head milling effect on timing and compression? Need advice on getting it right

[Mn_Z_Man]

when I first got my '73 the timing chain was stretched and compression was 160 PSI. I advanced the chain to #3 and it came up to 175 PSI. This winter I replaced the timing chain, and bent a valve in process. the 5 cylinders that were good showed 175 to 180 PSI. I then got a valve job to fix the bent valves- new exhaust guides, grind, new seals- AND the head needed to be milled. After the valve job, timing mark back on #1, compression is down to 155-160 PSI. The machinist didn't actually measure the amount he milled off ( :tapemouth), but he thought it was only a few thousands.

Now I've heard/read that head/deck milling will retard the valve timing, but never any discussion of how to fix it. the mark is at #1, but I'm wondering if the alignment marks don't work the same after the milling? I'm thinking about advancing to #2 dowel and see if that helps. Any suggestions? Would getting and adjsutable cam sprocket be a better idea? I'm gusing there is another tool to get to time the valves with out using the OEM marks. Any thoughts on this appreciated, I'm kind off the beaten path here, and not a lot of experience with custom engine mechanical.

I remember those 15-20 PSI made a big difference in how the engine ran, want to get them back!

thanks

Eric

[ozconnection]

There are a few things I'd like to mention to help you out a bit.

Decking the head is an important thing that needs to be done when the head is removed. It ensures that there is a good clean and flat sealing surface for the head gasket to work with. Generally, only a light skim is all that's needed to do this and rarely affects the compression ratio so much that you need to worry about it. When people purposely mill or shave the head to increase the compression ratio, we're talking 40 thou or more, that's when cam timing becomes an issue. I doubt you fit into the latter catagory.

You said that you advanced the timing of your cam and found your cylinder pressures went up. Try it again. For a stock cam, a stock cam sprocket is fine. Aftermarket cams deserve a multihole or 'dial in' type sprocket for best results.

Since your head was off, there are other things that may affect your final cylinder pressures other than the head skim (which would ordinarily raise cylinder pressures very slightly),

1)Was the new cylinder head gasket the same crushed or installed thickness as the old one?

2)How much carbon was there on the piston tops/head chambers?

3)How deep into the head did the guy machine the valve seats?

It only takes a few cc's variance to change the cylinder pressures. And one last question, were the pressures all taken with a hot engine with the throttle wide open?

Just some stuff to thing about.

Good luck.

[Mn_Z_Man]

thanks oz. The Carbon and head gasket came to mind for me too. There was a moderate amount of carbon- don't have much experience, but they did need some cleaning, much of the deposit was actually tan colored. Ditto for tops of pistons, but amount is hard to judge. Also don't know about head gasket, the one I used was a premium FelPro, generally well regarded, but don't know how it compares to what was in there (should have saved it). Depth of valve grinding, only enough to clean them up, again don't know the amount.

Yes the pressures were all taken hot, dry, plugs out, throttle wired open, 6-8 cranks, good new gauge. I'm going to check it agian this weekend now that I have ~1000 miles on since the build, and we'll if anything has changed.

Thanks

Eric

[Mn_Z_Man]

Ok, I avanced the valve timing to #2, adjusted the lash, and ran it ~250 miles. Rechecked compression, and it was maybe on average 2-3 PSI higher, still around 160 psi.

Put one shot of oil in the the lowest cylinder, and it went from 155 to 175. Doesn't run much different after the change, maybe a little smoother at 5+K, gas milage is still the same, about 16-18 around town. Another wierd thing is my plugs still look almost new after 1,000 miles.

conclusions- It seems my low compression is likely the result of worn rings, and cleaning of the carbon build up. That would explain why they were variable but higher before the valve job, and lower and even after. Advancing the timing made no real difference. I was assuming that since I don't burn any oil, by rings were OK, but I guess the oil rings and the compressions rings work independently.

Any thoughts? Will new rings and good hone bring her back to 185 PSI, and will I notice a difference?

I'm trying to decide whether to put the time (and $$) into this engine (with its new head) or start on the L28 build I've been thinking about. The problem is I'm not sure if my N42 head is a good candidate for an L28 build.

thanks

Eric

[FastWoman]

Yes, head milling will result in the sprockets being closer to each other, so the same number of links of of chain inbetween will leave the cam sprocket turned a bit and the tensioner ever so slightly more relaxed. The effect on valve timing would be somewhat like having a worn chain. I don't know much beyond that. I think I've read that the cam towers can be shimmed to add the height back, but I might be imagining that.

[jmortensen]

Retarding the cam timing makes more top end power and will show lower compression numbers most of the time. Advancing it makes more low end power, less high end, and shows higher compression numbers. Also compression gauges are really not that accurate and generally shouldn't be used against one another. A leakdown tester is a much better test of the engine. Just FYI.

If you put oil in the cylinders and the compression jumped, then rings is your culprit. The best test is a leakdown test, but with the jump you can pretty much assume that if it jumps 15 psi with the oil, there is something going on there. Nissan bottom ends are pretty strong, so you could probably get away with new rings and honing the block if there is nothing really obviously wrong. As to how much benefit that gets you, it's tough to say.

Yes, head milling will result in the sprockets being closer to each other, so the same number of links of of chain inbetween will leave the cam sprocket turned a bit and the tensioner ever so slightly more relaxed. The effect on valve timing would be somewhat like having a worn chain. I don't know much beyond that. I think I've read that the cam towers can be shimmed to add the height back, but I might be imagining that.

You've got it right. Cam tower shims usually come in .015" heights, and you add them underneath the towers to restore the distance between the cam and crank sprockets. An adjustable timing gear would allow you to fix the cam timing but wouldn't fix the loose chain issue. If the head really had "a couple thou" taken off, it probably doesn't matter, but if they had to take more than .015" I'd say get the cam tower shims because the loose chains tend to make noise. Cutting the head .015" actually adds .030" slack to the chain, because you lose length on both sides of the pulley. This makes the effect on timing more drastic and also the effect on slack in the chain more drastic. I don't know if by adding one shim you would need to look at the lash pads, but keep in mind when you start changing things in the valvetrain you are potentially messing with the geometry there and it doesn't take too much to throw everything off kilter and then you start having problems that are more difficult to fix.

The other option is to get a Kameari chain tensioner and an adjustable cam gear. With these two you can cut the head a good amount, take up the slack in the chain with the new tensioner, and correct the cam timing with the gear. That's probably $800 in parts, but it is a SWEET setup in that it allows a lot of flexibility. You could mill the crap out of the head to change the compression ratio, slap it back on and adjust the cam timing with the gear and take the slop out with the tensioner and you're good to go. http://www.zccjdm.com/catalog.php?ref=azcarbum&recview=11&dt=43033&disp=name

Edited September 9, 2009 by jmortensen

[Mn_Z_Man]

thanks all for the thoughts. I'm guessing if I could really get it back to 185 (new stock), it would run better. My mileage is not good, and the tail pipe stink is driving me crazy. I keep trying to lean out the SU's, but then it runs like doo-doo. Is 155 PSI enough to hurt the efficiency of the engine? The rings are definate maybe this winter. And yes, I'm not in any hurry to change the rocker geometry at this point- that getting inot more than I want to sink into the L24, really just want to run as good as practical.

thanks Eric

[jmortensen]

thanks all for the thoughts. I'm guessing if I could really get it back to 185 (new stock), it would run better. My mileage is not good, and the tail pipe stink is driving me crazy. I keep trying to lean out the SU's, but then it runs like doo-doo. Is 155 PSI enough to hurt the efficiency of the engine? The rings are definate maybe this winter. And yes, I'm not in any hurry to change the rocker geometry at this point- that getting inot more than I want to sink into the L24, really just want to run as good as practical

Again I would suggest a leak down test. Pay a shop if you don't have the tester. It's well worth KNOWING that you need rings rather than trying to use the numbers from a compression gauge, especially if you don't have one cylinder that is way down from the rest. The mileage issue is a not necessarily a function of bad rings, it could be a problem with the carburetors, bad tune, ignition timing, etc. Doing the leakdown test will tell you what is going on with the engine's ability to pump air efficiently. You can worry about the mileage after you get the engine mechanicals sorted.

http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/index.html

[Mn_Z_Man]

Thanks Jon, I will do leak down test before I commit to rings. Probably also time to to get the reman SU bodies form Ztherapy. Everything else is is in good shape, not too much more to do.

thanks

Eric