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Discussion Starter · #1 ·
MY93 9000 2.3FPT

I have hit the rev limter on infrequent occasions (mainly in 2nd) and it's coughed and then carried on as normal.

Recently however, since changing to a lower final drive and ECU mapping that holds a bit more boost higher up but has a slighlty lower rev limit than before, I've probably hit it a couple more times than "usual" as I've got used to the new shift points. It's been coming back with a slight misfire as I get back on the gas, but settles down again after a while. This has now developed in to a permanent rough running and lack of performance.

A leak back to the intake system on cylinder #2 has been diagnosed, so it's head off time
to see what the damage might be.

I'm wondering:

a) What the mechanism is such that hitting the rev limiter might cause damage, and what that damage might be (indications are clearly a bad valve seal)

B) Is it just coincidental that it's failed now, or is there anything in the changes that might have precipitated it (other than more frequent hitting the rev limit).

Any thoughts welcome.
 

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Rev limiter is actually set to protect your engine from hardware failures. Usually high revving does not offer any benefits on the road use with these cars. ( One can get enough power with 400 nm / 6000 rpm ). Of course hitting rev limiter tells that you have pushed your car very hard, but usually fresh engine can handle this without problems.

Btw, which software / rev limits you were using ?
 

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i have also wondered myself what the mechanism is that will limit RPM's.

could this be a computer controlled mechanism? i recall that, when my '92 900T was completely stock (as in when i made the purchase) that the rev limiter was set at 6k rpm or maybe even a tiny bit below, 5900-5950 or so. i hit it more than once, understandably, as i was switching from a pickup truck with a 1.8 litre engine to a turbo charged 2.0 (no noticable damage done).

i have upgraded the APC, EPROM, as well as other components, but nothing else that i could imagine might take part. the APC i doubt would have any effect as this unit receives no information about the engine's RPM (as i understand). the EPROM i might guess, that the fuel delivery map would have a fuel cut at a certain rpm = rev limiter. i glanced down on one of the occassions that traction broke loose while WOT and seen the tach somewhere in the 6200-6500 range... before i thought to lift the heavy foot (of course, this may have been a 'swing-reading' with the needle carrying beyond what it shoud be reading due to momentum).

it would not make sense to me that there would be any sort of a mechanical rev limiter with today's electronically controlled engines. i believe that any damage done would be due to lack of lubrication/balance at excessively high RPM's, or due to the excess of flow through the cylinder and along with it the increased pressure, heat etc.

the ECU may have created a fault due to hitting the rev limiter?

the DI may have been overstrained with the work to keep up with the accelerated spark rotation/increased resistance inherent with high turbo pressures?
 

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Certainly from what I have read that hitting the limiter on a regular basis will increase the wear to an engine, obviously the more miles, the greater the wear.

I assume the Saab to have an electronic limiter ?

I rarely have hit the limiter in 2nd, mainly because by then its all over by then anyway!

Although the Saab is a 16v engine, it doesnt appear to be a revver at all, I seem to remember someone telling me that the cams were of a very mild profile.

I hope you manage to sort the problem swiftly and cheaply !

I would be interested to know if the damage has been caused by kissing the rev limiter
 

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Discussion Starter · #5 ·
The limiter is electronic, and programmable within the ECU.

Mine doesn't drive like a regular Saab engine- it's got a big valve/flowed head, upgraded intercooler & intake, bigger turbo etc. which put together mean that it keeps pulling very strongly all the way to the red line i.e. there's no obvious drop off in torque that instinctively initiates a gear change.
 

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I have hit the rev limiter lots of times (usually on track days I hasten to add )
before and after my engine re-build with no ill effects
could it be a weakening valve spring allowing some piston to valve contact ?
Have you re-newed your springs at any stage ?
I took the precaution of changing them all at the 200k re-build and going for stronger ones as well
 

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Originally posted by Søren Hviid:
[qb]I belive the rev limiter in 2. gear is there to save the transmission not the engine! [/qb][/b]
Wrong assumption here. Low end torque is poison to gearbox, not the high end torque, especially with low gears ( 1. & 2. ). This is why the boost is usually limited by software with the 1. and 2. gears. Rev limiter function is purely to protect the engine.
 

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too much low speed torque is likely to cause failure of the main bearings in the engine. Similar effect to labouring an engine - it's related to oil pressure. Boost is reduced in 1st and 2nd to preserve the gear box (and as far as I am aware the problem is due to the size of cog selected)

Rev limiter....hit it once or twice a tad below 7000rpm (68mph or so in 2nd)...bad day at work. No damage! Although I would not recomend it as the drive train is shunted quite strongly. My 9-5 is nothing special and pulls cleanly and "sings" at higher revs up to the fuel cut off point - has never felt strained at the top end - seems to get much smoother over 5.5krpm
 

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To my mind the amount of stress the torque places on either engine or gearbox is the same regardless of what rpm it occurs at.
Also, if I put 300ftlb into the gearbox in 2nd gear or 3rd, the stress going into the gearbox is the same. (I agree that the output side of the gearbox will see more stress in 2nd than 3rd though due to the lower ratio)

To answer the question - if the engine held say 300ftlbs flat from 2k rpm up to 5krpm then the stress is the same all the way through the rev range.
In reality, however we tend to see a peak torque at about 2k+ rpms where the turbo can supply the max pressure. As the revs rise the turbo starts to loose the ability to hold the pressure due to the higher volume of air it needs to supply and the torque drops off.
 

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IIRC the rev limiter on street turbo cars is a fuel cut-out. This can cause a cyllinder to run lean for a couple cycles before and after. That could damage a few things.

Maybe your software programmer could change it to an ignition cut-out? That can damage the catalyst, which is why it isn't usually chosen on a street car, but if you don't have one it could work fine.

Terrible luck though. Hope it's not as major as it looks.

Dubbya~
 

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With T5's the fuel cut-out is activated, indirectly, by the MAP sensor. The pressure the sensor 'sees' in the intake manifold is converted to voltage. When the voltage gets too high (more than ~ 3.4 volts) the fuel is turned off. The rev-limiter, IIRC, is set at the T5 and can be reprogrammed, even on T5.2.
 

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Discussion Starter · #15 ·
No problem with fuelling at the high end. Having experienced overpressure fuel cutoff, I would say the rev limiter is ignition based. I don't believe it's MAP sensor activated either; it must be rpm linked otherwise it would not be possible to change the rev limit easily.

I'll know more after tomorrow afternoon; I suspect that a weak valve spring has allowed a bit of valve to piston contact. Here's hoping it's just the vlave that has suffered...
 

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hers an odd experience i had 2nite with the rev limiter. hit it very briefly in 2nd, then when i went into 3rd, max boost had tailed right of, when into 4th, gauge was only going halfway through yellow
This low boost situation continued for about 2 miles, now max boost seems to have upped itself, in each gear, the gauge is going right through red and into black in all gears,

ive tried various gaers and various throttle conditions, and it doesnt seem to be hitting over boost, but me thinks it cant be far off it.
Could this be some sort of adaption process as i generally dont go anywhere near red line.
 

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Discussion Starter · #17 ·
The good news: no damage


The blowback leak (combustion deposits all the way up the inlet elbow- yeuch!) was caused by a sticky hydraulic lifter.

Higher boost is known to cause problems forcing valves open; my previous software didn't hold boost so high so presumably didn't hit the problem.

Solution: a set of stiffer valve springs and newer lifters.

Thinking back, I've never been 100% happy with this head; it came off a 91 car of unknown mileage and although there were no cracks etc. it's never felt "right" and it could well be that it's had tired springs and lifters and we've just finally flushed the problem out. Fingers crossed...

The bad news: the cost
 

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Glad it's being sorted Bubbles- yes, the rev-limit is ignition based and is controlled by the T5.2, if that's what's on your '93. You didn't respond to the question about rev limits/software from 95power - I'm just wondering what you're boost pressures are near the rev-limit.
 

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Discussion Starter · #19 ·
Difficult to see exactly what the boost pressure is near the rev limit 'cos it's all happening a bit fast by then, but I'd reckon it's around 1.4 bar (it was around 1.2). I'm part way through a custom mapping by Abbott- we've still got more work to do, which hopefully can be sorted once we've got the mechanical problems out of the way.
 

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Wow - you said you had a bigger turbo - what kind is it? And by intake mod's, do you mean more fuel/bigger injectors? If you did use larger injectors, was Abbott able to remap the fuel for them?
 
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