What did I break?

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jos51700

jos51700

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The car: 72 Dart, 360 stock with mild cam, 4-speed

OK, drove the car to the muffler shop with open manifolds (trashed the cheapie headers), and they couldn't bend head pipes for it on the driver's side, so I said to leave it be and picked it up. This was a 6 block trip.

But,

I've moved about 40 miles away since I dropped it off, and had no choice but to drive it home. With open manifolds.

So,

I drove it carefully home. I wrapped the torsion bars and speedo cables in aluminum foil to stop some of the heat, I took as many breaks as I could to let it cool down (shut it off at lights, coasted with engine off, etc) and this was 10 miles in the city, 30 on the highway (I never went over 50mph) and I coasted a LOT.

I didn't think this motor ever smoked, but it does now, on the driver's side, one puff per engine cycle. It first puffed smoke on one of my many startups, about 2 miles into the trip, and it was leaving a trail by the time I arrived. The result? Only 100 psi compression on #1, and a very oily sparking plug. #3 had 135 psi, for comparison.



I'm thinking that there was some detonation that damaged a ring land or something, unless someone here has some better advice. I knew it was going to be lean so I was very gentle on the throttle whenever possible.

Any ideas? If it is a piston, I'll just pull it apart and bore it over and get some 9.5:1's, but the money is pretty snug right now...so a piston recommendation wouldn't hurt either. If you want to recommend a piston, toss in the cheapest you've seen them! This car is strictly street driven.

The cam is 400 miles new and so is timing chain and tensioner. I have magnum heads if you'd recommend them with what ever pistons. Cam is Comp XE268H.

Thanks
John
 
most likely, you burnt a valve due to no back pressure. i think the cheapo pistons would be the trw's????
 
It is possible I've burnt a valve, but that wouldn't explain oil in the cylinder...
 
jos51700 said:
It is possible I've burnt a valve, but that wouldn't explain oil in the cylinder...
Click to expand...

Yes it would as it could have mushroomed/tuliped the valve and caused valve guide issues....and in turn oil leaking past the guide. Tow truck would've been cheaper.
 
Try shooting some oil in the cyl, if it still has 100psi the valves are ok, last one I had was at 15-20psi and the rest were 155-160.

Just to make sure throw a vac gauge on the manifold, if it's an intake valve the needle will be all over the place, the exhaust valve is easy to check with open manifolds/headers, it will make a loud pfft pfft pfft sound when you shut the engine off.
 
try a leakdown test on that cylinder and see where the air is blowing out. great way to figure it out.
 
Did the leakdown a few days ago. 64% :(

Now I'm trying to figure out if the Sealed Power hypereutectic pistons are any good!
 
jos51700 said:
Did the leakdown a few days ago. 64% :(

Now I'm trying to figure out if the Sealed Power hypereutectic pistons are any good!
Click to expand...

OK 64% is vary bad (did you "0" the gauge before testing?)

Did the air come up through the carb? exh? Valve cover????
 
hyper pistons in my 360 for 9 years about 10K miles at 420hp. the pistons are not the problem. the problem most likely was it being on the lean side and shuting it off and restarting it on that trip causing very hot exh valve to heat cycle
 
He has stock pistons anyways..... The good ones. :)

Yeah, shutting it off didn't help, best way to run with open manifolds is to not let it decelerate, in other words, come to s stop, foot a bit on the throttle, so it doesn't suck back.
I ambetting a bad valve too.
 
........If it burnt a valve it would b missing like a sob......U cant burn a valve from no back pressure.........thats an old wives tale...........kim....
 
Welp, the valves are tight...the leakdown is coming out of the oil dipstick tube and valve covers. NONE out of the intake or exhaust.

It's piston time!
 
Hey JO,
The pistons are more than likely fine. Think overheated valves and you'll be closer to the truth I'd bet. Pistons only cause the valves to tighten up if the two come in contact and that is usually catastrophic to the entire assembly.
Pull the heads and have them repaired, have a look in the cylinders for scuffing. If it ain't broke, don't fix it.
 
oldkimmer said:
........If it burnt a valve it would b missing like a sob......U cant burn a valve from no back pressure.........thats an old wives tale...........kim....
Click to expand...

Exactly. Go to the race track some time and you'll see a whole bunch of guys running open headers and they ain't burning valves from it.
 
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