Big Cam = Lumpy Idle???

[Fal_325i]

Hi Guys,

Have acquired a vehicle with a 2.5 engine which consists of a bigger cam, at idle it sounds like it's missing, or like a an ols american muscle car. My wuestion is, is this normail?, if not how can it be rectified.

Thanks

[e30bmlover]

the higher the lift of the cam the more ropier the idle. so iv been told.

[Brianmoooore]

More the overlap on the cam than the lift, but "wild" cams = poor idle. Cams on a F1 engine are fairly wild, and they idle at about 4000RPM or more.

[Turbo-Brown]

Cams on a F1 engine are fairly wild, and they idle at about 4000RPM or more.

They also have a 25mm stroke or something silly leading to fuck all torque anywhere. Seem to recall they make about 183lbft at maximum power (say 700bhp@20000rpm), even I can beat that!

Of course what F1 cars also have is one throttle per cylinder which eliminates many of the idle problems associated with large lift-on-overlap cams

I'm amased by how many people in the magical world of the 205GTi consider ITBs to be a necessary part of the tuning procedure, especailly with big cams, and how many people who want more power from their E30 haven't even heard of ITBs!

[Morat]

Do F1 cars even have cams now? I thought they were fired individually via pneumatics and the ECU?
(Not that it affects the answer to the OPs question - yes, wild cam = lumpy idle)

[march109]

Morat love the pup, will he be taking over from the churchill dog any time soon, bet he adusts weight distribution, I've got a lab myself and he adjusts weight distribution by leapfrogging the seats for the whole journey.

back on topic:

yes wild and lairy cams = rough idle.

as wild cams are set to perform better at highter RPM they are less suited to lower RPM ie. idle speed. this makes their performance seem lumy at idle. Its not like twin cams or variable cam profile systems, where the cam profile is matched to the driving style.

The cam opens and closes the valves leading to the cylynders, the cam prifile dictates how long the valvle stays open or closed. at higher RP< it need to perform faster that it does at lower RPM so its all a balancing act.

[Gunni]

if you get a lumpy idle ,, raise the idle speed, it will stop beeing lumpy

[Brianmoooore]

Idle speed is set by the ECU software, so isn't that easy to raise, although there's an ECU pin that is earthed when aircon is switched on, that can be permanently earthed to raise the idle speed a little.

[Turbo-Brown]

Surely you could just jack the throttle open a little bit?

I know it'll stop the closed throttle switch from doing anything but I guess that's not a massive problem in this case.

[Brianmoooore]

If this car is going to be used on the road, I'd check that it can meet MOT emission requirements before worrying about the idle speed too much.

[Martinaston]

on the newer bm's the exhaust has a valve in the rear silencer that closes at idle to increase backpressure to smooth out the idle. you could see if an e46 back box fits
or just block one of your tailpipes for an experiment

[Globulator]

I'm amased by how many people in the magical world of the 205GTi consider ITBs to be a necessary part of the tuning procedure, especailly with big cams, and how many people who want more power from their E30 haven't even heard of ITBs!

ITB = Individual Throttle Bodies'? I don't know how much of an issue that is on the M20 2.5, as each inlet has a good long tube into the top plenum, so maybe the chance of one cylinders inlet flow disrupting anothers is fairly low in that engine.

Check the valve clearances are big enough - as even a standard cam has a rough idle below the recommended gap (in my experience).

[Templ8e30]

Do F1 cars even have cams now? I thought they were fired individually via pneumatics and the ECU?
(Not that it affects the answer to the OPs question - yes, wild cam = lumpy idle)

They still need cams but valve springs won't close fast enough so they use Nitrogen gas pressure in sealed pockets to close them. They need stripping and re-sealing after every event though. Big money.

Lightened flywheels make the idle lumpier too

Cheers,

Iain T

[Brianmoooore]

. Big money.
Iain T

I get to look around various F1 factories from time to time, courtesy of my brother-in-law. The way these guys spend money is unbelievable. Ԛ£100,000 pounds like we'd spend a tenner!