Ongoing Detonation Issue Help!!!!!

[Ishiftem]

Ongoing detonation issue with my stroked 409 Chebby. I’ve opened ring gaps to .040 and the top AND second ring. Went from c12 to q16 fuel with the same results. Still close examination of the ring ends show the shiny tell tale signs of the rings butting. Also black pepper spots on the plug porcelain with both 8 or 9 ngk plugs. Here are the specs:
409 W with factory iron heads.
4” stroke so about 480”
6.385 rod
13.3:1 compression
Rings 1/16 gapless top ring
Dual plane intake with 2 750 afb carbs
Cam is a solid roller
Lift is .661intake and .668 exhaust
Duration @ .050 254/260
Lobe separation 110
Installed @ 107.5 intake center line
@ .050 intake open 19 btdc close 55 abdc
Exhaust open 62 bbdc close 18 atdc
Comp cam Extreme street lobes 4876/4877
Timing 34 degrees
I thinking it’s either the cam causing the detonation or the extreme amount of crevice volume above the top ring. The top of the piston is nearly 1.200” above the top ring (not a typo). To make that even worse, there is a generous counter bore at the top of the cylinder. What can I do with this thing? It’s killing me! A different fuel? Retard the cam some? It makes good power for what is. I’ve gone 10.13 in a big 3850 pound 63 Chevy with a 4 speed. But every time I freshen it up and see the large amount of magnetic sludge on the drain plug magnet and dirty looking oil I know it’s loosing ring seal. Pull it apart and there are the shiny spots on the ring ends from them butting. Makes me want to lose my lunch! It’s not lean or over timed. I have asked every one I can think of and get blank stares and shoulder shrugs. Maybe because it’s a 409 and they are unfamiliar with the platform. Suggestions?

[Tom68]

I'd be surprised if your rings are butting from temperature growth, they may be fluttering and touching.

4.04 bore 40 thou ring gap oxy heat video.

https://youtu.be/WoX-RjDLCPE


Have you run it with normal rings ?

No manufacturers use gapless rings, not even Motorcycle manufacturers or Sports Car manufacturers.

P.S. I've seen an engine with 20 thou gaps running with preignition to the point the pistons grew and gouged the bores and the gudgeons turned blue it kept running, would have seized or pulled a ring land off if the rings butted.

[Tom68]

Is it viable to run it with flat tops ? No compression may not be as bad as we imagine.

Black spots may just be carbon from the obvious sources.

When does the detonation occur in the rev range ?

What damage is the detonation doing ?

It's a carbon accumulator with all that dead area, I'd try running it on Propane (no carbon) if nothing else worked.

[Tom68]

It's almost got to be what's different to everything else...

Maybe the piston dome can't dump its heat into the cylinder wall via the ring in time for the next combustion stroke.

Are those fuels cooling fuels ? Methanol may be a fix.

[Ishiftem]

Interesting video! Makes a guy think. Initially I had conventional rings gapped at .028. Same deal. Total seal said it’s lean or detonating. Well it’s not lean. So the idea was open the gaps up and use a gapless ring. If the gap is too big the gapless would take care of that. So opened the gaps to .035. Same. Now .040. Same. Interesting you mentioned ring flutter. A quick search turned up radial flutter. One cause is detonation. So the ends can touch from radial ring flutter without a ton of heat. Interesting. I will add that with new plugs and a clean shut off at the stripe the plugs are dry. If I drive back and pull them, they are wet with oil. A nice light coating. The pistons also rock a fair bit at tdc due to the lopsided dome. Basically the heavy side of the piston tries to keep going at tdc. Since the heavy side is to the outside of the block, the pistons hit above the top ring on the lifter valley side on both banks and beats things up. This time around I cut back the piston above the ring on the valley side of both banks to keep the piston from screwing up the top of the bore and did the Line2line coating to hopefully keep the piston a little more stable in the bore. I don’t think the rock has anything to do with the rings but I’ve been wrong before and readily admit it! I can’t run alcohol. Rules ya know.

[Tom68]

It'd take a lot of skirt length to control that tall top, but then add the fact that skirts are barrels and it's still gunna rock a lot.

I wonder if the pistons need to be balanced on the gudgeon, when the rod tries to yank the piston down the bore on gas exchange it's gunna smash over to the heavy side.

This has a visibly offset pin but otherwise it would have been equal weighted each side of the gudgeon.

409 Piston.jpg

[Tom68]

Interesting video! Makes a guy think. Initially I had conventional rings gapped at .028. Same deal. Total seal said it’s lean or detonating. Well it’s not lean. So the idea was open the gaps up and use a gapless ring. If the gap is too big the gapless would take care of that. So opened the gaps to .035. Same. Now .040. Same. Interesting you mentioned ring flutter. A quick search turned up radial flutter. One cause is detonation. So the ends can touch from radial ring flutter without a ton of heat. Interesting. I will add that with new plugs and a clean shut off at the stripe the plugs are dry. If I drive back and pull them, they are wet with oil. A nice light coating. The pistons also rock a fair bit at tdc due to the lopsided dome. Basically the heavy side of the piston tries to keep going at tdc. Since the heavy side is to the outside of the block, the pistons hit above the top ring on the lifter valley side on both banks and beats things up. This time around I cut back the piston above the ring on the valley side of both banks to keep the piston from screwing up the top of the bore and did the Line2line coating to hopefully keep the piston a little more stable in the bore. I don’t think the rock has anything to do with the rings but I’ve been wrong before and readily admit it! I can’t run alcohol. Rules ya know.

Ring flutter could simply be caused by a loss of seal to the bore blowing the ring into the groove, a rocking piston could initiate that.

I'm not a believer in the second ring causing top ring flutter, if the second ring seals and blows the top ring in who cares, the cylinder is still sealed.

[mag2555]

It sounds to me like it’s that 1.2” distance you mentioned.
The fuel in that space comes out too late to be burned as needed with the main mass of air and fuel.

When it does light off it does so due to the cylinder temps made by the main burn process.

Since this is taking place late the result is detonation, not a burn like when the main mass gets lit off.

Not burning that crevice fuel in and of itself is a loss of power since that makes for a lean condition, and the detonation is eating up potential power also.

[Ishiftem]

It'd take a lot of skirt length to control that tall top, but then add the fact that skirts are barrels and it's still gunna rock a lot.

I wonder if the pistons need to be balanced on the gudgeon, when the rod tries to yank the piston down the bore on gas exchange it's gunna smash over to the heavy side.

This has a visibly offset pin but otherwise it would have been equal weighted each side of the gudgeon.



409 Piston.jpg

Next set I think I will have as much weight left under the non dome side. I can also extend the skirts .200 easy without counter weight interference. Maybe even offset the pins to the dome side, anything to balance it. The heavy wear is on the non dome side at tdc and dome side at bdc.

[Ishiftem]

So what if anything can I do to alleviate this ring issue. Both detonation due to the crevice volume and the ring getting cocked in the bore are both good explanations. Both could cause radial ring flutter. I took 2 grams off the dome and line2line to help stabilize the piston. I had the intake and exhaust valve faces coated in hopes it MAY keep the incoming charge cooler. Would a lighter or heavier fuel be better? Looking for some out of the box thinking. Anything to lessen this issue. I saw a big gain from c12 to q16 and I suspect most of it was less detonation due to 116 mon vs 108 mon.

[Tom Walker]

Due to the radical nature of your Pistons as far as radial clearances are concerned, I am speculating if you are having some of the same difficulties as the serious users of nitrous are having?
They seem to have similar piston damage and at times talk about crevice volume detonation.
Maybe researching what they are doing to try and control that phenomenon will help.
Good luck and please let us know if you find anything to make it live. Interesting engine you have there, they have their own set of challenges designed in..

[Baprace]

So what if anything can I do to alleviate this ring issue. Both detonation due to the crevice volume and the ring getting cocked in the bore are both good explanations. Both could cause radial ring flutter. I took 2 grams off the dome and line2line to help stabilize the piston. I had the intake and exhaust valve faces coated in hopes it MAY keep the incoming charge cooler. Would a lighter or heavier fuel be better? Looking for some out of the box thinking. Anything to lessen this issue. I saw a big gain from c12 to q16 and I suspect most of it was less detonation due to 116 mon vs 108 mon.

Has the balancer been checked to make sure 34* is 34* ? also why not back the timming down 4* and see if that helps . The change in power with the higher octane is suggesting the engine is in detonation.

[abc]

I would start with listing to it with a set of knock ears. See if you can reliably replicate the issue. Assuming you can, then start backing the timing down and see if that solves it. I also believe a cam shaft change would be in my future if I were to keep running that combo. I obviously don't know the intended use but to me it looks like it needs one or several of these: more overlap to lower dynamic compression, lower static compression, or fuel more resistant to knock. This is all stated assuming you verify it is actually detonation.
http://www.contactmagazine.com/Issue54/ ... asics.html

[Ishiftem]

Due to the radical nature of your Pistons as far as radial clearances are concerned, I am speculating if you are having some of the same difficulties as the serious users of nitrous are having?
They seem to have similar piston damage and at times talk about crevice volume detonation.
Maybe researching what they are doing to try and control that phenomenon will help.
Good luck and please let us know if you find anything to make it live. Interesting engine you have there, they have their own set of challenges designed in..

It’s a love hate relationship! I’ll look into what the nitrous guys deal with.

[Ishiftem]

So what if anything can I do to alleviate this ring issue. Both detonation due to the crevice volume and the ring getting cocked in the bore are both good explanations. Both could cause radial ring flutter. I took 2 grams off the dome and line2line to help stabilize the piston. I had the intake and exhaust valve faces coated in hopes it MAY keep the incoming charge cooler. Would a lighter or heavier fuel be better? Looking for some out of the box thinking. Anything to lessen this issue. I saw a big gain from c12 to q16 and I suspect most of it was less detonation due to 116 mon vs 108 mon.

Has the balancer been checked to make sure 34* is 34* ? also why not back the timming down 4* and see if that helps . The change in power with the higher octane is suggesting the engine is in detonation.

Detonation is definitely what I’m leaning towards. Balancer is spot on. It has its best et at 34 degrees. At 35 no change. At 36 et falls off noticeably.