Any theories on cylinder compression psi?

[naviathan]

"Break In" is a controversial subject in the auto world. I've actually read studies where following proper break in caused premature wear after the break in period because things settled differently when "real" driving took place. Other studies show you should break the engine in the way you're going to drive it so that everything wears in to your driving habits. I'm no expert on it, but I hope to be soon enough.

 

Moving on to my real intent for this post. Cam timing isn't that difficult to understand. Obviously as the cam turns it opens and closes the valves. The cam timing ensures the cam begins opening, is fully open and closes the associated valve at the proper moment in the stroke. As the piston falls on the intake stroke the cam lobe controlling the intake valve opens drawing in air and fuel. As the piston reaches BDC (Bottom Dead Center) the valve should already be on the closed side of the lobe so that as the piston rises on it's next stroke, it builds compression. The spark plug fires before the piston reaches TDC (Top Dead Center) because the fuel takes time to ignite and burn to create the pressure for the power stroke. At this point both valve are closed and the pressure forces the piston down. As the piston reaches BDC again the exhaust valve starts to open and comes fully open as the piston rises to TDC. Now here we have what is called "Valve Overlap". Depending on the size of the cam as the exhaust valve closes the intake valve starts to open leaving a period where both valves are open. The larger the cam the longer the valve overlap time is. Hence the reason larger cams give lower compression numbers. And of course once at TDC we start the whole process over again. This is the best explanation I can give off the top of my head. It's really simple if you can look at it one step at a time and understand each step before backing up and seeing the whole picture.

[naviathan]

BTW there's places to get a degree wheel online really cheap and Harbor Freight has several nice micrometers cheap. I'm planning on getting mine there when I do the timing on my engine.

[mentallylost]

Woo Hoo, I figured it out! So it only took calling in sick to work for 2 days. But I got the psi #'s up to 215 and 220's.

 

When I assembled my motor a few months ago, the timing chain that I used did not have the "bright" chain links. So I counted the links instead. Apparently, my count was off by a link. Which is why when I advanced the sprocket from the #1 to the #3 link, my psi #'s increased by 5 psi instead of decrease.

 

I adjusted the sprocket location, put the cam back into the #1 location, and she now runs incredible. Quicker throttle responce, and a much smoother running motor. Now if only I could get more fuel in there...

 

naviathan, when I broke the motor in, I actually drove it nice for about the first 100 miles or so trying not to exceed 4250 rpms, changing the oil then for the first time. After then, I would run the motor hitting 5500 rpms for short bursts at a time. When I hit 400 or so miles, I just began letting into it spinning up to 6000.

[twoeightythreez]

Probably sounds awesome, too.

 

I am shivering in anticipation now with my Z.

Anyone want to buy an L28?

The guy who I traded the 510 for the turbo motor and some other trappings needs to clean out his garage, so we're gonna start the motor swap in my Z next weekend. I will try to put up some pics but since my dig cam. was stolen I don't know.

 

I bought a MSII yesterday and am waiting for it in the mail! Can't wait!

I've assembled stuff like that before and it's pretty exciting when it actually works!

 

I also got a speedhut boost gauge made to match my other faces (EL glo, blue faces, with blue numerals at night)

 

I'll pull the dash when I put the MS in because I have to fix my tach!

I plan on using the stock '78 N/A wiring harness with the MS, I still need a stock wastegate for the turbo, an I/C and BOV and some decent injectors (I have some new stock turbo injectors but they won't work with the custom fuel rail) and I'll be almost ready to break my tranny! Yummy, lotsa fun!