Hotter plugs or colder plugs for spark knocking?

[red69ss]

trying to resolve some spark knocking issues ,tried 93 octane and adjusting the timing . I'm thinking I might have to change springs in distributor? thouight I would try changing plugs first so hotter or colder? also I read somewhere that if the engines running lean this can cause spark knocking anyone experienced this?

[Sinister]

Try a step colder on the plugs. If your lean that can contribute to spark knock.

[drewdude92]

colder

[drewdude92]

also retard timing a bit

[Jetmech727]

How much compression are you running? If it is over 10.5:1 you may want to think about running e85 you will have to do some simple mods but it is 100 octain. retard timing is about the only thing you can do. Should be not more than 38 degrees total timing. Probably work better around 32 degrees. Also you can get away with about a 40/60 mix of e85. 60 gas 40 e85 with no mods. Hope that helps.

[red69ss]

thanks for the replies guys I believe I will try the colder plugs first. jetmech I am running at about 30 degrees total timing now and I've run it from 5 to 45 degrees and nothing helped. I had a friend suggest running a few gallons of 110 octane in it to eliminate it being fuel issues might try this. and I have no idea what compression it is motor was in car when I bought it.

[LONGHAIR]

Originally Posted by Jetmech727

Also you can get away with about a 40/60 mix of e85. 60 gas 40 e85 with no mods. Hope that helps.

I seriously doubt that, for several reasons.
First E-85 is 85% alcohol, so if you cut is with gasoline you still end-up with an "unpredictable blend" of around 60-65%. That is a real mess...

It is way too much alcohol for normal gasoline jetting and you are totally killing the octane rating of the alcohol itself by cutting it. Alcohol also has a much lower BTU rating, which is why it takes so much more to produce similar power, so the jetting must be changed to allow more flow to compensate. While this may indeed be possible, it is not practical. You would have to jet for a particular blend and then be able to consistantly maintain that mix. You might get by with that for short-run commuting purposes, where you mix your fuel at home, but buying fuel on the road?


red69ss, is this a converted HEI? if so, do you know the origin of the distributor? Is this a recent problem? if so, what changed?
In short....tell us more about what you have and what driving conditions casue the spark knock.

[mr48chev]

Also have you tried changing brands of gas or stations?
There are brands that I will not put in my rigs except as a last resort and one of those belongs to a popular chain that has mini marts with lots of gas pumps.

I also don't buy gas at those old mom and pop stations no matter how quaint or cool they look. Old tanks that may have crud or water in them.

I doubt that the problem is carbon on the tops of the pistons but if you lug it around town an it acts up when you get on it that might be one cause.

You didn't say what engine it has but there is always the possibility that the engine has a compression ratio that is higher than todays gas can deal with even with everything set right.

[red69ss]

longhair it is converted to hei its in a 72 lemans with a 400 pontiac motor I've had the car about 3 weeks now and it was doing it when I bought it ,I asked the guy if he ran 87 octane in it he said yes so I suspected this was the problem ,(I have to run 93 in my other 2 hot rods or they spark knock also). so after filling up with 93 at about 4 different stations and even adding octane booster had same results. the guy put this dist. in last year when he rebuilt the engine not sure where he purchased it.as far as driving conditions it does it when under strain (uphill) and some just at cruising speed but when you get on it it clears up, this is why I'm thinking it might be an advancement problem . mr48chev I have no idea what comp. it has but I would guess 10:1 or less ,but I could get in touch with the guy I bought it from for more info if I can't resolve it.....................today I pulled the plugs and changed them to an AC 45t plugs, and the wires on it are junk $2 parts house wires 2 of the clamps stayed on the plugs when I pulled them so I ordered some msd wires from jegs tonight and a advance curve kit just in case I need it. I put 2 wires I had laying around back on it and test drove it and it didn't seem as bad so hopefully the wires are the issue either way they needed to be changed, thanks for the help guys I'm certainly no master mechanic and I hate throwing parts at a problem till I get the right one so this is what I'm trying to avoid.

[racin69z]

Is your vacuum advance hooked up? If so, is it hooked to a ported vacuum or manifold vacuum? If its hooked to straight manifold vacuum, try a ported vacuum source. Or just un hook the vacuum advance (if its hooked up) and see if the knock goes away.

Lynn

[red69ss]

its hooked to the carb, I checked and its got vaccum I'll try unhooking it and see if it runs better... thanks

[racin69z]

Most carbs have both ported and full vacuum, depending on where you hook it. You will have to un hook it at idle and see how much its pulling to know just what you have.

Good luck
Lynn

[LONGHAIR]

Originally Posted by red69ss

longhair it is converted to hei its in a 72 lemans with a 400 pontiac motor

Okay, I would never have guessed that....

The important thing is not "what it is in" but what the distributor "came out of"...mostly because of the year.
I am not familiar enough with Pontiacs to know when they switched over to HEI, but I would imagine that it was around the same time as Chevy? The thing I really am not sure about though is the advance curve for these? Chevy did some "funny stuff" in the late 70s to early 80s with odd advance curves and "ported vacuum" advance units. The problem is that these parts are not just "drop-in" compatible. They will fit, but that is not good enough. The advance curve from those odd early "emissions years" when mixed with full vacuum and more initial timing.....will cause spark knock problems like you discribe.

[red69ss]

looked through the reciepts I got with the car last night and its a summit brand pontiac hei dist. that was put in last june it stated on thier website it comes with medium springs in it. hopefully I can adjust this dist and make it work.......after reading the installation instructions on summits website I believe its the vaccum advance that needs adjusting it talks about spark knock if not adjusted properly and how to resolve this ,so I think I'll try that,just not sure if I should wait till I get the new wires on it?

[dwcsr]

Originally Posted by LONGHAIR

Okay, I would never have guessed that....

The important thing is not "what it is in" but what the distributor "came out of"...mostly because of the year.
I am not familiar enough with Pontiacs to know when they switched over to HEI, but I would imagine that it was around the same time as Chevy? The thing I really am not sure about though is the advance curve for these? Chevy did some "funny stuff" in the late 70s to early 80s with odd advance curves and "ported vacuum" advance units. The problem is that these parts are not just "drop-in" compatible. They will fit, but that is not good enough. The advance curve from those odd early "emissions years" when mixed with full vacuum and more initial timing.....will cause spark knock problems like you discribe.

I had this same issue and did all the things you did. It turned out that the major gas supplier in our area was dropping regular in the 93 tank. Once the tv stations go wind of it and Texas now test gas for octane level along with testing the pumps. My knock majically went away.

Octant booster I learnd only bumps you up about 1 to 2 points so if you start at 87 you still only get to 89. I stared wit 89 and still only got 89 because the 93 which was really 87 is blended with the 87 tank.

Some times octane booster will decrease your total octane. So start with 87 and drop in 2 bottles and see what haqppens. If it goes away its the gas.

[LONGHAIR]

Originally Posted by red69ss

looked through the reciepts I got with the car last night and its a summit brand pontiac hei dist.

That's a good thing, it's the "un-known" junkyard units that are hard to deal with. It may indeed need some tuning to get it right though.

[red69ss]

so I turned the vaccum adv. counterclockwise 5 rounds since the instructions said to turn it that way till it locks to start and it is running much better and idling better, still doing it a little under hard strain so I'm going to try a couple more rounds and see if that clears it up or may have to adjust the timing some also, but I think I'm going in the right direction now hopefully.

[martindc1]

I was thinking it was something to do with either your vacuum advance or your carb. When your engine is under a load vacuum decreases. This in turn should retard your timing and richen your mixture. If this doesn't happen, you will get spark knock. Ported vacuum and manifold vacuum for your vacuum advance have no difference above idle. It will run better at idle with manifold vacuum though. When you set your timing you have to unhook your vacuum advance.

If, for instance, you run an Edelbrock carb, a stronger step up spring will make it run more rich under a load.