Fiddling with Cam Timing

[Xnke]

Ok, so in the search for a better idle quality and shifting the power band down some, I've finally gotten around to advancing my camshaft from the #1 postion, to the #2 position. I think I have enough data at position one that I can start playing in position two.

 

At idle, in position one, the following engine conditions were present:

 

Polite Idle: 1000RPM (A/C on: 1100RPM)

Minimum Stable Idle: 850RPM (950RPM)

Idle Ignition timing: 24 degrees

Idle Fuel Cell Value: 34

Idle Vacuum: 62Kpa

 

Clutch had to be feathered pretty sweet to pull away from a stop sign smoothly, 2000RPM seemed about right. Acceleration from 2500 to 6500RPM excellent in second gear, legal speed requirements limit third gear data. Third gear data shows much promise...Heheheh.

 

At idle, in position two, the following engine conditions were present:

 

Polite Idle: 850RPM (A/C on: 650 RPM, unstable)

Minimum Stable Idle: 560RPM (A/C on: No data available)

Idle Ignition timing: 26 degrees

Idle Fuel Cell Value: 31

Idle Vacuum: 52Kpa

 

Clutch still had to be feathered some, but not too bad. 1500RPM was enough to make a smooth start. Acceleration still good from 2300 to 6500RPM in second gear, Much better head-snapping motion. Caught me unprepared and had a P.I.O. moment on the clutch! No third gear data available at present.

 

Now, I just need to figure out a simple solution for idle control, I'm thinking a combination of a low RPM timing bump and perhaps a solenoid-type IAC valve tied in line with the A/C compressor clutch will do about right. Anyone got any good ideas for a solenoid type system for IAC? Getting megasquirt to control it isn't really needed, the engine starts and idles fine with simple warmup enrichment as-is. The only need for an increased idle speed is to counteract the extra drag at idle from the A/C compressor.

[ezzzzzzz]

I use a fuel cutoff solenoid found on many GM cars back in the seventies. It is mounted where a dashpot was located on my balance tube (SU carb'd). When the a/c is 'on' the solenoid gets 12 volts too. The plunger can be turned in or out to set idle as desired. I set mine so the idle remains identical whether the a/c is 'on' or 'off'. There are number of these solenoids in a varity of shapes. Cost is typically in the $65 to $75 range.

[ozconnection]

I'll just pass on a few observations I made when I was playing around with camshaft timing on my L28.

 

Idle fuel mixtures need to be tweaked when changing cam timing. Your poor idle with low manifold vacuum and the A/C on when the cam is in position two is indicative of poor torque, despite the new cam position and increase in idle speed ignition timing. Go to your tailpipe and listen to the tone. If your idle is splashy and sounds like "mmmmmmpmmmpmmmmmmpmpmpmmmm", then you're too lean. Richen it up a bit and see what happens. This is easy, easier than the solenoid option at this point and tweaking the mixtures costs nothing to try.

 

I'm running 22 degrees ignition advance at idle and my idle drops about 50 rpm when the A/C is turned on. I actually tune my idle with the trans in drive (auto transmission) to simulate real loads in traffic. Maybe you should tune your engine with the air on if its a manual shift car so it's never an issue when its turned on?

 

My AFR at idle is about 12.8 - 13.2 to one and has 16-18 inches of vacuum at 750rpm in gear (drive).

Edited July 6, 2011 by ozconnection

[Xnke]

Right now, I'm fighting a charging issue or I'd have more data ready for this thread. I think i've narrowed it down to a weak battery; so hopefully I'll get more data in here soon.