I have a Pontiac 400 in my 35 Chevy pickup that has recently developed a bad case of blow-by. The engine has dual quads, mother thumper cam, 670 heads or 10.5 compression for you non Pontiac guys. 1.65 rocker arms. Also a 4 speed and yes I beat on it some. I did a compression test and had 175# in 7 cyl and 100# in one cyl. Any ideas what may have caused this?
Bad rings, hole in piston or just basically it is time to pull it down and see what is wrong. is it blowing by or smoking? Smoking may mean that your heads need attention but blowing by means weak rings or hole in piston, hole in piston probably would show any compression though. Green car that is what caused it. Green car = bad luck
Excessive blowby in any engine usually means that you have rings that are worn out. The compression is going past the rings into the crankcase causing excess pressure in the crankcase causing the blowby when it excapes through the breathers. The extreme difference between the compression readings in the cylinders is a positive indicator of that as long as the compression test was taken correctly. Outside of worn rings it might have broken rings or rings that were lined up with all of the gaps in a row allowing the compression to escape into the crankcase. It looks like it's time for a rebuild.
No smoke out the exhaust. Blows oil out the breathers. Heads were recently rebuilt, new valves and seats. On compression test I put oil in the low cyl with no change in the reading. I plan to tear it apart soon.
99% of the time if you get low compression because of rings , repeat with oil will raise the compression. If oil doesn't raise the compression, its probably something besides rings.
well you have located the problem. Now you need to figure out what is wrong with that cylinder. You can guess a lot but until you pull the head and find out what broke, it's just a guess.
I had the same problem with a fairly fresh 389, it turned out to be a loose valve guide, lucky it didn't drop the valve.
Since oil in the cylinder did not improve the reading, I'd guess valve train issue on that cylinder. Pop the valve cover and turn the engine over to watch the valves.
When you tear the engine down, removing the piston in that cylinder, look at the top ring. If it is broken somewhere around its cir***ference, that will drop compression reading that amount. (simply #1 ring broken in half)
Pulled the engine apart and here's what happened. Seems I am the victim of poor gas and to much compression. #5 piston has a large portion of the ring land flopping around between top ring and second ring and several cracks in other places. The rod bearing is worn out on the top half indication detonation. the 400 block is headed to Ken's Speed and Machine Shop in Brooksville Fl. and will be returned a 455 with all new internals and less compression. An expensive lesson learned.
I'm a huge fan of Ken "the Dude" and I'm sure he has a handle on your new combination to keep your compression under control. When you get your new combination running, be prepared to hang on to your ***, cause that thing is going to move.. Good luck, -Abone.
If you don't mind me asking , and to further some of my research.... What was your quench measurement on this detonating engine? Piston in the hole at TDC plus the thickness of the compressed head gasket.
Pontiac Dude is awesome. He was running a 1970s Trans Am, stock suspension and drag radials into the high 8s in the 1/4. With a little help with nitrous.
was there any sign on valves of poor fuel? When I pulled the heads all cyl's looked the same. I am no expert but I thought they were rather clean, no sign of damage. The heads were rebuild about 1000 miles ago with new valves,seats and springs. I think my main problem was 10.5 to 1 compression and with loud exaust and a thumper cam hard to hear pinging in the engine.
Here is some info I got today. Crude measurements because I did'nt have the right tools. Piston to deck clearence at TDC .040. Old head gasket thickness .045. Bore if you need it 4.15 or stock 400 .040 over. Heads #670 72cc. I had a chinese set of calipers not sure how accurate. Hope this helps.
Holy cow thats 0.085 quench area. Everybody has an opinion on what's optimal but most would agree thats pretty high. The large quench area is supposed to be what causes most problems with compression related detonation. You just had the heads off, was there steel head gaskets in there before you changed them ? Most say that 0.040 is optimal, you can go tighter but unless things are really taken care with measured and tight the piston can hit the heads.
Pontiac's need to have the clearance checked between the valve spring retainer and the valve guide..Too much cam, and they will beat the seals out, or worse. Should have done a leak down first...
The heads were done 2 years ago, I don't remember what gaskets were used. The engine is at the rebuilder and I trust it will be done right. Should come out with a pump gas friendly 9.5 to 1 compression.
oh im sure it will, just trying to gather some information. lots of guy blame high compression alone, but a big/thick quench area seems to be the problem once compression reaches over 9+.
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