HD4mula

I never see this topic here and I'm not sure if it should go in the DIY Prom but I'll try here on TPI first. I would like to get some guidance or opinions on the timing advance curve for my moded TPI. I have the mods below and run 93oct fuel all the time. Base advance I run now is about 10°. Main interest is the WOT advance so I guess that would be the 90-100 KPA section. What do these engines run best with my mods? TIA.

My mods;
Roller 350 with 10:1 compression,
Short 1 5/8 headers with 2.5" true dual exhaust
52mm TB,
Plenum ported by Corvette Plenums,
AS&R runners,
Edelbrock base matched to runners,
AFR Eliminator 180 heads with 1.6 full rockers.
HyFire 6+ ignition box and MSD wires

Comp Cams XFI 268 Hydraulic Roller Camshaft
249-12-466-8
Operating Range: 1800-5800 RPM
Duration Advertised: 268° Intake / 276° Exhaust
Duration @ .050'' Lift: 218° Intake / 224° Exhaust
Valve Lift w/1.6 Rockers: .570'' Intake / .565'' Exhaust
Lobe Separation Angle: 113°

Mark 89Formula

You've got all the latest parts so it's a pretty safe bet that you're not going to find someone who has your exact combo. Even if they did the weight of your car, gearing, tranny choice, condition of the engine along with the environment could dictate a different answer. My first suggestion would be to make the initial timing constant in the PROM match the physical setting you dialed in with the distributor if you haven't do so already (couldn't tell if you had or not). The table you posted likes like a very nice starting point. I'd establish a baseline using those values, logging knock counts and air/fuel ratio, preferably at a strip with timing equipment. Then start bumping up the timing in 2 deg increments until you start getting repeatable knock counts and/or the car slows down. Target the trouble areas by backing off timing in the RPM region before the knock starts. It's a total WAG but I think you'll end up between 30-34 degs total timing once you clear 4000 RPM.

thirdgen88

I've got the same cam, and E-Tec 200 heads... Fairly similar setup.. So far, I'm at 34 degrees with it in by 2800 rpm...

My mods are here: http://gta.homedns.org/mods.html

Do you still have PE Spark Advance in addition to that table?

1989GTATransAm

iTrader: (3)

With the base advanced to 10 degrees you can add 4 degrees to those in the table. Why are are you at 10 degrees instead of 6 for the base?

Also do you have a table above 4800 rpm? Do you have a table that adds timing at WOT? I know mine does. Your table looks like mine but I'm at 6 degrees on the base and running 10.75:1 compression with a .035" quench.

One more thing the new heads with proper compression do not require a lot of timing.

HD4mula

Thanks all for the replies. Hey thirdgen88; those engine mods are so close to mines. Do you have any dyno #s of your car?

1989GTATransAm; I'm using 10° base (distributor) because that's where it idled better and felt ok the first time I started the car with the new parts and tested it. I'm still tuning the AFR (with Inovate WB) for WOT to ensure is not too rich. It was very lean (expected) with old engine tune and I added a lot of fuel for safety and now I'm taking it back slowly.
This will sound odd but I'm not using any of the common masks out there. Due to simplicity of install in my application I used the 165 ECM with the Australian $5D code for speed density. Tunercat & Tunner-Pro supports it but both show this table only to 4800 RPM so I assume values above would be the same as last line. Also I don't see the mentioned PE spark advance, nor the initial timing constant in the PROM. WOT I assume would be the 90-100 KPA columns so I don't know why values vary that much there. I was expecting like a 27 or 30 all the way up rather than jumping from 27.1 to 25 to 23.9 etc. I'm preparing to tune timing better soon. I was planning to take couple chips to the dyno with different timing/fuel and see which is better so i wanted to learn what would be best. Thanks again.

1989GTATransAm

iTrader: (3)

Gotcha.