The order of the compression test was 2-4-6-8-1-3-5-7. So the 165 was in the middle of all the cranking. Each cylinder was cranked at least 6 full revolutions. Even with the throttle closed I think it would read the same. The needle was not climbing anymore.
How does one suppose air can get past closed throttle blades? I think shop manuals dating back to 1904 [insert sarcasm emoticon] instruct to open the throttle all the way.
The general idea is that enough air will get through the open spark plug holes in the other seven cylinders to fill the manifold, and thus the cylinder being tested. That's what I've read in the past anyway...
I apologize for being a smartass. It was an attempt at humor Air does get in at idle...under a vacuum usually. It's been such a long time since I've read the procedure, I thought it was commonly accepted to open the throttles up. It may have been an oversight at the time and engine failures are rife with emotions. I truly wish you the best on your project.
Actually, I didn't use any editor, just copied and pasted from the online calculator, which used a smaller font, then went back into this message and resized the font. I was waiting to see if anyone was going to comment on the 1.5* specs that Steve gave shows different cam specs. I'll assume a typo error on his part. Gary
Gary, those were the actual readings from the degree wheel. Problem with these message boards is you can't tell when someone's joking. I do think you are correct about the recommended procedure for checking compression. I wasn't trying to relate back to any spec since there isn't one for this combo. Was just looking for the dead hole.
The cam specs with Steve's 1.5* from the cam card specs...shows different overlap and I/E durations. I guess I'm just tripping and/or brainfarting on something? Or since it's only 1* off, I guess it could be in the variation of actual cam grind specs vs the 'on paper' specs. Gary
It can be the variation of the actual grind or it can be a matter of letting go of the tool that is being used to turn the crank and it springs back a little. In any case you will rarely see the actual specs be spot on with the cam card specs. And even more interesting is if you take the time to look at the rest of the lobes on the same cam. Also the heal or base of the cam lobe can vary making it hard to decide where to zero the dial indicator. Most people run through the procedure several times just to get everything to settle in and triple check the degree wheel readings. If it is a symmetrical lobe I like to double check the lobe center by taking readings .050" down both sides of the nose as well as .050" up from the base. Paul
Makes sense. It's amazing just how 'off' custom grinds or small batch 'shelf' cams can be...not that you'll see perfection with other parts, but in the case of OEM parts, the batches are larger and more likely to be closer to spec (there will be exceptions, of course), since a small mistake in calculations could result in many more parts being bad. I've heard horror stories about some aftermarket cams being so far out of spec as to not even resemble the 'on paper' specs, or variations per lobe being too far out of tolerance between the others. I guess they figure it's going to have a rough idle anyway (and not last long enough to matter), so what the hell lol. Or maybe they hope the lobe wear will bring them closer to spec with each other? Kinda like wearing in some unevenly worn tires by doing some burnouts? :laugh: Gary
Since there is nothing you can do about 1 degree I was looking at the 4 numbers in general as an average sort of to determine where the cam was at. Since the timing set choices are in 2 degree increments that was as close as I care to get it. Got everything now except the custom Autotec pistons. Should see them in about a week. Then off to the machine shop. Bore will now be 3.845 if I didn't already say that. Also have a torque plate to use for the honing process. Should add BIG HP.
Sweet! Please post pictures when you get the pistons Your name should be engraved on the bottom of the pistons along with job #.
Well got around to CC'ing the chambers to see if #3 had abnormally high compression thus causing detonation. #1 53cc #2 53.5cc #3 54cc #4 54.5cc #5 52cc #6 56cc #7 52cc #8 53.5cc #3 was not the smallest and in fact one of the larger. These numbers really don't match up with the compression check numbers but keep in mind I was only looking for the bad cylinder NOT statistical data. Oddball #6 shows you why you should cc EVERY chamber not just a couple as I did. I randomly checked 4 chambers and they were all right around 53.5-54. Now I have a 4cc difference to try to equalize. Just so you can visualize what 4cc looks like, it's roughly a 5/8" column .800" tall. That's a lot of material. Never assume stock chambers are close to the same size, they're not. I had professor Roland look at my piston and bearing and he does not believe it was detonation and in fact said he would reuse that bearing. He does think the piston was broken for some time and finally just gave out. I tend to agree. We snapped off the cracked skirt by hand it was so weak. The edges were dark like it was that way for a while. Odd it wasn't missing and blowing oil into the air filter until that last run.
Glad you found visual evidence of corrosion in the crack. That says a lot right there. Any guess as what caused it? Where was the corrosion located, near the pin or originating from the ring area? Evidence of ring butting or any other calamity? What do your other pistons/ring grooves look like? The rings can't seal without the grooves being in good shape, and it's still a guess on the reason for the compression readings differences (a great guess, considering the parts aren't in hand) Are you familiar with what pre-ignition damage looks like vs. detonation? Maybe it happened on an earlier run? (I haven't scrutinized the thread) Any idea which cylinders might run leaner? Any signs on the piston of being dropped? Has anyone posted thermal imaging of a 350 Buick under full throttle/load to see where the true hot-spots lie? It's already been mentioned that duration differences would be more likely to cause such disparity in readings. Even crappy ring seal/prep/break-in might not spread as wide as that variance. A visual look at the bearings is not sufficient to determine whether or not they can be re-used. Yes, they look fine to me but... They would still need to be marked for location, felt for fit upon disassembly as well as re-assembly along with your rod bores measured for out-of-round. That's a lot of time spent vs simply throwing them away and re-honing the rods.
Steve, if you can measure the center to center distance on the number 3 rod then compare that with the rest of them. Perhaps even with #3 being one of the larger chambers the rod you use for #3 can be longer than the rest? I wonder how far off a set of stock rods are that probably have been reconditioned and ARP bolts installed? They would probably be more off from the factory than from being reconned though. Yeah the broken piston looked like it was cracked for a while with the ends the only thing holding it together. 4cc is quite the variance are you aloud per the rules to grind in the chambers? Derek
All other pistons and ring grooves look fine. Rings from that piston look fine. They held the broken chunk in place. Already bought the new bearings. Will be checking the rods diameter. Keep in mind the pistons/ short block has about 8000 miles. If the piston was broken on install then thats a pretty tough piston. Grinding chambers not allowed. Don't tell OK. It was obvious when it did break, there was a dead miss on the return road.
Oops, I meant measure/fit...not just visual inspection. Similar rules of geometric tolerancing apply to engine building practices.
Well finally got the machine work done. All looks good. Hope to start assembly around Jan 1. Heres a little piston info to add to the data base. Sealed Power Hypereutectic pistons H522CP at .040" over weigh .74 Kg or 1lb and 10oz Autotec forged at .045" over weigh .59Kg or 1lb and 4 1/2 oz Both with pins, no rings. Yes, they took some weight off the crank.
Piston weights are listed in grams. Sheesh, don't make everyone do all the math It's exciting getting them back together!
