Detonation Issue
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Detonation Issue
I have developed a really bad detonation issue this summer. Here are the specs:
1972 CE block - replaced original L46 motor
Correct 186 cylinder heads
Replacement 7029207 Quadrajet - Tuned by Lars 2012
Timing all-in @2600, set at 30°
Initial timing at 14°
I do not know the internals of the CE block, but for a stock 350 you cannot get higher than the 11:1 L46. I had started at 36° but have kept backing off until I am down to 30°. The detonation is still pretty severe - light pressure on the gas pedal at 1700 RPM in 4th gear gets that motor rattling.
Or have I misdiagnosed this as detonation ?
1972 CE block - replaced original L46 motor
Correct 186 cylinder heads
Replacement 7029207 Quadrajet - Tuned by Lars 2012
Timing all-in @2600, set at 30°
Initial timing at 14°
I do not know the internals of the CE block, but for a stock 350 you cannot get higher than the 11:1 L46. I had started at 36° but have kept backing off until I am down to 30°. The detonation is still pretty severe - light pressure on the gas pedal at 1700 RPM in 4th gear gets that motor rattling.
Or have I misdiagnosed this as detonation ?
Pro
Joined: May 2010
Posts: 737
Likes: 3
From: Pennington NJ
I could be way off, but if the age and condition of the harmonic balancer is of question that may be causing timing issues. I'm sure you know this but the elastomer between the two parts deteriorates as it ages possibly causing the two halves to "shift". This can cause the timing to be off and also cause vibration issues.
Check the balancer for the rubber coming out between the halves.
I just pulled my original motor and the balancer is way clapped out, to the point where I'm sure my timing was off.
Something to consider, my $.02
Will
Check the balancer for the rubber coming out between the halves.
I just pulled my original motor and the balancer is way clapped out, to the point where I'm sure my timing was off.
Something to consider, my $.02
Will
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
I could be way off, but if the age and condition of the harmonic balancer is of question that may be causing timing issues. I'm sure you know this but the elastomer between the two parts deteriorates as it ages possibly causing the two halves to "shift". This can cause the timing to be off and also cause vibration issues.
Check the balancer for the rubber coming out between the halves.
I just pulled my original motor and the balancer is way clapped out, to the point where I'm sure my timing was off.
Something to consider, my $.02
Will
Check the balancer for the rubber coming out between the halves.
I just pulled my original motor and the balancer is way clapped out, to the point where I'm sure my timing was off.
Something to consider, my $.02
Will
It was timed properly last summer after I installed the carburetor tuned by Lars and ran very well. The timing mark still showed 36° when the detonation began, so I do not think there is any issue with the timing mark on the balancer.
I haven't heard it at WOT in 1st or 2nd, but the roar of the motor could be drowning it out. It is mainly at cruise, low RPM and in 4th.
Burning Brakes
Joined: Apr 2013
Posts: 926
Likes: 2
From: Castle Rock CO
You may also want to look at replacing the entire Distributor Shaft Unit. And as the other dude says first start with the Vacuum Advance. If the Diaphragm is bad it will not allow you time the engine properly same with the Distributer Unit if the shaft is worn it will wobble a bit.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
My money is on too much vacuum advance.
If it's not detonating at WOT or throttle settings where the vacuum advance is not part of the game then my guess is that your vacuum advance is putting in too much timing at cruise. Check your timing at 2500-3000 rpm with the vacuum hooked up and see where it is at. You may need an advance can that puts in less timing. Some cans can add as much as 30 degrees.
How soon do you have all of your mechanical advance in? You can make it after 2500 rpm to limit total timing at lower cruise rpms. Although just a different vacuum can may fix the problem.
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Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Gents,
Good suggestion on the vacuum advance. I will check my timing w/ the vacuum advance connected and check to make sure I have the right connection at the carburetor. Thanks for the responses.
Peter
Good suggestion on the vacuum advance. I will check my timing w/ the vacuum advance connected and check to make sure I have the right connection at the carburetor. Thanks for the responses.
Peter
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Vacuum advance can replaced, replaced bubba's vacuum line from carburetor with stock unit... and a dead battery. Will start up and check timing/test drive in the next couple of days.
Pro
Joined: Nov 2008
Posts: 542
Likes: 3
From: Bloomfield Hills MI
I had the same issue recently. Replaced my vac advance can with one that has less advance at higher vacuum (B28 to B1) and my problem disappeared. My belief is that bad california gas is the issue as all gasoline in this state now has ethanol and I never had this problem 2+ years ago. I have now 32 degrees total with 14 at idle (vac disconnected). It's a big block which I know may have different curve needs. Also, I am running the second softest springs (blue) and if I try and go lighter I get detonation. Hope this helps.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
I slapped a Mityvac on my old advance can, lines up with stock specs perfectly - starts opening at 8 in Hg and is fully open at 15.5 in Hg. This is the #201 can which supplies 15° of advance.
Could this be a carburetor issue? Ignition timing is definitely not a forte of mine, but could I be running too lean? I don't have a problem at idle.
Could this be a carburetor issue? Ignition timing is definitely not a forte of mine, but could I be running too lean? I don't have a problem at idle.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
15* (at the crank) advance is too much, probably way too much. If it does not ping with vacuum advance plugged, and does ping with vacuum advance connected you have too much vacuum advance. People pull centrifugal out to compensate. This is wrong. You need a canister that is "all in" @ 2" under manifold vacuum. I believe 8* advance (or less) is all some engines will tolerate.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Jeff,
Per Lars' "How to Set Timing" paper:
I have the original 491 distributor with the original MS201 vacuum advance actuator.
I do have the specs wrong for the MS201 actuator, it should be 15° advance @ 12 in. Hg.
Per Lars' "How to Set Timing" paper:
16-18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the 16 to 18-degree initial spec with the 36-degree total without modifying the distributor advance stop system. It is more important to achieve the 36 total than to hit an exact 16 to 18 initial. However, if your initial timing is very low (below 10 degrees) or very high (above 24 degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine performance.
I do have the specs wrong for the MS201 actuator, it should be 15° advance @ 12 in. Hg.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
I was referring to vacuum advance. Sorry if that was confusing. Lars is talking about mechanical (aka centrifugal) advance. I suggest testing your car with vacuum advance plugged. It should not ping. I believe when you are adding in vacuum advance, you are chugging at low load. Low load is high manifold vacuum, which is pulling in the 15* vacuum advance on top of the mechanical giving too much lead. On my iron headed high compression 65 442, she will chug like crazy at low load. I have a B26 canister that has been modified to allow 8* vacuum advance, measured at the crank.
Last edited by Jeff_Keryk; Sep 18, 2013 at 10:32 PM.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
As long as I'm under 52 degrees of total advance at cruise speed I should be good, right?
Next week I will map my entire curve, both me mechanical and total. I think this will really help decipher the problem.
Next week I will map my entire curve, both me mechanical and total. I think this will really help decipher the problem.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
As long as I'm under 52 degrees of total advance at cruise speed I should be good, right?
Next week I will map my entire curve, both me mechanical and total. I think this will really help decipher the problem.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
I guess my point is, vacuum advance complements centrifugal advance. With vacuum advance disconnected, go for 34 to 36* all in by 2800 RPM. This means centrifugal advance only. Don't worry about initial right now. Road test car; push it hard and get into some serious rpm. If it pings back off 2* and retest. Then add in the vacuum advance. If it pings (especially at low load) you have too much advance (centrifugal plus vacuum) at that particular vacuum and rpm level.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Jeff,
My problem is that the car ran perfectly last year and early this year. I have developed this low load/cruise detonation over the summer. I have backed off quite a bit of centrifugal advance, and we know that the vacuum advance is fixed/maxed out.
I think I need to go back to square one, map out the entire spark advance curve, and try and get back close to OE settings. Then, I can tune from there. Maybe I will have some time this weekend and report back then.
Peter
My problem is that the car ran perfectly last year and early this year. I have developed this low load/cruise detonation over the summer. I have backed off quite a bit of centrifugal advance, and we know that the vacuum advance is fixed/maxed out.
I think I need to go back to square one, map out the entire spark advance curve, and try and get back close to OE settings. Then, I can tune from there. Maybe I will have some time this weekend and report back then.
Peter
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Maybe cooling issue. Fan, T-stat, shroud seals. Running a little hotter may make the difference. Try a short cruise with the hood off to check.
cardo0
cardo0