9

I'm driving a 1991 Citroen BX TGE16 which doesn't use fuel injection and I'm worried it doesn't have any fuel and spark cut off system so I don't over do it when downshifting or engine breaking.

Can I downshift without panicking every time my car revs up?

Edit: What happens to a car engine when giving max throttle while not in gear Older vehicles will not have this self-protection device. I defer to an expert with more information regarding when rev-limiters began to be implemented by the manufacturers to edit this answer with some of that information and remove this paragraph. Thanks.

  • I don't think that question is apliable since I was under the impression my car didn't have an ECU or if it did I don't know what features an old ECU would have compared to the modern ECU's we all know and love – EChan42 Apr 5 '16 at 8:31
  • I don't think this is a duplicate as it talks about non-fuel injected cars, and not modern computer injected cars – George Apr 5 '16 at 11:23
  • 1
    I've checked the parts catalogues for this car. Apparently it used either Bosch or Ducellier ignition systems. Neither the Bosch or Ducellier arms appear to have any kind of built in RPM limiter. – Steve Matthews Apr 5 '16 at 13:29
  • Thank you @Steve that was very helpfull, could you possibly share those catalogues with me so I can consult them in the future to resulve this kind of doubts by myself? – EChan42 Apr 5 '16 at 13:42
  • 1
    @EChan42 I used the online catalogue available as part of the Euro Car Parts website. A quick google should locate them. For non UK registered users, you can bypass the Registration Number search and pick the car make and model directly. – Steve Matthews Apr 5 '16 at 14:05
14

I'm not entirely sure a rev limiter will help protect you if you downshift at a speed higher than the gear you've selected is intended for. If you manage to lift the clutch, even without any fuel, throttle or spark, you could do some fairly significant damage.

To answer the question regarding the rev limiter. If a carburetor or mechanical fuel injection equipped car has a rev limited, it will take the form of a mechanical mechanism built into the rotor arm.

Typically rotor arms are fairly simple and compact in that they'll have a straight copper connector running along a straight arm and little else. If a rev limiter is built into the rotor arm they'll typically take on a more complex appearance. You may see springs built into them or they'll look more like a cam or snail shell.

Now, advice regarding downshifting. Take some time to learn the speed readout at the maximum RPM of each gear and simply adapt your driving style so that you don't downshift to a gear when the speed of the vehicle is close to or exceeds the known maximum for this gear.

For example, my Golf is bouncing off the rev limiter at 30mph in first. I therefore don't select first unless the speed of the car drops well below 30mph. With second it's around 55mph and third is approaching 80mph.

  • Thank you for your advice! I'll actually inspect the engine to see if I can discriminate anything that looks similar to what you pointed out. If I can't can I upload a photo and see if you can be of more asistance? – EChan42 Apr 5 '16 at 8:40
  • 1
    Simply popping the distributor cap off should reveal the rotor arm. Just be careful to put it back in the right orientation and don't disturb the order of the wires. – Steve Matthews Apr 5 '16 at 9:11
  • Your speed-based thresholds will change depending on load, weather and incline. – Lightness Races in Orbit Apr 5 '16 at 11:36
  • 1
    The relative engine speed against absolute speed in a given gear is a constant, defined by the gear ratio itself. What you're I think referring to is when you'd want to downshift which I accept is different in difference circumstances but the OP was worried about avoiding a downshift where, upon releasing the clutch, the engine RPM exceeded the redline. For example, shifting into 2nd at 75mph instead of 4th (which I've personally done, by accient, and can not recommend) – Steve Matthews Apr 5 '16 at 12:34
  • 3
    @BarryTheHatchet for a manual transmission vehicle, you are incorrect. As Steve M said, absolute speed in a given gear is a constant defined by the gear ratio. It is true that in a fully loaded vehicle, the engine will need to exert more power, and will therefore take more time to reach X speed in Y gear, but the speeds vs rpm will never change (so long as the clutch is engaged), because of the unbroken physical link between the crankshaft and driven wheels. – MooseLucifer Apr 5 '16 at 13:33
2

As stated by Steve, downshifting when the vehicle speed is too high for the gear selected will over-rev any engine. The rev limiter only stops the engine going too fast under its own power.

Non-injected engines do sometimes have rev limiters built into the ignition system, you'll have to investigate your own engine specification I think.

2

I would expect a mass market car from the 1990s to have a rev limiter of some sort.

But...as other posters have stated a rev limiter won't prevent over revving due to downshifting. Having said that it would require very aggressive downshifts to over rev your engine and it is fairly likely that you are going to damage or at least reduce the life of other parts of the transmission (clutch especially) before over revving as such is a big concern.

In general you don't want to do a harsh downshift when revs are close to the red line in any case, certainly from a reliability perspective and probably not even for performance.

For normal driving you want to try to make all of your actions have smooth transition as this will maximise reliability, fuel economy and car control.

Even in racing engine braking needs to be used carefully as it can be an abrupt load which risks locking the driving wheels and is best used in conjunction with softer deceleration ie lifting the throttle and progressive braking.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.