BTCC engines

[gmorris03]

I was watching the BTCC coverage from Brands at the weekend and was wondering why the BTCC cars seem to have such a lumpy idle? It’s much more than just a minor fluctuation in revs and is quite pronounced, especially when idling on the start line or finishing up at the end in the park fermé.

Any ideas?

 

[obewan]

I've wondered the same
This??
'Big Cams have more overlap. Meaning that both the exhaust and intake valves are open partially at the same time. At high rpm this gives better cylinder filling because the exhaust charge leaving the cylinder at high speed literally sucks in fresh air at a greater rate when the intake valve opens. However, at idle this means there is a constant variation in intake manifold pressure and thus cylinder filling. This is generally the reason they idle so poorly.'

Or this??
'The valvetrain will have been set-up for full-throttle peak power. A race/performance car will spend a lot more time being driven hard, so the low-end smoothness is sacrificed. On a run-of-the-mill road car, the valvetrain will be set-up with more leaning toward engine smoothness and low-end power'

Courtesy of Google, not my technical expertise

 

[mmm-five]

Like all race engines, BTCC engines tend not to like idling, and most will stall below a fairly high level (1500rpm or so).

This is mainly due to them wanting max power as high up the range as they can get it, as they’ll be spending most of their time up there. So they fit aggressive cams, and with this comes an very road-unfriendly idle.

Have a listen to a higher revving engine in one of the “formula’ cars, and you’ll find the idle is at a couple of thousand RPM, and if you don’t give it a good prod of the accelerator as you drop the clutch, you will simply stall. I’m not sure I’d want to have to sit at the traffic lights with the M idling at 2000rpm.

My old e34 M5 (and the e30 M3) had a lumpy idle due to the aggressive (standard) cams, but tickover was set at about 900rpm. The later M cars sorted this out and found other ways to have good peak power without using overly aggressive cams...and I was a bit worried that something was wrong when I first drove an M that didn’t have a lumpy idle.

 

[enuff_zed]

As above.
Had a 'fast road' cam in a 2 litre Capri a few years back and that was lumpy.
Tweaked the idle up to 1000 to compensate.
Could probably have left it at the standard idle, but being a 70s Ford it would have just shaken bits off!

 

[gmorris03]

Thanks for the replies guys! They sound so lumpy sometimes it’s nearly as if they’re stabbing the throttle quickly..!

 

[enuff_zed]

Thanks for the replies guys! They sound so lumpy sometimes it’s nearly as if they’re stabbing the throttle quickly..!

I think they may actually do that, to keep it from stalling, particularly if they're manoeuvring at low speed round the paddock.

 

[mmm-five]

I think they may actually do that, to keep it from stalling, particularly if they're manoeuvring at low speed round the paddock.

They end up looking like new Z4M owners trying to slowly get away from the lights, on a cold engine/gearbox oke:

The other issue will be the very heavy racing paddle clutch which is difficult to slip.