Learning from a mistake (cam install)...

[SadbutTrue]

Okay, so I tried to install a compcams 262xe 2 years ago. Short story, it was a failure.

Long story, we got it in, it was running. Didn't make any obviously terrible sounds but it clearly wasn't 'happy.' I believe my mistake was that we didn't setup the valvetrain correctly... I don't have adjustable rocker arms (screw-in), we didn't use a pushrod-length checker (and used the perscribed pushrods on compcams' site), and also didn't degree the cam. I know, it was dumb. Anyway...

When we got it back to the house after the initial drive, we did a compression test. All the cylinders were about 30psi lower than usual (we had just tested it a few months before and it was fine, and had been running fine up until the install), some a bit lower than that. My dad then suspected we had somehow damaged the piston rings at the time. My current theory is that because the valvetrain wasnt adjused properly, chances are the valves weren't closed when we took our readings and artificially lowered the cylinder pressure #'s... essentially saying that if we had simply torn it down and put the right pushrods in etc, the engine would have been fine (assuming no major damage to the engine).

Does this sound right? Trying to learn from my mistake before I repeat it in a few months...

 

[68stang351]

My current theory is that because the valvetrain wasnt adjused properly, chances are the valves weren't closed when we took our readings and artificially lowered the cylinder pressure #'s... essentially saying that if we had simply torn it down and put the right pushrods in etc, the engine would have been fine (assuming no major damage to the engine).

Does this sound right? Trying to learn from my mistake before I repeat it in a few months...

That was my first thought. I think you're right, lets see what the others think too.

 

[rbohm]

when you use a cam that has longer duration, and more overlap than the stock cam does, you will lose some low speed compression pressure. this is due to the valves remaining open longer, as well as both valves being open at the same time for a longer period. this is also the reason you lose low end torque as well. i rather doubt you have any engine damage though.

 

[2nd Mustang]

I'm no guru, but I've also read that different cam grinds will lower compression readings.

I have a mild Crane hydraulic cam and all cylinders are at 130 pounds. In comparison, some stock motors read around !60 pounds.

 

[SadbutTrue]

I'm no guru, but I've also read that different cam grinds will lower compression readings.

I have a mild Crane hydraulic cam and all cylinders are at 130 pounds. In comparison, some stock motors read around !60 pounds.

Those were the exact #'s we were seeing for the most part, iirc. 150-160 pre install, and then i think around 130 for most the cylinders after. I do recall one or two of the cylinders being well below the rest though. But again, don't know if that had anything to do with the cam install.

 

[FLStang]

The pressure reading on all cylinders should fall within 5-10% of each other.

Cam profile affects compression pressure tremendously, longer duration ("street and race") cams generally have more valve overlap as someone already mentioned, they stay open longer to let in more fuel and breathe out more exhaust. It leaves the piston less time to make pressure while the valves are closed, therefore you have lower compression pressure. The hotter cam "bleeds off" compression pressure.

This is a waste of fuel. What you should do when installing a higher duration/overlap cam, is to raise the static Compression Ratio (CR) of the engine, i.e. make the space above the pistons smaller. Either by replacing pistons with taller ones, or flycutting the heads so the combustion chambers get smaller. Or get different heads. This way you can bring the actual compression pressure back up to where it should be. In some cases you will need even more pressure than stock.

I wouldn't even bother with a cam if I wasn't able to raise compression ratio. Waste of money and fuel.

My opinions only of course.

Jan

 

[oscart]

Get the factory shop manual and check out Crane and Comp Cams website for tips. Modern oils are missing the lubricants needed to break in cams. Many new cam lobes are being wiped out because of this. Use Rotella-T oil and add one quart extra over the recommended amount for extra splash during breakin. Also, add a camshaft breakin lube to the oil. Crane, Comp Cams and others sell this. Have your engine tuned so that it will fire up right away. Run the engine at 2500 rpm for at least ten minutes. Change the oil and filter. You should be good to go. For high spring rate dual valve spring setups, remove the center spring during breakin. These are just a few tips so that your new lifters and cam will not wear out at startup.

 

[SoCalCruising]

Remove spark plugs, squirt in a little oil into the cylinders, turn the engine over a 5-10 secs to distribute the oil and recheck compression. If it jumps back up to 160ish (a big jump), then you can suspect the rings. If there is only a 5-10 psi change, your rings are fine. [Even a motor with good ring seal will benefit a little from the oil.] If only a little, you may have a valve adjustment issue. For example, I went from around 100psi up to 160-170psi by going from 1/2 turn preload to 1/4 turn preload on roller lifters.