Diagnosing hurt motor

[ST7317]

After my final pass last night, my nitrous motor was knocking. I am going to completely tear it down-what should I look for?

Compression test showed cylinders 1@150 3@145 5@145 7@140 2@150 4@140 6@110 8@150.

Vacuum reading is steady @ 4" Hg (normal is 9" Hg)

Oil pressure is normal.

No loose rockers-noise is coming from down low.

Knock becomes more prominent under load but can be heard at idle.

Last pass was off by .2 (1/8 mile).

What do you guys think? Burnt piston? Bearing? Something else?

Thanks

[needforspeed66gt]

Piston/rings/skirt - something along those lines would be my guess.

What's in the motor? Forged pistons? Ring gaps? Tuned with wideband O2 while spraying?


Were you monitoring your fuel pressure?

[bill jones]

-sounds to me like a broken top ring land--where the piston is all distorted---both on the top and quite possibly the skirt has collapsed---so the piston is real cozy with the deck surface of the head.

-a leak down test should how just how bad the debris has damaged the valves---and maybe to some extent how bad the cylinder wall is.

-Probably the only reason to do a leakdown test is for the educational value----so in case it ever happens again you have some history to refer back to.
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-It sort of sounds like maybe you got lucky and just hurt the one piston, rod bearing, rod, wrist pin and scarred or distorted that one cylinder wall.

-Quite possibly the other rod that's on the same journal----assuming two rods share the same journal-----could have some damage to the thrust surfaces on the big end.

-If it's a long stroke crank---the counterweights usually start banging the underside of the wrist pin bosses when things get squished.

-If the ring land and the top ring is broken those errant pieces of metal can migrate to any of the other cylinders if the engine has a common plenum for all the cylinders---and consequently you could find "pecker tracks" on piston tops & the head surfaces---and the valves & valve seats.
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-I've seen one BBC engine where stock GM steel rods were used----lots of nitrous---lots of power for a little while----and eventually the engine started slowing down and knocking-----turned out the rods were shortened up like an "S" on the beams----where the pistons and counterweights were banging on each other.

[needforspeed66gt]

lots of power for a little while

Key words there

[ST7317]

Pulled the motor but have not tore it apart yet. It blew oil out of the front and rear pan gaskets and the rear main seal. The crank is a Scat standard weight forged piece 3.75" SBC, Scat 6" H beam rods, JE nitrous series pistons, Speed pro rings gapped big (can't remember how big exactly). Dedicated fuel pump (return style) running 6 psi through 91 jet (plate system). Will post pics when I tear it down. Thanks for the replies.

[bigjimzlll]

Last N20 engine that let go on me I found piston peices under the carb in the intake...If it didn't seize it may not be too bad. How big of a shot and how much timing was taken out?

[ST7317]

31 degrees total timing spraying through a 91 nitrous jet (275 shot). The motor turns readily by hand. Nothing appears unusual by looking down the intake runners or in through the exhaust ports. I tune it by reading a plug after each pass-plugs looked good.

[Ron C.]

Since were guessing, I'd guess a lifted ring land at the intake relief touching the head. You might want to pull a few more degrees of timing.......next time.

Blessings.........Ron Clevenger

[63 Nitrous Ratt Vette]

31 degrees of timing may not have been enough out.....where was the heat mark on the plug? compression of the motor and fuel used?

Can you post a pic of the plug from the cylinder?

[ST7317]

After the previous pass, the heat mark was on the electrode side of the strap but I didn't see any aluminum specks on the plug. After the last pass, I didn't stop to pull a plug, I drove it straight to the trailer and loaded it up. I am using 50% VP C12 fuel mixed with pump gas. What do you think, too much timing and not enough good fuel?

[ST7317]

The next two pics are from the previous week. Running 33 degrees total, 82 nitrous jet, and projected tip plug. Everything else the same as above. From the looks of the last pic, 33 total was too much for an 82 jet, which means 31 total would have been too much for a 91 jet. Learning this the hard way.

[Ron C.]

Could be you have several things going on here. You show two different type of plugs? I would not run a projected tip plug with nitrous. Do not mix your own gas and you need to upgrade the gas to higher rating. Timing is most likely to much. Plugs look rich which is bad for nitrous and will burn the fuel down in the ring lands.....lifting them. Make sure you've got a decent tension oil ring, oil in the combustion chamber with nitrous is a disaster waiting to happen.

You can get away with a lot at 200hp nitrous level, when you start getting into the 300hp area you better have your ducks lined up and 400hp and up you better have the ducks skined-dressed and with a pretty bow tied around them.
That's my thought's anyway..........blessings............Ron Clevenger.

[ClassicComp]

I run 25° total w/a .082f/.083n jets w/C16 this is good for 210hp on the dyno (this is a rich tune)

[ST7317]

You show two different type of plugs?

I ran the projected tip plug on a small shot (82 jet) then changed to the non-projected tip for the larger shot (91 jet).

I run 25° total w/a .082f/.083n jets w/C16 this is good for 210hp on the dyno (this is a rich tune)

Thanks for the info, I am convinced that I am running too much timing.

Tore the motor down today. Pulled the intake and the lifter valley was littered with copper (bearing). Next I pulled the pan, the pick up was laying in the bottom . Pulled the rods, pistons, and crank.

#6 rod:





The #7 piston:




The crank, journals 5/6:




So, I am in for a new set of rods (unless they sell them as singles), at least one new piston, and I am pretty sure the crank is trashed. I would like to know why the pick up fell off. It had been tacked on the pump. Evidently the mig didn't take on the cast pump.

[rustbucket79]

scat does sell single rods, I would mag the crank and check it for straight, then have it ground. Cast iron doesn't weld to steel well, some detonation could have possibly rattled the pickup off.