BangScreech4-4-2

Hey, guys. Little help?


I have a '69 442 convertible (see avatar) with a pesky spark knock.


Let me back up a little. The engine is the original matching-numbers 400 G-block. It has about 6000 miles on a high-quality rebuild and I had the original Q-Jet rebuilt at the same time. The engine was done to factory specs with two exceptions: an 0.030 overbore and a Comp Cams camshaft (IIRC 218/225 @ 0.050). Oh, and I retained the mid-70s HEI the car had when I bought it.


This thing is smooth and powerful and pulls beautifully up to 5200-5300 RPM which is about as fast as I ever spin it. Starts right up after sitting all winter after about five seconds of cranking. All in all, it's been as good as a 400G could be. Only problem is, the last couple of summers, I've been getting some part throttle detonation, and I can't seem to shake it.


It only occurs when the engine is fully warm (like after at least 10-12 miles) and only at about 1/3-1/2 throttle -- it's fine once the secondaries open. At this point, I should add that I'm using Chevron 94 pump octane plus a bottle and a half of Lucas octane booster per tank.


The part-throttle aspect of the problem led me to suspect the vacuum advance, so last spring I put in an Accel adjustable canister. Because of the bulk of the HEI unit I have to remove the canister to adjust it as the hole where you stick the Allen key bumps right up against the firewall, so I initially installed it as it came out of the box -- mid-setting. This made no difference whatsoever, so I removed it and backed it off all the way. Again no difference, either in the spark knock or in the car's drivability -- I'm beginning to think vacuum advances are just a placebo!


I've tried two different spark plugs -- I went from the Accel 437 plugs (these COULD have started the problem, but it's difficult to pinpoint exactly when the knock developed as I only drive the car 6-7 months of the year) I had on there to an AC R44. This may have made a slight improvement; it's hard to tell. Does anybody make a colder plug?


So there it is. I'm sure I'm not the only guy experiencing this, but I couldn't find another thread that addressed this exact topic. Whaddya think?

oldcutlass

Lets get a little more technical. What is your total timing without vacuum advance, what is it with? Without having to go back and review all your threads, can you provide some specs for your engine, compression, cam, pistons, carb, gearing, etc.

As far as the position of your distributor, you need to restab it at tdc and get your advance canister away from the firewall.

MDchanic

This sounds exactly like a problem of too much advance at part-throttle.

As Eric said, we need more information, especially current timing specs.
It would also be good to know the mechanical advance at idle, and every 500 RPM up from there until it stops advancing.

Have you or anyone else ever changed the springs or weights in the distributor?

Do you have a vacuum gauge on the dashboard?

As Eric said, the first order of business is to re-stab the distributor so that you have enough clearance to properly adjust the timing and the advance, then work from there to reduce the amount of vacuum advance to the level of "just enough."

It is a very good idea to map out your vacuum advance, as well as your centrifugal advance, so that you can really see what's going on in there.

- Eric

BangScreech4-4-2

Thanks, guys. As I said, the engine was built pretty much to factory specs. Rather than post right now with just the details that I know in my head, I'll post again with as much specific detail as I can get as soon as possible.

MDchanic

... Except that an HEI distributor is very definitely not to factory specs, so that if the engine really is basically factory spec, the distributor is set up wrong for it, and if it is not, then... who knows?

This is why many of us advocate keeping the factory distributor.

- Eric

rustyroger

It could be as simple as weak springs in the distributor, allowing total advance in too early. How old is the distributor?.

Roger.

coppercutlass

Fwiw. My current set up really likes a lot of initial timing. 20 degrees. You get a lot of people who say the hei's work well with a lot initial but in reality most stock set ups where lucky to have more than 10 degrees initial. The stock advance had sometimes up to 23 degrees of total mechanical advance . So I you have more than 10 degrees initial you have wayyyy too much timing . That's just some food for though. There is a method I posted on how to check the total mechanical advance on a bench . The hei I checked If I remembered correct it had like 18 mech adv. but it was pretty crusty and probably wasn't properly advancing.

BangScreech4-4-2

'




Yes, well, it had the HEI when I got it, so that ship has sort of sailed.


Anyway, thanks for the help so far, everyone. I will come back with further specs and info as soon as I get a chance to go home and dig into my file.

BangScreech4-4-2

It's from the mid-70s and its history between then and the time I found it under my hood is unknown. I'm thinking it must be basically healthy because the engine runs so well (aside from the part-throttle detonation), but as Eric pointed out, who knows?

BangScreech4-4-2

Thanks. I will look for that thread.

coppercutlass

If not I can post up how to.

don71

This would be the first thing I would look into. HEI or points regardless.

Total mechanical advance, and how fast in comes in. At least one stiffer spring might cure it. Look for it to occur before or about 3000 rpm.

Vacuum advance occurs after that and should not me included in your figures.

oldcutlass

Most Hei's have about 16-18 deg's mechanical advance. Which is why most can get away with around 18-20 initial which will give 36-38 total without vacuum. Where the problem shows up is with the stock vacuum canister, its will run the total with vacuum way over 50. Generally when you get over 50 you get the symptoms the OP described.

BangScreech4-4-2

Okay, guys. Further details were requested, so here's what I've got:


-- The engine is a 400 G-block, rebuilt to factory specs, except for a Comp Cams XE 262H (218/224 @ 0.050, 0.462/0.469 lift [w/1.5 rockers], LS 110 degrees). It has Egge 0.030 cast pistons which should retain the factory 10.5:1 CR (less whatever the overbore takes away from that).


-- As discussed previously, there is an HEI of unknown vintage replacing the original distributor. At the time the engine was rebuilt, the builder saw no reason why we should change it. Timing is about 16 degrees initial, with another 18 in the distributor for 34 total (exclusive of vacuum). It appears to all be in just under 3000. The vacuum advance canister is an Accel unit set for minimum advance.


-- Trans is a Turbo 400.


-- Rear is a 3.08 Anti-Spin.


-- Tires are BFG T/A 235-60-14.


-- Carb is the original QuadraJet.

If there's anything I missed, please, post on up. I welcome any and all input or opinion. Thanks for your time!


-- Andy.

BangScreech4-4-2

Thanks, Eric.


-- Distributor springs have not been changed since the engine rebuild ten years (6000 miles) ago. They seem pretty snappy. The detonation problem developed two years ago.


-- No vacuum gauge in the car, but I do have a hand-held unit.


-- I have plenty of room to adjust the static and distributor timing; the canister just hast to be removed to adjust the vacuum advance, as there's not much clearance between the big HEI housing and the firewall. No biggie.


-- How would I go about mapping out the advances?

oldcutlass

What is the total advance with vacuum advance connected?

R-body_mopar

"It only occurs when the engine is fully warm (like after at least 10-12 miles) and only at about 1/3-1/2 throttle -- it's fine once the secondaries open. At this point, I should add that I'm using Chevron 94 pump octane plus a bottle and a half of Lucas octane booster per tank."



I'm just going to throw this out there, as I owned two bone stock '70 442's with 455 HC way back when; I always used Sunoco 260 leaded pump gas, which was advertised at 103.5 octane at the time.

If your engine is basically stock compression (10.5/1), it will require much more than 94 octane to prevent detonation, in my opinion. Couple that with the fact that today's pump fuel contains at least 10 percent ethanol (which requires a richer mixture), you may need to also increase jetting size to give it a richer mixture. And octane booster is basically just adding MORE alcohol to the already leaned-out fuel, not really a solution to the problem of TOO LOW octane fuel.

I gave up trying to drive my cars daily, as it was too much of a headache trying to mix Sunoco leaded Cam II racing fuel (or Avgas) with the lowered octane of the recently introduced unleaded low octane gas of the mid '70's. I was NOT a happy camper, at this point!

I know it's been a long time since then.......BUT....that was MY experience, for what it's worth!!

Sorry, just throwing this out there as a thought; I don't really know what all you people out there with older high compression engines do about the fuel situation, with lack of octane (and lead as a lubricant) ??? To say nothing about the corrosive properties of ethanol in older fuel systems.....BUT....that's another subject!

oldcutlass

We have less problems than you think. I drive mine mostly everyday year round with no issues. The only additives I use periodically are a pint of Marvel Mystery Oil to my fuel about 4 times a year and a qt to my oil 30 days prior to an oil change. I also run a can a seafoam in the tank twice a year.

Fun71

I don't even do any of that, just fill it with the 10% ethanol 91 octane fuel that is all we have available and drive.

BangScreech4-4-2

What's your compression ratio?

BangScreech4-4-2

52 degrees.

BangScreech4-4-2

Well, the Chevron 94 around here is ethanol-free, so that's not a worry. And I had the heads redone with hardened seats at the time of the rebuild, so neither is that. Now if I could just get rid of this damned pinging before the engine explodes ...

R-body_mopar

Like I said before, I gave up trying to run my '70 on unleaded low octane premium fuel many years ago, after struggling with mixing racing gas, Avgas, RealLead tetraethyl additive, etc.; just too expensive, and too much of a PIA!

Have you tried disconnecting and plugging the vacuum advance?? You may be able to run it without vacuum advance, and bump up initial timing a slight amount; trial and error, you know the drill....

And I agree, you are right; pinging WILL do lots of damage, even if it is not severe. Back in the day, I used to run colder Champion racing plugs, but can't remember the number (J63Y??or ?), but this was more to avoid the lead glazing of the porcelain on the J10Y's (UJ10Y's worked better, but probably hard to find today.....!), than detonation; high lead levels in the fuel would glaze the porcelain, and short out the plugs at high rpm's, making them useless without sandblasting/cleaning every 500-1000 mi. This was particularly annoying when daily driving the vehicle, and only nailing the gas on short bursts, after street/city driving....plugs would glaze in a flash.....

oldcutlass

I bet if you disconnect/ plug the line to your vacuum adv can and do a test drive you'll find your problem will go away. If it does, then limit your total with vacuum advance to around 48.

Fun71

That's a good question, but unfortunately I don't know for sure.
Factory heads, .030" over 6 cc dish Badger "10.25:1 replacement" pistons, Fel-Pro head gaskets.
Unknown head chamber volume, unknown piston to deck clearance, so unfortunately unknown compression ratio. It runs OK on 91 octane after I limited the HEI vacuum advance canister's original 24* down to 12* .

Oh yeah, it also has a 217/221 cam so the intake valve closing is much later than factory so the cylinder pressure (dynamic compression ratio) is less than it would be with the factory cam.

I have a question: Does the adjustment you made on the vacuum canister limit the total amount of advance or does it speed up/slow down the rate of advance? It really sounds like you need to limit the amount of part throttle advance.

I am unfamiliar with the adjustable canisters; I modified my original to limit the amount of total advance.

rustyroger

How much mechanical advance do you have at lower engine speeds, particularly at whatever speed you experience spark knock?.
It might be useful to measure mechanical advance over a range of engine speeds. If all the mechanical advance comes in too early perhaps the distributor needs re-calibrating.

Roger.

BangScreech4-4-2

Thanks for all the input, guys. I did in fact seal off the vacuum advance when the problem first surfaced and that definitely helped. I just can't help thinking that the car was designed to run with vacuum advance, so I should use it. But then again, it was also designed to run on 103 octane gas, so there's that ...


I guess I'll take a multi-pronged trial-and-error approach, you know, disconnect vacuum advance, fiddle with static, see where that gets me. At the same time, I'll see if I can find some colder plugs and maybe try some different springs in the distributor. If all that fails, I'll try to find somebody around here who knows what a distributor machine is and get the thing curved properly.

BangScreech4-4-2

My understanding is that it just limits the amount of total advance. There's no adjustment for rate of advance or where in the curve it comes in.

BangScreech4-4-2

Thanks, Roger. I will undoubtedly be spending lots of time with the timing light this spring.


But I thought that, all things being equal, the earlier the advance comes in, the better.

rustyroger

No.

Optimum timing varies with engine speed and manifold vacuum.

The faster an engine runs the earlier (more advanced) the ignition spark needs to be. The combustion flame travel takes time. It should be at maximum pressure at tdc. If it builds up to early it will try to force the piston into reversing its travel before tdc. That's what spark knock is, if it is way too advanced it can smash rings and piston lands, even knock a hole in a piston crown. Too late (retarded) will make the engine inefficient.
Your engine timing curve should advance as a function of engine speed, ideally it should always be just before spark knock occurs.
Some Audi and VW engines gave figures for the advance acceptable on various engines as rpm increased in the UK spec service manuals.
Some ecu's on British Austin Montego's and Maestros simply advanced the timing until a sensor in the block detected spark knock, then it would retard the timing a bit and advance it again.this happened several times a second, and worked surprisingly well. With the engine at idle you could tap the block with a wrench or small hammer and fool the ecu into retarding the ignition enough to stall the engine.
Early gasoline engines had manual advance and retard levers, but by the '30s most engines had automatic mechanically controlled advance distributors. Some air cooled VW engines just had a vacuum advance, fine for a low compression, low output engine, but not relevant here.

Roger.

BangScreech4-4-2

Thanks for the info!

billmerbach

I may have missed it in there but excuse my dumbness whatbspark knock? Is it like a miss or what?

BangScreech4-4-2

Detonation, AKA "pinging" or "preignition" or "spark knock". You most often get it when you combine high compression with low-octane gas. It's very, very bad.

rustyroger

I refer you to post #30. I'm not showing off, I was quoting from a guy with a bunch of engineering degrees who taught an engine management course I took years ago.

Roger

rocketraider

Best description I know is it sounds like popcorn popping or marbles rattling in a can. Usually very pronounced under load. Left uncorrected it can blow holes in piston crowns.

I was always taught that a very light detonation under hard part-throttle acceleration meant the engine was operating at peak efficiency, but I hate the sound of it because it makes me think the engine's getting ready to grenade.

This modern crap gasoline doesn't help; as pointed out ethanol leans the fuel curve and fuel too lean is as much a cause of detonation as screwy distributor advance.

billmerbach

Ok that helps sorry I missed it