# Can't seem to get the spark knock to go away.



## WhittP (Jun 19, 2013)

My problem is spark knock under heavy acceleration. I'll try to give all the details so here goes...

My car is a 1967 GTO with a 68 code 400 engine and TH400 trans. My impression is that the motor was rebuilt semi-recently to stock specifications. They certainly didn't spend any extra money when they did it on paint or a new cam. (I believe it has a worn cam lobe due to an interesting "tap-tap" when it gets warmed up good). The engine does, however, pull 24 hg vacuum at idle so I guess it's sealed up pretty well in there.

My first upgrade when I bought it a few months ago was an Accel billit distributor with separate Accel coil. I did this to replace the unit that the mechanical and vacuum advance wasn't working in while upgrading the electronics in the process.

I spent some time getting the springs right to give me 20 degrees of advance and I measured that it starts off at around 900 rpm and is all in by 2600. I had a spark knock issue under light throttle but I corrected that by limiting my vacuum advance to 10 degrees with that little stopper thingy. So now I have 12 degrees initial timing topping out at 32 degrees at 2600 with no vacuum advance. I know the engine would like a little more advance but I just can't do it with that spark knock.

I tackled the carberation issue after that. It's an Edlebrock 750 vacuum secondary model and I have it running pretty rich. I'm not a carberator guy by any means but I followed the Edlebrock books and did my best. I guess I need to look at the plugs but the car really does run well except for the knock.

I always put 93 octane and I've tried a few so-called octane boosters but I still get the knock.

I'd like some opinions from you guys that have been doing this a lot longer than I have. Thanks in advance!


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## mikea455 (Jun 1, 2013)

I'd experiment with timing just to see if indeed it is spark knock and not piston slap or too much side clearance on the connecting rods. If you still get knock with the timing retarded, there may be other issues. 

How about temps? Pontiac engine have a plate between the water pump and timing cover. If that plate is missing, rusted or missing the o-ringed sleeves that press into the timing cover, the engine may not get adequate coolant flow in the cylinder heads. 

Spark plugs play a small part in this. Around 1967 Pontiac changed from a closed chamber head to an open chamber head. I believe the extended tip plugs may cause a hot spot in the closed chamber heads.


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## WhittP (Jun 19, 2013)

Thanks for the tip. Back when I was running the old distributor there wasn't any spark knock issue. Like I said, it wasn't giving my much advance at all so I figured my issue was spark knock when I got my advance closer to where it needs to be and the noise appeared. My heads are 68 date coded so I assume they are the newer style open chamber. I'll post the numbers on here to be sure though. When I replaced the plugs I'm sure I probably asked for 1967 plugs not knowing that could make a difference.

I didn't know that about the plate behind the timing cover. The engine seems to be cooling effiently though. I've replaced most of the system including a 4 core radiator, fan, clutch, hoses, and upgraded it to a shroud like the A/C cars. (mine is no A/C) I have a 180 degree thermostat and it will cruise the streets at about 200 degrees but if I get on the freeway going 75 or so for an extended period it will heat up to 230. I always figured that was due to the 3.55 gears and tires I have. It spins at 3300 rpm at 70 mph.

I've noticed the knock is greater when the engine is nice and hot.


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## chuckha62 (Apr 5, 2010)

What octane boosters have you tried? They're not all the same. Have you tried Toluene? It's an excellent octane booster. You can also find race gas if you try. I'm currently supplementing 1 gallon of 110 octane Sunoco leaded racing gas to ten gallons of 91 octane (the best I can get at the pump locally) pump gas with good results. I've also run a lead additive in the past which helps with non hardened valve seats. 

It's too bad that you're suspect of the "rebuild" that you have. It makes it hard to nail down exactly what's happening.


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## geeteeohguy (Feb 2, 2008)

What Chuka62 said. Did it ping before you did the Accel ignition addition? My 2 cents, after owning multiple GTOs and driving them for the past 35+ years: There is no performance gain over the stock ignition with modified parts. (seriously....no horsepower gain at all) and, number two, if you are running a stock compression '68 engine, you need to run it on about 100 octane fuel to keep it from pinging. Period. There is no magic bullet or way to cheat it. If it's stock compression and you want to drive it, you need to use race gas or an octane booster that works, like TEL130. The stuff Autozone sells is a waste of $$$ and doesn't work at all. And I mean not one little bit. I run race gas in my '65 GTO with stock compression, and I finally got sick of the pinging issues on my '67 and swapped to low compression heads. Good luck.


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## WhittP (Jun 19, 2013)

Thanks for the advise guys. I didn't get the pinging issues before I replaced the distributor. I believe it was due to the mechanical advance not functioning and my timing was severely retarded. I went ahead and just replaced the stock unit with the stock-looking Accel unit since I don't know much about rebuilding distributors or tinkering with points. (I have all my old parts saved in case I ever want to get it back to stock though.)

Now that the timing is about right, I believe the engine is showing it's need for higher octane fuel. Where is the best place to buy the 3 things you mentioned? Race gas, TEL 130, and Toluene?


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## Rukee (Feb 8, 2007)

If you have a circle track or drag strip anywhere close call them and ask which gas stations sell the race fuel. Chances are there will be a few close.


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## geeteeohguy (Feb 2, 2008)

TEL130 can be purhased on-line. Race gas should be available locally if you search on line, also. None of this is cheap, but it's cheaper than a new engine.


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## WhittP (Jun 19, 2013)

Ok I found a bottle of Octane Supreme 130 with TEL on ebay. I'll let you guys know if this does the trick. Thanks again for the advise.

On a side note, after all the mechanical/electrical/cosmetic stuff is done then I plan on buying another engine to build. Something with a little more giddyup and that runs on 87 octane!


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## Oldeboy (Sep 17, 2012)

How did the Octane Supreme 130 work out for you?


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## WhittP (Jun 19, 2013)

Sorry I forgot to mention that! I ordered a bottle and added it to about 18 gallons I had in the tank. I took it for a drive and zero audible spark knock. I then readjusted the timing for 36 degrees total and still no spark knock. This product works but it sure makes a tank of gas expensive.

Anyone try mixing E85 to acheive higher octane? I've read it's naturally 105 and I can buy it down the street for $2.69.


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## Oldeboy (Sep 17, 2012)

Thanks for the reply. Glad to hear it is working for you. Where on ebay did you find it? 
I have heard of people buying it here but wasn't sure if it worked. 
Octane Supreme
Probably cheaper than ~5gal of race gas per tank.


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## WhittP (Jun 19, 2013)

It was Greg's Speed Shop on Ebay. I paid $125 for a 6 can case including shipping. One can gets a tank of gas to around 100 octane. An engine that runs on pump gas would be my first choice but until then I guess I'll have to bite the bullet.


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