Finally Pull Sc Motor Appart

[z-ya]

Finally had a chance to pull apart the SC motor that lost compression on 3 cylinders.

 

Specs:

- L28, std bore, stock P79 pistons

- 2mm metal gasket

- 8.1 CR

- Camden supercharger 8psi boost

- Dyno tuned to 11.5 to 12:1 AFR

- Advance at 8psi ~20deg

- 255 RWHP

- Intake temps reached 280F on a regular basis during track sessions

 

It held together for almost two time trial seasons. It failed on the way back to the pits after a session. Still was running, but on only three cylinders. The crank looks to be fine.

 

So hear are the results:

 

Piston 4:

 

Piston 5:

 

Pistons in order 1,2,3,4,5,6, Note all skirts broken on one side:

 

 

 

 

[1 fast z]

Yea all on the pressure side of the piston is where the broke. HEAT is the only thing that would cause that much expansion. Heat from intake temps, detonation, pre-ignition, etc. I would DEFINITLY add an after cooler. Water injection would also help.

[1 fast z]

After looking, if you had the pistons in corectly, they all broke on the NON pressure side of the piston.

[z-ya]

Yes, the pistons we installed correctly.

 

The damage is on the hot side (driver).

 

Heat, detonation was the cause.

[240hoke]

Holy smokes, that doesnt seem right. 8 psi and damage like that? it seems like the detonation would have sounded so bad you would have immediatly pulled into the pits.

[TravRMK]

I have seen pistons like that too many times on modified 2 strokes. Detonation is never good, especially in that amount.

[z-ya]

Holy smokes, that doesnt seem right. 8 psi and damage like that? it seems like the detonation would have sounded so bad you would have immediatly pulled into the pits.

 

Have you ever hear a Camden SC at 6500+RPM? You can't hear much else. All you can hear is the SC coming up through the TB. We dyno tuned it conservatively. My guess is that on the dyno the intake air temps never came close to what they did on the track. So it was detonating, and we couldn't hear it over the SC. Those old Camden superchargers are great at making heat. Plus I was running it dry. Having a couple of injectors upstream of the impellers would have cooled it a bit. Not a good setup for a track car, especially if run dry.

[Forrest]

Hey Pete, sorry to hear that - loved your car.

 

Have you ever considered using a J&S? I've talked to some guys that road race vettes, and they set their timing so they always get a little knock and let the J&S pull by 1 or 2 degrees constantly so that they are always on the bleeding edge.

 

http://www.jandssafeguard.com/

 

They're pretty cool and J&S has excellent customer service.

 

Might be worth a shot (on top of the obvious options for lowering intake temps.)

[jeffer949]

yeah ive pulled many pistons that looked like that out of my old race 125 two strokes. can we say detonation?? lol. i had one that actually had a hole worn into the center right below the piston. then i guess ive also had the pistion come out in chuncks also

[HowlerMonkey]

I've pulled a ton of L28 pistons that look like that including a 280zx that never experienced boost.

 

I've never seen this in an engine in florida (though it could happen) but have seen many in cold climates like virginia in the winter.

 

What I find knocks the skirts off on L28s is high revs at low temps.

 

On my car that lost all six, it ran fine, had no blow-by and only made slight knocks when cold. I only found it when going in to fit new rod bearings in prep, for a turbo.

 

If you have a thermostat stick open or don't run one in sub freezing climates, you can run the car down the highway as many miles you want, pull over at a rest stop, and feel the side of the engine is unbelievably cold for an engine you just ran hard.

[mario_82_ZXT]

http://www.jandssafeguard.com/

 

I've seen this posted here before, I'm pretty sure one of the OGs runs it, maybe even on an L28.

 

Mario