Extremely high RPM at relatively low speeds.
08-31-2016, 03:21 PM
#1
Heel & Toe
Thread Starter
Member Since: Aug 2016
Location: Huntersville North Carolina
Posts: 19
Received 0 Likes
on
0 Posts
Extremely high RPM at relatively low speeds.
Hi there. I just got my C3 roadworthy, and I've only got one real problem left to tackle before she's a dream to cruise around in. I was hoping some of you folks could help me out.
She's a 1976 Stingray with a three-speed automatic transmission. The issue I'm having is that she revs to ridiculous excess. At 50mph, she's already at 3,500RPM. Climb to 60, and you're well above 4,000RPM. She redlines at approximately 82mph. Obviously, highway speeds (60-80mph) feel like I'm stressing the engine beyond reason. It's uncomfortable, it eats a massive amount of gas, and it'll tend to wear the engine faster.
I had a transmission specialist examine the car today, and he was conclusive in his verdict -- there's nothing to fix in the gearbox. I initially took it there because I thought there might be a problem with the car's shifting into third gear, but there isn't -- it's a softer shift than first-to-second, but it's definitely there.
The only other suggestion the guy who inspected the car had was the axle ratio. He thinks it's got some crazy 4.xx:1 differential in it that was installed by a previous owner.
Here's my worry: After doing some research, I've yet to find any C3 that revs as high as mine does, even with the 4.xx:1 differentials. I think the most extreme one I've found so far was a guy who gets 3,700RPM at 70mph, which is what I get at 52-54mph, and that's ludicrous.
Is it likely that it's some monster differential with a rare ratio messing it up for me, or could something else explain the problem?
Thanks in advance for any advice.
Correction (update): The engine's at 4,100RPM when the car reaches 70mph, and 3,600 at 60mph -- in case the slight distinction from my original post is significant. Redline is definitely at 82mph.
She's a 1976 Stingray with a three-speed automatic transmission. The issue I'm having is that she revs to ridiculous excess. At 50mph, she's already at 3,500RPM. Climb to 60, and you're well above 4,000RPM. She redlines at approximately 82mph. Obviously, highway speeds (60-80mph) feel like I'm stressing the engine beyond reason. It's uncomfortable, it eats a massive amount of gas, and it'll tend to wear the engine faster.
I had a transmission specialist examine the car today, and he was conclusive in his verdict -- there's nothing to fix in the gearbox. I initially took it there because I thought there might be a problem with the car's shifting into third gear, but there isn't -- it's a softer shift than first-to-second, but it's definitely there.
The only other suggestion the guy who inspected the car had was the axle ratio. He thinks it's got some crazy 4.xx:1 differential in it that was installed by a previous owner.
Here's my worry: After doing some research, I've yet to find any C3 that revs as high as mine does, even with the 4.xx:1 differentials. I think the most extreme one I've found so far was a guy who gets 3,700RPM at 70mph, which is what I get at 52-54mph, and that's ludicrous.
Is it likely that it's some monster differential with a rare ratio messing it up for me, or could something else explain the problem?
Thanks in advance for any advice.
Correction (update): The engine's at 4,100RPM when the car reaches 70mph, and 3,600 at 60mph -- in case the slight distinction from my original post is significant. Redline is definitely at 82mph.
Last edited by Thymirus; 09-01-2016 at 04:44 PM.
09-02-2016, 09:59 AM
#2
Le Mans Master
Hi there. I just got my C3 roadworthy, and I've only got one real problem left to tackle before she's a dream to cruise around in. I was hoping some of you folks could help me out.
She's a 1976 Stingray with a three-speed automatic transmission. The issue I'm having is that she revs to ridiculous excess. At 50mph, she's already at 3,500RPM. Climb to 60, and you're well above 4,000RPM. She redlines at approximately 82mph. Obviously, highway speeds (60-80mph) feel like I'm stressing the engine beyond reason. It's uncomfortable, it eats a massive amount of gas, and it'll tend to wear the engine faster.
I had a transmission specialist examine the car today, and he was conclusive in his verdict -- there's nothing to fix in the gearbox. I initially took it there because I thought there might be a problem with the car's shifting into third gear, but there isn't -- it's a softer shift than first-to-second, but it's definitely there.
The only other suggestion the guy who inspected the car had was the axle ratio. He thinks it's got some crazy 4.xx:1 differential in it that was installed by a previous owner.
Here's my worry: After doing some research, I've yet to find any C3 that revs as high as mine does, even with the 4.xx:1 differentials. I think the most extreme one I've found so far was a guy who gets 3,700RPM at 70mph, which is what I get at 52-54mph, and that's ludicrous.
Is it likely that it's some monster differential with a rare ratio messing it up for me, or could something else explain the problem?
Thanks in advance for any advice.
Correction (update): The engine's at 4,100RPM when the car reaches 70mph, and 3,600 at 60mph -- in case the slight distinction from my original post is significant. Redline is definitely at 82mph.
She's a 1976 Stingray with a three-speed automatic transmission. The issue I'm having is that she revs to ridiculous excess. At 50mph, she's already at 3,500RPM. Climb to 60, and you're well above 4,000RPM. She redlines at approximately 82mph. Obviously, highway speeds (60-80mph) feel like I'm stressing the engine beyond reason. It's uncomfortable, it eats a massive amount of gas, and it'll tend to wear the engine faster.
I had a transmission specialist examine the car today, and he was conclusive in his verdict -- there's nothing to fix in the gearbox. I initially took it there because I thought there might be a problem with the car's shifting into third gear, but there isn't -- it's a softer shift than first-to-second, but it's definitely there.
The only other suggestion the guy who inspected the car had was the axle ratio. He thinks it's got some crazy 4.xx:1 differential in it that was installed by a previous owner.
Here's my worry: After doing some research, I've yet to find any C3 that revs as high as mine does, even with the 4.xx:1 differentials. I think the most extreme one I've found so far was a guy who gets 3,700RPM at 70mph, which is what I get at 52-54mph, and that's ludicrous.
Is it likely that it's some monster differential with a rare ratio messing it up for me, or could something else explain the problem?
Thanks in advance for any advice.
Correction (update): The engine's at 4,100RPM when the car reaches 70mph, and 3,600 at 60mph -- in case the slight distinction from my original post is significant. Redline is definitely at 82mph.
To determine the rear end ratio, you need to lift the rear of the car and support it with the tires just off of the ground. I would suggest supporting the frame (securely) on each side just in front of the tires where the frame kicks up. Then lift each control arm and support them with a block of wood or jack stand so that the tires move freely and the half shafts are close to horizontal and not at full droop. (At full droop, the U-joints will often bind and not allow the wheels to turn freely.)
Once the rear end is supported in the above manner, with the car in park, attempt to turn the tires. If they don't turn, you have a posi, a good thing. Now put the trans in neutral and place a mark on the tire where you can observe it while you manually turn the drive shaft. Count the revolutions of the driveshaft it takes to turn the tire one complete revolution. i.e. 3.5 revolutions of the drive shaft to one revolution of the tire equals 3.50:1 ratio
Let us know what you come up with.
Good luck... GUSTO
09-02-2016, 10:39 AM
#4
Le Mans Master
Yep....
If you have verified 100% yourself that its going into 3rd, and that its not just slipping horribly, then you need to figure out the rear end ratio.
Jack the car up, mark one side of the driveshaft with something (paint), put it in neutral and turn one of the tires 1 full revolution, then count how many times the driveshaft turns.... That will give you your ratio.
For those kinds of rpms, you would have to have 4.56 or 4.88 gears... Both of which I highly doubt. I think you have a transmission problem.
If you have verified 100% yourself that its going into 3rd, and that its not just slipping horribly, then you need to figure out the rear end ratio.
Jack the car up, mark one side of the driveshaft with something (paint), put it in neutral and turn one of the tires 1 full revolution, then count how many times the driveshaft turns.... That will give you your ratio.
For those kinds of rpms, you would have to have 4.56 or 4.88 gears... Both of which I highly doubt. I think you have a transmission problem.
09-02-2016, 11:07 AM
#5
Team Owner
I'd have to agree, if you are really in 3rd that comes out to 4.56 gears
09-02-2016, 11:29 AM
#6
Racer
I think if your rear gears were that low you'd be close to breaking the seat backs off every time you punch the throttle.
What torque converter are you running? Is it possible you have a bad TC? Obviously you've made sure the fluid is correctly filled?
What torque converter are you running? Is it possible you have a bad TC? Obviously you've made sure the fluid is correctly filled?
09-02-2016, 01:06 PM
#7
Heel & Toe
Thread Starter
Member Since: Aug 2016
Location: Huntersville North Carolina
Posts: 19
Received 0 Likes
on
0 Posts
All fluids (transmission fluid most importantly, obviously) are clean and at the right amounts. I checked them last week, and verified that they're still at the right quantity and quality today.
Additionally, I took the car out for a drive earlier today, and put some real pressure on her to test her out more thoroughly. There's a clear, distinct shift from first to second, and a shift almost as clear (it's, shall we say, somewhat softer) from second to third.
I don't know if this is relevant, but when you drive it very smoothly and accelerate slowly, shifting from second to third is hard to notice if you don't pay close attention.
Unless there's a known fault with these transmissions that mimicks shifting, or whose signs you could mistake for a shift, it definitely switches to third. I confirmed the over-revving problem by having it jump through to third gear, and then accelerating to the redline on a long, straight road. What could it be?
The car's very consistent, and very reliable on the rev range each time, but it's still doing exactly what it was before -- 4,100RPM at 70, redline at 82, and so forth, as described in the original post.
I'm getting my new set of jacks and stands tomorrow, so I'll measure the ratio and post it up here, but it would be good to eliminate anything else that could potentially be causing the issue.
Not a clue, to be honest with you. The guy who sold me the car reassembled it from crates and boxes, and everything else mechanical works fine. I've never even thought to look. Would a bad torque converter manifest in the way my car's behaving?
Thanks a lot for your help, and thanks in advance for any future advice.
Additionally, I took the car out for a drive earlier today, and put some real pressure on her to test her out more thoroughly. There's a clear, distinct shift from first to second, and a shift almost as clear (it's, shall we say, somewhat softer) from second to third.
I don't know if this is relevant, but when you drive it very smoothly and accelerate slowly, shifting from second to third is hard to notice if you don't pay close attention.
Unless there's a known fault with these transmissions that mimicks shifting, or whose signs you could mistake for a shift, it definitely switches to third. I confirmed the over-revving problem by having it jump through to third gear, and then accelerating to the redline on a long, straight road. What could it be?
The car's very consistent, and very reliable on the rev range each time, but it's still doing exactly what it was before -- 4,100RPM at 70, redline at 82, and so forth, as described in the original post.
I'm getting my new set of jacks and stands tomorrow, so I'll measure the ratio and post it up here, but it would be good to eliminate anything else that could potentially be causing the issue.
What torque converter are you running? Is it possible you have a bad TC?
Thanks a lot for your help, and thanks in advance for any future advice.
Last edited by Thymirus; 09-02-2016 at 01:09 PM.
09-02-2016, 02:26 PM
#10
Heel & Toe
Thread Starter
Member Since: Aug 2016
Location: Huntersville North Carolina
Posts: 19
Received 0 Likes
on
0 Posts
09-02-2016, 02:42 PM
#11
Race Director
Member Since: Apr 2007
Location: South Western Ontario
Posts: 11,061
Received 845 Likes
on
721 Posts
It's real easy to find out if it actually shifted to 3rd gear. Just accelerate to a speed PAST the point you think it shifted into 3rd. Then, pull the shifter back to 2nd and you should be able to easily tell it downshifted back to 2nd. You could also just drive it with the shifter in 2nd and see if you get the same rpm/speed numbers.
09-02-2016, 06:40 PM
#14
Team Owner
Someone swapped rear gear ratios.....need a set of 3.08 gears?? but you will have to set up your own trans/speedo gears, can't help you there.....the same problem, but in reverse happened to me some 20 years ago.....
my '72 was a Muncie stick shift 4 speed with 3.36 rear which is still in the car.....the trick is that around Wash DC/typical city traffic, I went from second to forth all the time....doing 80 mph was almost unheard of and IF it happened it was for maybe a mile before traffic/cops got you/me......
upon moving to Florida and driving the car down I95 I found that 80mph was a shifting point......seriously.....except for a couple of speed trap towns.....
Lawty and Lumberton.....at any rate I got tired of being passed by 18 wheelers and went with a 200 4r overdrive auto.....
80 is about 2400 rpm all day long.....
SO, two ways to go, get the 2.08 gear set and sell your drag race ration gear set for about an even swap, or go for the overdrive auto like I did many years ago....BUT you see I do most all my own work for some decades now, except machine work on engines.....
PM if you wish.....
my '72 was a Muncie stick shift 4 speed with 3.36 rear which is still in the car.....the trick is that around Wash DC/typical city traffic, I went from second to forth all the time....doing 80 mph was almost unheard of and IF it happened it was for maybe a mile before traffic/cops got you/me......
upon moving to Florida and driving the car down I95 I found that 80mph was a shifting point......seriously.....except for a couple of speed trap towns.....
Lawty and Lumberton.....at any rate I got tired of being passed by 18 wheelers and went with a 200 4r overdrive auto.....80 is about 2400 rpm all day long.....
SO, two ways to go, get the 2.08 gear set and sell your drag race ration gear set for about an even swap, or go for the overdrive auto like I did many years ago....BUT you see I do most all my own work for some decades now, except machine work on engines.....
PM if you wish.....
09-02-2016, 06:44 PM
#15
Team Owner
If you put the car in LO range (1st gear) and get it to a steady 15 mph, what rpm's are you turning? Then, if you stab the gas from that condition, does it accelerate from those rpm's as you would expect? Or, does it immediately rev a LOT higher rpms?
If it accelerates normally (from whatever rpm's at 15mph and then steadily upward), you just have 4.56 gears in the differential. If the rpm's immediately rev much higher as soon as you give it throttle, the torque converter on your car could be defective...or it could be a 'high rpm' torque converter, which is what would be used for drag racing.
In any event, I would think you would want to change the rear gear ratio. If you aren't into drag racing the car, use it for normal driving and frequent highway trips, you probably want a 3.08 (most economical) to 3.36 (better performance) ratio rear end.
If you also have a 'high rpm' torque converter or a converter problem, the torque converter can be changed out, but it would require removal of the transmission. And, if you need to remove the transmission, it will be time to have another discussion HERE about whether to keep the simple 3-speed auto you have now, or to replace with an overdrive unit.
Good luck and keep us informed of what you are finding and what direction you are taking.
____________
Mr. Vette... I think you meant to type 3.08 gears (not 2.08!).
If it accelerates normally (from whatever rpm's at 15mph and then steadily upward), you just have 4.56 gears in the differential. If the rpm's immediately rev much higher as soon as you give it throttle, the torque converter on your car could be defective...or it could be a 'high rpm' torque converter, which is what would be used for drag racing.
In any event, I would think you would want to change the rear gear ratio. If you aren't into drag racing the car, use it for normal driving and frequent highway trips, you probably want a 3.08 (most economical) to 3.36 (better performance) ratio rear end.
If you also have a 'high rpm' torque converter or a converter problem, the torque converter can be changed out, but it would require removal of the transmission. And, if you need to remove the transmission, it will be time to have another discussion HERE about whether to keep the simple 3-speed auto you have now, or to replace with an overdrive unit.
Good luck and keep us informed of what you are finding and what direction you are taking.
____________
Mr. Vette... I think you meant to type 3.08 gears (not 2.08!).
Last edited by 7T1vette; 09-02-2016 at 06:47 PM.
09-03-2016, 01:04 PM
#16
Safety Car
Sounds like slippage , Put it in manual low drive it 3000 and shift into second Get it again to 3000 and shift 3rd , does the tach rpm drop off ?
09-03-2016, 01:56 PM
#17
Heel & Toe
Thread Starter
Member Since: Aug 2016
Location: Huntersville North Carolina
Posts: 19
Received 0 Likes
on
0 Posts
I just climbed underneath the car and performed the check, and it's got 3.08. Confirmed it thrice. The driveshaft spins three times, and about 10% of another revolution.
I'm not sure if the '76 came with individually selectable gears originally, but the positions I can choose with the stick in my car are only P, R, N, D, and one more (LO?). There's not enough for there to be 1 and 2 separately. Could it be the guy who had the car before me used another gearbox entirely?
In any case, I placed the car in the lowest position possible on the shifter (LO?), and accelerated until it changed into second gear. Then, I allowed the revs to climb until it was straining just a little bit, and shifted into D, and it changed gears again, which must be third, right? Still revs just as high at high speeds.
As for testing how hard the revs climb from 15mph upwards, it's pretty fast, but nowhere near instant, and it does climb consistently. Perhaps I'm not registering the problem, and the speed at which the revs climb DOES indicate an issue with the torque converter.
This is starting to confuse me. I was sure it was going to be the differential.
I should also clarify that I won't be back in the United States until the early years of the 2020s. I live in England right now. A simple transmission swap might have been easily doable back home (God bless America), but over here, I'd have to import a 200R4, by the looks of it.
Like I said above, the only position below Drive won't allow it to sit in first gear -- it shifts into second when it wants. I can't force it like I would on a shifter with all three gears marked individually.
What do you folks think? Thanks a lot for your help, guys, and any future advice.
I'm not sure if the '76 came with individually selectable gears originally, but the positions I can choose with the stick in my car are only P, R, N, D, and one more (LO?). There's not enough for there to be 1 and 2 separately. Could it be the guy who had the car before me used another gearbox entirely?
In any case, I placed the car in the lowest position possible on the shifter (LO?), and accelerated until it changed into second gear. Then, I allowed the revs to climb until it was straining just a little bit, and shifted into D, and it changed gears again, which must be third, right? Still revs just as high at high speeds.
As for testing how hard the revs climb from 15mph upwards, it's pretty fast, but nowhere near instant, and it does climb consistently. Perhaps I'm not registering the problem, and the speed at which the revs climb DOES indicate an issue with the torque converter.
This is starting to confuse me. I was sure it was going to be the differential.
I should also clarify that I won't be back in the United States until the early years of the 2020s. I live in England right now. A simple transmission swap might have been easily doable back home (God bless America), but over here, I'd have to import a 200R4, by the looks of it.
Sounds like slippage , Put it in manual low drive it 3000 and shift into second Get it again to 3000 and shift 3rd , does the tach rpm drop off ?
What do you folks think? Thanks a lot for your help, guys, and any future advice.
Last edited by Thymirus; 09-03-2016 at 02:00 PM.
09-03-2016, 02:01 PM
#18
Burning Brakes
Is it possible for you to post some pictures and videos? I'd like to see pictures of the shifter and if possible, the transmission. A video of the camera pointing to the speedo and tach as you read out what you're doing with the gas pedal and shifter would also be fantastic.
09-03-2016, 02:36 PM
#19
Safety Car
Your car should have a selector for P R N D 2 1 .. You may also have the linkage out of adjustment which would NOT allow a manual low but if you have a stock shiftier YOU For sure have detentes for D 2 1
Disconnect shift cable , by hand move trans linkage to park, now slowly one click at a time move it to r n d 2 1 Make sure you feel clicks for all of them. Now put trans in N by hand , go up into car set shifter in N , reconnect cable.
Test car when done
Disconnect shift cable , by hand move trans linkage to park, now slowly one click at a time move it to r n d 2 1 Make sure you feel clicks for all of them. Now put trans in N by hand , go up into car set shifter in N , reconnect cable.
Test car when done
09-03-2016, 04:06 PM
#20
Heel & Toe
Thread Starter
Member Since: Aug 2016
Location: Huntersville North Carolina
Posts: 19
Received 0 Likes
on
0 Posts
Your car should have a selector for P R N D 2 1 .. You may also have the linkage out of adjustment which would NOT allow a manual low but if you have a stock shiftier YOU For sure have detentes for D 2 1
Disconnect shift cable , by hand move trans linkage to park, now slowly one click at a time move it to r n d 2 1 Make sure you feel clicks for all of them. Now put trans in N by hand , go up into car set shifter in N , reconnect cable.
Test car when done
Disconnect shift cable , by hand move trans linkage to park, now slowly one click at a time move it to r n d 2 1 Make sure you feel clicks for all of them. Now put trans in N by hand , go up into car set shifter in N , reconnect cable.
Test car when done
Last edited by Thymirus; 09-03-2016 at 04:09 PM.