Need 4.0L compression standards for comp. test. Please help!

jrotten · #1 (**permalink**) 07-20-2003, 08:00 PM

Well, after scanning my factory repair manuals, and even my old Haynes (thppt!junk) manual for some standard compression readings on my 89 4.0L TPI, I have concluded that they are not there even though both publications make reference to em. I have 170 psi across the board, which SOUNDS healthy, but I cant tell with nothing to compare it to. Anybody know? I have low power after an engine rebuild and was mainly doing this to eliminate the possibilty of having the cam off one tooth. thanks. jrod :-? [addsig]

dingus · #2 (**permalink**) 07-20-2003, 08:00 PM

haynes junk? i don't think so! i found it in less than a minute...

section 2D-3 says for an inline-6, compression should be between 120-150 psi.[addsig]

jrotten · #3 (**permalink**) 07-20-2003, 08:00 PM

oops. sorry, that was a Chiltons manual. :-O [addsig]

jrotten · #4 (**permalink**) 07-20-2003, 08:00 PM

Took it around the block last night with the stock air box removed just for a minute. Bad thing to do, but it proved one thing: big time restriction in the intake tract. I think my cam is trying to suck through a straw. Looks like a K&N and Rusty's cat back is in order.[addsig]

MudDawg · #5 (**permalink**) 07-20-2003, 08:00 PM

Nice and even... looks real good to me.. Hmmmm..those I-6's must really be choked up....but then again, the engineer's have to deal with making the intake tract nice and quiet too...while you are doing the cat-back...install a hi-po cat...like one off a big-block ford...they are long and skinny if memory serves me...or maybe the stuffins will fall out of your cat when you take the back half of the exhaust off ;-) On my 2.5, I had a new pipe built out of 2.5 inch tubing and installed a muffler made for a 500 inch Caddy...nice and quiet...no more restriction..

The reason you need larger pipe is that as the exhaust travels down the pipe it cools off..cooler denser vapor needs a larger pipe in order for it to flow easily. As for the muffler, the noise is reduced by absorbing the energy in the exhaust pulses...a large muffler absorbs the energy without having too much flow restriction. The number of chambers in the muffler determine how quiet and restrictive it is...racing mufflers have 1 or 2 chambers...but they are loud....but a I-6 at full song sounds better than a V-8 8-) [addsig]

jrotten · #6 (**permalink**) 07-20-2003, 08:00 PM

Hmmm. 170 psi kinda makes me wonder about the machine work I had done. The head was shaved but the block was not decked. I am wondering about my compression now, 170 psi seems awfully high. A wrong deck height could also affect lifter preload and possible keep the valves from fully closing, not to mention the risk of spark knock on less than prem. fuel. Time to check lash this weekend and do a leak down test. Thanks for the responses. J. [addsig]

remember5 · #7 (**permalink**) 07-20-2003, 08:00 PM

You may also borrow a compression guage from a friend to be sure yours is reading correctly. [addsig]

MudDawg · #8 (**permalink**) 07-20-2003, 08:00 PM

Shoot, put you a big ole cam in that rascal.....that'll bring the cranking pressure number down....

Just kidding!!!!! that would make it mighty hard to live with...but it would decrease the effective swept volume at cranking speed and lower the cranking pressure....by the way...if you have a motorcraft ignition...did you know that you can easily make it retard the ignition??? the retard is triggered when you crank the motor to make it easier to turn over...also the knock sensor "tells" the control system to retard the ignition when it "senses" preignition. If the motor pings...try some "octane booster".. might be cheaper than buying premium...

Dawg[addsig]