Detonation: Timing Plugs Compression Heads Etc

[beanhead]

Ok, based on the stats...nothing jumps out as a real obvious cause. There's a possibility you're hitting lean for a moment at WOT, with the smaller secondary carb and your combo. I'd pull a spark plug and cut the the insulator off so you can get a real good look at the fuel ring at the very bottom. If the tune is close there should be about .100" of ring down there. Thinner than about .080", or too lightly colored and it's lean WOT for sure.
I'm slightly surprised at the 165 psi, with that cam, and quench distance..I'd think it would crank a little higher than that. My 440 + .060"--roughly the same cubes as yours--cranked 160-165, with a bigger cam, installed 4 degrees advanced, just south of a true 9.5 SCR with the pistons down .060 in the hole (and a .039 gasket) and it never pinged..and I'm down in the valley, it be hot here! Same junky CA pump 91. 850 speed demon, bigger jets than stock, same heads as yours. I ran a 750 QF carb for awhile and even dialed to the max, that engine wanted more.
The disclaimer here is, this all assumes the numbers you have numbers are accurate..and that what you're hearing is definitely detonation.

 

[68 Sport Satellite]

Thanks for that Beanhead. I also recall the dyno guy being surprised the cranking pressure was only 165 psi. I just did an online calculator and it says with all of my info I should have a cranking pressure closer to 239 psi. Wow, that seems high. Anyone have pressures that high?
EDIT - I forgot! That 165 psi was measured at the original dyno shop with my old 906 heads when I was at 9.6:1. That makes sense. I don't think they checked cranking pressure with my new heads and .027" gaskets.

Disclaimer - I'm not that knowledgeable on all of this at all. It's my first ever build and I'm just judging from consensus of what people tell me and I read.

Are your pistons flat top like mine? I'm running forged JE's with -6cc valve relief. Also curious what size carb did you end up going with instead of the 750 QF? My car is 99.9% street driven and I don't want to lose low end torque with my 727 by going too big on carb size. My QF 735 is basically a QF 750 with smaller secondaries. I wonder if it can be converted to a 750 cfm?

I too thought that with this motor it would be higher than 391 rear wheel hp. The 523 ft-lb rear wheel torque seems good though. Best ET quarter mile run with me as novice driver in 90 F summer heat was 12.80 sec out of 8 runs. Not bad, could be better with better reaction time and cooler ambient temp.

Do you think if I upped the carb size it could help with the pinging issue?
Good recommendation on checking the plugs. I have plugs I pulled a few months ago with no change in tune and I can check. I believe my motor is also set to 4 degrees advanced on the timing chain. Calculated compression ratio is 10.2:1.

 

[beanhead]

Are your pistons flat top like mine? I'm running forged JE's with -6cc valve relief. Also curious what size carb did you end up going with instead of the 750 QF?

Yes they were ICONs with I believe 6cc reliefs also. The carb was an older speed demon 850 which reportedly flowed more like a 950 holley due to rating methods (still using it on my just-commissioned 505). I've modified it over the years for adjustable air bleeds, PVCRs, etc. These were all fixed holes from the factory, but that wasn't cutting it...it's a really good working unit now.

Do you think if I upped the carb size it could help with the pinging issue? Is torque influenced at all by carb size?

I wouldn't say just going bigger would help your issue, even if it does turn out to be fuel related. Any carb needs to be dialed in to suit the combo...check through your stuff first. If you have it mixing properly, and it's still not 'enough' for the engine it might be down some HP but should't be pinging. Personally, I would use a bigger carb on your setup, but that's just me, and only from a performance standpoint. Some folks like 750s on 440-450 inch engines; I've always found that they like more if they've been upgraded with things like aftermarket heads and cams. So much depends on the combo and how you're using it. You go to the track, so.....I'd comfortably bet that your times would pick up with some more carb, once tuned.

 

[biomedtechguy]

Keep going to the track!
With that kind of power, better times and MPH are definitely available!

 

[68 Sport Satellite]

Wish I could go to track more but it’s a 2 hr drive. So only once or twice a year.

here are pics of the plugs. Not sure what you said to measure.

 

[beanhead]

Wish I could go to track more but it’s a 2 hr drive. So only once or twice a year.

here are pics of the plugs. Not sure what you said to measure.

View attachment 987393 View attachment 987394

You want to cut the threaded portion off at the base. Of course, that means a new plug, or plugs...

this isn't mine, just a pic I grabbed off the 'net but shows what I mean.

The dark ring at the very bottom of the porcelain, indicates WOT. These in the picture look to be too wide = rich, but gives you an idea.

And, not really relevant to your issue, but yours look rich at idle and close to being too hot..if you can re-curve the distributor to get 20 at idle with the same 32 total, you may get some improved response..just for future thought.

 

[beanhead]

Thanks for that Beanhead. I also recall the dyno guy being surprised the cranking pressure was only 165 psi. I just did an online calculator and it says with all of my info I should have a cranking pressure closer to 239 psi. Wow, that seems high. Anyone have pressures that high?
EDIT - I forgot! That 165 psi was measured at the original dyno shop with my old 906 heads when I was at 9.6:1. That makes sense. I don't think they checked cranking pressure with my new heads and .027" gaskets.

Well HELL I don't know how I missed this!! For SCR I got just under 10.2, which is okay, but the cam and how it's installed can throw your cylinder pressure into unfriendly territory. And oh yes, you can get carbon build-up on a low mile engine. Your old iron heads *most likely* were nowhere near your new heads in chamber size, as in they were bigger = less compression.
Do a cranking compression test; its easy. I don't know all the criteria you used but a cranking psi of 2-anything is way too much on our pump gas. If it was actually close to 239 I'd be shocked you only got a little WOT ping in hot weather, I'd think it would rattle anytime you got on the throttle...!
If you aren't comfortable testing or don't have the stuff, bug the **** out of Greg to help you if he's in your neck of the woods

 

[68 Sport Satellite]

My buddy that lives nearby has cranking compression of 200 psi with 10.2:1 and it never pings on 91 octane.

I will investigate further, thanks!

 

[68 Sport Satellite]

You want to cut the threaded portion off at the base. Of course, that means a new plug, or plugs...

this isn't mine, just a pic I grabbed off the 'net but shows what I mean.
View attachment 987408
The dark ring at the very bottom of the porcelain, indicates WOT. These in the picture look to be too wide = rich, but gives you an idea.

And, not really relevant to your issue, but yours look rich at idle and close to being too hot..if you can re-curve the distributor to get 20 at idle with the same 32 total, you may get some improved response..just for future thought.

I tried before to advance initial timing but the motor won’t start if it’s above 18. I have it set at 15 initial now but have a duplicate dizzy set at 17/32 that I can swap in.

 

[beanhead]

I tried before to advance initial timing but the motor won’t start if it’s above 18. I have it set at 15 initial now but have a duplicate dizzy set at 17/32 that I can swap in.

Ahh okay. Yes it needs to be able to turn over! Honestly if 18's the limit that may be an indicator of too much pressure..

 

[68 Sport Satellite]

Ahh okay. Yes it needs to be able to turn over! Honestly if 18's the limit that may be an indicator of too much pressure..

Good stuff. I didn’t know that could be a sign. My cam is very mild and should have no trouble up to 22 I have heard. So in that case maybe bumping my head gasket from .027” to .038” to bring it from 10.2:1 to 10.0:1 could help? I am also looking into snow performance water/alcohol injection.

 

[beanhead]

Good stuff. I didn’t know that could be a sign. My cam is very mild and should have no trouble up to 22 I have heard. So in that case maybe bumping my head gasket from .027” to .038” to bring it from 10.2:1 to 10.0:1 could help? I am also looking into snow performance water/alcohol injection.

Well, if you threw on some good 'ol Fel-Pro 8519s it should lower you to just under 9.9, but you then lose any quench benefit. And at this point, your good quench might be what's saving you from rattling more than you are. I'm getting the feeling even with an 'okay' compression ratio the cranking psi is too high, and that's what gets you in trouble..the pressure created during operation. Static compression doesn't take the cam and pressure into account, just the physical build specs that you've already punched into the calculator. But I could be wrong! So, the only way to know is to test it. Remember, milder cams make more pressure...(more pressure but less power, LoL go figure!) so after you know for sure what the cranking pressure is, if it's too high honestly I would look into a cam swap before all the extra injection stuff, or maybe just back off the cam timing a few degrees. It would be nice to know if/where it was degreed in.

 

[Cranky]

@Moparfeind did you ever do a cranking PSI check? If you did, I didn't see it....

 

[BSB67]

Good grief, just do a cylinder pressure test.

 

[Moparfiend]

@Moparfeind did you ever do a cranking PSI check? If you did, I didn't see it....

No I have not done it. I seemed to have been able to tune out the detonation. The rear gears 3:55 helped as well. I have since started the bodywork and not touched the motor in months. I probably will run it and evaluate it more after I get the car back together.

 

[Cranky]

No I have not done it. I seemed to have been able to tune out the detonation. The rear gears 3:55 helped as well. I have since started the bodywork and not touched the motor in months. I probably will run it and evaluate it more after I get the car back together.

Was just curious what it was. You said it had what you called a radical cam in it too, right?

 

[wyrmrider]

do you have a good mini starter or a 4 field big starter = are you cranking slow
did you degree the cam
you could run it straight up and see what difference that makes
wasting time here without compression numbers

 

[68 Sport Satellite]

thanks to all for your input on my pinging issue. I don't have access to my buddy's shop any longer where we did a lot of work on this car (he moved) and my home tools are limited (as well as my current time). I was just researching here to learn how to find out the issue. Once I get my trans cooler leak addressed and get the compression check gauge and some free time, I'll check compression and report back. I'm also verifying with my engine builder if my cam was indeed installed a few degrees advanced. I didn't realize if it was and my cranking pressure is too high it could cause this issue. Big thanks for all of the input.

 

[68 Sport Satellite]

I got word back from my engine builder for the install of my Lunati Voodoo 60302 cam. It was installed straight up, but Lunati builds some advance into the cam profile.

 

[Cranky]

Lots of cam mans do that....but is it enough for what you are doing and your combo? I have no idea what Lunati does but have some experience with other main stream cams makers.....