Detonation stumping me

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JoePorting
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Re: Detonation stumping me

Post by JoePorting » Tue Nov 13, 2018 4:51 pm

Maybe check your firing order to see if you have some plug wires on wrong.
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Re: Detonation stumping me

Post by Newold1 » Tue Nov 13, 2018 5:01 pm

I am still a little bit confused from your descriptions of the advance, but I am reading that you have 14 degrees of initial and you are getting 16 degrees of mechanical advance in the distributor and then you can use additional advance supplied by the vacuum diaphragm on the distributor.
How much mechanical advance are you getting at lets say idle to 3000 rpms, because if you are getting 25-30 degrees somewhere in the lower part of that range you might be getting to much there.
How much advance is in the diaphragm?

As a lot have said here, Have you checked TDC on the damper? What is the measured cranking compression in pounds?
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Re: Detonation stumping me

Post by Kevin Johnson » Tue Nov 13, 2018 5:22 pm

The power brakes can still function even with a vacuum leak in the diaphragm.

... use a vacuum pump with a gauge and connect it to the booster reservoir, pull a vacuum of 20", turn off the vacuum supply (close the valve) and see if it holds the vacuum. It should hold for over one minute without dropping from the 20".

41apache
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Re: Detonation stumping me

Post by 41apache » Tue Nov 13, 2018 7:27 pm

Make sure its not straight antifreeze I have seen stock motors ping from it wont take the heat out of the chambers.

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Re: Detonation stumping me

Post by gtca9014 » Tue Nov 13, 2018 9:35 pm

Newold1 wrote:
Tue Nov 13, 2018 5:01 pm
I am still a little bit confused from your descriptions of the advance, but I am reading that you have 14 degrees of initial and you are getting 16 degrees of mechanical advance in the distributor and then you can use additional advance supplied by the vacuum diaphragm on the distributor.
How much mechanical advance are you getting at lets say idle to 3000 rpms, because if you are getting 25-30 degrees somewhere in the lower part of that range you might be getting to much there.
How much advance is in the diaphragm?

As a lot have said here, Have you checked TDC on the damper? What is the measured cranking compression in pounds?
Without the vacuum advance connected, I have initial timing set at 14 degrees. When I rev engine to 3000 rpm I have 30 degrees (14 degrees initial plus 16 degrees mechanical. If I hook up the vacuum advance, I have 34 degrees at idle (14 initial plus 20 from vacuum canister). I am using full manifold vacuum at the baseplate.

What I found tonight was that while I have full manifold vacuum at idle, when I rap the throttle, the vacuum drops off and then climbs higher. This doesn't work for vacuum advance as it adds timing at part throttle and pings. I thought the vacuum dropped off and stayed off when opening the throttle. Am I on the wrong port? It is one in front of baseplate on an angle.

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Re: Detonation stumping me

Post by gtca9014 » Tue Nov 13, 2018 9:54 pm

Compression test was 175 psi +/- 5 psi.

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Re: Detonation stumping me

Post by gruntguru » Tue Nov 13, 2018 11:26 pm

JoePorting wrote:
Tue Nov 13, 2018 4:51 pm
Maybe check your firing order to see if you have some plug wires on wrong.
A more subtle version of this is cross firing between leads. Make sure the leads are well separated - especially for cylinders that are adjacent in the firing order.

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Re: Detonation stumping me

Post by Geoff2 » Wed Nov 14, 2018 2:49 am

When you 'rap' the throttle, the vac adv does indeed drop off, but the centri curve then comes in with the extra rpm & this is what you are seeing.

Keep using the vac adv [ connected to manifold ], that is the right thing to do. Double check that you have the correct carb port. Because it is in the carb base does NOT mean it is below the t/blades. To check: remove VA hose at carb port. If it is man vacuum, you will feel suction at idle when you put your finger over the port.

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Re: Detonation stumping me

Post by naukkis79 » Wed Nov 14, 2018 1:57 pm

gtca9014 wrote:
Tue Nov 13, 2018 9:35 pm

What I found tonight was that while I have full manifold vacuum at idle, when I rap the throttle, the vacuum drops off and then climbs higher. This doesn't work for vacuum advance as it adds timing at part throttle and pings. I thought the vacuum dropped off and stayed off when opening the throttle. Am I on the wrong port? It is one in front of baseplate on an angle.
It works just as it should. You lose manifold vacuum only at WOT, at part throttle you got vacuum, much more than at idle. Initial+mechanical advance is for WOT, for part throttle vacuum canister adds timing - and it should not ping even total timing is about 50 degrees.

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Re: Detonation stumping me

Post by Kevin Johnson » Wed Nov 14, 2018 2:26 pm

Here is a Jegs installation manual for a Holley 750; It shows the different vacuum ports (page 6):

https://www.jegs.com/InstallationInstru ... -3310S.pdf
Holley.jpg
Good luck.
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gtca9014
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Re: Detonation stumping me

Post by gtca9014 » Wed Nov 14, 2018 2:59 pm

naukkis79 wrote:
Wed Nov 14, 2018 1:57 pm
gtca9014 wrote:
Tue Nov 13, 2018 9:35 pm

What I found tonight was that while I have full manifold vacuum at idle, when I rap the throttle, the vacuum drops off and then climbs higher. This doesn't work for vacuum advance as it adds timing at part throttle and pings. I thought the vacuum dropped off and stayed off when opening the throttle. Am I on the wrong port? It is one in front of baseplate on an angle.
It works just as it should. You lose manifold vacuum only at WOT, at part throttle you got vacuum, much more than at idle. Initial+mechanical advance is for WOT, for part throttle vacuum canister adds timing - and it should not ping even total timing is about 50 degrees.
It does have nearly 50 degrees at part throttle and pings like crazy. Best combination I had was PCV disconnected, without vacuum advance connected with slight pinging at WOT. I covered the coke to shut off, it took 3-4 seconds before it died. Is that normal or should it have taken less time?

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Re: Detonation stumping me

Post by gtca9014 » Wed Nov 14, 2018 3:00 pm

Kevin Johnson wrote:
Wed Nov 14, 2018 2:26 pm
Here is a Jegs installation manual for a Holley 750; It shows the different vacuum ports (page 6):

https://www.jegs.com/InstallationInstru ... -3310S.pdf

Holley.jpg

Good luck.
Thanks! I am connected to the full manifold port as described.

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Re: Detonation stumping me

Post by Kevin Johnson » Wed Nov 14, 2018 3:54 pm

gtca9014 wrote:
Tue Nov 13, 2018 9:35 pm
...
What I found tonight was that while I have full manifold vacuum at idle, when I rap the throttle, the vacuum drops off and then climbs higher. This doesn't work for vacuum advance as it adds timing at part throttle and pings. I thought the vacuum dropped off and stayed off when opening the throttle. Am I on the wrong port? It is one in front of baseplate on an angle.

Here is a discussion about whether you should use the timed spark vacuum port versus the full vacuum port (this second one is the one you are currently using):

http://theamcforum.com/forum/timed-spar ... 11416.html

Geoff2
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Re: Detonation stumping me

Post by Geoff2 » Thu Nov 15, 2018 4:33 am

WOT ping is dangerous. Many causes. CR too high for fuel octane, engine too hot, too much total timing, etc.
What many people do not understand is that once cruising, Man Vac Adv [ MVA ] acts the same as Ported Vac adv. Both are receiving the same engine vacuum. The extra benefits with MVA are that you get a smoother idle, more vacuum, better mileage & the engine runs cooler.

More on MVA benefits by a GM engineer: scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance ... 47495.html

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Re: Detonation stumping me

Post by naukkis79 » Fri Nov 16, 2018 11:58 am

gtca9014 wrote:
Wed Nov 14, 2018 2:59 pm
It does have nearly 50 degrees at part throttle and pings like crazy. Best combination I had was PCV disconnected, without vacuum advance connected with slight pinging at WOT.
Have you checked cam timing? To get engine pinging at part throttle and 50 degree timing it needs plenty of heat in chambers, one way to have it is extremely late cam timing so intake stroke have time to reverse flow much more than normal amount exhaust gases back to cylinder.

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