hommer

Hello, i have a 1977 car just put on a new stock set of vortec heads and weiand intake on the original block, it has flat top pistons, mild 2102 edelbrock cam(420 intake and442 exhaust lift) 600 cdfm edelbrock carb, headers and side pipes, it is only used as a street car , i am getting conflicting story's regarding the ignition timing and setting,, right now i have the original distributor on and using the constant vacum hooked up , very close as the original setting was, any know what might be the best for this engine set up, maybe a new dist. with adjustable advance's? new moduale? any suggestions? thanks homer

MelWff

get a spring/weight kit, use the stiffest springs with your stock weights and install the metal advance stop bushing, set total mechanical for 36 degrees. Your vacuum advance should add about 8 to 12 degrees.

billla

It's still just a GEN I SBC - the heads didn't change anything in terms of what the engine wants for timing

Everything you need to know is right here:

http://forums.corvetteforum.com/c3-t...long-post.html

gregs74

I have my timing set at 12 degrees intial timing and 36 degrees total timing which is what the total was when i set the intial at 12. That is where the vortecs make best power. I have about the same setup except for a 750 edelbrock and an edelbrock eps performer intake. I have hooker sidepipes with the 2.5 inch hooker straight through inserts and love it. Makes great street power.

cv67

interesting, many Vortec users run 30-32 as they dont seem to need more. Others....

SH-60B

Yes, mine is set to 30*. That's where they made best power on the dyno. Hot Rod, Car Craft, etc. went over all this in the late '90s. Vortecs have a very efficient combustion chamber that doesn't want alot of lead when used with flat tops at about 9.5 to 1.
Lower compression seems to want more lead than 30*.

billla

I've built a fair number of Vortec head 355's and 383s and done quite a few tops. One mild (truck) Vortec 355 we put on the chassis dyno locally definitely wanted 36. CR was 9.1. Experience from the dyno operator noted that they usually see 34-36 - but there are a lot of variables to be sure.

Curious what the power difference was between 30 and 36 all-in on the dyno?

hommer

thanks for the input, i have tried all different settings within reason,with all the info i am getting and reading, i am not sure which is the best by seat of my pants feel, i have spent more on gas in and out of the shop testing, i think i could have bought some dyno time ,LOL , it runs ok i just want to get the most of what is tied up in the new motor with these stock vortec heads

Solid LT1

I think your guys are pretty SMART! Based on the better combustion chamber design of the Vortec, I would use around 32 degrees max ignition advance on a iron Vortec equipped motor.

hommer

ok right now i have it set at 36 total, it feels pretty good there, but today i will go from the high 36 down to the low , 31, and see if i notice a big change , i am running out of street dyno time, lol. as we had some snow last nite and if they put any salt on the roads i am finished until spring as it will be stored

cv67

Read your plugs might be a better gauge of whats best. Porcelain can tell a story or two.

hommer

the plug is light- medium grey, looks good maybe a touch rich, it is a new motor so that.s ok , but i will see if there is a colour change after i drop the timing all the way down to 31, the carb 1406, it is new also and have it calibrated for good feel also , i want to leave it alone right now and finish up my timing then go back to the carb, look like i will get out in a couple hours and see if i like the feel at 31 and also see what the plug looks like in comparison to the 36

billla

In the end, the only way to determine what's going to work *best* is on the chassis dyno, but I have to think on a mild 355 we're talking a max of a couple ponies one way or the other over 4-6 degrees of full advance.

BKbroiler

I have Vortec heads and I don't do too much tuning at the track since when I'm there it is usually for bracket racing but I believe my car runs better with total mech advance at 32/33 versus 35/36. Not enough difference to feel in the seat of the pants on the street.

cv67

Will go out on a limb and say if theres no gain with more leave it on the lower end, probably where it "wants" to be

69427

Your thinking is correct. Advancing the timing further than necessary (missing the ideal peak pressure point at 15*ATDC) just results in additional negative work done by the crank/piston as they have to compress a higher pressure traveling to TDC.

Shark Racer

These two posts tell the entire tale!

hommer

ok i went to 31 degrees from 36,.... i THINK, i noticed more torque i think, but the off idle was a little hesatant i was asumming lean, so when i got back sure enough the plug i pulled was pure white so when i retarted the timing it went to a leaner mode so if i leave it at a lower advance it needs more fuel, i thought it would be less fuel, so less advance = more torque, maybe,but it needs more fuel, interesting!!! don't forget i am going by feel only, ,ineed to get some more needles for my carb now to really fine tune, i,know a dyno is the only real way to get it spot on but i will get it close, maybe in spring i will dyno tune maybe$$$$$$ i am very perticular on how my own car runs and like to get it as close to perfect as possible, nothing like a good running 350 no matter what the output,

billla

Honestly, there's just no practical way you can do this SOTP. Split the difference at 33 degrees total and just leave it - then start working on tuning the carb. Even there, an A/F meter is really the best way to do it if you can borrow one.

I am a bit worried about how you're reading the plugs - coloring is a lot more subtle that many realize, and you can't just make a change and then go drive for 5 minutes and expect to see something different. I would go back to the factory config on the Performer carb if you've been swapping springs, rods or jets. The hesitation is a lot more likely to be due to a timing error or the springs on the rods than a huge difference in the A/F ratio. You can download the tuning guide from Edelbrock if you don't have one - but if the issue is at idle, then you'll also want to check for vacuum leaks and set the idle mixture/speed properly. Start from ground zero and work methodically.

You're following the timing process in the sticky or Lars' paper, right?

Shark Racer

You don't know what you're talking about.

If the car stops running and the plugs are brown, put in jets the next size down.

If the car stops running and the plugs are white, put in jets the next size up. If it doesn't start, replace the pistons and try again.



(and I agree with this 100% - nothing beats a wideband for carb tuning. Other than a wideband and a dyno...)