[acannell]

let this be a warning to you all

i know some of the more experienced of you may not find this surprising or may feel that you would not have suffered the same consequences for whatever reasons, but i think everyone should be aware just how much damage an engine can suffer at 14 psi even with a big intercooler, about 11 air-fuel ratio, stage 3 computer and new injectors

1990 daytona es 2.5 turbo
hybrid T3/T4
grainger valve set to 14psi
big front mount intercooler (roughly equivalent to 3 stock cores)
+40 injectors with AFPR set to be equivalent to stock injectors
STAGE 3 fwdperformance 90/91 computer
innovate XD-16 wideband O2
stock exhaust
greddy type RS bov
rebuilt engine with less than 3000 miles
rebuilt swirl head with less than 3000 miles
stock timing

these were NEW cast mahle pistons

its not clear to me exactly when the detonation occured or why. AFR ratios consistently showed about 11 at WOT at 14 psi.

nevertheless, given that this computer still uses a 2 bar sensor (stock), whatever caused this happened below 15 psi for sure

my guesses as to what happened:

i had recently unplugged the knock sensor because i thought the computer was indicating false knock (for several reasons, this is detailed in another thread). perhaps the knock was real and this was the result.

i am using a grainger valve instead of letting the computer control the wastegate. see a recent thread where the pope discusses why this is not a good idea. perhaps this contributed.

my EGT's were probably above 1500F at WOT. its hard to determine this due to the slow response of my EGT gauge.

for a detailed thread on how i came to discover this piston had cracked see:

http://www.turbododge.com/forums/f4/f15/164157-aww-man-noise-head-block.html#post1271122

 

[bhazard]

If youre gonna rebuild an engine, why bother with cast pistons, why not just use forged?

 

[turbokid]

and did you run premium???? Thats another factor. If you are running a g-valve at over stock boost you need to run premium because it can prevent detonation.

 

[acannell]

yes i was running premium

i would have put in forged to begin with but they were out of stock for the oversize size i needed.

this still didnt have to happen. RIGHT?

 

[turbokid]

essentailly no forged is not necessary but super nice to have.
Theres gotta be something cause I am running 14 psi with a stock turbo, intercooler, premium and a gvalve with not an issue in the world. So who knows

 

[crazymadbastard]

meh, not impressed Here is 18psi+ and cel was blinking (egts previously hit 1600+ and went past 28psi before wo damage on a super60):
Boost is dangerous kids M-kay??

 

[phantomrt]

I might have to disagree on the cause of the piston damage. Detonation could have been the issue, but possibly not. I'd suspect a piston ring end gap issue. I'd think the top ring land would bust off first during detonation.

I have a pretty well modified head on my 2.5 with a S60 turbo, no intercooler (for about 15k miles), stock calibration and stock injectors and stock fuel pressure at 11 PSI of boost and I run 87 octane gas all day long with it and I rarely have detonation issues. This engine is pretty powerful for 11 PSI. Its faster than some people I know who had stock engines running 15 PSI. You had premium gas, plenty of fuel, probably not too much spark advance--I don't see why you had a problem with detonation.

I've used nothing but a drill glaze breaker instead of 400 grit sand paper. Every engine I've put together never had a piston ring issue, ever, and I've assembled a couple with some very questionable cylinder walls, but its never been a problem.

 

[acannell]

what are you saying, that you think my 0.020" clearance is too big and may have caused this?

 

[phantomrt]

Nope. .020" should have been plenty. If anything, this damage would be caused by the gap being too small--as the steel piston ring expands, that gap gets smaller, and if it gets to zero clearance, any further expansion is going to cause the ring to bind up in the cylinder and require some mega force to move it--mega forces that the ring lands cannot take.

 

[Juggy]

I popped a piston running 94 octane at only 5 PSI boost.....the cylinder had ZERO compression...

you can detonate at 1 psi if your not careful....

 

[phantomrt]

I can make any one of the engines in my garage detonate with some uber ignition timing advance.

 

[speeduphoria]

what are you saying, that you think my 0.020" clearance is too big and may have caused this?

yeah but what was the second ring gap?

 

[Bbneon95]

So what can you do to be sure to not get detonation? Should I be running 90+ octane all the time?

Sorry, I know others probably don't know either.

Kev

 

[bn880]

Your timing was probably just too advanced. Timing advance just isn't worth it in high boost applications. (i finally blew a mahle (cracked)) after 7 years at upto 30psi due to ridiculous advance on a custom ECU. Dumb move i guess, NEVER heard any detonation when it cracked.

 

[crazymadbastard]

I am dropping my timing to 10btc just to be sure.

 

[fiater]

Even in a NA engine, adding anymore than a few degrees to stock timing is only worth a few hp, and anymore timing brings diminishing returns, not even accounting for the damage detonation does. Highly advanced timing is never worth it.

 

[speeduphoria]

Even in a NA engine, adding anymore than a few degrees to stock timing is only worth a few hp, and anymore timing brings diminishing returns, not even accounting for the damage detonation does. Highly advanced timing is never worth it.

But G heads sure love the timing

 

[acannell]

okay wait what are you talking about exactly? im trying to keep up with crazymadbastard and bn880:

so timing advance is BAD.

but why would I have any advance? my timing is set to stock. are you saying the stage 3 computer is advancing the timing? so this performance computer I bought is killing my engine?

crazymadbastard: are you dropping your timing because of this thread?

 

[speeduphoria]

2.5 pistons are weaker because they are smaller and they are more sensative to knock.

How the timing works:
Idle should be aroung 10-15 deg BTDC, then when driving the computer has a pre programed timing like 25deg that + the base mechanical timing= total timing, so would be like 35 deg in your case while driving.

So in boost it uses the same formula except it has a boost retard so like at 5psi it will be 35-2 so 33deg, then 10psi would be 35-5=20deg.

In vac the computer would have 35 + it advances the timing for fuel economy and performance since its hard to blow up just cruising, you can run 60deg of timing and 17A/F in cruise and not hurt a thing.

So all of this is pre programed in the computer, every setup is a little different and different people do cals a little different.

So this leaves the owner/operator with a couple options to fine tune with, fuel pressure, base mechanical timing,, plugs and fuel octane.

So on these cars if you see knock on the scanner the 1st thing you should check is triple check that the timing is set correctly, then retart the dist timing and/or add some better octane gas to see if its real or fake(allways assume its real till you test so be careful). If its audible then its obiviously real knock!


acannell you said you had a audible @ 7psi w/ no IC, you should be able to run 14psi on 91 octane gas w/ no IC and no knock, so since you think your timing is right and your A/F is OK, try colder plugs and retarding the base timing. Its either your cal is made for a 2.2 or a G head or your timing is NOT Correct

 

[T2 shadow]

I run 8 degrees at idle and the van seems to like it A LOT and I have no idea where mine is advanced,but I have run 13 psi with 92 octane intercooled and it runs HARD. I set mine at 8 and have never had any issues maybe I am lucky.