timing or fuel first?

With a carb and distributor engine, the adage was "Get your timing right FIRST, then refine your Fuel..." (and sometimes you went back and forth a bit).

Dave's working on tuning a $12P BIN on my Indy Pace Car I've outlined here:
http://www.gearhead-efi.com/Fuel-Inj...working-on-it-)


We had a hiccup with fuel pressure problems, but that's sorted out now. It's driving pretty good now, but if we change the ignition tables, aren't we going to have to "start over" to some degree with the fuel?

I been doing lots of reading and can't find an information on how to create a timing table based on how your weights/springs/vacuum HEI distributor was set up. With my Indy SD4 engine was carb'd, I had an HEI distributor with a base timing of 16 degrees BTDC, and 38 degrees by 2800rpm. Vacuum advance was limited to 12 degrees. With the big cam and high compression, it idled pretty good there, and ran like a scalded cat.

We started with a BIN that had a basic Camaro TPI 350 timing table that's fairly safe and won't melt my engine while we were getting the fuel map sorted out. The Camaro's base timing is 6-8 degrees BTDC (running it at 10 and Dave's pulled some a bit based on Knock Sensor counts).

Will it simply be a matter of subtracting from the timing table if we move my base timing up by 6 degrees (to 16)?

actually if you have your distributor timing down, that's a really easy timing table to put together as a baseline. set every row up to start with 16 degrees of timing. then make a nice gentle ramp from say 1000 rpm up to 35-36 degrees @ 2800 rpm. (always start conservative with timing).

that's your distributor with vacuum advance connected.

to 'reconnect vacuum advance', simply add 12 degrees to your first couple low MAP rows (low map = high vacuum).

then blend between low map and high map, so vacum advance decreases as rpm increases.

also make your 'out of reasonable operating range' cells fairly retarded. for example 100kpa @ 400rpm should never happen, since that's some serious lugging, but if it does, you'd want freaky low timing advance so your engine doesn't explode.

kinda like this chart?

also about the effect of timing on fueling, yeah it'll do a bit. especially in cruising range it's a big..big deal.

they're kind of in a symbiosis

but if you do your fueling while your timing is 'imperfect' you'll find the tweaks necessary to get your fueling back in like is minimal.

of course if you're running rich, it'll want a bit more timing to burn all that fuel, as well as you'll have more tolerance to the added timing due to rich mixtures providing cooler combustion. so as you perfect your fueling, i usually notice that a bit less timing is required.

Quote:

---

Originally Posted by steveo

also about the effect of timing on fueling, yeah it'll do a bit. especially in cruising range it's a big..big deal.

they're kind of in a symbiosis

but if you do your fueling while your timing is 'imperfect' you'll find the tweaks necessary to get your fueling back in like is minimal.

.

---

I'm thinking that's what Dave has in mind - to get the fueling closer to "right" before fiddling with the timing.

Quote:

---

Originally Posted by steveo

of course if you're running rich, it'll want a bit more timing to burn all that fuel, as well as you'll have more tolerance to the added timing due to rich mixtures providing cooler combustion. so as you perfect your fueling, i usually notice that a bit less timing is required.

---

That should also cut down on all the popping in the exhaust (like a real race car LOL ) when I back of the throttle?

oh thats a combination of decel enleanment and timing. when you have very free flowing exhaust and a bigger cam, those very low MAP cells are critical, and decel enleanment too. in EE (which is mostly what i tune) decel enleanment can't even be controlled, so i just dick around with timing until it burns all that fuel off. funny that if you go too advanced, though, it actually starts making power during decel again and you dont get much engine braking at all.

base timing should remain at 10 degrees if the engine cranks into life without windmilling while cranking.it only uses the base timing at crank and in diagnostic mode.base timing in diagnostic mode and referance angle in the tune must match to make the tables read real commanded advance.also the time demain correction setting is there to adjust timing LAG of the ignition module to keep timing correct as rpm rises .if you old tune was 16 base and 32 all in set the 100kpa colomn to 16 degrees at low rpm raising to 32.if you have longish runners or have knock at max torque you make the timing table dip at max torque then start feeding the advance back in as VE drops off not uncommon to run more advace down low then lower it round max torque then ramp back up to match the ve being low at low rpm highest at max torque then falling again.