Well, I'm looking further into this nasty predetonation thing. I thought I'd take a look into the fuel/air mix. I'm not an expert but looking at this picture if anything I'm running a bit rich! Someone here was suggesting I might be somewhat lean. This is cyl #7 and it was mostly a nice dark, maybe the inside is a little light??? I'm not positive how to read these.

Waddya think?


[Modified by Rons67, 6:14 PM 6/20/2002]

 

Wouldn't want to say it was TOO lean, but it sure as hell ain't too rich! :eek:

 

The plug does not look too bad. If you are running a stock ignition system I would think the dark ring around the base of the plug is from oil (Picture is a little hard to see good detail). Sometimes the intensity of an aftermarket ignition will make a plug harder to read. What is your compression, timing. have you thought about a octane booster, if you have tried one did it help?

I don't think the plug is that bad, I would check a few more of them.

 

Yea, Stock ignition, and you're right there was just a trace of oil on the base of the plug. Also I've tried octane boost and 'bout all I can ever find is 104+ so that's what I use. Timing I've gone back to total advance of 36deg which leaves me at about 12deg BTDC. Comp Cams says that I might want to try 8BTDC. Number 5 looked pretty much the same. I'll look at some others. Compression is probably 10:1 based on the pistons we used.

Cold the engine does pretty good, but it'll still ping. As it warms up it's bad.

 

Does not look rich at all to me.
I bet you can get rid of it by playing with your timing.
Its amazing what can be done with junk fuel and a twist of the dist.

 

I would take a couple of degrees of timing out of it. There is a lead additive out of Utah that myself and a couple of other guys on the forum are using that is working well for us. One guy has a 12 to 1 motor and it works good for him. It is a lot cheaper than the over the counter stuff. If you are interested shoot me an email and I'll give you some info on it. :chevy

 

What rev range are you experience detonation - low, mid, high, or is it across the entire range.

Duke

 

Detonation is "generally" in the 1800-2500 range. But, again, with heat it goes on up the rev range. The other day coming back from BG I couldn't stomp on it AT ALL ( :cry ) when I was running near 200degF. So it's in the lower RPMs. Thanks for listening.

 

Do you have a Timing Light with a degree future. If not can you borrow one? I would detail your advance system in all aspects. Disconnect and plug the vacuum advance and check and see at what RPM your centrifugal advance is coming in at, and what RPM increments it increases and finally what RPM the total centrifugal is achieved. Next, tape the flyweights down and check the vacuum advance and detail (with a Mighty Vac) at what inches of Hg it starts to activate and the total achieved. If you have very light weights and you are getting full advance in at 1500 RPM this may be to soon for your application. This may help eliminate the timing issue and if the problem persists you can check other avenues.

As an example here is how my 65 327/365 breaks down.

Base Timing: 15 Degrees.
I disconnected the vacuum advance and plugged the carb off. I idled the car at 800 RPM and used my digital timing light and a timing tape on the balancer. It read 15 degrees advanced. I idled the car down to about 700 before it quit to ensure the flyweights were not activating.

Vacuum Advance: { readings are without base timing added}
@1"Hg to 4"Hg no advance.
@5"Hg, vacuum advance added 5 degrees of timing for a total of 5 degrees
@6"Hg, vacuum advance added another 5 degreed for a total of 10 degrees
@7"Hg, vacuum advance added another 6 degrees for a total of 16 degrees
@8"Hg, vacuum advance added another 4 degrees for a total of 20 degrees
I used a Might Vac with a gauge on it, hooked to the vacuum advance and took reading that way.
I recorded the amount of advance and the amount of vacuum pressure.
I ran the vacuum advance to a total of 15"Hg and no more advance was obtained after 8"Hg.

The Fly-Weights- Added 26 degrees
The fly-weights started to activate at around 900 rpm. With the vacuum advance disconnected, I revved the engine to 2400 RPM and had a total advance of 41 degrees. Subtract the 15 degrees for the base timing and I concluded the fly-weights were adding 26 degrees of advance.

Totals:
15 degrees base
20 degrees vacuum advance
26 degreed from fly-weights
total advance at 2500 RPM is 61 degrees.


Mark

 

Try going to a colder spark plug. What are you using now ?

 

I'll assume you have a small block. Best power is usually achieved with total WOT timing (initial plus centrifugal) set at 36 to 38 degrees. You should test for total timing as Mark explained. You can reduce the low rev detonation by reducing the initial to limit the total to about 32-34. If that doesn't cure it then install stiffer springs to slow down the centrifugal. This will reduce low end torque a bit, but you want to establish an igntion curve that keeps you out of signficant detonation most of the time.

It comes down to experimenting with each engine - takes some time and a well thought out test plan, but it's not expensive.

Detonation problems are typically in the lower end of the rev range. If an engine detonates all the way across the range you have to reduce total timing. If it only detonates at low revs, slowing the centrifugal curve a bit will usually cure the problem.

Detonation is primarily a low rev phenomenon because the last of the unburned charge is exposed to high temp and pressure for a longer period of time at low revs than at high revs. At all revs, combustion takes about the same amount of crank rotation to complete, and it typically ends at about the same number of degrees ATC than the total WOT timing, so if the plug fires at 36 BTC, combustion is complete at about 36 degrees ATC. At 2500 RPM combustion takes double the clock time as at 5000, and that's why there is more propensity to detonation at low revs.

Duke


[Modified by SWCDuke, 8:52 AM 6/21/2002]

 

Aren't there two different timing tag setups available for small blocks, with matching dampers? Could it be that you have a mis-match and timing isn't really what you think it is? Might want to find true TDC and see what the damper/tag is saying. 36 dgrees "shouldn't" be that much.

Could also try running some cleaner through the motor to make sure you don't have a lot of carbon buildup in it. I use Berryman Chem Tool, trickle through the motor while at high idle. It started life as a carbon remover for cleaning electric motors (yes, I'm that old) so will do a great job at cleaning carbon from the engine. Will also trash your plugs, so put in a fresh set when your done.

If your running unleaded fuel, than I'd say your plugs are rich. Unleaded plugs burn a lot "lighter" color than we remember from the leaded days. Look deep into the base of the insulator (with a light) to get the most accurate reading.

-Greg


[Modified by GregP, 12:35 PM 6/21/2002]