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Discussion Starter #1
408 BBC stock closed chamber heads, 9.67 to 1 comp, comp cams 270 magnum flat tappet w/224*@ 0.050". holley 770CFM carb, 1984 Olds Cutlass, turbo 400 trans w/2400 stall, 3.42 gears, 27" tall tire. The car weighs almost 4000 pounds with me in it. (I'm 325#)
I rebuilt the distributor with a complete Pertronix kit and have initial set at about 20*BTDC. Total is about 38* all in by 3000 RPMs.
The engine wants more initial, (about 24*) but when set there it pings if you floor it at lower RPMs then clears up. I was wondering if I slowed the rate of mechanical if I could sneak a couple more degrees of initial into it?
Don't get me wrong , it runs like a scalded dog now but I can feel the idle labor a bit in gear which clears up with a couple more degrees initial.
What do you think?
 

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What is your vacuum advance setting at? Do you have it limited to no more then 12 to 14 degrees at the most or do you have it allowing up to almost 20 degrees of extra timing? Do you have it on full time or timed port? A lot of times if you have slight pinging when you get lightly on the gas more times then none you can have to much vacuum advance coming in and you need to have an adjustable canister so you can adjust it and also use a vacuum advance limiter to keep it from giving to much extra timing at cruise.

I run only about 16 degrees initial with a hydraulic roller cam similar in size like yours but run my vacuum advance on full time manifold which gives me around a total of 28 to 30 degrees of timing at idle in park. I have a msd limiter plate to limit my vacuum advance to no more then 14 degrees at the most so I don't get into pinging and I have my timing like yours all in about 3000 rpm or a little over. I use a set of medium weight springs for my mechanical advance so its not starting to advance to around 1300 rpm and up.

I think 20 degrees of initial timing is plenty for that cam at least in my experience. Run your vacuum advance on full manifold timing and see how it does from that. My s10 is very similar to your setup with a set of 3.42 gears and similar cam size but I have a small block chevy dart shp build.
 

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Discussion Starter #3
My vac can is adjustable and is set at 12*. It is hooked up to full manifold. The pinging is at lower RPMs floored, not light throttle cruise. The car likes a lot of initial due to the closed chamber heads and domed pistons in my opinion.
 

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I see the choices as stiffer gears, perhaps a bit more stall, or aluminum heads, or higher octane fuel. You can add some richness to the mixture, play with timing and timing curved, run it cooler and feed the carb outside air if where you live isn’t super summer hot.

You’re banging on the limits of modern gas station fuels. There is no doubt that high compression detonation problems are subdued by aluminum heads, the OEMs have gone this way with EFI as well, that is a combination that opens the door to old time compression ratios with modern gas station fuels.

The additional gearing and stall gives the engine better mechanical advantage against the load it has to move, this does subdue lower end detonation. Beyond that it’s tinker around the edges with mixture ratios and cooling of inlet air and engine temps.

Bogie
 

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Discussion Starter #5
I'm running 91 octane now. I would like to put in a good 3000 RPM stall but budget won't allow right now. Just dropped a grand last month on distributor rebuild, carb rebuild, shocks all around, new windshield and new wires. I live in southern Wisconsin, so heat and humidity can be ugly. The problem with a head change to aluminum is it will require new pistons too. My stock heads are 101cc closed chambers. Edelbrock is the only ones with a 100cc head but still requires new pistons due to chamber design. Gears are a no go as I don't want to be spinning more RPMs at 60 mph. The cheapest is obviously a new torque convertor but for now I'm just wondering if I can tweek the timing to get a hair more initial. The end game over the winter is acually a 454 I have on my shop floor. It has less than 4000 miles on a rebuild. Came out of a wrecked truck. Even has the 781 heads if I choose to go that route.
 

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I would do some of the suggestions that bogie said and I would maybe try to enrichen your carb tune and see if that helps some. I did not see your running iron heads I am taking it. Since your running domed pistons that can cause a lot of more headaches there. I ran high dome pistons once with a set of iron heads with 93 octane and even a pretty stout cam of 244/[email protected] hydraulic flat tappet back in early 2000's and had around 10.5 to 1 and it was a beast to get to not ping and I ended up tearing it down and replaced the pistons with flat tops and put it all back together and it no longer had the sensitivity issues like it once had. That is why I never use domed pistons.
 

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Discussion Starter #10
Okay, so here's the deal. I took the distributor out and flipped the center piece
for the weights around so it was just like the picture in the Pertronix instructions and put the weights back in with the medium springs. When the center piece was the other way I had about 20* of mechanical advance. After I flipped the center piece around I then got 10* of mechanical advance all in at about 2500 RPMs and then it slowly gained another 4-6 degrees heading to 6000 RPMs. I set the initial at 24* and mechanical gave me 34* by 2500 RPMs and I had 10* on the vacuum can at full manifold vacuum. Floored the gas at 3000 RPMs and it would ping. If floored at 3600 RPMs it would barely ping then clear up. I took out the medium springs and put the heavy ones in. I then had 34* of advance at about 3200 RPMs. I Floored it at 3000 RPMs and no ping. The car goes like crazy now and the idle is much cleaner and feels stronger when put in gear. When I get back to the shop I'm going to up the total vacuum advance and see if it likes that.
 

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I very strongly suggest that you get this Longacre spark plug magnifier/light with holding fixture and begin looking at your plugs after a full-power run from zero to whatever speed you can attain without being arrested. Of course, this is best done on a drag strip or other dedicated roadway that isn't governed by speed limits, but sometimes those things are not available to us and we have to use what is available, like a long, straight stretch of country road with real good visibility. Begin with a fresh set of plugs.....

When you have achieved max velocity, go to neutral and chop the motor. Coast to a stop and pull the spark plugs RIGHT THERE. This will obviously take some planning of the place where the vehicle stops. You might make arrangements with a resident along the road to pull into his driveway to pull the plugs. Whatever. I'm just saying do not drive the car after you chop the motor. That will re-color the plugs and give you a false indication of what is going on in the motor.

The pinging that you hear is detonation, which is akin to bringing a sledgehammer down onto the piston crown. It sounds tinkly and cute from the outside, but is wrecking your pistons and cylinders on the inside of the motor. Of course you don't need to pull the plugs to inspect them for detonation as long as you can hear it. It's after you have found the proper timing and fuel delivery and eliminated the sound from the outside that you need to begin the power runs and inspecting plugs to see if the motor is detonating and you can't hear it. Light detonation will smash engine oil out from the top of the top piston ring, from between the ring and the top land of the piston. These little specks of oil will be deposited onto the ceramic insulator of the plug and baked on there as a little black dot, which you can easily see with the Longacre spark plug reader linked at the bottom of this post.

As detonation is allowed to continue, the beating and heat will begin to disassemble the piston material and you will begin seeing little silver specks on the ceramic. This is the aluminum material from the piston crown that is melting and being deposited onto the plug. It won't be long until the game is over when you see the silver specks. You will soon have holes in the pistons and compression will drop to zero.

I suggest that the best way to see all the way up to the top of the ceramic is to use a hand-held 4" grinder with a cut-off wheel mounted on it, to cut away the threaded part of the spark plug when you get back home, where you can mount the plugs in a vise. Take a spare set of plugs with you and keep the ones that you just did the power run with separated and numbered in the sheet metal organizer that I have shown above with the light. Install the spare plugs into the motor for the drive back home.

Scour the web for photos and explanations of spark plug conditions and colors and you will become expert at reading your plugs and finding out what's going on inside the motor.
https://www.speedwaymotors.com/Longacre-52-50886-Spark-Plug-Viewer-with-Holder,61348.html

.
 

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Discussion Starter #12
Tech, I will take your advise very seriously. I actually bought the Longacre tool you mentioned about a month ago. I had to finish other things on the car before getting to where I'm at now. This week I will be buying new plugs and there is a back road that is rarely traveled that is level and long and the sight is good. I have looked up as much info as I could on plug reading today and learned more than I thought possible on the subject. The more I learn the dumber I feel about how I used to tune engines. It's a wonder I never blew one up. As has been said I'm at the edge compression wise with iron heads and to compound the issue I have a miserable almost .070" quench. As I indicated I know much, much more than when this engine was built 16 years ago. This engine is a pet project of mine as I just want to see what I can wring out of it in it's present form. Last time I was at the track this engine had a comp 280 magnum with 230*@ .050" and with a lousy amature tune it managed 13.5 seconds in the 1/[email protected] 102MPH. I feel that with the milder cam I can come out of the hole harder and knock a couple tenths off. New 454 going in over the winter so just want to see what I can make this baby rat do. I'll post plug reading and final timing later this week.
 

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I had a very similar setup:

402 BBC, 781 Heads, 9.8:1, Comp 270h magnum, Edelrbrock 750, 67 Camaro, Muncie M-20, 3.42 gears, 27" tall tire. 3750 pounds.

i felt it ran best with 20 initial, 38 total @2500k, plus 12 Vaccuum Advace.

Total of 34-38 @ 2500-3000 is the window most Chevys run best (according to Lars Grimrud and Henry Olson).
With static initial (without vacuum) @ WOT between 10-20 depending on dynamic compression ratio / bleed off / cam-duration to displacement ratio.
With dynamic intitial (with vac.) for an idle timing of around 25° +-5°, again, depending on individual pressure situation of the build.

In your case it seems like the first move is to reduce static initial timing down to maybe 16°, change to the equivalent bushing/springs to achieve same old 38° @3000k.
See if it still knocks when floored.
Every combo is different, 781 heads act differently.
 

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Discussion Starter #14
I had a very similar setup:

402 BBC, 781 Heads, 9.8:1, Comp 270h magnum, Edelrbrock 750, 67 Camaro, Muncie M-20, 3.42 gears, 27" tall tire. 3750 pounds.

i felt it ran best with 20 initial, 38 total @2500k, plus 12 Vaccuum Advace.

Total of 34-38 @ 2500-3000 is the window most Chevys run best (according to Lars Grimrud and Henry Olson).
With static initial (without vacuum) @ WOT between 10-20 depending on dynamic compression ratio / bleed off / cam-duration to displacement ratio.
With dynamic intitial (with vac.) for an idle timing of around 25° +-5°, again, depending on individual pressure situation of the build.

In your case it seems like the first move is to reduce static initial timing down to maybe 16°, change to the equivalent bushing/springs to achieve same old 38° @3000k.
See if it still knocks when floored.
Every combo is different, 781 heads act differently.
You must have had one heck of a dome on your pistons to get 9.8:1 with a 781 head! I have a 21cc dome on my pistons with a 101cc closed chamber head and only got 9.67:1 compression, measured. Anyway my timing is now at 22* initial and about 36 by 3000rpms. Goes like hell. No ping.
 

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Discussion Starter #15
I very strongly suggest that you get this Longacre spark plug magnifier/light with holding fixture and begin looking at your plugs after a full-power run from zero to whatever speed you can attain without being arrested. Of course, this is best done on a drag strip or other dedicated roadway that isn't governed by speed limits, but sometimes those things are not available to us and we have to use what is available, like a long, straight stretch of country road with real good visibility. Begin with a fresh set of plugs.....

When you have achieved max velocity, go to neutral and chop the motor. Coast to a stop and pull the spark plugs RIGHT THERE. This will obviously take some planning of the place where the vehicle stops. You might make arrangements with a resident along the road to pull into his driveway to pull the plugs. Whatever. I'm just saying do not drive the car after you chop the motor. That will re-color the plugs and give you a false indication of what is going on in the motor.

The pinging that you hear is detonation, which is akin to bringing a sledgehammer down onto the piston crown. It sounds tinkly and cute from the outside, but is wrecking your pistons and cylinders on the inside of the motor. Of course you don't need to pull the plugs to inspect them for detonation as long as you can hear it. It's after you have found the proper timing and fuel delivery and eliminated the sound from the outside that you need to begin the power runs and inspecting plugs to see if the motor is detonating and you can't hear it. Light detonation will smash engine oil out from the top of the top piston ring, from between the ring and the top land of the piston. These little specks of oil will be deposited onto the ceramic insulator of the plug and baked on there as a little black dot, which you can easily see with the Longacre spark plug reader linked at the bottom of this post.

As detonation is allowed to continue, the beating and heat will begin to disassemble the piston material and you will begin seeing little silver specks on the ceramic. This is the aluminum material from the piston crown that is melting and being deposited onto the plug. It won't be long until the game is over when you see the silver specks. You will soon have holes in the pistons and compression will drop to zero.

I suggest that the best way to see all the way up to the top of the ceramic is to use a hand-held 4" grinder with a cut-off wheel mounted on it, to cut away the threaded part of the spark plug when you get back home, where you can mount the plugs in a vise. Take a spare set of plugs with you and keep the ones that you just did the power run with separated and numbered in the sheet metal organizer that I have shown above with the light. Install the spare plugs into the motor for the drive back home.

Scour the web for photos and explanations of spark plug conditions and colors and you will become expert at reading your plugs and finding out what's going on inside the motor.
Longacre® 52-50886 Spark Plug Viewer with Holder

.
Tech, I did a post about a month ago called spark plug reading and got only one response. The plugs showed bluing on the ground strap at the bend. There was dry carbon on the ring. I had some peppering on some of the plugs but no shiny specs. On #2 I had buildup on one side of the ground strap. I'm guessing oil? I still haven't cut any open to see down inside. What do you think?
 

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You must have had one heck of a dome on your pistons to get 9.8:1 with a 781 head! I have a 21cc dome on my pistons with a 101cc closed chamber head and only got 9.67:1 compression, measured. Anyway my timing is now at 22* initial and about 36 by 3000rpms. Goes like hell. No ping.
correct: 38cc Dome pistons. So the solution was retarding initial 2°? Good to hear. I guess you used different non-MSD 14° Bushing to achieve the total 36°?
 
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