Welp, I blew my head gasket last night. It was running great right up until it wasn't! I'm going to yank the head off this weekend and have the head checked/resurfaced, but I am hoping to get another set of eyes on my timing table as well to look for any issues. I'm guessing I got too aggressive somewhere. Engine is a stock M20B25 with an OEM head gasket and ARP Studs torqued to 65 ft lbs. Running a Comp 6262 turbo at ~17psi. Unfortunately I wasn't datalogging when it let go so I am not sure of those exact details, but previously my AFR's had been consistently in the high 11's-low 12's under boost. I had the car dyno tuned (which was a whole other fiasco) and the tuner sent me home with a table running like 2* of advance at 200kpa/4,000 RPM. In my infinite wisdom I determined he left a lot of power on the table, and switched to the table shown below. I am now realizing perhaps that was a bit too much. Any thoughts?
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Looks like a typical conservative turbo M20 table to me. Above 140kPa or so you're running around two degrees less advance than I am. A couple degrees more below that. I don't feed as much advance in at the limit as you but I also don't pull it as early. I pull advance from ~4000 to 5500rpm with the bottom of the trough in the high 4,000s. You probably hurt the head gasket with the untuned boost spikes and dyno pulls on super late timing, because I don't see your current advance numbers being a problem on 93 octane. -
Thanks for the reply. I definitely don't think my boost spikes and mega-retarded dyno pulls helped anything. Probably will dial it back one or two degrees once its back together just to be safe. Do you have any input on the torque of studs? Everyone has their own opinion but I think I am going to try 75ft lbs instead of the 65 I was at previously.Comment
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I don't remember what I torqued mine to, but it was probably 70ftlbs per the instructions.Comment
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While taking the head off I noticed my vacuum line from the intake manifold to the ECU was split nearly in half, and I hadn't touched the car since "the incident" so I am guessing this failure caused it to lean out and detonate. It pushed the fire ring all the way to the coolant channel on 5 & 6. I'm of course going to put some heavy duty silicone vacuum line on there, but it is going to keep me up at night worrying about that happening again.
Car should be going back together next Saturday with a cut ring gasket.
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Maybe your crank trigger angle was offset? What were you running?
How did your tuner tune to decide on 2* advance?Comment
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Evan your timing does look to be quite conservative. What fuel are you using? You verified your timing with a light?
Cylinder 6 is usually the hottest so I'm not surprised that you pushed the head gasket there, it really wouldn't hurt to keep the AFR below 12 at max boost the cooling is better than the small amount of power loss. Id suggest you o-ring the block, it can be done with the block in the car. Rent the isky groove-o-matic. Use the goetze head gasket. Torque your fasteners to ARP's spec and make sure you use the lube and don't bottom out the studs in the block.
Edit* I'd have to say a faulty vacuum line feeding the ECU is a good culprit for your issues, use high quality hose and protect it.-NickComment
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Not that it would make a difference, but the ARP instructions say 70fl/lb:
Risky move adding 10° to a dyno tune without first feeling out WHY it needed to be so low. Sometimes the raw numbers in the maps don't correspond to real world numbers if you have any settings mixed up. As others have suggested, even though the map "says" 2°, actual timing at the crank could be different.
Once back running, go into the timing settings and lock it in at 10° after starting (sometimes won't like to start at 10, but the adjustment can be made after running), and verify with an adjustable timing light that you are in fact seeing 10°. If you are reading 20° at the light to find TDC, then all those numbers in the map are off by that same 10°. This also holds true with injector sizing and VE tables. If you "tell" the ECU that you have 25lb injectors when you have 19 and tune it,the VE "numbers" will look very different in each case, but the curves will match.
Last edited by ForcedFirebird; 07-23-2020, 06:33 AM.Comment
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