Tick over speed

[WokingWedger]

Why do our engines tick over so fast ?

Mine sits happily at the regulation 850 rpm, but even my wife's diminutive Jazz (all 1400 cc 4 cylinders of it)ticks over happily at 500 rpm.

We have twice the no. of cylinders for crying out loud!

I love the big yank lumpy blub blub blub blub sound on slow tick over.

I know somebody is going to come on here now and say 'mine sits nicely at 300 rpm' or something equally annoying, but its not the norm is it.

Is it the old Lucas ECU that is the problem ?

Higher state of tune (cam etc)? But then some drag cars tick over slowly !

Thoughts anyone ?

[Steve_T]

It'll be the choice of cam I'd have thought.

[WokingWedger]

Hi Steve, how are you and Karen ?

I must try and get along to the mucky duck now my car is all back together.

I an sure choice of cam comes into it , but you hear slow tick over on engines with really wild cams. It sounds nice and lumpy and uneven. Mind you I wouldn't want to always be worrying about it cutting out at a set of lights.

[blitzracing]

Steve_T said:

It'll be the choice of cam I'd have thought.

This- The Range Rover can run down to 600 rpm on its much softer cam.

[WokingWedger]

blitzracing said:

Steve_T said:

It'll be the choice of cam I'd have thought.

This- The Range Rover can run down to 600 rpm on its much softer cam.

Hi Mark. So are you saying that the tick over could be adjusted to 600 rpm , but with a typical TVR cam, it would not 'pick up ' revs very well ?

[Steve_T]

WokingWedger said:

Hi Steve, how are you and Karen ?

I must try and get along to the mucky duck now my car is all back together.

I an sure choice of cam comes into it , but you hear slow tick over on engines with really wild cams. It sounds nice and lumpy and uneven. Mind you I wouldn't want to always be worrying about it cutting out at a set of lights.

We're good thanks Peter. If you're looking at cam choice don't chase it too far, you'll loose more mid range torque than you will gain at the top end.

[ClassiChimi]

I'd say it's a combination of higher spec cam and the fact the cam doesn't receive much oil at lows revs,
Hense a higher tickover to compensate.

[blitzracing]

WokingWedger said:

Hi Mark. So are you saying that the tick over could be adjusted to 600 rpm , but with a typical TVR cam, it would not 'pick up ' revs very well ?

Its more complex than that- a long duration sports cam is great for high RPM performance, but at low gas velocitys you tend to get exhaust gas back up the inlet tract at the point the exhaust valve is closing, and the inlet opening (the overlap period). This has the effect of stalling the air that going through the AFM and down the inlet tract, so the idle becomes very rough due to poor airflow and exhaust contamination. The only fix is to raise the tickover to a point where there is enough air velocity in the inlet tract to fill the combustion chamber fast enough to get a reliable burn.