Intermittent miss and knocking coming from exhaust

KCTurbos · Nov 6, 2014

Yes sir it was hissing. I have narrowed it down to an exhaust valve. I was still running stock valve springs so I am guessing that a valve kissed the piston... The weirdest part of the whole things is that the miss was intermittent???

Here is the ghetto leak test that I did that showed where the hissing was coming from.

https://www.youtube.com/watch?v=rR6qw80SVJk&list=UUoKpxoPYQfFYZjgyA1kOsPg

What are the chances that the valve just barely "kissed" the piston and that all I need is a new head? I had plans to take the truck back to "near stock" levels in about 3 weeks and then build a rclb truck. I just had the motor out last year with all new seals, main and rod bearings, new heads, etc... Any chance I can throw on a new head without having to pull the motor and change out the piston?

suprdzlduty · Nov 6, 2014

It will be in the head most likely. It doesn't take much of a kiss to tweak a valve. Unless the valve drops totally the piston will be fine. Most cases that I've had is replacing the valve will take care of the problem, with no head damage.

Bustedknuckles · Nov 6, 2014

Should just need a valve job actually.

strokin6L · Nov 6, 2014

Charlie....sounds like you have the exact same symptoms as me with my #3 cylinder. Mine happens to be an intake valve instead. When I pulled the valve cover off and had it running....everything that was moving(pushrods, rocker arms, valve springs) looked perfect. I bet if you did a leak down test you'll know its the valve.

valvetick · Nov 7, 2014

That sucks Charlie. Hope its just a valve and you got lucky. My old cobra dropped a valve and trashed everything.

swinky · Nov 7, 2014

High back pressure?

MrOneEyedBoh · Nov 7, 2014

That's another reason to go non-vgt. Doesn't vgt have considerably lower back pressure?

strokin6L · Nov 7, 2014

Vgt higher back pressure and non vgt lower. Depends on how turbos are sized tho.

KCTurbos · Nov 7, 2014

In my experience it has shown that it as a fallacy that vgt turbos "always" run higher ebp... You just have the "ability" to adjust the vanes and use back pressure to drive the turbo harder. (kinda like being able to switch between .54 A/R to .90 A/R turbine housing by simply adjusting the vanes).

I have about 20 different tunes and have been testing the truck a lot over the last few months. I was using a manual boost/ebp gauge because the Map and EBP gauge max out too early. But about 2 weeks ago my ebp gauge started acting up so I don't know exactly what EBP was doing.

On average at WOT I ran a 1:1.2 boost/ebp ratio (30-40psi boost and 35-50psi EBP). The turbo runs very efficient at that range but only made around 630-650hp (unloaded dyno jet with 3000# roller which reads low on average).... BUT... I wanted to push the limits so I asked a couple of tuners to turn the boost up all the way and see what happens.

I had one tune that pushed 50+ psi of boost... It was crazy. Truck pulled super hard and brought down. Anything below 50-60mph was useless and would just spin the tires in 2wd so I did most of my testing above 70mph. But my back pressure gauge was broken so I was not accurately able to monitor EBP.

I wrote myself a tune that also built over 50+ psi of ebp but anything past 50psi and the turbo would stall out, lose power, and boost/ebp would skyrocket and max out my gauges so I don' know how high they ever got. That was last week and the truck seemed to be running fine afterwards.

The dyno run in question when the problem started I was pushing around 45psi of boost and no idea what back pressure was... I would guess 50-60psi from previous runs on the street when my gauge was working. I was pushing around 670hp on what seemed like 7 cylinders (you could see the dead cylinder on the dyno graph). I could not really feel anything out of the ordinary until the truck came back to idle and had a miss...

So I am still unsure of what exactly happened... Especially since the problem would come and go on the drive home.

swinky · Nov 7, 2014

Good luck man! I'm looking forward to seeing one of these things make a pass. High 11s in a crew cab would be awesome.

KCTurbos · Nov 7, 2014

swinky said:
Good luck man! I'm looking forward to seeing one of these things make a pass. High 11s in a crew cab would be awesome.

Thanks for the support... but... sadly this truck is going to be taken down to a lower hp build... probably 500-550hp with 190/30 and a stage 2. It sucks having my DD down all the time (this was also our test truck)

I did not think I would quite make it into the 11s on this setup fuel only, stock suspension and tires (but that would have been awesome). I was thinking more around 12.2 - 12.4

I bought a rclb 2wd and I am pulling all the parts off and gonna do a big build on that truck. Probably built motor, 350/100s, twin pumps, and a 74mm vgt turbo to start with and then hopefully building compounds down the road... I would love to chop it and make a rcsb... it will be a lot easier to get that truck into the 11s as opposed to the ccsb 4x4

KCTurbos · Nov 10, 2014

Well...

I finally found out exactly what was going on and it was not good news

The custom head gaskets that I was using was a two piece fire ring design. One inner, smaller, fire ring that was not embedded in the head. Then one outer thicker, fire ring that was cut into the head. What I believe happened is that the inner fire ring somehow broke down... then somehow worked its way into the cylinder. It bounced up and down for a while and then got pushed out through the exhaust valve.

I never found the little pieces... but I did have a blown turbo about 2 weeks ago and it looked like something went through the turbine wheel. I would guess that little pieces of fire ring were bouncing around and were slow exiting through the exhaust valve which is what was causing the intermittent miss.

Surprisingly the cylinder walls look good and I cant even find one mark on them. The tops of 2 pistons are pretty banged up.

Here are some pics of the carnage.

This is what was left of the inner fire ring on cyl 4 and 6

Here is what the tops of the pistons look like

Here is what the cylinder walls look like... still have the cross hatching after 205,000 miles

Here is a pic of the head... actually not that bad

Now I am trying to decide what to do... I was about to take the truck back to a smaller hp build, so I am not looking to build a bottom end. The cylinder walls look good so I was debating on reusing it... but I am not sure that is a good idea with how the pistons look.

webb06 · Nov 10, 2014

is it just the one piston that is trashed? if so i'd just throw a new piston in and roll with it.

KCTurbos · Nov 10, 2014

Well I finally got the other head off today... more bad news. This side has more signs of wear on the cylinder walls. One of the cylinders has a nice long vertical scratch in it the I can catch with my fingernail. Some of the others have excessive signs of wear. Which is funny because the pistons look much better on this side.

webb06 · Nov 11, 2014

KCTurbos said:
Well I finally got the other head off today... more bad news. This side has more signs of wear on the cylinder walls. One of the cylinders has a nice long vertical scratch in it the I can catch with my fingernail. Some of the others have excessive signs of wear. Which is funny because the pistons look much better on this side.

Dang!!!! That's not so hot..........

TheReelMuhcoy · Nov 11, 2014

KCTurbos said:
Well I finally got the other head off today... more bad news. This side has more signs of wear on the cylinder walls. One of the cylinders has a nice long vertical scratch in it the I can catch with my fingernail. Some of the others have excessive signs of wear. Which is funny because the pistons look much better on this side.

Im really sorry to hear the bad news, that's a real bummer.

But please tell me that these Head gaskets are not on the market! I know you said custom but you never know... I wouldnt want to put those onto the race motor I am building...