Issues After A New Cam Fitted To 8 Valve Xu9

[welshpug 1,622]

wasn't sure whether this should go in Aftermarket management as I've got Motronic MP3.1 on the engine, or XU engine and gearbox modifications due to the new cam!

 

main issue is that it doesn't idle anywhere near as cleanly as it did before, quite understandable having gone from a standard D6B cam to a PTS "gravel" cam.

 

I have wound out the air screw on the throttle body to try and get it to idle better, which it does, just!

 

When cold it seems to be running rich, not sure if that's simply because its not burning? I'm going to double check that the Coolant Sensor is ok, but I had no issues previously.

 

At low revs (under 2k) it does not like part throttle openings, tending to kangaroo quite badly! I'm sure this is caused by the same thing as what is causing the bad idle.

 

above 2k, and all the way to the limiter, its fine

 

 

What I'm basically asking is what can I do to make it idle better? short of a remap which I can't afford right now, and I think I would rather find a second hand DTA system or similar as remapping the Motronic system isn't as cheap.

[Baz 421]

What's that cam specs?

[welshpug 1,622]

I believe its as follows:

 

288 duration, ~12mm lift and 3.6 lift at TDC, don't know at what angles the valves open and close though.

 

got it on a standard cam pulley as I was told that they were made to run using the original pulley, obviously a vernier would allow fine tuning if necessary.

[petert 550]

3.6mm @ TDC is a reasonably FAT cam. If you haven't got at least 11:1 you're pushing sh*t up hill trying to make it behave under 2000. Even with 11:1, it will need to idle at 1100-1200. With correct fueling it should be drivable on the later ECU.

[Tom Fenton 1,525]

Lots of valve overlap meaning at low revs the trapping efficiency isn't at all great leading to the problems you mention. It will be running rich on the old settings as it will be blowing some fresh charge straight out of the exhaust. What exhaust system is it running, free flowing one will help reduce backpressure.

[welshpug 1,622]

its a pretty decent Longlife exhaust, 2.25" bore mated to a standard phase 2 downpipe, certainly not a restriction as it had been RR'd with the standard cam at 129.8 bhp, and 124lbft (up a few over standard figures of 128 and 119 lbft)

 

all makes sense thanks guys, shall have to see what I can do with my pile of bits, and decide to live with it or go back to standard till I can afford to do some more with the engine/management.

[welshpug 1,622]

It just dawned on me why I've heard of rally 1.9's running 1600 pistons and recurved dizzy's to get the compression up, this ruddy cam!

 

what I need to do, is to work out how much clearance @ TDC I have, and how much extra compression, and less clearance, the 1.6 pistons give, more reading+searching for me

Edited January 2, 2009 by welshpug