My '69 BB has straight chambered side exhaust (Allen's) which is pretty loud under hard acceleration. So loud that if the engine was pinging it's doubtful if I'd be able to hear it. I was wondering if there's some way to verify whether or not detonation is occurring?

 

Install a air fuel ratio sensor

 

Although an A/F ratio would show a rich or lean condition leading to pinging, it would not show a way too advanced IGN Timing event. Such as the case of an over-powering vacuum advance canister on the distributor.

Having the skills to read plugs and know what the signs are will be your best bet.

I have noticed pinging is more pronounced in the header tubes as opposed to cast iron manifolds that muffle the sound.

So, what is your Total Timing w/ Vac hooked up and 3,000 rpms?

 

I haven't measured it with the vac. adv. connected. It's timed per Lars' "How To Set Timing" article. It doesn't hit his recommended numbers exactly. It's currently set to 19 deg. initial, 35 deg. total all in @ ~3,000 RPM with limiter bushing installed and vac. adv. disconnected.

AFAIK it has a stock L71 bottom end with "990", 118cc heads that should, I believe, drop the CR to ~10:1.

 

I’ve never used one, but I believe there are standalone knock sensors out there. I think MSD makes one.

 

Looking at the porcelain on the plug if you see black specks on the white that is evidence of detonation. The black specs are bits of aluminum from the piston.

Eroded away ground strap is advanced detonation.

Both those with cracked porcelain is a sure sigh as well.

Light detonation at cruise with light throttle is not uncommon and acceptable.

I haven't seen an aftermarket detonation detection system on the market for several years, not sure where to find one now.

 

MSD USED to make one many years ago. I still have one installed in my C3 with it's 427 running 12.25-1 compression. I used it because with headers and a hearing loss I couldn't differentiate without some form of help. The MSD unit I had used a **** to adjust it to the proper noise range for your particular engine. They sold the device with a general purpose sending unit and you had to adjust it while driving. The various sensors available are designed for specific dynamic ranges. The ranges vary from engine to engine and it's design.

I have Allen's Chambered Exhaust connected to Headman Header and with all the noise out of the back I can't tell. It sounds Good however just out behind me...

J&S Electronics makes KNOCK Retard Gauges. They offer a variety of useful items for anyone running high compression, turbo or supercharger. They make some great items! They are online at: http://www.jandssafeguard.com/safeguard.html

On my C4 with it's L98 the knock sensor is simple to test, while idling with the hood open just tap an engine's cylinder head (gently) with a wrench or ball peen hammer while running and the idle speed should drop for a short period. With a bad knock sensor you loose a lot of timing and it won't come back until you replace the sensor. When I bought my 1988 C4 (used) the knock sensor was bad and the performance loss was significant

 

OK, so you have 35 total no vac. Assuming the vac can adds another 10-12 you should be alright. But if it adds more and you creep up on 50-52-54 degrees total, that could be your pinging. Of course, that will vary under load, mid-throttle cruising.

To limit vac canisters maximum advance, you can purchase one with lower max. Purchase one that is adjustable. Or, install a limit bracket for the canister arm.

To test what the canister might be doing, just watch your timing light marks with vac plugged off, then hooked up to full manifold vac. The difference in the two results is what the canister is adding. Too much vac adv causes "trailer hitching". Feels like the car is pushing you then pulling you.

Also, which octane gets your money? Nothing but premium at 10:1. CR.

 

Here in California 91 octane is the highest available at the pump. 100 is obtainable if you're willing to travel a ways to get it and pay $10/gal. for it.

 

the vacuum can is supposed to add 0 degrees under heavy throttle. and i think people have full advance coming in too low also. a lot of distributors show full advance way before 3000 rpm.it would be nice if there was an affordable electronic ignition that controlled ignition like efi. with MAP (aka vacuum), throttle position and rpm. with maybe a knock sensor thrown in...

 

You could also wear a pair of ear plugs. They will cut the volume of the exhaust and you can still hear the pinging or rattle of detonation. The audio frequency of detonation is much higher.

 

Derek, have you seen these? Built in MAP, programmable curve with smart phone app, not exactly cheap though.

​​​​​​https://progressionignition.com/shop...ap-distributor

 

I'm surprised I couldn't quickly find an Arduino or Raspberry Pi based solution. Megasquirt has a module, out of stock. I think a Holley Terminator X system will detect pinging and pull timing. But both of those are full EFI systems that also do ignition timing. Perhaps a knock sensor and an oscilloscope?

 

You need something like this with a visual alert.

https://tunertools.com/products/link...20499021594688

 

That's perfect! I'd love to see what's inside.

 

there was an old thread with a youtube vid
where a guy made a knock system with a couple cheap parts.
a dist with the common module and when it knocked it pulled timing

sorry I can't remember any search terms

 

The J&S one mentioned in post 7 uses a bosch knock sensor,retards timing, works with points or MSD too. for $89.
What more could you want?

 

Sorry, the $89 gauge is just the display. You also need the SafeGuard knock controller. I'm the designer.

 

My guess is with 19° initial advance, that's part of your problem. That means the adv. is never going to be below 19°. I'd take at least 5° out of that and ideally get it down to 10-12°. The 35° total is marginal with iron heads on 91-oct fuel.

You also need to install an adjustable vac. adv. and limit it to 8-to-10 degrees.

You need to limit the centrifugal advance, too. Now that is going to be tough if you are using a stock distributor. When I used a Delco dist., I used to disassemble it to remove the distributor cam, fill in the limit bushing slot with epoxy then, using a distributor machine to check advance, file the epoxy just to where I had the appropriate amount of adv. which, in your case, would be a limiter slot that holds the centrifugal adv. to 22-24°. Set your initial at 12° and add your adj. vac. adv limited to 8-10 deg. That's the set-up I used before I want to an aftermarket computer-controlled ignition computer, an MSD ignition box and an MSD distributor.

Also, make sure your center carb tuning at part throttle is not too lean because that also can make the engine more susceptible to detonation.

Those "990" heads were service replacement rec. port heads for 454s. Since they are cast iron, another reason your engine might be prone to detonation is your 10:1 CR. On 91-oct fuel, you're better off down around 9.2 or 9.5 because, with an iron head BBC, it's better to be able to run a healthy advance curve rather than high compression when fuel quality is compromised.

With headers, it really tough to hear detonation, so you need to get a knock sensor. Reading plugs can find detonation, too, but, because you're relying on the appearance of molten aluminum on the plug tips to "see" detonation, knock is happening way before it starts softening piston tops. In lieu of a KS, you can go more conservative on your spark curve and hope for the best.

Resist the temptation to use octane boosters. The vast majority of them use MMT as the active ingredient and long-term, regular use of MMT boosters tends to cause metallic deposits on plug tips, piston tops, exhaust valve faces and combustion chamber walls. The best choice, if you are going to increase octane, is to mix 2:1 pump 91 to 100 octane unleaded which will get you to about 94-oct. Given tuning of the three main circuits that's spot-on, that ought to keep you out of detonation with an aggressive spark curve and 10:1 CR even if you get some heat into the combustion chambers with a longer pull at WOT.