a 15% error. That was considered acceptable, this according to an article on the guage. Im using an autometer mechanical guage. I tested it to be within 2 degrees of my digital meter.
Thread Starter
8th Gear
Joined: Aug 2017
Posts: 8
Likes: 0
I have a 3:55 ratio in my diff. With a 27" tall tire, it runs about 3200 at 70. The engine temp would be what it should be, 190 or so. It was a 383. New engine going in soon, much more performance. I too run a Dewitts. Ive been using the stock mechanical clutch fan. That will soon change to dual 11" electric fans. How accurate is your temp indicator? The factory sender and indicator can have a 15% error. That was considered acceptable, this according to an article on the guage. Im using an autometer mechanical guage. I tested it to be within 2 degrees of my digital meter.
Could be that previous owner is wrong about ratio. I will have it checked
Thread Starter
8th Gear
Joined: Aug 2017
Posts: 8
Likes: 0
I have a 3:55 ratio in my diff. With a 27" tall tire, it runs about 3200 at 70. The engine temp would be what it should be, 190 or so. It was a 383. New engine going in soon, much more performance. I too run a Dewitts. Ive been using the stock mechanical clutch fan. That will soon change to dual 11" electric fans. How accurate is your temp indicator? The factory sender and indicator can have a 15% error. That was considered acceptable, this according to an article on the guage. Im using an autometer mechanical guage. I tested it to be within 2 degrees of my digital meter.
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Thread Starter
8th Gear
Joined: Aug 2017
Posts: 8
Likes: 0
Melting Slicks
Joined: Oct 2008
Posts: 2,154
Likes: 69
From: Regina Sk Canada
I have an 80 vette with a 350hp 355 sbc, Th350, just got rid of the 3.08 for 3.55 gears. My car runs 195-200 all the time, normal operating temp. I have a copper 3 core rad, shroud, 18" fan and a 195 thermostat. motor has 25 initial timing with 35 total. Headers and a 2.5" exhaust. Rear tires are 27" tall. 295/50/15 cooper cobra tires.
Last edited by Grumpy 427; Aug 9, 2017 at 08:59 AM.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Work on your cooling system
How old is the radiator, when is the last time you blasted between the fins with a water hose? Run your motor at 3k for a minute or so see if your lower hose is gettting sucked shut
If youre running a mech fan consider adding an electric hooked to a thermostatic switch that comes on at say 200. On the hwy though you shouldnt need it.
List your location...I have a fantastic trans guy who builds bulletproof 700rs installed for WAY under 4k. Sounds like you need to work on your cooling system first.
How old is the radiator, when is the last time you blasted between the fins with a water hose? Run your motor at 3k for a minute or so see if your lower hose is gettting sucked shut
If youre running a mech fan consider adding an electric hooked to a thermostatic switch that comes on at say 200. On the hwy though you shouldnt need it.
List your location...I have a fantastic trans guy who builds bulletproof 700rs installed for WAY under 4k. Sounds like you need to work on your cooling system first.
Last edited by cv67; Aug 9, 2017 at 09:08 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
A Chevy engine running temps of 210*F or less is no big deal. You likely have a 195*F thermostat. Running at 3000 rpm for an extended period would cause the engine temps to rise; but that amount is no big deal to the engine (unless you are putting only water in the cooling system).
But, I do understand the concern about running those revs at highway speeds. Even if you changed to a 3.08 rear gear, the revs will only drop to about 2500 rpm. To make both you and the engine 'happy', you really need to install an overdrive transmission with a .67 high gear. You will get better 'off the line' acceleration with the OD trans low gear; and you will get much better mileage and car/driver comfort at highway speeds (2000 rpm; 15-20% better mileage on the highway).
If you do a lot of highway driving, the OD trans may actually pay for itself over several years. Otherwise, you would be doing it to save engine life and driver comfort, which is what I did. And it was definitely worth it.
But, I do understand the concern about running those revs at highway speeds. Even if you changed to a 3.08 rear gear, the revs will only drop to about 2500 rpm. To make both you and the engine 'happy', you really need to install an overdrive transmission with a .67 high gear. You will get better 'off the line' acceleration with the OD trans low gear; and you will get much better mileage and car/driver comfort at highway speeds (2000 rpm; 15-20% better mileage on the highway).
If you do a lot of highway driving, the OD trans may actually pay for itself over several years. Otherwise, you would be doing it to save engine life and driver comfort, which is what I did. And it was definitely worth it.
Last edited by 7T1vette; Aug 9, 2017 at 10:07 AM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Running hot at idle indicates too little air flow, running too hot at cruise indicates a radiator that is inadequate. The being said all seals and air dam must be in place. I don't think 200*-210*is too hot, especially on a hot day.
I run a 3.55 in my 77 with a TH350 and it runs 3200 or better at 65. I can run it indefinitely at 80 MPH screaming away with side pipes at 3700 RPM and it still stays cool.
Stay with the 3.55. I think you would regret getting a 3.36 or lower ratio just to lower the highway RPM's.
I run a 3.55 in my 77 with a TH350 and it runs 3200 or better at 65. I can run it indefinitely at 80 MPH screaming away with side pipes at 3700 RPM and it still stays cool.
Stay with the 3.55. I think you would regret getting a 3.36 or lower ratio just to lower the highway RPM's.
Melting Slicks
Joined: Jun 2016
Posts: 2,979
Likes: 332
From: Woodinville WA
Others have said it, and I'll agree: a modern OD transmission or a 5 speed.
You can get a torquey, fun 1st gear for more UMPH off the line AND a much lower cruise speed. -Less noise, less heat, more MPG, and less engine wear-and-tear. A lock-up TC will get you even a few RPM lower still and will reduce transmission and engine coolant temps, if using a stock AT radiator integrated trans cooler. The transmission determines your actual "gear ratio" between the engine and the tires and a modern transmission can give you a much wider "spread" so you can have your fast-off-the-line cake, and eat it too (low cruise RPM).
With 365HP and an automatic, you don't need a super "built", expensive transmission and you can EASILY save a bundle with a used transmission although you need to be careful.
Adam
You can get a torquey, fun 1st gear for more UMPH off the line AND a much lower cruise speed. -Less noise, less heat, more MPG, and less engine wear-and-tear. A lock-up TC will get you even a few RPM lower still and will reduce transmission and engine coolant temps, if using a stock AT radiator integrated trans cooler. The transmission determines your actual "gear ratio" between the engine and the tires and a modern transmission can give you a much wider "spread" so you can have your fast-off-the-line cake, and eat it too (low cruise RPM).
With 365HP and an automatic, you don't need a super "built", expensive transmission and you can EASILY save a bundle with a used transmission although you need to be careful.
Adam
Last edited by NewbVetteGuy; Aug 9, 2017 at 01:08 PM.
Drifting
Joined: Oct 2005
Posts: 1,615
Likes: 150
From: Forth Worth TX
I'm amazed that a possible over heating issue is met with recommendations to switch to an OD transmission. Really? That's the optimum fix?
Your 3.55 gears have nothing to do with your engine running hot. It's your cooling system that needs attention.
Your 3.55 gears have nothing to do with your engine running hot. It's your cooling system that needs attention.
