Hey George....

Sean · February 25, 2004, 04:24:38 PM

You run a ZZ4 don't you? What kind of fuel are you running in it? Mine isn't technically a ZZ4, but it should be pretty close and I can't get the spark knock out of it.

I filled my tank up(16gal) with 91 octane plus a can of that "Outlaw" octane booster. Noticed the spark knock right off the bat, so I decided the Outlaw stuff was a joke and added a can of the NOS "off-road" on top of it. Still getting spark knock.

We had the timing set at 12 degrees initial originally, which is where my buddy ran it in his Dirt Car. He was running Race fuel though, so after hearing the knock, I decided to do it by hand. I have been messing with it for two days now and if I back the timing off far enough to get the spark Knock out, it severely cuts the power back on the engine.

I think the most compression I have ever run on anything was 9.5:1 and that ran fine on premium. Does that extra .5 on compression make that much difference? I am wondering now if his compression calculations were off and it has more than 10:1 compression. Or am I missing something else?

It only seems to do it at part throttle, under a load. Would heavier springs in the distributor get rid of it? I am running the "Medium" set out of the three that Pertronix sent with the HEI kit.

purplepickup · February 25, 2004, 05:23:16 PM

I run 92 or 93 octane. When I'm out west where the octane of premium is lower, I get pretty bad spark knock. ZZ4 compression is 10:1 but mine is a little higher because I've taken .010" off my heads. I used to run 12 degrees spark advance but I've had to back that off to around 8 degrees.

There's a couple things about your engine that might be working against you. Your pistons are flat top and ZZ are dished, & your heads are cast iron with straight plugs and ZZ are aluminum with angle plugs. Your compression might be higher than you think and aluminum angle plug heads can handle more compression without detonation. Your distributor might have a different curve...not sure what yours is. At one time you said you timed by ear and you might have to back off the timing a little. You need to get real fussy (use a light) because detonation will cause big problems if you keep driving it that way. A light that shows your total advance would be nice. Around 32-34 degrees total is about all you want. If it knocks, back it off a couple.

One other thing to check is your fuel mixture. Too lean can cause detonation too.

Sean · February 25, 2004, 06:02:55 PM

We did use a light to set the timing at first and forgot to unhook the Vacuum advance while doing it. It didn't have any spark knock, but it didn't have much grunt either. When we put a light back on it after realising what we had done, the initial advance was just barely over 0 degrees.

After setting it at 12 degrees initial, it turned into a real Hog. When I hammer on it It mashes me back hard enough to feel the seat springs bottom out on the fuel tank behind the seat. I was pretty happy with it until I got the exhaust put on and was able hear the spark knock. I guess I'll put a light back on it and start backing it off 1 degree at a time until I get the knock out.

91 octane is the highest I can get locally, other than AV gas, or buying Racing fuel at the dirt track. The NOS Octane booster claims to raise the octane 7 points. Does that make 98 octane, or 91.7?

Sean · February 25, 2004, 08:57:29 PM

I am beginning to wonder if it is spark knock that I am hearing. I retarded the timing back to around 5 degrees initial and it did not lessen the rattling sound at all. It is not a loud sound, just a very light rattling, almost un-noticeable. It only does it under light acceleration. Under hard acceleration it doesn't seem to make the sound. Or maybe I just can't hear it anymore.

The guy I bought the Engine from told me right off the bat that it made noise when it was cold because of the pistons. He has another engine, with the same Pistons and Rings, but different Cam and Heads that makes the same rattling noise. He said it was because of the Pistons not having full skirts like a normal piston. But, he also said it went away as soon as it got oil through the engine.

Somebody over on the HAMB mentioned GM having a problem with their newer engines. Something about the Hyperutectic pistons making a rattling sound at certain rpm's. I wonder if the combination of the un-skirted vette pistons with the low tension rings is making the noise I am hearing?

I guess tomorrow I will back it off even more and see what it does.

enjenjo · February 26, 2004, 12:45:09 AM

I've had the same problem on a late model SBC, it was the ignition curve. On OE setups, they have a knock sensor that will keep the engine just out of detonation. With an older style ignition the advance curve is such that it will knock at part throttle under load. There are two ways to easily cure it, depending on how bad it is, and when it knocks.

If it only knocks in high gear, not in first or second, you can install a high gear spark retard, it's just a vacuum solenoid that cuts off the vacuum advance in high gear. From experience, it doesn't seem to hurt acceleration or mileage, many engines will run faster with the spark retarded to some extent in top gear. The solenoid I use comes from the scrap yard, I believe off a early 90's Olds.

The second way, a bit more expensive, Jacobs I believe makes a electronic spark control that has provision for a knock sensor. This retards the spark when needed, but puts the advance back in when the engine can handle it.

Pope Downunder · February 26, 2004, 05:27:49 AM

Quote from: "Sean"I am beginning to wonder if it is spark knock that I am hearing. I retarded the timing back to around 5 degrees initial and it did not lessen the rattling sound at all. It is not a loud sound, just a very light rattling, almost un-noticeable. It only does it under light acceleration. Under hard acceleration it doesn't seem to make the sound. Or maybe I just can't hear it anymore.

>>>

If it's piston skirt rattle it will be very noticeable at idle. My 283 has this also, so I know exactly what it sounds like (cluck, cluck, cluck!). This sound in yours sounds like pinking to me. I had a similar problem with the 283 when I first got it going. (It was up there at around 10:1). My problem was the ignition curve. The vacuum canister was far too strong. At light throttle, and on trailing throttle under load, like easing out approaching a crest, it would rattle. It also had a stumble, but under full throttle went OK. I got the dizzie recurved and no problems.

Sean · February 26, 2004, 08:17:49 AM

The Vacuum advance that came with my Pertronix Kit is adjustable. Would that help anything?

I never messed with it, but from what I understand you stick the supplied allen wrench in the vacuum hole and turn a screw inside of it.

enjenjo · February 26, 2004, 11:14:00 AM

Changing the advance curve may help, try it.

Sean · February 26, 2004, 12:36:02 PM

Quote from: "enjenjo"Changing the advance curve may help, try it.

I looked through all my parts boxes and apparently I lost the instructions that cam with the Pertronix Kit. Anybody know what to do with the Adjustable Vacuum advance?

I looked on the net and found this site that explains timing pretty good, and mentions the adjustable advance Can, but doesn't say how to adjust it. Pretty good article though.

http://www.chevyhiperformance.com/howto/97438/

Sean · February 26, 2004, 02:48:20 PM

I went and bought a Curve Kit and put the heaviest set of springs in my Distributor. Still rattling. Backed the timing off to 0 and along with running like #$@# it still rattles slightly.

I turned the screw inside the vacuum advance 3 turns in both directions and neither way made any noticeable difference. Switched the Vacuum advance from "full" vacuum to "ported", that made it run a little better, but didn't affect the rattle.

I poured a gallon of xylene in the fuel tank, which according to the internet should have bumped me up to just over 93 octane. Didn't change anything other than the smell of the exhaust. And that is on top of 2 cans of store bought octane booster, so the octane should be well over 93 by now.

I guess i'll run half this fuel out and add 1/2 tank of Turbo Blue to it. I think its 115 octane. Xylene is supposedly 117 Octane but it isn't any cheaper than racing fuel, even if you buy it in a 5 gallon can. With Turbo Blue at 115, I should only need 3 or 4 gallons mixed with 91 octane in my 16 gallon tank to get the octane up in the Mid 90's. That is still going to be expensive as a daily driver considering it only gets about 10 mpg.

Sean · February 26, 2004, 10:34:04 PM

I don't think its an Octane issue. I added 8 Gallons of Race Fuel to the Tank late this afternoon, drove it about 15 miles back to my house and it was still making the rattling noise. I ended up getting something called Track Tech. I never heard of it before but its 111 octane.

With 8 gallons of 111, and approximately 8 gallons of the 91 and octane booster that I was running earlier, the Octane should be plenty high enough for 10:1 Compression.

I have the timing set at 10 degrees initial. The heavy springs in the curve kit I put in this afternoon are supposed to come on at 2500 and limit the mechanical advance to about 12-13 degrees. At this point, I have no idea whats making the noise. It doesn't seem to affect the way it runs, so I'm just going to run it and quit worrying about it.