RPM won't increase past 3.8k in 5th

SpartanRonB

Registered User
Has anyone had any personal experience with their SC or otherwise know a reason why my 1990 Thinderbird SC won't increase RPM's past 3.8k in 5th gear? In 4th with full power, shift to 5th and car felt like a limiter kicked in. At that point it won't climb past the 3.8k RPM. I have no idea why. Also, just to explain the RPM in perspective, it has 4.10 rear end.
 
Few questions;
-Why do you have a 4.10 rear with a 5-speed SC?
-Was this car originally an automatic and converted?
-Did this just start, or has it always been this way?
-If it just started, was anything changed or done prior to it starting?
 
Few questions;
-Why do you have a 4.10 rear with a 5-speed SC?
-Was this car originally an automatic and converted?
-Did this just start, or has it always been this way?
-If it just started, was anything changed or done prior to it starting?


I don't believe the exact gear is relevant to the problem per se so don't want to get to off topic. Just wanted people to understand rpm so high because bigger gear. But since you're wondering... I may be mistaken as to the exact gear. I just know my dad had put a much larger gear put in and the largest speedo gear from supercoupeperformance is still 10% off. It's always been a manual.

I haven't really been able to open it up for quite a while to go that fast in 5th gear for it to need to pull more rpm. (You can do your own math as to how fast that might be). I believe it to be a sudden thing. The only modification that I've made in last year is cold air intake, maf, and throttle body. I believe I've been able to achieve beyond 3.8k in 5th since that modification but not 100% certain since its been about a year.

I have no problems pulling rpm in any gear other than it not pulling past 3.8k in 5th. And it's a sudden thing at that rpm not like a struggle to climb.
 
The reason that was question #1 is because that is WAAYYYYYYYY too much gear for an M5R2. It may be the cause of your issue, but even if it isn't, it is still an issue that you should fix. 1st gear in your transmission is 3.75. To put that in perspective, a world-class T5 has a 2.97 1st gear, so as far as 1st gear is concerned, any rear end gear behind an M5R2 acts like it is 25% steeper than it would in a Mustang. So imagine putting a set of 5.13s in a fox body Mustang and trying to drive it on the street. If you have 4.10s, that is effectively what you are doing, and 1st gear is completely useless. 3.27s or possibly 3.55s is really as far as you want to go with an M5R2 trans. 3.27s came stock in automatic SCs, so you can probably buy a used 3.27 diff for around $150, swap it out, and sell your 4.10 diff to someone for probably $400. This will put some money in your pocket, give you a whole bunch of drivability back in your car, likely actually make your car faster, and also possibly avoid this ever coming up again since with 4.10s at 3800rpm you are going 97mph (fast, but easy to accomplish opening it up on the highway), but with 3.27s 3800rpms would be over 120mph.

Moving onto the CAI. You say you changed the MAF: what MAF did you install? Was the car tuned? Installing a different MAF without tuning it can and likely will throw the fuel curve way off, which can cause all kinds of drivability issues. 3800rpms in 5th is a heavier load on the engine than in 4th or 3rd, so if it is running rich, or worse lean, it may be able to pull past that in the lower gears, but not in 5th.

If the car has always been a 5-speed, then that rules out my other theory. I know some automatic SCs have speed limiters at 120mph, and since 3800rpms with stock gears would have been just over 120mph, I thought maybe the speed limiter was kicking in. The only way this could be the case though would be if you swapped an automatic SC ECM into this one, and even then, I'm not sure it would work that way.

Another possibility is it could be some kind of an ignition fault, possibly a bad crank or cam sensor, or DIS module.


At this point, I would start by swapping the stock MAF and airbox back into the car, since that is the easiest/cheapest thing to do. If the problem still persists, put 3.27s back in the car. If you still have the problem after that, start looking into your ignition system.
 
Traditional reasons for not being able to wind up in top gear:
- drivetrain drag...low fluid in trans; bad trans/diff internals, bushings etc.; bad u-joints; bad wheel bearings (front and/or rear)
- dragging brakes
- plugged cats
- overfilled engine oil
- low compression
- slipping clutch
- low tire pressure
- gauges not reading correctly
- too much gear... puts engine outside power band
- intake blocked/collapsing
- throttle binding/restricted by floormat, busted pedal, engine compartment mods, failed motor mounts, etc.
- and on the SC, bypass malfunction
 
The EEC has an rpm limiter for 5th gear vs. time. It's designed to protect the engine, it starts with the upshift light, then it starts pulling power. I've never engaged it because I don't drive that fast and/or have the right gears to make it happen, + I have tuning to turn that feature off. But I know it's in there, just haven't experienced it myself (other than the upshift light in 5th).
 
Last edited:
The problem is that the speed limiters are kicking in. The M5R2 speedometer gears will only calibrate to 3.55's. You can defeat this with a chip, or I think if you want to you can change the internal gear in the transmission so that you can get more speed sensor range, but that means taking the tail housing off and I've never done it so I can't verify how well it works or what speed sensor gears to put with it. I'm assuming your upshift light comes on well before the speed limiting occurs.

Actually, what was I thinking. The EEC has an rpm limiter for 5th gear vs. time. It's designed to protect the engine, it starts with the upshift light, then it starts pulling power. I've never engaged it because I don't drive that fast and/or have the right gears to make it happen, + I have tuning to turn that feature off. But I know it's in there, just haven't experienced it myself (other than the upshift light in 5th).

There is a limiter on 5-speed cars? I thought that was only on the autos. Years ago, I topped out an old 89XR7 I had, and it was just over 4K RPMs in 5th. I only did it once, and immediately slowed right down afterwards because I didn't want to die or go to jail, so it might have been a fluke or something, but I just figured if there wasn't a speed limiter by that point, then there wasn't one there.
 
The reason that was question #1 is because that is WAAYYYYYYYY too much gear for an M5R2. It may be the cause of your issue, but even if it isn't, it is still an issue that you should fix. 1st gear in your transmission is 3.75. To put that in perspective, a world-class T5 has a 2.97 1st gear, so as far as 1st gear is concerned, any rear end gear behind an M5R2 acts like it is 25% steeper than it would in a Mustang. So imagine putting a set of 5.13s in a fox body Mustang and trying to drive it on the street. If you have 4.10s, that is effectively what you are doing, and 1st gear is completely useless. 3.27s or possibly 3.55s is really as far as you want to go with an M5R2 trans. 3.27s came stock in automatic SCs, so you can probably buy a used 3.27 diff for around $150, swap it out, and sell your 4.10 diff to someone for probably $400. This will put some money in your pocket, give you a whole bunch of drivability back in your car, likely actually make your car faster, and also possibly avoid this ever coming up again since with 4.10s at 3800rpm you are going 97mph (fast, but easy to accomplish opening it up on the highway), but with 3.27s 3800rpms would be over 120mph.

Moving onto the CAI. You say you changed the MAF: what MAF did you install? Was the car tuned? Installing a different MAF without tuning it can and likely will throw the fuel curve way off, which can cause all kinds of drivability issues. 3800rpms in 5th is a heavier load on the engine than in 4th or 3rd, so if it is running rich, or worse lean, it may be able to pull past that in the lower gears, but not in 5th.

If the car has always been a 5-speed, then that rules out my other theory. I know some automatic SCs have speed limiters at 120mph, and since 3800rpms with stock gears would have been just over 120mph, I thought maybe the speed limiter was kicking in. The only way this could be the case though would be if you swapped an automatic SC ECM into this one, and even then, I'm not sure it would work that way.

Another possibility is it could be some kind of an ignition fault, possibly a bad crank or cam sensor, or DIS module.


At this point, I would start by swapping the stock MAF and airbox back into the car, since that is the easiest/cheapest thing to do. If the problem still persists, put 3.27s back in the car. If you still have the problem after that, start looking into your ignition system.

Thank you very much for your detailed response. As for the size of the gear, I agree that 4.10 isnt the optimal gear for this car but it is what my dad put in years ago and I'm going to keep that way unless necessary. I lost my parents in a motorcycle accident 9 years ago (only a year or two after he put the rear in). My dad was an air craft mechanic for 20 years in the air force and lead mechanic at the Ford dealership for almost another 20. Our goal was to get this car in the 11's but keep stock appearance sleeper theme. When we talked about rear end, I had researched this car a ton and told him it wasn't going to need the 4.10's and he agreed but it's just what he wanted. lol. He was known for going overboard all the time. He once sold an engine from a vega he had built when i was in elementary school to someone who just dropped it into an impala, added a delay switch and went to quarters at daytona nationals. So, I didn't fight him too much on it since he's the expert. Even more so because he did it when I was away at law school after he lost the rear to a corvette at a stop light and while he was at it rebuilt the engine, ported charger, plenum and heads. I haven't opened up to see if rockers but probably a good bet those were upgraded too. So, it adds even more of a sentimental story for me and won't swap out rear unless absolutely necessary. It can pull from stop in 2nd but if just driving passively can make it through intersection before needing to shift to 2nd.

As for the MAFS, throttle body, tuning questions... Bill at supercoupeperformance sold all of that to me at the same time so he matched it up. If my memory is correct it was 70 and 76 mafs and sampling tube. Don't quote me on that though. I havent bothered tuning chip because I'm still adding things often. Just trying to limit to around $500 a month so not going to waste money on a chip burn until necessary. Fuel ratio is off and will want to stall at startup but that is solved with just a little feathering throttle and then any noticeable issues for that go away. So, your reasoning about this mod has some merit but I don't personally see this as the real culprit because it'll pull 4th fine even up to 6k but sudden drop when shifting to 5th.

Dave's answer seems to be the most likely culprit and would be dead on to my first reaction when it happened. it was a sudden resistance like deceleration rather than an inability to perform and climb past 3.8k in 5th if you can envision what im saying. I know that back in the day this car did 140 mph but dont recall what rpm that would have been on the stock gears in 5th. So, with Dave's reasoning that EEC has a limiter vs timing in 5th gear, it makes perfect sense and explains the feel of the car of what is happening. I'm guessing this never happened or at least noticed with stock gear since, if i recall correctly, there would be no need to be in 5th gear in the stock gears to reach terminal speed whereas the 4.10 runs out of gear before 140 requiring shifting to 5th.

I let the car sit for about 8 years before finally making the decision last year that i was going to stop procrastinating and start budgeting money each month to do modifications so that I can start toward the goal my dad and I shared and i can achieve it in memory of him. It will be an 11 second car so going back to stock stuff is out of the question lol. Sorry for going on and on. Just sentimental and sharing my little father-son car story. I'm a lawyer now and want to make my dad proud that I do this myself since I didnt follow in his footsteps and he probably would have never in a million years thought I was going to take such an interest in doing something like this myself since I always had him to do it before.

I've also got issue with the brakes but didn't bother doing a post on that sense there are so many previous post about this damn break system that there are several different possibilities and it's just a matter of figuring out which. I'm gonna have to break down and actually have a mechanic diagnose what the hell is actually going on there.
 
The EEC has an rpm limiter for 5th gear vs. time. It's designed to protect the engine, it starts with the upshift light, then it starts pulling power. I've never engaged it because I don't drive that fast and/or have the right gears to make it happen, + I have tuning to turn that feature off. But I know it's in there, just haven't experienced it myself (other than the upshift light in 5th).

See my long-winded response to madmike. But, essentially I think you have nailed it assuming you are correct that EEC has limiter vs timing in 5th gear as opposed to a mph limiter. Which, with you being the guru when it comes to these cars, I have absolute faith you are right (I rarely post but have been reading these forums for years. I'll be coming to you when it comes to cam selection when I get to that point eventually). It is exactly the feeling a limiter has. I'm guessing this was never experienced with the stock gears years ago even up to 140 because maybe wasn't a need to shift to 5th gear since 140 was able to be achieved in 4th gear. I just don't recall if ever went above 3.8k in stock gears in 5th. I just remember the thrill of driving 140 mph at that young age. But that was back in college years and not paying attention to rpm stuff without anything happening to cause me to. Much more attentive to everything with this car now that I'm trying to do it as much as I can by myself in memory of my dad. I guess I'll just leave the EEC alone for now until after I've got my mods worked out for my EEC programs. When I do get an EEC chip burn I'll leave that as that limiter setting on the default program but remove the limiter for additional program for racing road track course. Never know when someone might be driving my car on the public highway without me and tempted try to drive it too fast and get in trouble ;)
 
Well now that you add that its been sitting for 8 years. It could have a fuel pump going bad, fuel filter clogged, plugs and wires needing attention. Like Mikey said. There is less load on the engine at a lower gear especially wit 4.10s. So if you have an ignition system or fuel system that isn't up to par it do what you are describing.
 
Well now that you add that its been sitting for 8 years. It could have a fuel pump going bad, fuel filter clogged, plugs and wires needing attention. Like Mikey said. There is less load on the engine at a lower gear especially wit 4.10s. So if you have an ignition system or fuel system that isn't up to par it do what you are describing.

It sat for 8 years but has been driven frequently in the last year or so. That stuff has been changed. Those things wouldn't be the cause of this particular issue. Again, Dave seems to have the correct answer. I'll know for sure whenever I get done with mods and reburn EEC with removing that limiter against timing in 5th gear. For now it's not really an issue since not going to be driving it over 100 on public roads. But thanks for your input.
 
Back
Top