Engine gurus... Detonation

[Mountain Ram]

So I built a 347 last year and have between 2-3k mile on it. it has performed flawlessly for me since built. I typically drive it like I stole it and have only ever heard any detonation when I put 87 in it. I rarely drive on the interstate or really much above 60mph.

I got on the interstate this morning and decided to let loose coming down the on ramp. This thing hauls ass... Anyway- In third gear, doing around 75-80mph, I could hear some random detonation. (3500rpm). Starting to pull a grade at the same speed, gave it a little gas and the detonation got a lot worse. I immediately backed out of it and it cruised happily at 70mph with no issue.

I don't normally drive the interstate and rarely do I exceed 75 in any of my vehicles, but this seems like its indicative of a possibly bigger problem. I don't want it to run lean and damage the engine.

Specs- 347 with edelbrock performer rpm heads, Comp XE266HR cam, 1.6 rollers, ~10:1 compression, 93 octane, 76mm MAF & TB, 30lb injectors, A9L computer. C4 trans, 37" tires and 4.56 gears. Two fuel pumps with the high pressure pump being a Stock '90s model truck external pump. I have no way to check AFR and no way to get the A9L tuned at the moment.

It runs slightly rich at idle. with the 30lb injectors, I would have thought I had enough fuel to feed it. I don't hear it in any other gear, but don't normally hold 3500rpm in other gears for very long prior to shift.

Thoughts? I have had issues with the fuel pump holding pressure when off. It drops pressure in about an hour, but I thought this may have been a bad check valve...

 

[Quick & Dirty]

How much initial timing are you running?

May need to pull some of the part throttle, higher rpm timing out of the map if you are running more than stock initial timing.

 

[Fairlane514]

Have you tried backing your timing down?

Sorry Was typing while Qucik and Dirty was posting

 

[TwoDalesDad]

To much Timing....YOU REALLY DO 75-80...WITH 37" TIRES...MY BRONCO WAS SCARY AT 65..

 

[Steve83]

...I could hear some random detonation. (3500rpm). Starting to pull a grade at the same speed, gave it a little gas and the detonation got a lot worse.

That alone suggests it was detonating from heat. If it was detonating from a lean mix, romping on it causes it to go rich, and the pinging should stop. But if it's caused by heat, romping it makes it worse. If the engine wasn't running hot, then it could be combustion chamber deposits. Have you checked compression recently?



If it's abnormally HIGH, it's because of deposits raising compression, which results in hotspots & detonation. Either seafoam it, or just steam-clean it by dripping water into the FULLY warmed-up engine at ~2KRPM. I prefer water because it doesn't stink like seafoam, and you know when you're making progress by the level of soot, and when you're done by the soot abating. It's also a lot cheaper.

Of course, timing chain wear & ignition timing are important, too.

 

[Mountain Ram]

Have you tried backing your timing down?

Sorry Was typing while Qucik and Dirty was posting

Have not touched anything yet... Been working on my crawler all day. Initial timing was 14 and I bumped it back down to 12- don't remember why... Will check it and see where I am when I can get ahold of a timing gun.

To much Timing....YOU REALLY DO 75-80...WITH 37" TIRES...MY BRONCO WAS SCARY AT 65..

I rarely drive it on the highway, but I wanted to go WOT and when I heard the rattle I looked at the tach and speedo... Surprisingly enough- it drove great at speed. I have to credit the 37" Toyo radials and a spot on alignment...

Engine has very few miles on it and while I have not ruled out the timing chain, brand new double roller and gears, so they are unlikely the culprit.

Engine temp was 190. Running a 195 TStat and even in traffic on a hot day, the truck has never gone over 210... I guess deposits could be an issue but its got very little mileage and does not run that rich at idle to cause that kind of issue...

 

[73azbronco]

If you are running hyperT pistons, need to make sure your max all in timing is under 35. If forged disregard.

With your setup, I highly recommend a trip to the tuner and let them tweek it into specs. Best $500 you'll ever spend. I'm using a carb and still letting the tuner message mine when rolling.

 

[Ourobos]

What MAF meter are you running? A 24# injector should be enough for that combo also FYI.

 

[Mountain Ram]

If you are running hyperT pistons, need to make sure your max all in timing is under 35. If forged disregard.

With your setup, I highly recommend a trip to the tuner and let them tweek it into specs. Best $500 you'll ever spend. I'm using a carb and still letting the tuner message mine when rolling.

HyperT pistons. Had not heard a total timing max with those. Interesting information- I will check it though.

What MAF meter are you running? A 24# injector should be enough for that combo also FYI.

MAF is for 30lbs. They are probably a little big, but I picked up the MAF and injectors as a set and got them cheap.

 

[Ourobos]

I figured it was 'calibrated' for 30#, but what BRAND?

And on the hypereutectic pistons.. Never heard that silliness before.. They are simply a different alloy that expands better than a forged aluminum.. For a naturally aspirated engine, they are actually a better choice for a street engine and should make no difference in the amount of timing in the engine.. Don't see where that logic comes from honestly.

 

[73azbronco]

Recommend you stop driving it if it is detonating until you figure out the problem. Hypert pistons come apart under detonation in a most dramatic fashion.

 

[73azbronco]

I figured it was 'calibrated' for 30#, but what BRAND?

And on the hypereutectic pistons.. Never heard that silliness before.. They are simply a different alloy that expands better than a forged aluminum.. For a naturally aspirated engine, they are actually a better choice for a street engine and should make no difference in the amount of timing in the engine.. Don't see where that logic comes from honestly.

Well, maybe you need to do some research about the silliness with KB. The logic comes from the manufacture who makes the pistons.

 

[Ourobos]

KB? Keith Black? Is he using Keith Black or are you assuming so? I'll go from my hundreds of engines I've built experience, enlighten me smart guy.

 

[Mountain Ram]

Hypereutectic pistons are actually pretty common in today's engine and are the standard choice for NA motors. Mine are not KB, if its important, I can dig out the receipt and figure out which pistons they are.

I do know detonation can cause a piston to come apart... Which is my concern. But why doesn't it do it whenever the engine is at WOT and 3500rpm? It shouldn't make a difference what gear the trans is in or what speed I am going. The only times I have heard this are when I use 87 octane (every time I use it) and then today at WOT and 3500rpm in third gear...

I have a C&L MAF with the calibration tube an stock mustang sensor. I also have access to a NIB BBK for 30# injectors if it will make this problem go away...

 

[Broncobowsher]

Back the timing back to 10°
Start looking around for a shop that can properly tune the combo.

At that speed you are doing a fairly decent load. For the "calibrated" MAF to work it lies to the computer about how much air is actually entering the engine. It under reports the amount so the computer will fire the (oversized according to the computer programming) injectors less. In this case the computer thinks the engine is only getting about 2/3s that air that is actually flowing through it (19/30 is close enough to 2/3). So you have the high cylinder pressures of a lot of air but the computer thinks you are running a light cruise and adds in a bunch of timing. Add in a bump in compression, you have detonation.

For a combo that is this far away from stock computer parameters, get it tuned correctly.

 

[Mountain Ram]

Back the timing back to 10°
Start looking around for a shop that can properly tune the combo.

At that speed you are doing a fairly decent load. For the "calibrated" MAF to work it lies to the computer about how much air is actually entering the engine. It under reports the amount so the computer will fire the (oversized according to the computer programming) injectors less. In this case the computer thinks the engine is only getting about 2/3s that air that is actually flowing through it (19/30 is close enough to 2/3). So you have the high cylinder pressures of a lot of air but the computer thinks you are running a light cruise and adds in a bunch of timing. Add in a bump in compression, you have detonation.

For a combo that is this far away from stock computer parameters, get it tuned correctly.

Now this makes sense to me... I have had little luck finding a tuner within a reasonable drive... Guess I will get back out looking. You would think living in a town with a NASCAR track AND an NHRA track that I couldmfindnhalf a dozen of them...

 

[Ourobos]

From my experience C&L MAF meters have been JUNK.. It would be odd that it just started doing this though. You could test if it was the MAF meter by unplugging it and making a spirited run, if the issue goes away, throw the BBK meter on there.

 

[Broncobowsher]

From my experience C&L MAF meters have been JUNK.. It would be odd that it just started doing this though. You could test if it was the MAF meter by unplugging it and making a spirited run, if the issue goes away, throw the BBK meter on there.

Doubt that will work. Without a MAF the computer goes into a limp table. Makes massive assumptions on airflow based on the original engine parameters, including the 19# injectors. It will be dumping 50% extra fuel in an engine that is only 15% more displacement. It would be surprising if it even started.

But on a stock setup, that is a good test for a wacked MAF

 

[Ourobos]

Well true, it slipped my mind about the larger injectors, that is the way to do it on a stock setup, good catch.

 

[EFI Guy]

Broncobowsher speaks the truth here. But it is even further off than the roughly 2/3, you can figure an extra 15% for the additional displacement of the 347 in to the load calculation.

I will also second the advice of a different MAF. Don't waste good money having someone tune it with that MAF. Personally, I'd upgrade to a slot MAF anyway, just don't go crazy with the sizing, resolution will be key to good drivability.