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couple of engine questions


ClioSport Club Member
trying to futher my knowledge on engines here to help get more power from mine, now i already know the basics on how an engine works and what part does what etc but couple of bits im confused on.

when you rev an engine too high it eventually loses power and you need to change gear to get back on the power (mine does it when it still has about 300rpm before the limiter, around 3,000rpm) what causes this? is it the injectors struggling to get enough fuel in? does the engine need headwork, is it down to the standard exhaust/cat?

next, on the same subject as above where the engine drops off the power band, i find that in first gear it dies at about 5,000rpm and your better to go into 2nd but in 2nd gear it pulls right the way to the limiter (6,250rpm) is this some sort of power limiter on the ECU?

  Chocolate Bar™
my 1.2 16v pulls all the way to the limiter if i wanted it to. no noticeable drop off in accleration at all. got a rolling road where it had a perfectly smooth torque curve which i have always put it down to.

cant help you on the rest :clown:


ClioSport Club Member
it pulls to limiter ok in 2nd and in 3rd but not 4th but by the limiter in 4th its pretty fuked anyway doing 100+mph so suppose thats why but the first gear thing is really annoying me, in 2nd it pulls and pulls then hits the limiter, first it pulls, pulls then dies for a second or 2 then limiter
  2005 Nissan Navara
What your asking in your two points is one in the same.

There is a WHOLE big can of beans that can be explored here, but in simple terms...

when an engine is running throught the rpm range when ur foot is flat to the floor (or at any throttle angle for that matter), imagine the engine taking in air through the inlet. a standard na engine is never going to be able to take in 100% capacity of air that the cylinder will geometrically allow...the technical term for this is volumetric efficiency (the % of volume of air taken into the cylinder compared to the theoretical maximum).

basically as the revs increase so does volumetric efficiency, usually upto around half maximum engine speed, then it starts to drop of. The torque produced by an engine directly follows volumetric effeciency, as simply the ammount of air taken in dictates the ammount of fuel which can be burnt and so can be converted to chemical energy, and finally force on the piston....torque.

So to your question...when your reaching maximum rpm, the volumetric efficiency has dropped to its minimum, this is to say that the engine cannot take anymore air in as its simply going to fast...there is reduced time period for intake valve opening mainly...ammongst other factors. therefor it drastically drops its torque output and you need to change gear.

You can then go onto say, that this is where cams/hedwork/throttle bodies etc come in...they increase the volumetric efficiency of the engine, and so its ability to produce torque.


ClioSport Club Member
cheers mate thats a great help! but one thing i dont understand is why does it drop off the power band in 1st gear at about 5,000rpm but not in any other gears? is this some sort of limiter in the ecu?