I am concerned that you may be premature in wanting to drop cylinder pressure to solve this problem. I think you have one lean cylinder...so.... how about: Let go of the idea of maintaing excellent fuel economy for just a month, and see what fattening up your fuel ratio does...

Cylinder pressure of 180 PSI is not really that high..... IF everything else is in order... the following is copied from a Chevy High Perfomance Magazine ariticle that talks about tuning WITH an eye toward cranking compression.

"From our rather limited research, it appears that building a performance street engine that combines a decent-duration camshaft with enough static compression ratio to create 175 to 190 psi, will reward you with a very snappy engine that is not only responsive and fun to drive, but makes decent power as well"

http://www.superchevy.com/how-to/81679- ... mpression/

Of course I could be TOTALLY wrong.. and you may call me an idiot.. but I would hate for you to drop your compression ratio and still have a problem because your compression ratio may only be compounding the issue but not Causing it...

No, I get it. I can order some jets and richen up the mixture, but...

As I read about Webers they have two circuits, an idle circuit and a main circuit. The idle circuit apparently is dominant up until about 2500 rpms and then the main circuit engages. I have 2.76 gears, so at highway speeds, I'm only running about 2000 rpms, does that mean I'm only on idle circuit? Recall, at idle my AFR is 12.5, so pretty rich already, if I richen up the idle circuit for cruise, then at idle it'll be way too rich.

I may be totally misunderstanding what I've read about Webers, perhaps I can increase the main jets and that'll richen up the cruise AFR.

Anyway, it runs fine now except I'm having a "fall off" or stumble while cruising. I don't think this is related to AFR, it's never done that before. I believe that solving the detonation issue with the colder plugs and dialing back the timing either uncovered an underlying issue or started a new one. This sporadic rough running during cruise is new, but I'm running at 5 degrees advanced (which is new) and these colder plugs (which is new). The issue is the same in that I'm getting crankcase pressure, I noted fresh oil on the valve cover and some around my dizzy - same as when the detonation was occurring, just not as much. Seems like a valve issue to me, but the detonation at cruise was never an issue, only under load up hill or when I punch it down full throttle.

Could it be that I really do have a sticky valve as I first thought? Might be worth trying the other head just to narrow the field. If it acts the same way, then it's pretty strong evidence that it's another issue than the head, right?

I can change the head in about 8 hours; seems worth the effort to me, just to get a new data point.

Brian

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If you only have detonation under heavy throttle than your AFR is not correct at that instant. maybe at cruise the AFR is close enough to where it needs to be and so it does not rattle..... but if you get detonation when climbing a hill then at that moment your AFR is not where it NEEDS to be..

Change the head if you want... or take the car to a dyno and get it super-tuned and keep the high compression head, the fuel economy, and get back to enjoying the car as you built it.

I agree, my compression is a good thing; I don't think that's the issue. I'm trying to be methodical and scientific about this, so please hear me out.

Since I installed this new build, I had a sporadic detonation issue, otherwise the car ran great, idled smoothly, lots of power, etc. During this sporadic phase, I noticed at least once (even with the hotter plugs) an episode where it had a miss or power fall off on the highway, ran rough, seemed like an intake leak or bad misfire, no detonation or funny noises, just loss of power and I could feel that the engine was struggling, stumbling, out of balance somehow - same as what I'm reporting now, but I shrugged it off as bad fuel, water in the gas, or something, since it was a very short episode (like 10 seconds) and went away. Still, the thought passed through my head that it might be a sticky valve.

Since that time, months later, the detonation issue became chronic. It was so bad, it burned through a head gasket. When I changed the head gasket, it was clear that the damage was in #4, it burned through to #3 and also burned a good way past the metal ring in another spot. You can see that in this picture: http://s785.photobucket.com/user/67dart ... ort=3&o=19

That evidence suggested to me that #4 had a sticky or burnt valve. Also, another indicator that the detonation issue seems to be in one cylinder is that during detonation I don't hear multiple noises out of sync, but rather a regular "rap" "rap" "rap" which to me suggests only one cylinder has the issue.

OK, so I changed the head gasket, and ran a compression test, nothing out of the ordinary except #1 was a good bit lower than the rest. That good compression test got me away from the bad valve theory, I figured it must be something else.

I checked AFR, intake vacuum (good, no leaks), exhaust back pressure (fine), ECU, distributor, exhaust leaks (none), cap, rotor, wires, etc. - all seem normal. Perhaps #2-#6 were running too hot a plug given the AFR at cruising (14.7, but still normal range). Indeed, I changed to colder plugs and dialed back timing about 5 degrees (from 10 advanced), the detonation issue seems to be much better (I still get a hint of it under load). It runs great now, except I am having that miss and/or power fall off more regularly now. After the head gasket change, I drove it for about 100 miles or so without issue, but the miss or dead cylinder or whatever it is comes and goes regularly now, I can feel the power drop and a vibration in the steering wheel; then it goes away again.

The thought occurred to me that perhaps the timing and colder plug adjustments merely are masking another issue. This miss or stumble or whatever it is, along with that picture above, is leading me back to #4 valve(s) being the issue.

Before I pull the head, I suppose one more test that I could run is to install those hotter plugs back into #1-3 and #5-6, but leave the colder plug in #4 and see if I get detonation. I'm betting I won't see it. I don't think it's that #4 is running leaner than the rest, the plugs all look the same after the head gasket was changed. Greenish tint indicates lean condition; I did see a hint of green tinge on #3 and #4 but those plugs were from when I drove home 50 miles with a blown head gasket, where those two cylinders were in direct communication, so I can't make anything out of that. Since then plugs look about the same, AFR in front and rear bank are identical (I don't think one intake runner is leaner than the other two in the rear bank, but I suppose that's possible).

I'm going to pull the head and swap out for this fresh one that I have. It should have almost the same compression according to the head thickness, but I'll measure the compression once I have it installed to be sure. Being as scientific as I can, at least some evidence is pointing to #4 valve(s) as the bad actor. If I do that and nothing changes, at least I'll have another data point.

Brian

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OK, so now I'm back to the compression theory, AFR too low for load conditions...why, you ask?

I pulled the head today and I found this: http://s785.photobucket.com/user/67dart ... y.jpg.html

Use the magnify tool on photobucket and look closely. The metal has pulled away from the gasket and is nearly burnt off. Also, the metal ring is split about an inch or so on either side of that detached piece.....BUT, it's not #4, this is #6! None of the other cylinders' gasket ring had this condition.

OK so where am I?

Well, maybe I just put her back together with a new gasket, now that I have the colder plugs and dialed the timing back. Maybe that weird stumble was this thin detached piece of metal getting hot and causing weird pre-ignition that I can't hear but causes the rough running. Remember, I had driven the car for a few hundred miles with this gasket, and there was a lot of detonation during my trial runs, before I got to colder plug and timing back. That pic shows how powerful the heat/compression is during detonation (I call it detonation, but I probably should use "pre-ignition" since detonation is usually associated with broken pistons/bent push rods etc. and I have none of that).

OR, as an added measure, I richen up the mixture as well under load to avoid burning through gaskets....the old engine didn't have this problem at all with these carbs and setup; so it's definitely the higher compression along with these carbs/jetting.

The head that I have prepped has had a bit less metal milled off it than my current one, like 1/16" thicker or 0.063" more metal on it, so I will go ahead with that swap, perhaps the compression will be a bit lower with this one, that can't hurt. It'll be interesting to see what compression numbers I get. That's why I was asking about copper head gaskets in the other thread, wondering if I might lower the compression with a thicker gasket.

Science is great, am I right?

Brian

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..would be to use 100+ octane in her full time, but that would get expensive. That octane boost leaves a reddish orange residue on everything, even in the header down pipes, I don't like using that stuff...

brian

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Brian I forgot but what is your cranking pressure?

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12.70 @ 104.6

cold cranking I'm running about 180 psi in #2-#6, #1 is lower at 150 psi. It's not like super high, but it's enough with this crappy "summer blend" of gas in SF Bay area. My dynamic compression ratio was calculated to 8:1.

b

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I installed the replacement head today: http://s785.photobucket.com/user/67dart ... 2.jpg.html

I ran a compression test after setting the initial (cold) valve lash to specs:

#1 180 psi
#2 120 psi
#3 150 psi
#4 150 psi
#5 180 psi
#6 150 psi

Seems like I'm in decent shape with this head, except #1 and #5 are 180 psi, like 5/6 cylinders on the other head.

Can I play with valve lash to lower compression or is that a pipe dream?

Brian

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I don't want to burn my valves, but maybe I can tweak the compression lower in two cylinders with some adjustment. Anyone ever done this?

b

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It seems as if something is a mis. The other head posted a 150PSI in #1, and 180 in the rest.. this head shows 180 in #1 and wide variations through out... I have no idea why...... but something is out of adjustment it would seem.

zipped it back up, it seems to run fine. No signs of preignition/detonation.

On the compression test, yes, well I assume the difference is in the head, as I didn't change the bottom half in any way. I tried loosening the lash up, but the readings were the same...

I have the timing at TDC and it runs fine, has lots of power, idles smoothly. I only drove it for 20 minutes, but I did highway, traffic, steep hills, heavy acceleration, all seems OK.

I did flush out the tank to remove the octane boost laced gas (pumped it into my wife's minivan ) and put in fresh 91 octane. I wanted to see how it runs on pump gas with the new head. I am sticking with the colder plugs as well. The old head didn't seem to run well below 5 degrees advanced, this one seems OK at TDC or even a few degrees retarded. I can't figure how that makes sense, but at this point I don't care, as long as it's running good.

I'll test drive it this week, see how it goes.

Brian

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Drove it all week, ran fine, smooth idle, quick starts, etc. Today, I stopped to put gas in her, drove her about 10 miles and then it started running really rough, I had a noticeable shaking in the steering wheel. Pulled over, seemed OK when I revved it, seemed smooth, drove rest of the way to work, about 3 miles, wasn't itself, but not shaking like it was. Checked under the hood, I had that same fresh oil on the valve cover coming from my breather cap as before.

Later in the day, after it cooled down, started up, idled fine. After driving about 15 miles, really rough running again. Driving on the highway it started shuddering, my steering wheels was visibly shaking, any chance this is the transmission? I pulled over and adjusted my kick down cable, backed it off a bit, then some more later, didn't make any difference. I was thinking maybe I had set the kick down cable too aggressively, as it shifts pretty abruptly (I like that), so I adjusted for softer shifts, didn't make any difference. I read this old thread that made me think of this as a possible issue: http://www.slantsix.org/forum/viewtopic ... db7f15c367

The symptoms seemed similar to that described in that thread, but that doesn't explain fresh oil on valve cover, that's crank case pressure, seems like I have an issue related to combustion.

NOTE: The preignition or detonation was completely gone, and I'm not noticing any similar sound when this shuddering happens, it's been running very quietly. For example, I noticed all week that I could hear my turn signal where before I could not. But that doesn't mean there isn't some timing issue or something going on. I did note that my idle seems to have increased, like advancing the timing, but I didn't change it since last Sunday when I set it to TDC. Also, when I shut her down she's doing that run on/dieseling thing (recall I said that was gone when I changed to the colder plug, well it's back and I still have the colder plugs in).

When I get it home, if I can nurse it home 60 miles, the first thing I'll check is the timing, see if that's gone off, check the (second now) dizzy again, etc.

I'm stumped unless it really is the gas supply and the higher compression on (now only two) cylinders. Very frustrating, perplexing...

Brian

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I wonder if you have a broken ring land.

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Joshua

Started out from south bay with cold engine, runs fine for the first say 10-15 minutes on the highway, then it started running rough. Vibration felt like wobbly drive shaft or torque converter or band issue, but definitely in the motor. Puttered home at about 50 mph for about 30 miles, then it smoothed out for the rest of the way. There was a noticeable difference than when I first put the head on, it was super smooth up until this morning's episode, on the way home, after the shuddering smoothed out it I couldn't feel any vibration in the wheel, but at stop light when I pulled off the highway it felt like a misfire, but not a strong one. Climbed up the hill to my house no problem, so it hasn't lost much power to speak of, no more detonation ping. Certainly it feels like a misfire, something's out of sync. I figure it's burned up another head gasket or something.

I think I've been covering up an underlying issue. When I changed the head gasket and this time another head, new gasket, colder plugs etc. it ran great both times, for a few hundred miles, then bad again, so something's up that I haven't figured out yet.

I can't figure it running so roughly, then smoothing out, unless its fouling plugs and they clean out again with driving or something weird. I'll check the timing tomorrow, pull the plugs see what they look like after a few hundred miles.

brian

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