Let's talk engine timing

[warren57]

Hey Chevynut, please give me your thoughts.
I installed a GM Performanve 330 HP engine in my 57.
Starts right up and runs pretty good. Not to thrilled with the Holley carb and may try some sort of EFI in the future.
So let's talk timing. Instructions said set at 10 degrees BTDC. Ok, starts and runs good. Idle a bit rough and a bit of off idle stumble, which certainly could be a carb issue.
I set timing a couple more degrees advanced, like we did it in the old days, for altitude here (Denver)
Idle is better, off idle lag better.
Now if I advance it a few more degrees, idle smooths out and off idle issue much better.
As kids we used to drop in the distributed, advance it until the starter started dragging, then back it off until the started spun freely and that's where we left it. Don't recall ever having an issue ( but that was nearly 50 years ago, ha ha) and that likely equated to 16 or more degrees advance.
So my question... Is recommended timing more an issue with the factory meeting emissions standards? And, is there any issue involved running a little more advance?
Appreciate everyone's thoughts.

[chevynut]

Well there are guys here that know more about engines than I do, but I'll give you my thoughts.

First of all, do you know if the timing mark on the damper coincides with TDC? I would think if this is a new engine it would, but it might be something to check.

Secondly, as I understand it you need to check the timing with the vacuum advance (if it has one) disconnected and the hose plugged.

It seems like the engine should run fine at the recommended 10 degrees advance.

[Rick_L]

At some elevation like you have, you lose some cylinder pressure because of the less dense air. Also the octane requirement decreases - the engine has less tendency to ping. Increasing the ignition timing helps recover some of that cylinder pressure and doesn't run the risk of pinging that you would have at closer to sea level. So a few degrees of extra timing would be a logical thing to do.

This is not an emissions thing. A factory EFI setup will have a timing map with one of the variables being atmospheric pressure (MAP). At higher elevations, the timing is increased automatically with factory EFI - for just the same reasons.

The only downside of this would be if you took a road trip and came down off the hill.

[markm]

I know several guys from around here that have attended the NHRA ET Finals when held at Bandimere, if I recall correctly they had advance timing several degrees and drop jet sizes and still ran like crap.

[chevynut]

they had advance timing several degrees and drop jet sizes and still ran like crap.

They probably ran as good as they could at 5000+ feet. It makes a big difference in power as I found out long ago with rental cars. I rented the same car in two locations on a business trip, one in Denver an one in Dallas, I believe. I couldn't believe the difference.

[55mike]

I agree .. double check TDC to the markers on the balancer and pointer. Once you know for sure that 0 is just that, then I believe 15-17 degrees (vacuum advance hose plugged off) is OK especially at high altitude. Just don't let it ping... With my aluminum head 355, it likes 17 degrees initial and about 34 degrees at 3K.

[Rick_L]

You're going to lose about 20% of your sea level power at 5000' elevation. The only good news is that the air is drier at elevation. That's after you get the tuneup (fuel and timing) right. It's just physics.

[Rick_L]

I saw on the other site (chevytalk) where Ignitionman suggested 1 degree of initial timing per 1000' of elevation, in response to Warren posting there. Makes sense to me as a safe rule of thumb recommendation.

When I was racing at Amarillo and Denver, we did a bit more. We also didn't change the jet as much as you think you first think you "should". (First thought is that you should lean it out.) Part of that may have been due to drier air.

[warren57]

Thanks everyone, I'll check TDC, but sounds like a few extra degrees isn't bad. And It didn't ping, started fine and seemed to perform better.

[markm]

On the flip side I was told some of the Bamdimere boys had to slow things down a bit to stay within their class when they showed up at Topeka.