Cam angle changes and effects on compression

[Jeff N.]

Last night, Martin and I adjusted the cam advance angle on my engine. The adjustment was from +8 degrees to +4 degrees.

With 8 degrees advance, car was pinging at various RPMs at 4k+ with the stock and tuner chips such as EAT and Conforti. We suspected the car was making too much compression with the 8 degrees advance.

We did quick before and after compression tests on the #1 cylinder to see how a 4 degree angle change effected the cranking compression.

At 8 degrees: 195lbs
At 4 degrees: 175lbs

By retarding the cam from 8 to 4 degrees advance, the intake valve now closes slightly later during the compression stroke thereby preventing the cylinder from making as much pressure.

Likely the car was pinging at the upper RPMs due to excessive pressure as the VE increased as the car came "on cam".

Thought the gearheads in the group out there might be interested.

Cheers!

Jeff

PS - JoeS - you are right about the Korman advance gear. POS, it's going back. Wouldn't assemble right and you need a no-longer-available BMW part to capture the pin. Too bad as I really wanted to run the 5 degree setting.

[Robin-535im]

Last night, Martin and I adjusted the cam advance angle on my engine. The adjustment was from +8 degrees to +4 degrees.


I love the feeling when you get it all back together and zip off to try it out. Usually that happens for me at 2:00 am so that past-tired adrenaline kick makes it even more fun.

BTW- how do you tune the MAF on the car? Do you upload a different curve and drive it for a while and see how it is, or is there a dyno procedure that helps make it faster? I recall Martin took his to the dyno, but I didn't follow what you measure to tell you which way to tune the MAF. Perhaps you advance the setting until it pings then back it off, assuming more advance = more power.

I'm eager to hear how the trout drives now! :)

http://www.bimmer.info/bmw/Robin.Ritter/MyCarJeffsCar.jpg

[Jeff N.]

This is basically resetting to an advance angle I was previously running. The Metric Mechanic cam gear provides a set range of choices - 2 retard, 0, 2 advance, 4 advance. To get to 8 advance, I had moved the chain forward 1 tooth (10 degrees) and put the gear to 2 retard.

Effects are a smidge less low end power (due to less compression) with less advance. Obviously, the removal of the ping tendency was more important.

The MAF only changes the mixture, not the timing. I'm currently running the AFM, not the MAF, because I want to keep the number of variables low. I'm planning to re-install the MAF sometime next week.

Tuning the MAF seems to be best done on a dyno with a wideband A/F meter. While I think you can get it generally close without doing that, only dumb luck would get it spot on and maximize HP and torque.

How are things your way Robin?

[Robin-535im]

Tuning the MAF seems to be best done on a dyno with a wideband A/F meter. While I think you can get it generally close without doing that, only dumb luck would get it spot on and maximize HP and torque.

How are things your way Robin?

Well - I love that cold air that's been blowing in. It really seems to make a difference when you're at 60F ambient vs. 95F.

My latest car plan is to take the rear subframe off the old 535, replace all the rubber (pitman arms and subframe bushings are already new) and swap it onto the new 535. RD springs and bilsteins with all new rubber on the back should tighten things up.

Re: MAF tuning... Wouldn't the DME just make whatever mixture changes you incurr via MAF tuning get set back to the nominal 14.7:1? Assuming you're at steady state and the car is all adjusted, if you up the load reading, the car injects more fuel and advances the timing because the general trend on the maps is for more fuel and more advance as RPM's increase, right? So as you drive around after that, the pulse duration gets reset to whatever gives you 14.7, but the timing should still be advanced.

I ask because OldgreenlookatmenowIhave100moreHPe34's post about the MAF
made me more interested in getting one, but I'm not sure how to optimize. Currently I would think you tune it up to get the best power from your car, then ship the curve to Mark so he can take the load readings you spoofed from the MAF curve and program that into the chip via changing the maps, then you can set the MAF back to linear and have all your smarts in the maps.

[Jeff N.]

Re: MAF tuning... Wouldn't the DME just make whatever mixture changes you incurr via MAF tuning get set back to the nominal 14.7:1? Assuming you're at steady state and the car is all adjusted, if you up the load reading, the car injects more fuel and advances the timing because the general trend on the maps is for more fuel and more advance as RPM's increase, right? So as you drive around after that, the pulse duration gets reset to whatever gives you 14.7, but the timing should still be advanced.


I'm not 100% sure how the corrections work when the mixture's off. It would be a good Mark D essay. I think there's a limited amount of correct that the O2 sensor can do and I think it's tied to a "lambda table". Also, I'm not sure that DME is programmed to always keep the car at stoich - max power is often made on a slightly leaner mixture. Or, the DME can be programmed to run a bit rich - as ours our at WOT - to ensure the car doesn't go overly lean.

The MAF voltage, I think, will effect the lookup on the main and adjustment tables but I'm not sure the delta of a curved MAF signal vs. a stock AFM signal would make for a significant timing adjustment.

You know what would be really cool? I'd love to see a pseudo-code type flowchart of the Motronic decision tree and rules. Suppose that might be the keys to the chip programming kingdom and thus a tightly guarded secret. I think something like that would answer most all our questions. :)

Cheers!

Jeff