Real Fiero Tech

Dropped a valve seat

How much boost was it making? Details

It was idling in the driveway when it went. Engine was warming up

Stock 3900, right? They're cheap.

Always sucks though... Car's down and now you gotta swap it.

How much boost have you run?

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When designing the LC3 supercharged Northstar, GM changed the mix of the powdered metal valve seats to include more copper for better heat conductivity. Also, higher performance applications typically need a tighter press fit on the valve seats, as the greater heat input into the combustion chamber will make the aluminum right around the valve seats expand more.

Jaguar XK inline 6's were known for problems related to this. If the engine was EVER overheated--even once--the aluminum around the valve seats would creep, relaxing the valve seat press fit. The engine would then drop a seat sometime later. If you repaired just that seat, it would drop another one later. The necessary repair was to replace ALL the valve seats... basically a complete cylinder head overhaul, just for one overheating event.

This was the built forged engine. I had it up to 6800 rpm and 20 lbs on certain tuning sessions.

I fired it up to let it warm up and it stumbled and died. I fired it back up and it had a npticeable misfire and didnt sound right.

I pulled the plugs and cyl 1 had a closed ground strap. Did a compression test on that cylinder and it was at 0. I pulled the head to find the intake port valve seat was missing.

The whole thing needs torn down. The intake valve wasnt closing since it had no seat so the metal pieces were flying back up into the plenum and getting sucked down into the other cylinders.

Here is some of the metal bits pulled from the plenum

Car is already back up and running with the junkyard lz9 with lnj camshaft.

Do you think the engine overheated recently?

Shaun41178(2) wrote: ↑Sun Sep 19, 2021 11:56 am This was the built forged engine. I had it up to 6800 rpm and 20 lbs on certain tuning sessions.

The whole thing needs torn down. The intake valve wasnt closing since it had no seat so the metal pieces were flying back up into the plenum and getting sucked down into the other cylinders.

Here is some of the metal bits pulled from the plenum

20210319_152721.jpg

Car is already back up and running with the junkyard lz9 with lnj camshaft.

Yeah, there's FOD visible on the quench pad of the next chamber in your first photo.

20 psi? That's a lot of heat load. As you build engines for that much power, you may need to have heads rebuilt with a greater interference on the valve seat press-fit.

There's definitely something to be said for cheap disposable engines. You're Johnny-on-the-spot with that engine R&R.

pmbrunelle wrote: ↑Mon Sep 20, 2021 8:42 am Do you think the engine overheated recently?

Not technically but you are on the right track.

I took the car for a spin and before I left my street I noticed water droplets being flung up on the rear window and some steam. I turned around and drove about 1/4 mile to get back to the house.

Temp gauge showed 210 peak.

So where was I leaking coolant? From my coolant fill behind the drivers seat. One of the welds was weak, cracked and completely broke off. So the whole fitting and hose was off and the water pump was just pushing as much water through as it could.

I got it home and put water in it. Took almost 3 gallons. The water pump had pushed nearly all the coolant out. So the gauge said it never overheated but it had no coolant for about an 1/8th mile

I filled it back up and bled the system. Got it up to temp and it was fine. Next start after that is when it dropped.

Yes, I had the junkyard engine installed in about 4 days

Got a dyno appt first week of October. Only gonna be on 8 psi on the stock engine. I expect around 330-350 whp. No new records will be broken that day.

The Dark Side of Will wrote: ↑Mon Sep 20, 2021 9:41 am 20 psi? That's a lot of heat load. As you build engines for that much power, you may need to have heads rebuilt with a greater interference on the valve seat press-fit.

Besides discussing the anti-detonant effect of water injection, NACA Report 756 (freely downloadable) also discusses the reduction of engine component temperatures with the water.

They measured the temperature of different engine parts, such as on the cylinder head between the valves. The water reduced the temperature of many parts by a decent amount.

Shaun41178(2) wrote: ↑Mon Sep 20, 2021 9:05 pm
Temp gauge showed 210 peak.

So where was I leaking coolant? From my coolant fill behind the drivers seat. One of the welds was weak, cracked and completely broke off. So the whole fitting and hose was off and the water pump was just pushing as much water through as it could.

I got it home and put water in it. Took almost 3 gallons. The water pump had pushed nearly all the coolant out. So the gauge said it never overheated but it had no coolant for about an 1/8th mile

That was probably your overheating event. If there's no coolant backing up the metal, the metal in the chambers heats up fast. The heat then has to conduct through the deck and upper surface of the cylinder head to get to the CTS in the outer wall of the cylinder head before you see it on the gauge. With water in the system, the gauge will read the temperature of the water, which will be close to the temperature of the metal making up the combustion chamber. Without water in the system, the chamber metal will be MUCH hotter than what the gauge reads.

bummer, what's the game plan? throw in a stock LZ9 with a cam and send it?

ericjon262 wrote: ↑Tue Sep 21, 2021 8:54 pm bummer, what's the game plan? throw in a stock LZ9 with a cam and send it?

Stock lz9 and no cam. Running just a factory GM lnj cam in it. I'll be down about 100 hp or so without the custom cam. Oh well.

Shaun41178(2) wrote: ↑Tue Sep 21, 2021 9:30 pm

ericjon262 wrote: ↑Tue Sep 21, 2021 8:54 pm bummer, what's the game plan? throw in a stock LZ9 with a cam and send it?

Stock lz9 and no cam. Running just a factory GM lnj cam in it. I'll be down about 100 hp or so without the custom cam. Oh well.

I probably should have gone that low key after the rod bearing, but I guess I'm just a masochist.

Shit….