'65 GTO Spark Knock? - Pontiac GTO Forum
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post #1 of 14 (permalink) Old 07-19-2017, 06:49 PM Thread Starter
 
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'65 GTO Spark Knock?

Ok guys and girls, I need some advice on eliminating spark knock. At low speed the 389 with Turbo 400 likes to loaf along in third gear. When I hit the gas it has some initial spark knock, actually more like a rattle, it then downs shifts and takes off! The temp gauge is a little worrisome on these 90+ degrees days in NC... tops out at about 212 degrees, I'd rather it be 190-195 degrees. Anyway, the engine is the original 389 block, bored 30 over, Ram Air III heads, Hooker headers, Edelbrock intake Holley 600 cfm, mid-rpm power cam. Timing is set at 6 BTDC, which is the stock spec... as more advance just makes the spark knock worst. I've Added 2 bottles of STP octane booster to 93 octane gas. Any advice would be greatly appreciated. Thanks, Roqetman!
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post #2 of 14 (permalink) Old 07-19-2017, 08:38 PM
 
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Engine temp has some influence on "pinging." 212 is in my opinion is too hot. I'd rather see 180 if it were me.

aluminum radiator, fan shroud, flex fan, 160 thermostat, would be how I would set up my cooling system.

Lean jetting may be a possible thing to look at. The alcohol laced gas runs leaner and I would bump up my jetting 2 steps from the stock setting and then play with it from there by reading the plugs.

Colder plugs can sometimes help - another area you can experiment with.

Distributor advance, both mechanical and vacuum will have a big influence on "pinging." This has been well covered in other posts here. Assume 6 degrees was set with vacuum advance hose plugged? 6 degrees might also be "retarded" for the cam you are using. Retarding the engine timing can cause the engine to run hot, but it is like a double edged sword when you have high compression. You get "pinging" if you put the timing where it will run best, but then you retard it to eliminate "pinging" which can eliminate the "pinging" but now your engine timing is retarded to the point that the engine will run hot.

93 Octane may not be enough for your engine. A quick calculation shows your compression at 10.25-10.5 IF you did not use dished pistons to lower it. You probably want to try a half a tank of some high octane 98-104 racing gas to see if that helps and I might even advance the timing to something more like 9 - 12 degrees and observe if it runs better and cooler.

Also, the cam events can be a contributing factor. The static compression ratio is what you have with all the parts assembled, ie 10.25. Dynamic compression is the amount of air let into the engine. As an overstated example, the cam timing could turn a 10.25 compression into a dynamic compression of 6.0 or 9.0 based on the opening and closing events of the intake valve. Lower is always better and internet info says you don't want to really go over 8.0 on the 93 pump gas. The Wallace website has both the Compression Ratio Calculator Compression Ratio Calculator - Wallace Racing and the Dynamic Compression Calculator Wallace Racing: Dynamic Compression Ratio Calculator . Knowing these can be very helpful - and you can play around with them to get assorted results.

But first, I would look into getting the engine temp down a little.

You might also want to revisit one of your old posts which I think covers the same question with several responses: https://www.gtoforum.com/f170/65-gto-...engine-103042/
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post #3 of 14 (permalink) Old 07-19-2017, 11:06 PM
 
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Jim covered this pretty well...you are in a bind...the hot temps contribute to the pinging....more advance would help cool the engine, but then you are inviting more spark knock...if you can get the engine to run cooler with other mods such as coolant, radiator, pulley sizes, more efficient airflow, etc., you could then run more advance which would help power and be an additional help for running cooler.

you get the rattle on initial acceleration because the load on the engine increases...once the trans downshifts the load on the motor is lessened...good luck with this frustrating problem....
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post #4 of 14 (permalink) Old 07-20-2017, 03:15 AM Thread Starter
 
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Thanks for your replies. I will review the answers to my previous post and Wallace Racing. I agree, I'd rather see 180-190 on the temp gauge too. The previous owner had the engine rebuilt in 2004 with 10.75 compression. It has a newly rebuilt 4 row radiator and a new water pump. I set the timing at 6 degrees with the vacuum port plugged. I played with timing and it likes 10-12 degrees but spark knock is worse. It has GM electronic ignition. I tried using 2 medium weight springs on mech advance, 1 med & 1 heavy, and now I'm back to 2 heavy weight springs that are more like the stock springs.
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post #5 of 14 (permalink) Old 07-20-2017, 04:06 PM
 
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Those responses are very good information. I had the same issue several years ago with my 69 400 ram air 3. I went through a long period trying all the above options and finally got it down to a mild pinging but now the engine performed like a slug with retarding the ignition, changing springs, weights Etc. So what finally worked for me was to buy 2 sets of standard head gaskets from Butler, put some spray copper between the sets and installing 2 gaskets on each side of the engine which reduced the compression. I was then able to turn everything back up for performance and have been driving it like that successfully for the last 8 years. I would try every thing else first if I were you but that finally fixed it for me and was under $100 and a weekend to install. Good luck.
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post #6 of 14 (permalink) Old 07-22-2017, 06:34 PM Thread Starter
 
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I thought of that too but trying to find another way. I'm wondering if full manifold vacuum to the vacuum advance on the GM HEI distributor would help or not? Thanks!
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post #7 of 14 (permalink) Old 07-25-2017, 11:53 PM
 
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wow...good idea....and it has worked well...any idea what an extra head gasket thickness dropped the compression to?
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post #8 of 14 (permalink) Old 07-26-2017, 08:14 PM Thread Starter
 
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John23, I have no idea what thickness the head would be to lower the compression enough to eliminate rattle. I rather not pull the heads. Thanks!
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post #9 of 14 (permalink) Old 07-26-2017, 09:45 PM
 
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wow...good idea....and it has worked well...any idea what an extra head gasket thickness dropped the compression to?
Just for fun, I plugged in some numbers in the Wallace Compression Calculator. If you doubled up on the gaskets, it looks like you will drop the compression about 3/4 of a point.

The trade off is now going to be a rather large quench/squish area above the piston which can contribute to detonation. "All the engine builders we spoke to mentioned that tightening the quench (reducing the piston-to-head clearance) to get it under 0.050 inch will increase the static-compression ratio, but this tighter clearance also creates a more powerful squish effect. This additional turbulence creates a more homogenous “soup” in the chamber, reducing the harmful effects of lean air/fuel ratio pockets. With all other variables being equal, this (tighter quench/squish area) contributes to creating an engine that is less prone to detonation."

So adding a second gasket in an attempt to drop compression and rid the engine of spark knock may only do what you are trying to cure - you are trading lower compression for more detonation.

The preferred way to lower compression is either use pistons matched to your heads to lower the compression, or get heads with the chambers cc'd to match your pistons to lower compression, or both.

You may be able to "tune" out the rattle, but this can cause problems in other areas.

You can use high octane racing gas or a good octane booster for higher compression engines. Another solution is a water/alcohol injection system. Here is an interesting article on Octane, additives, and water/alcohol injection: Everything You Need To Know About Octane

You might want to try playing around with the vacuum advance and going to manifold vacuum vs ported as you stated. Been reading more on the Crane adjustable vacuum advance can AND the Crane vacuum advance plate limiter #99619-1 https://www.summitracing.com/parts/crn-99619-1 . Using these together might be an option. I did a little reading on various forums and it seems the Crane directions should not be used for the limiter plate, but rather, it should be positioned in a reverse manner so as to truly limit the amount of pull on the vacuum advance rod. I grabbed a bunch of info and will try to cobble it all together for a better understanding, but it may work for you, at least it may be something to try that will not require you to tear into the engine and is not too expensive.
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post #10 of 14 (permalink) Old 07-27-2017, 07:00 PM
 
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Roquetman, A very good discussion here by the gang. Lot's of great points

and of course detonation is an evil circle, an engine that runs hot causes detonation and detonation causes an engine to run hot. I like 160 thermostats, and of course all of the cooling system fundementals as discussed, 4 core rads, and correct spaced water pump plates etc. But if you have all that right,,,,,,,you get back to timing.

If you set timing at 6 degrees and you don't what the centrifigal advance is....then you don't know if it is way too retarded or way to advanced. Sometimes the Distributor has as little as 16 degrees advance in it, in fact I pulled one last week that did.....or as much as 34 degrees,...add your base of 6 or even 12, it knocks.....if the dist has 16 inside it will run hot....

. Lot's of vac cans have 30 degrees of advance, some 25....way too much. If one hooks that to full manifold vac.....constant knock...thoughout the power band.

so start at the beginning, find out your Centrifigal advance. Maybe you already know it. Then skip to the next paragraph. if not do this, get a dial back timing light or timing tape on the balancer. Chock the wheels park or neutral, parking bake on. Have a safety helper,....remove dist cap and rotor take off one dist weight spring. replace rotor and cap. Now rev the engine while watching the timing mark until it stops advancing (removing the spring brings the total in fast and you won't have to rev to high or too long.....Replace the Spring

Now subtract the base timing from that total and that is your centrifigal advance. let's say it is 24. Now you will set your base at 12...for a total of 36 degrees.

Now to vacumn advance, You vac can is very likely pulling in way too much advance and all at the wrong times, a weak dithering vac can can also mess you up. you can try to check it if you care to, use a handheld vac pump and the timing light or tape to see how much timing it will pull. But I will save you lots of time and effort, it is most likely not right.

Get this vac can;Standard Motor Parts SMP# VC 302....NAPA sells it as a VC 1703,...same can. put that in your dist and hook to full manifold vac, iT will pull 10 degrees timing at the crank. Hook it to full manifold vac.

on this example you will get 22 BTDC at idle which should cool you off and also make your light throttle cruise cool at 46 BTDC. your strong springs retards the timing as the rpm's advance, and don't come all in until 4000 or 4500 RPM when you engine needs it earlier, as you pedal down to accelerate (when it pings) vac advance drops and your centrifigal stays in or comes in depending on RPM of engine.....

One possibilty is your vac can has way too much advance, the ones I pull do, that causes pinging and pinging causes heat,...then when you accelerate and drop vac your advance springs are so strong the engine gets no timing advance from them and it gets sluggish and hot from retarded timing....

everything is working against the other....

Try the way I said and try one light and one medium spring...and see it you can get some improvement....this may not get you perfect,.......... but lots closer...

we are pulling for you...Perseverance pays off.

(PS cars that had specs for six degrees of timing had a lot more baked into the centrifigal advance to make up for that retarded timing at idle...it was less that ideal.


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