Detonation

[Pentroof]

I recently put a 390 together for the F100 I built for my daughter. A bit of a budget build, in that I used a rotating assembly I had on hand: 390 GT stuff that results in 10.5 CR.
Motor has Edelbrock heads, Sniper EFI controlling timing and this juice roller:



Initial timing is 16, total set at 36 and cam was installed at 110 ICL. Transmission is a C6.

I was very happy with the combo's initial results. Runs great, sounds great (for a truck) and had no pinging whatsoever. Running best pump gas I can get.

Recently, the temps and humidity have risen and I'm getting pinging under load. It's very controllable, meaning it's fine if I keep my foot out of it. However, I'd prefer to be a little further from this edge. I do plan on buying the full Holley software package to add some detail to my curve, but I'm looking for some thoughts on cam and ignition timing.

I'm considering retarding the cam, lowering the total timing, or both. Any thoughts?

I'm also hoping I can modify total timing based on temperature, but not yet sure I can do that with the Sniper, even with the full software package.

[blykins]

Jim, I'd pull 2 degrees total out of it and see how it likes that. 

I wouldn't move the cam timing unless it's a very last resort.  Retarding it further will take some of the low-end snap away from it. 

Did you calculate the actual compression ratio, or is that a guess based on parts? 

[gt350hr]

Jim, I'd pull 2 degrees total out of it and see how it likes that. 

I wouldn't move the cam timing unless it's a very last resort.  Retarding it further will take some of the low-end snap away from it. 

Did you calculate the actual compression ratio, or is that a guess based on parts?

+1 Do not retard the cam. Reduce total timing or at least initial timing as Brent suggests or mess with the vacuum advance. Retarding cam timing will make things worse for sure. "To me" 36* is more total advance than you need with that head.
    Randy

[Katz427]

Brent and Randy have much more experience than I have. I can only back up their suggestions, by saying I have an SDS computer from Western Motorsports​, I find that pulling 2 degrees out of the total works wonders. All we have here is 91 octane. I run 34 degrees total, it's hot and humid, but pulling out just 2 degrees worked.

[Falcon67]

Echo.  I'd dial total back to 32 and see how it does.  If you run a curve you want to keep as much initial as it can stand.

[blykins]

Jim, did you use factory pistons? 

[gt350hr]

Echo.  I'd dial total back to 32 and see how it does.  If you run a curve you want to keep as much initial as it can stand.

  +1 always limit the total . Initial is your friend.

[Drew Pojedinec]

Randy/Brent, wouldn't it be easier and better to just install a stiffer spring and slow the advance curve down?

[blykins]

Randy/Brent, wouldn't it be easier and better to just install a stiffer spring and slow the advance curve down?

That would be an option depending on what the curve is like right now.  Could have a slow curve in it already. 

There's several variables here and I'm kinda reluctant to give a firm answer one way or the other until we get some answers.  The quick answer is if he's wanting a quick reprieve, he can pull a couple degrees total out.

I'm curious about the rotating assembly and I'm waiting to hear what pistons are in it. 

[Pentroof]

Thanks for discussion so far....

No vacuum advance or springs involved: timing is mapped and controlled by the ECU for the EFI. I entered initial and total and Holley translates that to a curve. I'll be able to tweak that if necessary once I get the PC software, otherwise it's pretty linear. Watching timing on the handheld display, the timing jumps to total VERY quickly.

Yes, pistons are stock style and compression was not directly calculated, just estimated based on the GT specs as publicized by Ford.

[blykins]

Jim,

Edelbrock's 72cc heads usually come out closer to 74cc.  However, if we were to use the 72cc number, a .040" gasket, and put the pistons at zero deck, it would take a 2cc valve relief to get to 10.5:1 SCR.   Most factory 390 pistons were around the 9-10cc mark.  Even if we used an 8cc valve relief volume, you'd be at 9.8:1. 

When I grind cams for others, I usually am pretty conservative on the numbers that affect the octane requirements. 

My guess is that you're sitting at around 9.7:1 SCR with around 7.8:1 DCR.  It should run perfectly fine on 90-91 with those numbers, even with a heavier truck and a higher rearend gear ratio.

You may want to double check your ECU settings and make sure that it's not an issue with the timing curve.  I do assume that you degreed the camshaft and installed it on the 110° ICL.

[Pentroof]

Thanks.

Yes, installed at 110 ICL, as I noted above. I may still have my notes to verify. I do remember there were no surprises, meaning I think the sprockets ended up installed without any adjustments.
Head gaskets are FelPro 1020 and pistons are stock, 4 reliefs.

Also for the record, and before it gets asked, cooling is not a problem.

I should probably verify that when the LCD is telling me 36 that it's not actually 40 or something. I did have to slightly adjust the IAC to get the initial timing up to the 16 that I wanted. At that time, the setup has you also verify that general timing movement and total timing at the balancer jives with what was typed in. I remember thinking it was fine, but I should make sure. Balancer is marked up to 45, so there should be no questions.

So, are you saying 36 degrees should not be an issue, given what you now know? If so, I'll keep digging with the ECU and put a hustle on getting the software in hand.

[blykins]

No sir, I don’t think 36 should be a problem.

[Pentroof]

More info:

I've found that there are 3 timing inputs, not just the 2 initially input during the setup configuration:
Idle
Cruise
WOT

From these, the software blends the curve (unless you define the map using the software). Currently, my cruise and WOT settings are both 36. I'm thinking I should be increasing the cruise (as would a vacuum advance) and retarding the WOT.
It would make more sense to me if I could see the map with manifold pressure (vacuum). Maybe the software will let me do that.

[blykins]

Is it a factory balancer or a new one?