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Discussion Starter #1
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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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: http://www.gtoforum.com/f170/65-gto-389-timing-specs-w-modified-engine-103042/
 

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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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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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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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Discussion Starter #6
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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wow...good idea....and it has worked well...any idea what an extra head gasket thickness dropped the compression to?
 

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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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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. :thumbsup:
 

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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.....:nerd:Replace the Spring:nerd:

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.


:nerd::nerd::nerd::nerd:
 

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OK, as promised. First off, I have never done anything like this or even messed around with vacuum advances, either factory or the aftermarket adjustable types. I simply ran the cars I had by factory tune-up specs and tweaked them from there. So this is new stuff to me and seems interesting enough to investigate further.

Sifting through the internet looking for solutions to "spark knock" and ignition timing problems I read about this vacuum advance limiting device in the form of a Crane Vacuum Timing Limiter Plate, Part #99619-1 . Summit sells just the plate for $2.83 if you want to use your factory non-adjustable vacuum can and play around with it. Cheap enough to give it a try. Pic #1 is what it looks like. Here is the description: These Crane vacuum timing limiter plates are designed to actually change the amount of vacuum timing. This is very helpful in high-compression or heavy engine load applications. The Crane vacuum timing limiter plates have 10 different notches that will shorten the amount of vacuum timing by 2 degrees. They will also change the advance of the initial timing by 2 degrees because of the change in the starting position of the breaker plate or magnetic pickup.

So it seems to offer adjustability to the vacuum advance can by limiting the amount of travel the arm of the vacuum advance can is able to move, thus affecting the timing through the vacuum of the engine. Now this is NOT the same as an adjustable vacuum can which I will get into later, but is to be used with the Crane Adjustable Vacuum Canister which comes in a kit form, #99601-1 for points https://www.summitracing.com/parts/crn-99601-1?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-crane-cams&gclid=EAIaIQobChMIzKaqu8-q1QIV1YKzCh2omwU-EAQYBCABEgJQdfD_BwE for $35.43 or #99600-1 for HEI https://www.summitracing.com/parts/crn-99600-1?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-crane-cams&gclid=EAIaIQobChMI85r5wdCq1QIVlI2zCh21MgFuEAQYAiABEgKrfPD_BwE for $35.40. The limiting plate device acts just like a stop which many do through fabricating means like brazing up the hole and filing to meet their needs or tacking a stop onto the vacuum can arm, or using a simple screw. Pic #2 shows the vacuum can arm, the pull rod, and a small piece of metal tacked into place to limit the travel of the arm to meet the users desired needs for total vacuum advance.

The Crane piece seems simpler and ready made with adjustment. HOWEVER, the intended attachment to the vacuum advance arm per Crane instructions seems to be better attached on the opposite side of the arm than on the side Crane instructs you to do. Pic #3 shows how the attachment is recommended by several forum users. In this manner, the plate actually limits the amount of travel as the engine vacuum controls the movement/stroke of the pull rod towards the can side - just like the home made stop in Pic #2 . The Crane plate is supposed to mount in the open hole you see at the end of the vacuum can arm and on the opposite side of the pull rod. The forum posters say what this does is push the arm forward against the spring found inside the vacuum can and as you increase the notches/degree settings, becomes more difficult. I also suspect by increasing the degree settings of the Crane plate, you are also moving the distributor plate it attaches to and actually advances your timing through the distributor which may require you to re-adjust your crank timing back to its initial setting. Sound right?

Pic #4 shows it installed in a distributor. You can rotate the plate to create the amount of limiting stop you want to use to restrict the movement of the pull rod. From one of the forums, "The Crane instruction say to install the limiter cam so as to compress the spring in the vacuum canister and limit travel. This makes the spring very stiff as you compress it and the canister that comes with the kit already has an adjustable spring. Plus, it's harder to install and adjust the cam against the compressed spring.
It only takes a drill, tap and 8/32 hex head screw and #8 flat washer, and a bit of time to get the plate on the OTHER side of the pull pin, and working correctly. What I did was tap a 8-32 hole in the bracket and install the adjustable limiter to stop the advance in the normal direction of movement. (Caution. Install it with the adjustable teeth as shown in the picture or you will have interference problems.) Now, I can adjust the total amount of vacuum advance to some reasonable number and hook it to the manifold vacuum port and get a nice smooth idle."

With regards to the Crane Adjustable Vacuum Canister, the Allen adjustment of the canister only controls the rate or the speed at which the vacuum advance changes. It does not change the number of degrees of advance - so two different things. By using the Crane kit, you get two separate adjustments you can play with. You can control the speed at which the pull arm operates and you can limit the travel of the pull arm by selecting different settings on the wheel. You also get the mechanical advance springs as well, so another adjustment (your timing curve) which has been very well covered here on the forums.

Another option is to try different vacuum advance canisters. GM made an assortment of them based on the engine, cam, and engine's vacuum. Grandma's Pontiac would not have the same vacuum can as an L88 BB Chevy, but you can interchange them. These canisters have different vacuum ratings. One might have a vacuum advance that begins to pull at 3"-5" of vacuum and only gives you 8 degrees maximum advance on a big cam engine pulling 10" of manifold vacuum while another might have the vacuum advance begin to pull at 6"-8" of vacuum, gives 14-16 degrees of maximum advance at a high of 24" of manifold vacuum. So it is possible to find a vacuum canister that will work if you are willing to experiment, but I think the Crane adjustable vacuum can might be easier to work with and adjust.

You can read more at these sites: http://www.stevesnovasite.com/forums/showthread.php?t=200004 , crane cam limiter plate?? - Chevelle Tech , Tried to get a B28 vac can to work - 70 Z28 - Page 3 - Team Camaro Tech

Hope this might help, and as always, it is all open to comments, corrections, or other. :thumbsup:
 

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Pontiac Jim said is correct in everything said about the adjustable Vac cans and the Crane limiter plate, I have used the crane limiter plate, have one in a box under my Distributor Machine long with a number of adjustable vac cans.

As I recall the limiter plate goes down as low as 8* distributor, which is 16* crank timing...for limiting vac advance. That is too much timing for today;s gas, you may get detonation, it would work at that lowest setting in 1965 or so, now not so good.

Also you are spending a whole lot of time adjusting it and fiddiling with it when it will never take you where you want to go.

The can I described for HEI has 8 to 10 Baked in Crank degrees, perfect companion setting for your 36 to 38 Total.

It pulls in the vac low...which is good here is the spec..... 3-6 hg 5* @7-9....

What that means is the can starts to move at 3-6 hg,...it pulls in 10* advance at the Crank and all 10 are in with 7-9 hg..

so for lively cams it will keep your timing in, it will maybe raise vac a little with good timing, and when hooked to full manifold will run cool and strong...

Perfect vac setting for a can is 2 hg below idle vac,...unlessyou are using full manifold van that lower is ok.....

It is really hard to cut that 2 hg accurately anyway as outside temp, gas ethanol or not, changes it a bit anyway and if you cut it too close your vac can will dither the timing in and out,...making it run stupid....

This is a great can for an HEI, and if you set up the rest will run good.

Look the Crane is OK and the adjustable cans for vac are better than having no idea what is going on...

Also MSD has some real ez to adjust dist with different bushing for Centrifigal set-up and even a different stop than crane has.....

but the last MSD I did I took out there adjustable vac can and put in the one I described......all the adjustments you are doing probably won't get you better and steadier than that,..

lot?s of ways to do it,....this is just one way...:nerd::nerd::nerd::nerd::nerd:
 

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Lemans guy, was hoping you would contribute as this is your area of expertise. As stated, never used either of these items, but both looked interesting as a kit and figured it could be something to play around with to see if it changed anything.

I believe the factory advance canister you speak of, and is recommended by many GM guys having HP cars/engines is the B-28 vacuum advance canister. This is the one that begins to pull at lower vacuum and all in by 8" of engine vacuum so as to work with those low vacuum/big cam engines. It also limits the amount of vacuum advance.

The can is listed under several numbers and it may still be available at local auto stores IF they have a listing for it. Many auto parts stores will not show this can, let alone the guy behind the counter know what you are talking about. However, RockAuto does list these and has them listed under several part numbers. They are inexpensive. I purchased a couple just to have on hand to play with in the future. Ebay of course has them, but more money. They do have it listed (and it has the DV1810 number as well) 7232 | RockAuto

That might help a few of the readers if they want to try just the can and not play around with the Crane Kit with its adjustable can and limiter plate.

Here is a link to the PDF by Duke Williams and presented by Lars Grimsrud (whom has been cited on this forum in the past) which explains all this and may be of help. Everybody should download it for future use even if you do not need it now. http://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf

:thumbsup:
 

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Spot on right Jim about Lars,..and I go by everything he lays out,..he is the expert not me, and great info in his tuning papers but he has updated some of that info.

So with today's gas numbers change a bit and what used to be 52 degrees BTDC at light throttle cruise is now 46 to 48.

Also the B-28 can is hard to find, but it will not work for Roquetman as he has an HEI distributor and the B-28 can is for points distributors only. They are different lengths and will not interchange. It matters not if you have a Petronix or other module, it is the size of the vac can.

The one I referenced for Roquetman, the standard motor parts SMP VC 302, pulls in the correct advance 10 degrees at the crank for HEI.

The B-28 can, for points distributors pulls in 16 at the crank. So you will have to correct it with a vac stop. Lars sells those. and it will make even a common B-26 work like a B-28, as it limits timing to 10 or 12 degrees. I have put the B=28 can on Corvettes and others, and have a couple. But when I put the B-26 on with Lars vac corrector it shows just like a corrected B-28 on the distributor machine.

Vac timing 10-12 and pulls in real fast and low, makes cars run great if hooked to full manifold vac and the other set-ups are correct.

read Lars papers to understand the timing concepts. and I always pay close attention to what you say Jim as your experience and knowledge go far in my book,....and we all benefit from our collective wisdom and experiences and failures......

and Hope Roquetman get his timing duked in!:nerd::nerd::nerd::nerd::nerd::nerd:
 
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