Any body have any suggestions for the timing curve on the 260 HP Gooderench 350? I'm currently running 36 total mechanical advance all in by 2000 RPM. Distributor is hooked to ported vacuum and and adds another 20 degrees of vacuum advance. The engine seems a little unresponsive off idle.

8.5:1 CR
Edelbrock Performer RPM manifold
Holley 600 CFM vacuum secondary
Shorty headers with 2.5" exhaust tubing

This is a low compression very mild cam engine and from what I'm hearing mechanical advance should be all in by 3000 not 2000 rpm. Any thoughts?

 

Unresponsive off idle = low static timing + too much centrifugal advance, to get to 36

Typical stock distributors (or stock PROM programming: same difference, just accomplished by the one or the other method) typically are set up with 4 to 8° at idle, and 30ish of "centrifugal" (RPM dependent) advance, to add up to 36° or so.

A MUCH better setup is to use MORE static timing - especially with a no-compression motor like that one, at 8.0 - 8.2:1 since it won't ever get into pinging - and LESS RPM advance. Like, 20 - 24° static, then only 12-16° of RPM advance, plus the vac adv at low load.

Also you can often get ALOT more off-idle "zip" by running the vac adv off of hard manifold vac (which will make it active at idle) instead of ported (which makes it inactive at idle). Instant 15 - 20° more advance off idle.

Try all possible combinations. The word for this is "tuning". That means, the process of methodically adjusting EVERYTHING until the engine's demands are best satisfied. Just setting things to the numbers for your sheet metal's stock motor is worthless for any other motor besides that; and sometimes can be improved on substantially even in stock ones, especially if all of the factory's compromises for altitude, the full range of weather conditions, emissions certification, and on and on, don't apply to YOUR situation. Which is not to say that the factory is "wrong"; only, that their setup is not necessarily optimum for YOUR usage.

Once you get the timing right, go through the Holley tuning sticky on the Carb forum, step by step, FOLLOWING THE INSTRUCTIONS FOR EACH STEP BEFORE MOVING ON TO THE NEXT.

 

Ok i guess I can bump up the static timing to 24 and then mechanically limit the centrifugal advance so it does not exceed 36 degrees mechanical. Do you recommend playing with the springs so that the centrifugal advance is all in by 3000 or is 2000 rpm ok?

I did notice the car idles smoother with better off idle transition using manifold vacuum instead of ported. But I'm afraid of detonating the pistons with that much total timing at idle and steady cruise.

I'm not sure I've ever heard a Chevy 350 ping so I wouldn't know when it starts to knock. Is there a maximum timing (static plus vacuum) that you recommend I stay below on the motor, like 54 degrees or something?

I think a lot of the factory/standard specs should apply to my specific case. I'm pretty much at sea level, (46 feet) and it's around 75-85 degrees these days when I drive the car.

Oh and BTW, i have read your Holley tuning sticky back to back several times, AND followed it. It helped, but I was able to pick up another 1.5-2 MPG using a wide band AFR gauge. I think there is still room for improvement though as I'm only getting around 12 MPG in the city conservative driving...

 

Yea do what sofa said and that should help a lot. What distributor are you using?
There should be no problem with detonation from the high timing at idle and cruise from the vacuum can because once you move the gas pedal it lowers the timing back down.
And forget the factory specs, like sofa said before. they can be greatly improved on.

And when you say its unresponsive off idle. Do you mean wot or part throttle or both?

 

So i switched over from ported to intake manifold vacuum and it idles much worse now! It's not taking well to the additional advance. I've uploaded a YouTube video where you can see a vacuum gauge, wideband AFR gauge and clearly hear how rough the idle gets when switching over to manifold vacuum. Nothing else was changed on the car.

I'm not sure what to do now. If I retard the timing by rotating the distributor I will get less than 36 degrees at WOT and sacrifice performance.

The video is in 2 parts, first one shows ported vacuum, next one is full intake manifold vacuum.

 

I'm using a Mallory HEI vacuum advance distributor.

By unresponsive at idle I mean part throttle. It seems sluggish to rev up.

 

Problem solved!

The carb base plate was cracked creating a huge vacuum leak at one of the vacuum ports. Replaced the base plate and the engine idles smooth on manifold vacuum.