I have a ZZ502 that I am putting a Holley Sniper EFI unit on. I'm wondering if the car would actually benefit much from ECM timing control. Right now I'm not planning on it, and will just continue with the mechanical distributor.

The cam in the ZZ502 is 224/234. In theory, the computer should be able to "smooth" the idle out by advancing/retarding individual spark pulses.

What I don't know is how well it does that and whether it's worth it on a motor with this cam.

Would I like it to idle smoother? Sure, I suppose I would. But I have no idea what kind of difference if any, it would make. Has anyone experimented with enabling/disabling computer control of the timing to see what difference it makes?

 

Absolutely go with ECU controlled timing. In addition to more accurately controlling idle and transition to idle, it will be much better at controlling advance accurately across the entire operational rpm/load map.

 

Seconded. The problem I faced is that these computers have a lot of logic in them to control things like slamming the throttle shut. When you slam it shut, they do a bunch of calculations to try to keep the motor from stalling out. At the same time, vacuum advance kicks in and adds a bunch of timing. This causes double-feedback and you end up with some wonky conditions.

Let the computer control everything. It makes tuning easier too, as you can control timing changes from the handheld rather than with a 9/16 and a timing light.

 

For the record, I drove my EFI setup without computer controlled timing for a while. Drivability is so much better with it. You can get things like starting timing retard, timing kickup for AC, engine fans, etc.

Will you see extra WOT performance? No.

 

The Holley ECU has a function called 'idle spark' where it constantly makes small timing adjustments at idle, much faster than the idle air control can react. With idle spark enabled you'll have rock solid idle speed, even with a fairly radical cam. That alone makes it worth it, IMO.

 

Is camshaft selection a consideration? I read that non-computer camshaft profile may not play nice with a computer.

https://www.summitracing.com/search/...nd-lifter-kits

 

Only an issue with factory EFI. Aftermarket EFI uses same cams as carb.

 

That's the -only- feature I actually care about, and the one I may try to implement with the magnetic trigger. First I'll get it running without it to see how well it drives.

What I was hoping was someone had installed it on a similar big block and had tried it both ways to see if there's much difference.

I don't think the idle is/was very rough with the ZZ502 cam, but I'll decide that when I get it running.

Most of the stuff people are talking about with the throttle closing is handled by the IAC valve, not timing. But the idle control in terms of -rhythm-, not speed, is timing.

Nah, I don't believe that last part. Cam selection is important and esp. with a throttle body you can't have a super wild camshaft or you'll get a bunch of reversion in the intake paths, that's for sure. But short of the old mechanical L88/ZL1/LS6 grinds I doubt it's a problem. Mine's the top hydraulic (390hp) so the same is fairly tame.

In fact I'm not sure whether to tell the wizard whether it's "stock" or "street/strip". I'm thinking the latter with the ZZ502.

 

'Stock' most likely refers to cams below 200 dgr duration @ 050, so 'street/strip' is what you want.

 

Here's a pic of my lines, install, and a walkaround. Sorry, I know it's not my Vette, but at least it's an L36 427 M20 so shared powertrain!

The fuel tank isn't very pretty, as I had to flatten an area out to mount the pump module and no one will ever see the top of the tank in-car!

I've decided not to go with ECM control over timing at this point because it idles and drives really quite well. You can hear the idle quality in the attached video, I think it's fine with that cam!

Looks better fullscreen of course if you click the expand box [ ]

 

I have never seen a 69’ 2+2! I never knew they even made them after the mid 60s. Yours is gorgeous!!

Ironically, I also have a 69’ Grand Prix with a 454, 4 spd....my dad has owned it since 85’.

On the timing control thing, as good as I’m sure the aftermarket controllers do at controlling timing, I just don’t think it’s needed. A good setup distributor, with the correct curve, dialed in vac advance or running it “locked out” works fine.

No reason to reinvent the wheel. The 502/502 cam is very mild, the engine makes plenty of vacuum and will idle fine. Lots of people put FI on 502s.

 

Yeah we’ve caught some flak over the years for the Chevy motor but......it works for us.