Hi everybody. 86 GT, automatic, V6, all stock. What is the normal engine rpm's cruising flat let's say 75mph. The reason i am asking: mine is spinning around 4K. Is it normal? If not, tach?, tranny?
Tnx
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11:20 AM
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chrishahn87 Member
Posts: 1584 From: East Berlin, Pa - USA Registered: Dec 2004
Maybe your TCC is not locking up. It shouldn't be that high. As you accelerate through the gears, does it feel like you have a 4th gear. If not, your TCC is not locking up.
Originally posted by voron.308: 86 GT, automatic, V6, all stock. What is the normal engine rpm's cruising flat let's say 75mph.
I also have an `86 GT, automatic, V6, all stock, except for a 3.4L swap, which, contrary to the popular belief of many, is something that doesn't affect engine RPM.
Basically, I just finished repeating the "real-world" experiment of KurtAKX:
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Originally posted by KurtAKX: I think I am seeing about 33-3400 RPM at 75 with an 86 SE V6 Auto.
I repeated his experiment, but with my `86 GT, automatic, V6, the same model you have, voron.308, and it runs stock diameter, 215/60R15 rear tires.
In my case, the result was essentially the same as that of KurtAKX, an RPM reading of just under 3400 RPM while cruising at 75 MPH.
Therefore, IMHO, your reading of 4000 RPM at 75 MPH seems to me to be much too high if everything about your automatic '86 GT Fiero is indeed stock.
Accordingly, before you get all of us arrested for speeding while trying to help you out on this issue ("Yes, officer, I know exactly how fast I was going, 75 MPH, while on cruise control, to help out fellow Pennock's Fiero Forum member, voron.308" ) --- why do you now say:
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Originally posted by voron.308: Thank you guys. Looks like i am Ok.
In short, I don't get it.
Given that there is nothing "OK" about that RPM level at 75 MPH, your clarification regarding this issue likely would be of value to others here who've had a similar RPM question, would it not?
ha, that's nothing, I'm working on wiring in a new connector for my ECM becuase the people that owned the car before me used tinfoil to complete the connections inside the plug... after messing around with the plug, I had the car IDLING at 3500 RPM... I didn't even have to press the gas to leave a stop... ugh, the things people do to cars, just hope this new plug helps my problems :P
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03:33 PM
Gwain Member
Posts: 460 From: Titusville, Florida, USA Registered: Dec 2004
I have an 85 GT stock, 2.8. At 70 - 75 expressway I'm turning 3 grand.
There was a problem when I first bought the car that the TCC wasn't working. Fixed that but it's only worth a couple hundred rpm. 4 grand a 75 sounds high to me.
Have you given the thing a good tune up and checked the timing?
Originally posted by voron.308: 86 GT, automatic, V6, all stock. What is the normal engine rpm's cruising flat let's say 75mph. The reason i am asking: mine is spinning around 4K.
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Originally posted by chrishahn87: Yes thats right around where it should be!
The stock automatic does not have an "overdrive" gear, allowing the car to cruise at high speed while the motor is at a lower rpm.
I agree with chrishahn87 in that the stock automatic Fiero GT does not have an "overdrive" gear.
Nonetheless, as both KurtAKX and I have demonstrated with real-world examples, 4000 RPM at 75 MPH still is WAY above normal RPM for an all-stock, automatic, `86 Fiero GT doing 75 MPH.
Normal revs for an `86 Fiero GT automatic should be much closer to 3400 RPM than to 4000 RPM at that speed, even if it's "all stock," as voron.308 asserted his was at the start of his thread here.
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07:30 PM
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Oct 5th, 2008
serealport Member
Posts: 286 From: Memphis TN USA Registered: Apr 2007
check timing for idle. Also, does your temperature gauge seem to go up any, or does it stay really low. If your thermostat is bad (stuck open), your car will never heat up to proper operating temps, and your Torque Converter won't kick in, causing high revs and rpms along with bad gas mileage. I don't remember my rpms at 75, but at 60 I'm doing about 2500-2600 rpms with my 85GT and auto. I would assume I would be right around that 3400rpm mark.
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01:28 AM
2farnorth Member
Posts: 3402 From: Leonard, Tx. USA Registered: Feb 2001
i just got my car on the road two weeks ago and im runnining in the range or 4000 at 75 '86 se V6 th125 all stock
i thought this was odd but concluded that it was because there are only three gears (not a car aficionado) thought i'd toss in my .02
ps could running rich cause high rpm and really high idle(13-1700)
3200-3400 is the norm for the a properly functioning TH125C at 75 mph. That includes the TCC lockup (like a 4th gear) . Look on the front of the trans for a square wiring connector facing the driver's seat. This is the TCC (torque convertor clutch). Is there any wiring connected to it? Many times when the TCC fails they will just get unpluged and left like that. But that is only good for a 200-300 rpm reduction. Those that are running at 4000 have a malfunction somewhere or the final drive in the trans has been changed. Other causes are bad tach, torque convertor slipping, or trans slipping internally.
Originally posted by serealport: i just got my car on the road two weeks ago and im runnining in the range or 4000 at 75 '86 se V6 th125 all stock i thought this was odd but concluded that it was because there are only three gears (not a car aficionado) thought i'd toss in my .02
serealport, the RPM levels you're seeing do indeed seem odd, and more specifically, seem unjustifiably high (4000 RPM) for a speed of 75 MPH.
You suggested the number of forward gears in your TH125 automatic transmissiion (three) as the potential cause. Although the lack of a fourth, "overdrive" gear in that transmission technically speaking is a factor, that is not the real issue because your Fiero's RPM is a reasonably straightforward, mathematical function of its speed (expressed here in MPH), its final drive ratio (FDR), and its tire diameter (TD) expressed here in inches:
RPM = [336 x FDR x MPH]/TD
Unless your car's numerical FDR has been increased substantially for better off-the-line acceleartion, and/or its rear tires' outside diameter is downright dinky in size, your Fiero simply shouldn't be revving as high as 4000 RPM at 75 MPH.
The post by 2farnorth immediately above this one suggests some potential causes for that problem.
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12:27 PM
americasfuture2k Member
Posts: 7131 From: Edmond, Oklahoma Registered: Jan 2006
last time i drove it, not being tuned yet, i was spinning around 3k at 70mph
------------------ 1987 Fiero GT built by my brother, merlot566jka, 3500 LX9 from 06 Malibu, WOT-TECH.com 1280 grind stage 3 cam, LS6 valve springs, 1227730 ECM conversion, Darrel Morse solid aluminum cradle mounts, Truleo headers modified to fit the 3500, 36# inectors, 70mm 4.3 throttle body adapted to 3500 intake, ported heads, upper and lower intakes, lightly polished, tcemotorsports.com crank trigger wheel, CenterForce dual friction clutch, Flowtech Afterburner muffler, 2.5" piping, cat deleted, EGR deleted, SinisterPerformance tuning, C6 Corvette exhaust tips. projected to be 35 MPG with a guesstimate of 250 hp at the motor
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12:45 PM
serealport Member
Posts: 286 From: Memphis TN USA Registered: Apr 2007
Originally posted by project34: You suggested the number of forward gears in your TH125 automatic transmissiion (three) as the potential cause. Although the lack of a fourth, "overdrive" gear in that transmission technically speaking is a factor, that is not the real issue because your Fiero's RPM is a reasonably straightforward, mathematical function of its speed (expressed here in MPH), its final drive ratio (FDR), and its tire diameter (TD) expressed here in inches:
RPM = [336 x FDR x MPH]/TD
Unless your car's numerical FDR has been increased substantially for better off-the-line acceleartion, and/or its rear tires' outside diameter is downright dinky in size, your Fiero simply shouldn't be revving as high as 4000 RPM at 75 MPH.
im going out on the highway in about thirty min ill double check
what is normal FDR im 99% sure my car is not modded and i have stock rims plus i just got this car back from the trans shop less then a month ago(in for a rebuild)
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01:40 PM
cptsnoopy Member
Posts: 2587 From: phoenix, AZ, USA Registered: Jul 2003
Originally posted by serealport: what is normal FDR
It depends on the model of Fiero. The highest numerically in stock, automatic-equipped Fieros is 3.33:1. (I don't know what the FDRs are for Fieros with manual transmissions.)
For automatic-equipped Fieros, since 3.33:1 was the numerically highest FDR in a stocker, closer inspection of the formula I'd mentioned above makes it clear that you shouldn't be running as high as 4000 RPM with stock GT tires (outside diameter around 25.2 inches),when your stock, automatic-equipped Fiero is doing 75 MPH.
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Originally posted by serealport: im 99% sure my car is not modded and i have stock rims
Just remember that it's the outside tire diameter --- not the rims' diameter --- which affects your RPMs.
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Originally posted by serealport: im going out on the highway in about thirty min ill double check
Please post us on the RPM reading you saw at 75 MPH.
If it was indeed around the 4000 RPM you'd reported earlier, you've likely a problem with your Fiero, unfortunately, regardless of whether it's the tachometer reading (certainly a possibility, as cptsnoopy just suggested), the speedometer's reading, or something more serious.
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03:14 PM
serealport Member
Posts: 286 From: Memphis TN USA Registered: Apr 2007
Originally posted by project34: If it was indeed around the 4000 RPM you'd reported earlier, you've likely a problem with your Fiero, unfortunately, regardless of whether it's the tachometer reading (certainly a possibility, as cptsnoopy just suggested), the speedometer's reading, or something more serious.
the speedos close to right (tested on mile markers)
these are the results i got from the tach and speedo approx idle 1500 40-2500 50-3000 55-3500 65-4000 70-4100 to 4300 75-4500
Wow! A 4500 RPM reading is way too high for 75 MPH in your car.
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Originally posted by serealport: the tach may be high becuas if i floor it the tach goes to approx 7000 before shifting 1st to 2nd
In a way, that is almost good news because it strongly suggests to me that the problem is only a faulty tachometer reading, as opposed to something more serious.
One reason I say that is because if the engine really was revving to 7000 RPM whenever you floored it, the engine very likely should have grenaded itself l-o-n-g ago. Another reason I say that is because I think the fuel cut-off limiter normally kicks in at a bit under 6000 RPM.
In a way, that is almost good news because it strongly suggests to me that the problem is only a faulty tachometer reading, as opposed to something more serious.
One reason I say that is because if the engine really was revving to 7000 RPM whenever you floored it, the engine very likely should have grenaded itself l-o-n-g ago. Another reason I say that is because I think the fuel cut-off limiter normally kicks in at a bit under 6000 RPM.
When flooring it in first my 85 GT will get up to 6600-6700 rpms before it shifts and I don't have any problems. been driving the car ever since I got my license. Although an engine rebuild could change the rpm limit of the engine if anything was done to bore or anything. The problem I have is that the car doesn't accelerate very fast after 5k rpms (under the impression it's a common problem with fieros), so to get my maximum acceleration, I can't floor it in first lol. I idle at about 900 rpms when warm, and run 2500-2600 rpms at 60. If I recall rpms go up about 300 per every 5 mph after that pt. which would put me at 3400-3500 at 75.
Peak power for the stock intake/cam is about 4500 rpm. It drops off very fast after 5200 rpm. Running the car to 6000+ does not do any good either for power or longevity of the engine.
Peak power for the stock intake/cam is about 4500 rpm. It drops off very fast after 5200 rpm. Running the car to 6000+ does not do any good either for power or longevity of the engine.
Yes, I know this, but after a rebuild if the engine was bored out or some internals were changed/replaced the red line won't be in the same spot.
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12:08 PM
cptsnoopy Member
Posts: 2587 From: phoenix, AZ, USA Registered: Jul 2003
i havent driven the car much except for the past few weeks after the tranny was worked on
the tach may be high becuas if i floor it the tach goes to approx 7000 before shifting 1st to 2nd
these are the results i got from the tach and speedo approx idle 1500 40-2500 50-3000 55-3500 65-4000 70-4100 to 4300 75-4500
how do you test tach reading
See if you can get a hold of an AutoXray or other logging scan device. You can plug it into the ALDL and check the RPM that way. There may be other easier ways to check it but I do not know of them at the moment.
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03:06 PM
Nov 5th, 2008
fierofrog77 Member
Posts: 27 From: murray, ky, usa Registered: Oct 2008
I am also having the same issue with my 87gt i am running about 4300 at 70 and 4500 at 75, oh and when i floor it i don't shift till about 7000 or more, cant really tell the tach stops at 6500. the car as far as i know is stock, but i have had a suspicion that my tach has been off since i bought the car. It idles around 12-1300 rpm. if anybody figures it out let me know. thanks kris
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10:37 PM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
) --- why do you now say:
