Check this out. I contacted someone here on the thirdgen.org who has the Desk-Top-Dyno program. He's been kind enough to run some estimates for me on my current build up. I had my timing set at 10*. The cam is Comp Cams XE-262-H. He said that according to the program, which I understand is just a rough estimate, that I was losing about 10 to 12 horsepower due to the timing.

I figured more advance right...Nope. He ran the same configuration with the timing at 0*, and got 12 more horsepower. At 4* he showed 10 more horsepower than at 10*, or something along those lines. That goes against what I understand about advance, but I'm certainly no expert. He seemed to be intelligent and experienced, so I decided I didn't have anything to lose, so why not give it a shot.

He also suggested to set the timing of course with the engine warm, but also with the RPM's at 1000, and the car under a load. I can see the philosophy behind setting the timing under a load. I was a diesel mechanic in the army, and charging systems were the thing I used to end up fixing more than anything. The tanks (I can tell stories about torque that you won't belive) were equipped with external voltage regulators and external rectifiers. If you didn't adjust them while the engine was under a load it wasn't done properly and the system wouldn't fully charge.

I do know that at 4*, with the cam I have, vacuum is a little on the low side, especially on steep inclines or declines, making the power brakes a little tough. 6* is what I decided to try. So I warmed it up, revved it to 1000 rpm, turned on the lights, the stereo and the fan. Disconnected the vac advance and set the timing at 6 at first. Re-set the idle at about 750 and I had lunder 10 pounds of vacuum. I did it the same way at 8*, and it idled just fine, with enough vacuum. I checked it again at 750 without a load and it was still at about 8*.

Again, I was very skeptical about the whole thing. So I took it out for a seat of the pants dyno test. It seems that the guy was right, my over all power band, according to my pants, was wider, and it did seem to be stronger and quicker. I always run the same course, and it is always and empty road.

Very interesting. Actually, I don't just think he is right on this one, I know it. The car is faster, and responds better. Why is that? With the cam timing, could 10* advance be a little too much?

Any thoughts?

 

I have that cam too and I run about 6-8 deegrees. My vacuum advance is not adjusted just right, so I get pinging around 2K rpms if it is around 10. I'm assuming you have a non-computer controlled HEI and carb? Have you done anything to the weights/springs?

Your vacuum is too low. I pull about 13-14" at 900rpm or so. I would bet you have a vacuum leak somewhere.

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

Is he talking ignition timing or cam timing? There's no option to set ignition timing in Desktop Dyno.

 

Biochem,

Yes, carbed and an old HEI vac advance, no computer.

I don't have a vac leak anywhere, and pull about 11 to 12 pounds at 750 to 800 rpm.

I just meant that at 6*, my vac pull was too low.

 

If it's timing in Desktop Dyno, it's cam timing. Advancing the cam tends to increase the low end power of the engine, while retarding the cam timing tends to increase the upper end.

-Mark W.
'88 SC Camaro w/ 305 TBI

 

I don't really know, that's just what he told me. It makes more sense though, that I would have more over all power with my CAM timing at 0...but not with the ignition timing at 0. We apparently misunderstood each other I guess.

At any rate, the car does seem to run better at 8* instead of 10*.

 

They are right. Desktop Dyno only lets you advance the cam.