Pinging question...

[ChevyThunder]

OK I have a good one here... :) I have a small block 400(406) that I'm having a little
problem with pinging on warmer days under moderate to heavy load,
usually 75 degrees or more. yesterday was 87 degrees and it was
pretty bad.
The engine was rebuilt last year,
-.030" over
-9.62:1 compression,
-stock crank and rods
-Mildly ported iron 400 heads
-Lunati cam 224/224 duration at .050",
-Edelbrock performer intake and 600cfm performer carb.
-The ignition timing is set to 4 degrees with a total of 28 degrees of advance at 3000 rpm.
-stock HEI distributor with a new vacuum advance unit.
It has an adjustable timing gear that I believe is set at 4 degrees advanced. I've been using 93 octane fuel but there seems to be very little difference. The engine is lucky if it sees 180 degrees operating temp. even when it's real hot out.. I'm about at my wits' end with this thing, If I back off the ignition timing so it doesn't ping, the motor barely runs. :( ... it seems kind of weak now but at least it's liveable. What I was wondering was if the cam timing being advanced 4 degrees will cause something like this to happen.... Tomorrow I'm going to install a air/fuel gauge to see if it's running too lean but if it's not then i don't know what else to do... :confused:

Any insight on this would be appreciated.
Thanks in advance,
Jason

[dq409]

Jason, Running too lean will cause this so I think your headed in the right direction.

Your carb book should have all the info you need once you determine what the mixture is now. You can also find that info on Edelbrock`s website.

[jim_ss409]

Advancing the cam will increase the likelyhood detonation but not all that much. You might consider doing a compression test, even if you only check one cylinder. If you've got much over 190psi. you're getting into the thin ice area where your timing and fuel-air ratio will have to be pretty right on in order to avoid detonation. As DQ said that Edelbrock book will give you the nessissary info to set your carb. You could also try heavier springs in the dist. so that the total advance didn't occur untill about 3500 or 4000rpm. but with only 28deg. total advance (36deg. or more is comon) I'm also thinking that you're running too lean.

[jim_ss409]

I re-read your post and noticed that you were going to install fuel-air gauge. Let us know how well it works. I think that jets and or metering rods will solve your problem and they aren't that hard to change once you get into it. You will probably end up with an engine that runs better than ever. :cheers

[fatride]

When the engine was built did the rebuilder pay attentoin to the quench distance? too much quench distance will result in detonation no matter what the air fuel ratio and will not respond too timing adjusments or octain rateings.
;)

[MRHP]

Try dicsonnecting your vac advance and see if that helps. If it does, it is coming on too soon or too much. A 160 thermostat should help some. I would think 32 to 34 degrees of full advance would be about right. Which vac port do you have the vac advance port on? I find that installing full vac to the advance diaphram will give you extra advance at idle which helps clean up the idle and under acceleration the advance is taken away for full power. Hope this helps! :D

[CDNpontiac409guy]

Jason, the guys have just about everything covered :deal :cheers

What I'm wondering is.... At what RPM is the pinging going on ?
I LOATH the GM HEI distributor. Unless the cetrifugal advance mechanism has had a limiting "stop" put it in, it will invariably over advance as you rev the engine. You may have 28 degrees at 3000, but you could have 38 at 3500. Add the vacuum advance to the equation, and you may have your answer.

Also, the timing running that far behind ( 34-38 being average ) will somtimes cause th ecombustion chambers to run hotter. Detonation being the result.

[Southtowns27]

What do your sparkplugs look like?

[dq409]

Jason, I just got an email from Dave about your problem and this is what he recommends :

Use center plate marked 375, early 80's 350 Suburban
Use weights marked 41

Same settings used in crate, ZZ series small and big block dists, 350 thru 572.

This setup will give 21/22 crankshaft degrees, with a 10 degree initial, 31/32 total mechanical degrees. Spring to start curve 100 rpms over out of gear idle speed, limit to between 2,600 to 3,000, whatever feels best.

Stock vacuum advance, add stop for 8 crankshaft degrees, .089 drill bit is the right distance.
Connect vacuum advance to full manifold vacuum after setting initial.

He can try 10 or 12 degrees initial.

This should fix him up on the pinging


Boy we miss you Dave,,,,, Come on back !! ,,,, dq

[ChevyThunder]

ok I got the mixture gauge in and working and the mixture seems ok... a little on the rich side but not too bad.... towards the high side of stoich cruising and goes a little into the rich side on hard accel. It wasn't all that warm outside when i was driving it so it didn't ping at all. It's supposed to be around 80 degrees tomorrow and sunday so I'll mess with it more this weekend if it doesn't rain. I checked the compression and I got about 165-170 at operating temp with all the plugs removed.. I think the vacuum advance might be moving too far. With it disconnected it definatly does not ping but it seems to run real crappy. The 28 degrees of total timing was with the vacuum disconnected and it doesn't go any farther at higher rpm's... I checked it again this time at 3500 and still 28. I'll try adding a stop so I have less vacuum advance and see what that does. It seems to only ping with ambient temp above 70 and between 2000-2400 rpm on a light incline or during normal accel. Also I don't know if it means much but this car is absolutely horrible on gas, MAYBE 11mpg on a good day, and it's really not that faster than the same car with a 275hp 350.... I'll see what happens this weekend I'm gonna mess with the timing curve and the vacuum advance a little... The kit is about $8 for weights and springs and about $12 for an adjustible vacuum advance unit, so I figure it's worth a try... I'll post back with the results...... :)

[ChevyThunder]

I had a little more time to spend tinkering with this thing today... I had another distributer with the center plate and weights that dq409 had mentioned so I tried using that distributer and it seemed to run better(a lot better) but it still was pinging around 22-2500 but now only with a light load.. It turned out that the vacuum advance was giving me almost 25 extra degrees when it was plugged in.. :eek: . I made a small blocking plate much like the one Summit gives you with their adjustible vacuum diaphram and I got it down to about 10 degrees of vacuum advance. I set the base timing at 12 degrees and connected the vacuum hose to full vacuum and bada-bing.... no more pinging.. :) . Thanks everyone for your help.. I couldn't have done it without you.. :D

[fatride]

Ahhh, success! I'm happy that you could get the detonation under control without taking the engine apart.

[dq409]

IgnitionMans sharp !!! Isn`t he !!?? Come back Dave,,,, WE LUV YOU MAN !!! :D :bow :D